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TRANSCRIPT
WhitakerAudio
Stereo Audio Preamplifier
User and Assembly Manual
Copyright 2013 WhitakerAudio LLC, Morgan Hill, California, USA.
No part of this document may be reproduced without the express written consent of
WhitakerAudio.
Specifications subject to change without notice.
Any trademarks used in the manual are the property of their respective owners.
Note:
This document is intended to assist readers in building an audio product for personal use. No
warranties are expressed or implied. Always use good engineering practices and appropriate
safety precautions.
Table of Contents
1 Circuit Description 82 Parts List 123 PWB Design 174 Construction Techniques 215 Step-by-Step Instructions 23
5.1Assembly of Right Channel PWB 245.2Assembly of the Left Channel PWB 365.3Chassis and Final Assembly 45
5.3.1Front Panel Components 455.3.2Back Panel Components 495.3.3Cable Clamps and Related Hardware 515.3.4Install the PWBs 515.3.5Back Panel Audio Connections 525.3.6Front Panel Connections 605.3.7Install Interconnecting Wiring 705.3.8Final Assembly Check 74
6 Initial Checkout 766.1Functional Tests 806.2Setting Operating Levels 816.3Important Operational Note 83
7 Performance Measurement 847.1RIAA Equalization 877.2Final Touches 88
8 Troubleshooting Guidelines 919 Safety Considerations 9210 Notes and Tube Data 93
10.1Tube Characteristics 9310.1.1 6EU7 9310.1.2 5879 9410.1.3 7025 (12AX7) 97
Stereo Audio Preamplifier
The Stereo Preamplifier is a high-quality device intended for discriminating listening. The
product features quality components throughout. The preamp is available either as a kit or
assembled as a part of the signature series of WhitakerAudio products. This product shares the
craftsmanship and impressive industrial design of the other offerings in the WhitakerAudio line.
This unit, like the other products, is limited in number
The WhitakerAudio high-quality classic vacuum tube based preamplifier provides five
available inputs:
• Phonograph
• Microphone (high impedance)
• Tuner (100 kΩ input impedance)
• Auxiliary (100 kΩ input impedance)
• Front panel auxiliary (100 kΩ input impedance)
An input switch selects the desired source, which is fed to a tone-control stage and finally to
an output buffer amplifier. An isolated “Tape Out” connector is provided for side-chain
applications, such as recording on audio tape or on a computer. The Tape Out port is connected
prior to the tone control stage.
WhitakerAudio
The “Front Aux” input position connects to a front-panel jack for connection of a personal
music device to the preamp.
No power supply is included in the design, as the assumption is that this unit will be paired
with a separate power amplifier, such as the 20 W stereo amplifier or 40 W stereo power
amplifier. A multi-pin connector ties the power supply of the amplifier to the preamp chassis.
This has the benefit of reducing cost and at the same time reducing noise from the supply that
could be picked up by the low-level preamp circuits.
The stereo preamplifier is built around two printed wiring boards (PWBs), engineered for top
performance. A ground plane covers the component side of the PWB. Component placement and
board traces have been engineered to minimize hum and noise. The boards feature a unique
mounting technique for the five vacuum tubes used in each channel that keeps heat away from the
PWB and minimizes hum in the amplifier. The PWBs are engineered to reduce off-board
connections, thereby simplifying layout and providing for controlled characteristics from one unit
to the next.
6
Stereo Audio Preamplifier
The phonograph circuit uses a 7025 (12AX7) tube as the active stage. The microphone circuit
uses a 5879 tube. The tone control stage is built around a 6EU7. The output buffer amplifiers are
7025 tubes.
This product is offered in both kit form and completely wired and ready to go. This document
provides a technical description of the product and complete assembly instructions. The user
manual is the same for both kit and assembled products.
Physical dimensions: 19-in wide by 12-in deep by 6-in high. Note that the depth specification
does not include back panel cables. Weight approximately 16 lbs.
This product has been designed and built for discriminating consumers, using classic
technologies and new components. Beyond the great specs, the preamplifier sounds very nice.
Clean. Warm. Interesting. Like a tube amp should sound.
The preamplifier generates little heat and (for a tube amplifier) consumes little power. Turn it
on and let it run. Put on some music and enjoy audio as it should be heard.
The stereo preamplifier shown with the companion 20 W stereo audio amplifier.
7
WhitakerAudio
1 Circuit Description
A schematic diagram of one channel of the preamplifier is shown in Figure 1.1. Two such circuits
are used for the stereo preamplifier.
The phono preamp is a two-stage high-gain circuit intended for use with a magnetic
phonograph pickup device. A nominal load of 47 kΩ is assumed1. The 7025 twin triode features
low hum and noise, and is designed specifically for use in high-fidelity circuits that operate at low
signal levels. Typical voltage gain of this stage is in excess of 130.
The audio signal from the phono pickup is applied to the grid of V1a via the rear panel RCA
connector. Interstage coupling between the two phono amplifier sections includes an RIAA
equalization network consisting of C3/R6, C6/R10, C7, and C8. The output of the preamp is
coupled from the plate of the second stage by coupling capacitor C9 to the input of the tone
control amplifier through output level control R16 and input switch SW1. Capacitor CP is used to
optimize the high frequency response of the circuit if needed. This is a “select on test”
component. A typical value would be 20 pF, if used.
The microphone preamp stage (V2) is intended for use with high-impedance dynamic
microphones. Optionally, an external impedance transformer may be used to interface a low-
impedance microphone. The circuit uses a 5879 low-noise sharp-cutoff pentode in a conventional
amplifier circuit with high voltage gain (approximately 60). The output of V2 is coupled through
C10 to output level control R17 and to input switch SW1.
The Tuner and Auxiliary inputs are routed through level control potentiometers R18 and R19,
respectively, to input switch SW1. The level-adjust devices are located internal to the
preamplifier.
The tone-control amplifier (V3) uses a 6EU7 low-noise twin triode in a two-stage cascade that
consists of an input cathode-follower connected to a triode voltage amplifier through a frequency-
sensitive (tone control) interstate coupling network. The network consists of the following
components:
• R25, R26, R27, R28, and R30
• C16, C18, C19, C21, and C22
1. This value can be modified if needed by changing R1.
8
9
Stereo Audio Preamplifier
Phon
ogra
phin
put
0 V
V1, 7
025
(12A
X7)
C2
C5
C1
C4
C7
C6
C9
C8
C3
R1
R2R3
R4
R5
R9
R6
R10
R8
R7
+275
V
+260
V+2
35 V
+175
V
+1.5
V+1
.5 V0 V
++
++
1
2
3
45
6
7
8
9+195
V
Mic
roph
one
Inpu
t
V2, 5
879
+250
V
+110
V
+70
V0
V
+1.7
5 V
R11
R12
R13
R14
R15
C10 C
11C
12C13
+
+
1
37
8
9
4
5
Out
put
+245
V
0 V +1 V
+140
V
+140
V
C26
+
C23
C25
C24
R33 R
34
R35
R37
R36
R38
V4, 7
025
(12A
X7)
16
3
4
59
R32
Volu
me
Con
trol
Tune
rIn
put
Auxi
liary
Inpu
t
Prea
mp
inpu
t: P
hono
Mic
T
uner
Aux
Fro
nt A
ux
R16
R17
R18
R19
SW1a
/b
V3, 6
EU7
+250
V+2
20 V
+140
V
+12
V
+1 V
+17
V
0 V
C14
C15
C16
C17 C18
C19
C20
C21
C22
R20
R21
R22
R23
R31
R25 R27
R28
R29
R30
Treb
leC
ontr
ol
R26
Bas
sC
ontr
ol
1
2
45
67
8
9
R24
++
CP
1
C27
+C
28
+27 8
Fron
t Aux
Inpu
t
+27 8
Tape
Out
+245
V
0 V +1 V
+140
V
+140
V
C29
+
C30
C32
C31
R43 R44
R45
R39
R46
R41
V5, 7
025
(12A
X7)
16
3
4
59
R42
Hum
Bal
ance
To h
eate
rco
nnec
tions
of
V1 –
V5
23 4
Gro
und6.3
V he
ater
R40
Line
Out
Leve
l
H H
SW2
To m
icin
put o
not
her c
hann
el
Mic
mix
D1
D2
R47
C33
CR
-1
C28
+
ac in
ac in
—
+
Inpu
t Pre
amlif
iers
Tone
Con
trol A
mpl
ifier
Out
put B
uffe
r
Opt
iona
l6.
3 V
dc
reci
fier
outp
ut
6.3
V he
ater
Figure 1.1 Schematic diagram of the preamplifier.
WhitakerAudio
The bass and treble controls in the coupling network can be adjusted to provide for up to
approximately 15 dB of boost or attenuation (cut) at 20 Hz and 20 kHz. With the bass and treble
controls set at the mid-range positions, the amplifier provides a reasonably flat response curve and
a nominal voltage gain of 3.
A two-stage buffer amplifier in the form of V4 is used to transform the high-impedance output
of the tone-control circuit to a lower impedance that is less susceptible to capacitive loading from
interconnecting cables, thereby simplifying the connection to external devices (usually an audio
power amplifier). The first stage of the 7025/12AX7 is a conventional voltage amplifier, with the
plate connected directly to the grid of the second stage. The cathode-follower output is taken
across a 100 kΩ load. Voltage gain is nominally 30.
The V4 buffer stage is duplicated in V5 and serves as a buffer amplifier for the Tape Out jack.
This stage separates the two signal paths and ensures that actions on one will not impact the other.
The input B+ voltage is filtered and reduced in potential as needed for the various circuits by
the following components:
• R4 and C2, and R9 and C5 for the phono preamp
• R15 and C13 for the mic preamp
• R23 and C15, and R29 and C17 for the tone-control circuit
• R38 and C26 for the output buffer amplifier
Volume Input Power Bass TrebelPhonograph
Microphone
Tuner
Auxiliary
Front Aux
Stereo Preamp J C Whitaker 2012
Front Aux
Figure 1.2 Preamplifier front panel.
10
Stereo Audio Preamplifier
• R41 and C29 for the Tape Out buffer amplifier
• C27 and C28 on the B+ input line, with bleeder resistor R47
Typical operating voltages are shown in Figure 1.1 for a maximum input B+ of 275 V dc.
Zener diodes D1 and D2 limit the B+ input voltage to 400 V dc. These devices are intended to
protect the preamplifier against excessive voltage input due to a power supply problem. Note that
the range of acceptable input power supply voltages is 250 V ± 25 V.
The circuit boards are designed to accommodate either ac or dc heater voltages. To minimize
hum, a dc heater supply is used in the stereo preamplifier. The external heater supply of 6.3 V ac
is applied to a chassis-mounted bridge rectifier and filtered by a large-value electrolytic capacitor.
The 6.3 V dc (approximate) supply is then distributed to both PWBs and all heaters. The Hum
Balance control (R42) provides a means to vary the resistance to ground of each leg of the heater
supply. Note that only one Hum Balance control may be installed on a stereo circuit.
It is important to remember that a capacitor-input dc power supply, such as that used for the
heater circuit, will rise in output as the load is reduced. For this reason, do not operating the
preamplifier with any of the tubes removed as this may allow the rectifier circuit output to
increase beyond the recommended 6.3 V dc.
The front panel controls are shown in Figure 1.2. Back panel connections are shown in Figure
1.3.
Right Left Right Left Right Left Right Left Right Left Right Left
Auxiliary Power ConnectorDanger: High Voltages
Do Not Probe
WhitakerAudio Stereo PreamplifierBuilt in California, USA.Phonograph Microphone Tuner Auxiliary Tape Output Line Output
MicMix
StereoMic
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
TE shell 17
Center
0.15-in
1.12-in
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix Combo
0.88-in
0.13-in
Center
Neutrix Combo
0.88-in
0.13-in
Center
Figure 1.3 Preamplifier rear panel.
11
WhitakerAudio
2 Parts List
A complete parts list for the stereo preamplifier is given in Table 2.1. The table includes
manufacturer part numbers and stock numbers from one of the major parts houses2. These parts
are, of course, available from a number of manufactures and suppliers. The details given here are
for the convenience of the user. Substitutions of certain components may be made based on
availability..
2. Allied Electronics, 7151 Jack Newell Blvd. S., Fort Worth, Texas, 76118, USA,. www.alliedelec.com.
Table 2.1 Parts List for Stereo PreamplifierPart Description Quantity Manufacturer Part No. Allied
Stock No.
C1, C4, C11, C24, C31
25 μF, 50 V, electrolytic
10 Vishay TVA1306-E3 507-1089
C2, C5, C15, C17, C26, C28, C29
22 μF, 450 V, electrolytic
14 Illinois Capacitor 226TTA450M 613-0367
C3, C16 0.1 μF, 400 V 4 Vishay 225P10494XD3 507-0247
C6 0.0033 μF, 600 V 2 Vishay 715P33256JD3 507-0337
C7 0.01 μF, 400 V 2 Vishay 225P10394XD3 507-0642
C8 180 pF, 500 V, mica 2 Cornell-Dubilier CD15FD181JO3F 862-0547
C9, C12, C20, C25, C32
0.22 μF, 400 V 10 Vishay 225P22494YD3 507-0251
C10, C14, C33
0.047 μF, 400 V 6 Vishay 715P47354LD3 507-0342
C13 47 μF, 450 V, electrolytic
2 Illinois Capacitor 476TTA450M 613-0369
C18, C22 0.0022 μF, 600 V 4 Vishay 715P22256JD3 507-0332
C19 0.022 μF, 400 V 2 Vishay 225P22394XD3 507-0643
C21 220 pF, 600 V, ceramic
2 Vishay 562R5GAT22 507-0815
C23, C30 100 pF, 1000 V disc 4 Vishay 561R10TCCT10 507-0889
C27 0.01 μF disc, 1 kV 2 Vishay 562R5GAS10 507-0721
C28 4700 μF, 50 V, electrolytic
1 Illinois Capacitor 478TTA050M 70112239
CP 25 pF, 1000 V ceramic
2 Vishay 561R10TCCQ25 507-0725
R1 47 kΩ, 0.5 W 2 Ohmite OL4735E 296-4788
R2, R7 2.7 kΩ, 0.5 W 4 Ohmite OL2725E 296-6170
12
Stereo Audio Preamplifier
R3, R5, R12, R25, R28, R31, R35, R37, R39, R45
100 kΩ, 0.5 W 20 Ohmite OL1045E 296-4773
R4 39 kΩ, 0.5 W 2 Ohmite OL3935E 296-4784
R6, R14, R20
470 kΩ, 0.5 W 6 Ohmite OL4745E 296-4789
R8 680 kΩ, 0.5 W 2 Tyco LR1F680K 437-0431
R9, R15, R29, R38, R41
15 kΩ, 2 W 10 Ohmite OY153KE 296-2333
R10, R23 22 kΩ, 0.5 W 4 Ohmite OL2235E 296-4779
R11 2.2 mΩ, 0.5 W 2 Ohmite OL2255E 296-6466
R13, R24, R36, R46
1 kΩ, 0.5 W 8 Ohmite OL1025E 296-4772
R16, R17, R40
250 kΩ potentiometer, linear
6 Honeywell/Clarostat 308N250K 70153217
R18, R19 100 kΩ potentiometer, linear
4 Honeywell/Clarostat 308NPC100K 70153229
R21 1.5 kΩ, 0.5 W 2 Ohmite OL1525E 296-4774
R22 15 kΩ, 0.5 W 2 Ohmite OL1535E 296-4775
R26, R30 1 mΩ potentiometer, audio taper, dual gang
2 Precision Electronic Components
KKA1051S28-ND DigiKey1
R32 250 kΩ potentiometer, audio taper, dual gang
1 Precision Electronic Components
KKA2541S28 DigiKey
R27 10 kΩ, 0.5 W 2 Ohmite OL1035E 296-1483
R33, R34, R43, R44
220 kΩ, 0.5 W 8 Ohmite OL2245E 296-4780
R42 100 Ω, 1 W, potentiometer3
1 Honeywell/Clarostat 381L100 753-8428
R47 100 kΩ, 2W 2 Ohmite OY104KE 296-2322
D1, D2 Zener, 200 V, 5 W 4 ON Semiconductor 1N5388BG Newark
CR-1 Bridge rectifier, 25 A, 50 V
1 Vishay 26MB05A 70078708
SW1 Rotary switch 1 Electroswitch C4D0206N-A 747-6690
SW2 Toggle switch 1 Cannon 7101SYZQE 676-3000
Table 2.1 Parts List for Stereo Preamplifier
13
WhitakerAudio
4 terminal barrier strip
2 Molex 38720-6204 607-0071
V1, V4, V5 7025/12AX7 tube 6 Electro-Harmonix, Genalex, others
Tube Depot2
V2 5879 tube 2 NOS Tube Depot
V3 6EU7 tube 2 Sovtek, others Tube Depot
1/2-inch #6 threaded standoff
4 Keystone Electronics 2210 70181896
3/4-inch #4 threaded standoff
20 Keystone Electronics 1895 839-0781
9-pin tube socket 10 Belton P-ST9-601 AES3
Main chassis, 10-in x 17-in x 3-in, steel
1 Hammond 1441-32BK3 806-0534
Main chassis baseplate, 10-in x 17 in, steel
1 Hammond 1431-30BK3 806-0544
Front panel, 19-in x 3.5-in
1 Front Panel Express custom
Rear panel overlay 3 FastFrame custom
Front panel handles 2 RAF 8128-832-A-24 219-8007
Chassis feet 4 Bud F-7264-A 736-7264
Large knob 4 Davies 1110 543-1110
Handle, side 2 Bud H-9174-B 736-4374
RCA connector 10 Neutrik NF2D-B-0 514-0004
Mic connector 2 Neutrix NJ3FP6C-BAG 70088266
3.5 mm connector 1 Switchcraft 142AX Mouser
Power connector, pin
1 Tyco 211767-1 512-8561
Power connector, receptacle
1 Tyco 211766-1 512-8869
Power connector, shell
1 Tyco 206070-8 374-1260
Power connector, contact pin
10 Tyco 1-66099-5 374-1075
Power connector, contact socket
10 Tyco 1-66101-9 374-1076
12 terminal Molex header
4 Molex 22-03-2121 863-0352
12 terminal Molex housing
4 Molex 22-01-2127 863-0334
Table 2.1 Parts List for Stereo Preamplifier
14
Stereo Audio Preamplifier
Consumables for the stereo preamp are listed in Table 2.2.
4 terminal Molex header
16 Molex 22-03-2041 863-0309
4 terminal Molex housing
16 Molex 22-01-2041 863-0424
Molex crimp terminal
128 Molex 08-50-0114 863-0292
Pilot light 1 Dialight 249-7841-1431-574 511-0276
PWB 2 ExpressPCB custom
Top chassis decal 1 custom
Plexiglas overlay 1 TAP Plastics custom
9-pin tube socket shield, black
4 Antique Electronic Supply
KU:P-SS9-325-BK
Notes:
1 Digi-Key Electronics (www.digikey.com).
2 Vacuum tubes are available from a number of suppliers, including: Tube Depot (www.tubedepot.com), The Tube Store (www.thetubestore.com), Tube World (www.tubeworld.com), and other suppliers.
3 Antique Electronic Supply (www.tubesandmore.com/).
Table 2.2 Consumables for the 20 W Stereo AmplifierQuantity Description
6 ft Hookup wire, #18 black, solid
2 ft Hookup wire, #18 green, solid
2 ft Hookup wire, #22 black, solid
20 ft Hookup wire, #22 green, solid
2 ft Hookup wire, #22 red, solid
30 ft Audio cable, 1 conductor shielded, stranded
2 ft Audio cable, 2 conductor shielded, stranded
10 ft Audio cable, 3 conductor shielded, stranded
2 ft Expandable sleeving
4 ft Heat-shrink tubing, small
1 ft Heat shrink tubing, large
10 Spade terminal, #6 screw, #22 wire
4 Quick-disconnect terminals, female, #18 wire
1 Ground lug terminal, #6 screw, 3/4-inch
68 Phillips head screw, #4, 3/8-inch
28 Nut, #4 with captive lock washer
16 Lockwasher, #4
4 Phillips head screw, #6, 3/8-inch
5 Phillips head screw, #6, 3/4-inch
Table 2.1 Parts List for Stereo Preamplifier
15
WhitakerAudio
Because of the large number of interconnecting wires needed for this preamplifier, expanded
sleeving is used where practical to organize the cabling. This tends to simplify cable routing and
provides for a cleaner appearance. Wherever possible, cables should be dressed either tight to the
chassis, or well above the chassis so as to stay as far as possible away from signal-carrying lines
and to rest against the bottom cover plate, which further improves shielding.
1 Nut, #6, fiber locking
1 Wing nut, #6
10 Flat washer, #6
4 Lockwasher, #6
4 Standoff, #6 female-female, aluminum
4 Sheet metal screw, Phillips head, #6
4 Standoff, chrome, 0.75-in. tall, 0.62-in. outside diameter, 0.32-in. inside diameter
12 Phillips head screw, #8, 3/8-inch
1 Phillips head screw, #8, 5/8-inch
5 Nut, #8 with captive lock washer
8 Lockwasher, #8
11 Tie wrap, small
10 ft Rosin core solder
4 Cable clamp, 1/2-inch
1 Paint, black, hammered
3 ft Hookup wire, #16 stranded, red (for external power supply cable)
6 ft Hookup wire, #16 stranded, black (for external power supply cable)
6 ft Hookup wire, #16 stranded, white (for external power supply cable)
6 ft Hookup wire, #16 stranded, green (for external power supply cable)
3 ft Large expandable sleeving (for external power supply cable)
6 inch Large heat shrink tubing (for external power supply cable)
Table 2.2 Consumables for the 20 W Stereo Amplifier
16
Stereo Audio Preamplifier
3 PWB Design
The PWB implementation incorporates all components for each channel, except for the front
panel controls and the rear panel connectors. The overall component layout is shown in Figure
3.1. A ground plane is used on the component side of the board to reduce hum and stray noise.
Input level control potentiometers R16, R17, R18, and R19 are mounted on the board and
accessible from underneath the chassis. These controls are typically adjusted during initial setup
and are not changed thereafter. The vacuum tubes are mounted on the chassis and connected with
short jumpers to the PWB. Connections from the boards to the chassis-mounted components are
accomplished with removable connectors.
Figure 3.1 Component layout for the preamplifier PWB.
17
WhitakerAudio
The wire connection codes from the PWB to the chassis components are given in Table 3.1
Table 3.1 PWB Connection Wiring CodesConnector # PWB Code Function Connection Color Code
Conn1 GND Phonograph input ground Shielded cable, shield
PHONO Phonograph input Shielded cable, white
GND Phonograph input ground
GND Tuner input ground Shielded cable, shield
TUNER Tuner input Shielded cable, white
GND Tuner input ground
GND Auxiliary input ground Shielded cable, shield
AUX Auxiliary input Shielded cable, white
GND Auxiliary input ground
GND Microphone input ground Shielded cable, shield
MIC Microphone input Shielded cable, white
GND Microphone input ground
Conn2 GND Tape Out ground Shielded cable, shield
TAPE2 Tape Output to back panel
Shielded cable, white
GND Tape Out ground
GND Ground
Conn3 GND Phono preamp output ground
Shielded cable, shield
PH-OUT Phono preamp output to SW1
Shielded cable, white
GND Phono preamp output ground
GND Ground
Conn4 GND Tuner output ground Shielded cable, shield
TUN-OUT Tuner output to SW1 Shielded cable, white
GND Tuner output ground
GND Ground
Conn5 GND Auxiliary output ground Shielded cable, shield
AUX-OUT Auxiliary output to SW1 Shielded cable, white
GND Auxiliary output ground
GND Ground
18
Stereo Audio Preamplifier
Conn6 GND Microphone preamp output ground
Shielded cable, shield
MIC-OUT Microphone preamp output to SW1
Shielded cable, white
GND Microphone preamp ground
GND Ground
Conn7 GND Tape Out ground Shielded cable, shield
TAPE1 Tape Out input from SW1 Shielded cable, white
GND Tape Out ground
GND Ground
H 6.3 V ac heater to tube sockets
Green
H 6.3 V ac heater to tube sockets
Green
Conn8 GND Tone control stage ground
Shielded cable, shield
INPUT Tone control stage input from SW1
Shielded cable, white
GND Tone control stage ground
GND Ground
Conn9 R26a High terminal of R26 Shielded cable, red
R26arm R26 wiper Shielded cable, white
R26b Low terminal of R26 Shielded cable, black
GND Shield for R26 cable Shielded cable, drain wire
R30a High terminal of R30 Shielded cable, red
R30arm R30 wiper Shielded cable, white
R30b Low terminal of R30 Shielded cable, black
GND Shield for R30 cable Shielded cable, drain wire
R32a High terminal of R32 Shielded cable, red
R32arm R32 wiper Shielded cable, white
R32b Low terminal of R32 Shielded cable, black
GND Shield for R32 cable Shielded cable, drain wire
Conn10 GND Buffer amplifier output ground
Shielded cable, shield
BUF-OUT Buffer amplifier output to back panel
Shielded cable, white
GND Buffer amplifier ground
GND Buffer amplifier ground
Table 3.1 PWB Connection Wiring Codes
19
WhitakerAudio
The PWB is connected to the chassis through the mounting holes for the vacuum tube sockets.
The tube sockets are conventional chassis-mounted devices, with jumper leads extending from the
active pins to the PWB. The sockets are held away from the PWB using standoffs. This approach
permits the sockets to be firmly mounted on the chassis, rather than being physically supported by
the PWB. This mounting method also takes advantage of the shielding capability of the steel
chassis.
Heater connections to the vacuum tubes are made directly on the sockets using
interconnecting wire tightly twisted together and dressed against the chassis to minimize hum.
The pinout for the back panel power connector is given in Table 3.2.
One channel of the stereo preamplifier is shown in Figure 3.2.
Table 3.2 Back Panel Power Connector PinoutPin No. Function Connects To Notes
1 Ground Terminal 2 of PWB (black)
2 Pilot lamp PL1 115 V ac (white)
3 Pilot lamp PL1 115 V ac (white)
4 Auxiliary B+ Terminal 1 of PWB Approximately +275 V dc (red)
5 Filament Heater 6.3 V ac for heaters (green)
6 Filament Heater 6.3 V ac for heaters (green)
7 Not used
8 Not used
9 Ground Chassis ground (black)
Figure 3.2 The right channel preamplifier PWB.
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Stereo Audio Preamplifier
4 Construction Techniques
Most of the construction work on this amplifier involves populating the two PWBs, and then
connecting the chassis-mounted devices to the right and left channel PWBs.
There is no mystery to populating a PWB. Having said that, some general guidelines are
worth noting:
• Proceed in a logical manner, usually beginning with the smallest components; these are
typically resistors. Begin at R1 and move through to Rx.
• As each component is installed, check it off the list before moving on.
• Confirm the value of each component before it is installed on the PWB. While rare,
incorrect packaging or labeling of components can occur. Therefore, verify the marked
resistor and capacitor values before installation. If you are a bit rusty on the resistor color
codes, check the resistors with an ohmmeter. Capacitors can be checked with a
capacitance measurement bridge. While this is perhaps overkill, it is instructive to see the
variation in component values, which measurement before installation will reveal. The
resistor color code is shown in Figure 4.1.
• Observe proper polarity of devices such as electrolytic capacitors and diodes. A distinctive
pad usually denotes the positive terminal, in the case of an electrolytic. The silk screen
legend may also provide guidance.
• For devices that may dissipate some amount of heat, such as 5 W resistors, allow extra
lead space between the device and the board so as to keep the device above the board. This
will minimize heat-caused damage to the PWB and provide for better cooling of the
Figures Multiplier Tolerance Temp. Coeff.(Ω) (0.10 /K)-6
10
23456789
0.010.11101001 k10 k100 k1 M10 M
10%5%
1%2%
0.5%0.25%0.1%
200100501525
1051
SilverGoldBlackBrownRedOrangeYellowGreenBlueVioletGreyWhite
Figure 4.1 Resistor color code.
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device. For resistors in the 2–5 W range, 1/4-inch should be sufficient. For higher-power
devices, more clearance should be provided.
• Use only the amount of heat and solder necessary to do the job. Once a PWB is damaged
by excessive heat it is very difficult to repair. Excessive solder flux on the board will tend
to attract dust, which—at high voltages—can lead to arc-over problems.
• Inspect each solder connection to be certain that the mounting pad or via has been filled.
• Mount large components last. These typically include barrier strips and high-voltage
electrolytic capacitors.
All audio cables of length greater than about 6 inches should use shielded cable. Generally
speaking, the shield should be connected to ground at the load end. Grounding both ends of a
shielded cable to the same ground plane is not recommended as it may lead to circulating currents
that will adversely impact the noise floor.
Efficiently making the socket connections to the PWB requires a bit of practice. The
recommended procedure is detailed in Section 5. Be certain to wear protective eye glasses during
this process, and at other steps in PWB assembly.
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Stereo Audio Preamplifier
5 Step-by-Step Instructions
The following steps are recommended for assembly of the stereo preamplifier. As each step is
completed, check it off the list. The project is divided into three major elements—assembly of the
right channel PWB, assembly of the left channel PWB, and assembly of the chassis. Builders are
encouraged to follow the specific instructions provided in the following sections.
An examination of the shipping box will show that the right channel and left channel parts are
contained in separate boxes. It is recommended that only one set of components be opened at a
time as this will reduce the possibility of component mix-ups.
Each section heading that follows marks a logical start/stop point in construction of the
amplifier. Do not rush through the project; instead, consider setting modest goals for progress
over a period of some days. A more relaxed pace often results in a better end-product, and a more
enjoyable overall experience.
Organize a work space that provides ample room and light. The following tools are minimally
required for assembly:
• Common screwdriver
• Phillips screwdriver
• Wire cutters
• Pliers
• Long-noise pliers
• Solderless terminal crimp tool
• 25–35 W soldering iron
• Solder
• Heat-shrink gun is recommended, although not strictly required.
While there are certainly a number of approaches to building a vacuum tube product, the
following procedure has proven to be efficient and repeatable.
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5.1 Assembly of Right Channel PWB
Check the shipping box for the major component groups. Find the right channel preamplifier box.
Set the other boxes aside for now. Organize the components for the right channel preamp in a
logical manner, grouping resistors together, capacitors together, and so on. Note that a schematic
diagram of the right channel preamplifier block is provided, along with a copy of the PWB
component layout. The component layout sheet is a useful reference as many of the part numbers
are obscured when the components are installed. The estimated time required to complete
assembly of the right channel PWB is 4 hours.
After reviewing the Construction Techniques section of this manual, begin populating the
right channel PWB. As the packing envelopes are used, set them aside for recycling. Before
beginning, clean the soldering iron tip.
– Install R1 – 47 kΩ, 0.5 W. Solder in place, leaving some extra lead space so as to place
the resistor about 1/4-inch above the board. This will facilitate replacement should it be
necessary to use a different value of magnetic cartridge load resistor.
Caution: leads may tend to fly toward the face when they are cut. Wear protective glasses. For
all steps that follow, excess lead length should be trimmed after the component is soldered in
place.
– Install R2 and R7 – 2.7 kΩ, 0.5 W. Solder in place.
– Install R3, R5, R12, R25, R28, R31, R35, R37, R39, and R45 – 100 kΩ, 0.5 W. Solder in
place.
– Install R4 – 39 kΩ, 0.5 W. Solder in place.
– Install R6, R14, and R20 – 470 kΩ, 0.5 W. Solder in place.
– Install R8 – 680 kΩ, 0.5 W. Solder in place.
– Install R9, R15, R29, R38, and R41 – 15 kΩ, 2 W. Position the components about 1/2-
inch above the board and solder in place.
– Install R10 and R23 – 22 kΩ, 0.5 W. Solder in place.
– Install R11 – 2.2 mΩ, 0.5 W. Solder in place.
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– Install R13, R24, R36, and R46 – 1 kΩ, 0.5 W. Solder in place.
– Install R21 – 1.5 kΩ, 0.5 W. Solder in place.
– Install R22 – 15 kΩ, 0.5 W. Solder in place.
– Install R27 – 10 kΩ, 0.5 W. Solder in place.
– Install R33, R34, R43, and R44 – 220 kΩ, 0.5 W. Solder in place.
Find the potentiometers (R16, R17, R18, R19, and R40). Remove the mounting nut and
lockwasher from each device. Inspect one of the devices. Note that a mounting “key” tab is
included on one side of the component. Using a pair of wire cutters, clip-off the tab so that the
device will mount flush against the circuit board. Note also that the leads from the device will
need to be reoriented in order to mount properly on the board. Bend the three leads straight down
toward the shaft of the device. Using the long-noise pliers, spread the outer two leads slightly
away from the center lead. Fit the shaft of the device through the mounting hole and then guide
the three leads through the PWB pads. When viewed from the component side of the board, the
leads of the device should protrude a short distance above the board.
– Install R16, R17, and R40 – 250 kΩ potentiometer, linear. Secure with the hardware
provided. Solder the leads in place from the component side of the board.
– Install R18 and R19 – 100 kΩ potentiometer, linear. Secure with the hardware provided.
Solder the leads in place from the component side of the board.
Find the 100 Ω potentiometer. Inspect the body of the device and find the mounting tab on one
side. Using a pair of long-nose pliers, bend the tab down toward the device. Remove the mounting
hardware.
– Install R42 – 100 Ω, 1 W, potentiometer, linear. Connect a 1-inch piece of bare wire to
each terminal of the device and solder. Fit the leads through the PWB pads and then fit the
shaft of the device through the mounting hole. Secure with the hardware provided (the
locking ring nut should be finger-tight). Solder the leads in place from the component side
of the board.
– Install R47, 100 kΩ, 2W. Solder in place, leaving approximately 1/2-in spacing above the
board.
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All resistors have now been placed on the right channel PWB. Clean the solder iron tip before
proceeding with placement of the capacitors
– Install C1, C4, C11, C24, and C31 – 25 µF, 50 V, electrolytic. Solder in place. Be certain
to observe proper polarity.
– Install C2, C5, C15, C17, C26, C28, and C29 – 22 µF, 450 V, electrolytic. Solder in
place.
– Install C3 and C16 – 0.1 µF, 400 V. Solder in place.
– Install C6 – 0.0033 µF, 600 V. Solder in place.
– Install C7 – 0.01 µF, 400 V. Solder in place.
– Install C8 – 180 pF, 500 V, mica. Solder in place.
– Install C9, C12, C20, C25, C32 – 0.22 µF, 400 V. Solder in place.
– Install C10, C14, C33 – 0.047 µF, 400 V. Solder in place.
– Install C13 – 47 µF, 450 V, electrolytic. Solder in place. Be certain to observe proper
polarity.
– Install C18 and C22 – 0.0022 µF, 600 V. Solder in place.
– Install C19 – 0.022 µF, 400 V. Solder in place.
– Install C21 – 220 pF, 600 V, ceramic. Solder in place.
– Install C23 and C30 – 100 pF, 1000 V disc. Solder in place.
– Install C27 – 0.01 µF disc, 1 kV. Solder in place.
– Find CP – 25 pF, 1000 V ceramic. Set aside for now. This capacitor may be installed
following frequency response tests to pad out the high frequency response of the phono
preamp to more closely match the RIAA transfer curve.
All capacitors have now been installed on the board. Clean the soldering iron tip before
proceeding.
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Stereo Audio Preamplifier
Note that D1 and D2 diodes installed in the steps that follow are sensitive to static discharge.
Take appropriate measures to minimize the possibility of component damage due to static
electricity. Note that the electrostatic protection packaging is not recyclable.
– Install D1 and D2 – zener diode, 200 V, 5W. Position each device about 1/2-inch above
the board. Solder in place. Be certain to observe proper polarity.
– Install the 4-terminal Molex terminal strip. Solder in place.
– Install the 12 pin miniature Molex connector headers (male) at the following positions:
Conn1 and Conn9. Note that the pins are closely spaced. Be careful to not create a solder
bridge between pads. Solder in place.
– Install the 4-pin miniature Molex connector headers (male) at the following positions:
Conn2, Conn3, Conn4, Conn5, Conn6, Conn7, Conn8, and Conn10. Solder in place. Note
that it will be necessary to hold the headers in place from the component side of the board
until the first connection point is soldered. Be careful to not create a solder bridge between
pads.
– Mount the standoffs on the foil side of the board using 4-40 hardware provided. Use a
lock-washer on the screw head (component side) of the board. Standoffs are placed on
each side of the five tube sockets. The screws should be finger-tight for now.
– Find the five tube sockets. On three of the sockets, use a short piece of bare #22 hookup
wire (found in the Tube Socket Hookup Wire package) to connect pins 4 and 5. Crimp in
place. Solder only the pin 5 connections.
– Mount the three sockets with the jumper wires between pins 4 and 5 on the standoffs for
tubes V1, V4, and V5 (the 7025 devices). Pin 1 of the socket can be identified by a number
stamped on the bottom side of the socket. Pin 1 on the PWB is identified by a square pad.
Place the socket in the proper orientation over the PWB so it rests on the standoffs. The
top of the socket (the side the tube plugs into) should face outward. Loosely tighten the
mounting screws. See Figure 5.1a.
– Repeat this procedure for the remaining tube sockets (those with no jumpers between pins
4 and 5), which are V2 and V3.
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Note that V3 does not have a square pad on the board denoting pin #1 because for the 6EU7
pin #1 is one of the heater connections (which are wired off the board). Use the same orientation
for V3 as for the other tubes on the board.
In the following steps the tube sockets will be connected. These steps will require a
considerable amount of time and patience. Successful completion is critical to proper operation of
the amplifier.
– Install connecting wires from the PWB pads to the tube socket pins. Solder in place.
Materials for these steps can be found in the “Tube Socket Hookup” package. Note that
the PWB contains pads only for the active pins used on the socket. The heater connections
are made separately. The recommended procedure for installing the connecting wires is as
follows:
(a) (b) (c)
(d)
Figure 5.1 Installation of sockets on the PWB: (a) mount the socket on the standoffs, (b) insert connecting wires from the PWB to the socket, (c) board-to-socket connections completed, (d)
finished job with heater wires installed.
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Stereo Audio Preamplifier
1) Feed a bare wire from the component side through the pad and into the socket
soldering tab.
2) Crimp in place (see Figure 5.1b).
3) From the component side, pull the wire tight and solder in place.
4) Trim the excess lead length.
5) Solder the socket tab.
Table 5.1 shows the pins that should be connected to the PWB.
– Repeat the connection process for each PWB-to-socket connection listed in Table 5.1,
being careful to match up the right pad on the board with the right connection on the
socket. (See Figure 5.1c.) This is best done by starting at pin 1 on one side of the socket,
and on pin 9 on the other side. Check to make sure sufficient clearance is provided for
wires and connecting points on the sockets.
After all of the connecting wires from the PWB to the sockets have been installed, double-
check your work. Make sure all of the appropriate pins have been soldered. Note that no
connections are made to the PWB pad in the center of the tube sockets.
The final step in assembly of the right channel PWB is connection of the heater pins.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V1 tube socket. This will
facilitate access to the pins.
– Open the “Heater Circuit Wire” package and cut two 7-inch pieces of #22 green heater
wire. Place one piece of heat-shrink tubing over one end of each wire.
Table 5.1 PWB to Tube Socket ConnectionsSocket No. V1, 7025 V2, 5879 V3, 6EU7 V4, 7027 V5, 7025
Pin 1 yes yes no, heater yes yes
Pin 2 yes no, NC no, heater yes yes
Pin 3 yes yes no, NC yes yes
Pin 4 no, heater no, heater yes no, heater no, heater
Pin 5 no, heater no, heater yes no, heater no, heater
Pin 6 yes no, NC yes yes yes
Pin 7 yes yes yes yes yes
Pin 8 yes yes yes yes yes
Pin 9 no, heater ct. yes yes no, heater ct. no, heater ct.
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– Connect one wire to pin 4 of V1. Crimp and then solder (note there is already one wire on
this pin, placed there before the socket was installed). When soldering, make sure the heat-
shrink tubing is pushed back on the wire so it is away from the soldering point. If the heat-
shrink tubing gets hot it will be very difficult to slide into position later.
– Move the heat-shrink tubing along the wire to the base of the pin. Apply a stream of hot air
to shrink the tubing over the wire. Be careful to not apply too much heat.
– Replace the standoff removed from the V1 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
– Crimp the other green wire on pin 9 of V1. Solder in place, keeping in mind the caution
about the heat-shrink tubing.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not apply too much heat.
– Twist the wires together and route to socket V2. The wires should be placed at socket-
level. In this way they will be resting against the chassis when the board is inserted in the
chassis. Do not dress the wires against the PWB as this may increase hum in the output of
the preamplifier.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V2 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V2. As before, slip a piece of heat-
shrink tubing over each wire. Crimp one wire on pin 5, and then crimp the other on pin 4.
Do not solder.
– Cut two 7-inch pieces of #22 green heater wire. Slip one through the heat-shrink tubing on
the lead already connected to pin 5 of V2. Route the wire to the pin and crimp in place.
Solder, keeping the heat-shrink tubing away from the hot wires and the soldering iron.
Slip the other wire through the heat-shrink tubing on the lead already connected to pin 4 of
V2. Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing
away from the hot wires and the soldering iron.
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Stereo Audio Preamplifier
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V2 socket. The screws should be finger-tight. The
heater leads should straddle the standoff. Remember to include the lockwasher on the
component side of the board.
– Twist the wires together and route to socket V5. The wires should be placed at socket-
level. Do not dress the wires against the PWB.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V5 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V5. Note that one lead goes to pin 4
and the other to pin 9. As before, slip a piece of heat-shrink tubing over each wire.
– Connect one wire to pin 4 of V5. Crimp in place (note there is already one wire on this pin,
placed there before the socket was installed). Crimp the other green wire to pin 9. Do not
solder.
– Cut two 7-inch pieces of green heater wire. Slip one through the heat-shrink tubing on the
lead already connected to pin 4 of V5. Route the wire to the pin and crimp in place. Solder,
keeping the heat-shrink tubing away from the hot wires and the soldering iron. Slip the
other wire through the heat-shrink tubing on the lead already connected to pin 9 of V5.
Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing away
from the hot wires and the soldering iron.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V5 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
– Twist the wires together and route to the H pads on the PWB. The wires should be placed
at socket-level. Do not dress the wires against the PWB.
– Trim the wire lengths as needed to comfortably reach the pads on the foil side of the
board. Insert one wire in each H position. Do not solder.
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– Prepare two 7-inch pieces of #22 green heater wire. Place one piece of heat-shrink tubing
over one end of each wire.
– Connect one wire to pin 1 of V3. Crimp and then solder. When soldering, make sure the
heat-shrink tubing is pushed back on the wire so it is away from the soldering point. If the
heat-shrink tubing gets hot it will be very difficult to slide into position later. Connect the
other green wire to pin 2 of V3. Crimp and then solder.
– Move the heat-shrink tubing along the wire to the base of the pins. Apply a stream of hot
air to shrink the tubing over the wire. Be careful to not apply too much heat.
– Twist the wires together and route to socket V4. The wires should be placed at socket-
level. In this way they will be resting against the chassis when the board is inserted in the
chassis. Do not dress the wires against the PWB as this may increase hum in the output of
the preamplifier.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V4 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V4. As before, slip a piece of heat-
shrink tubing over each wire. Crimp one wire on pin 4 (note there is already one wire on
this pin, placed there before the socket was installed). Crimp the other wire on pin 9. Do
not solder.
– Cut two 7-inch pieces of #22 green heater wire. Slip one through the heat-shrink tubing on
the lead already connected to pin 4 of V4. Route the wire to the pin and crimp in place.
Solder, keeping the heat-shrink tubing away from the hot wires and the soldering iron.
Slip the other wire through the heat-shrink tubing on the lead already connected to pin 9 of
V4. Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing
away from the hot wires and the soldering iron.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V4 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
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Stereo Audio Preamplifier
– Twist the wires together and route to the H pads on the PWB. The wires should be placed
at socket-level. Do not dress the wires against the PWB.
– Trim the wire lengths as needed to comfortably reach the pads on the foil side of the
board. Insert one wire in each H position (there will be two green wires in each H pad.
– Solder both H pads from the foil side of the board. Time off any excess lead length.
– Find tubes V1, V2, V3, V4, and V5. Set them aside in a safe place. The tubes will be
installed during the final checkout process.
– Check the PWB for proper installation of all components. Look for bad solder connections
or a solder splash that might cause a short-circuit.
Assembly of the right channel PWB has now been completed. The finished board is shown in
Figure 5.2. Set the board aside; it will be used later.
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Figure 5.2 Preamp PWB: (a) component side, (b) foil side.
a
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5.2 Assembly of the Left Channel PWB
Check the shipping box for the major component groups. Find the left channel preamplifier box.
Set the other boxes aside for now. Organize the components for the left channel preamp in a
logical manner, grouping resistors together, capacitors together, and so on. Note that a schematic
diagram of the left channel preamplifier block is provided, along with a copy of the PWB
component layout. The component layout sheet is a useful reference as many of the part numbers
are obscured when the components are installed. The estimated time required to complete
assembly of the left channel PWB is 4 hours.
After reviewing the Construction Techniques section of this manual, begin populating the left
channel PWB. As the packing envelopes are used, set them aside for recycling. Before beginning,
clean the soldering iron tip.
– Install R1 – 47 kΩ, 0.5 W. Solder in place, leaving some extra lead space so as to place
the resistor about 1/4-inch above the board. This will facilitate replacement should it be
necessary to use a different value of magnetic cartridge load resistor.
Caution: leads may tend to fly toward the face when they are cut. Wear protective glasses. For
all steps that follow, excess lead length should be trimmed after the component is soldered in
place.
– Install R2 and R7 – 2.7 kΩ, 0.5 W. Solder in place.
– Install R3, R5, R12, R25, R28, R31, R35, R37, R39, and R45 – 100 kΩ, 0.5 W. Solder in
place.
– Install R4 – 39 kΩ, 0.5 W. Solder in place.
– Install R6, R14, and R20 – 470 kΩ, 0.5 W. Solder in place.
– Install R8 – 680 kΩ, 0.5 W. Solder in place.
– Install R9, R15, R29, R38, and R41 – 15 kΩ, 2 W. Position the components about 1/2-
inch above the board and solder in place.
– Install R10 and R23 – 22 kΩ, 0.5 W. Solder in place.
– Install R11 – 2.2 mΩ, 0.5 W. Solder in place.
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Stereo Audio Preamplifier
– Install R13, R24, R36, and R46 – 1 kΩ, 0.5 W. Solder in place.
– Install R21 – 1.5 kΩ, 0.5 W. Solder in place.
– Install R22 – 15 kΩ, 0.5 W. Solder in place.
– Install R27 – 10 kΩ, 0.5 W. Solder in place.
– Install R33, R34, R43, and R44 – 220 kΩ, 0.5 W. Solder in place.
Find the potentiometers (R16, R17, R18, R19, and R40). Remove the mounting nut and
lockwasher from each device. Inspect one of the devices. Note that a mounting “key” tab is
included on one side of the component. Using a pair of wire cutters, clip-off the tab so that the
device will mount flush against the circuit board. Note also that the leads from the device will
need to be reoriented in order to mount properly on the board. Bend the three leads straight down
toward the shaft of the device. Using the long-noise pliers, spread the outer two leads slightly
away from the center lead. Fit the shaft of the device through the mounting hole and then guide
the three leads through the PWB pads. When viewed from the component side of the board, the
leads of the device should protrude a short distance above the board.
– Install R16, R17, and R40 – 250 kΩ potentiometer, linear. Secure with the hardware
provided. Solder the leads in place from the component side of the board.
– Install R18 and R19 – 100 kΩ potentiometer, linear. Secure with the hardware provided.
Solder the leads in place from the component side of the board.
– Install R47, 100 kΩ, 2W. Solder in place, leaving approximately 1/2-in spacing above the
board.
All resistors have now been placed on the right channel PWB. Clean the solder iron tip before
proceeding with placement of the capacitors
– Install C1, C4, C11, C24, and C31 – 25 µF, 50 V, electrolytic. Solder in place. Be certain
to observe proper polarity.
– Install C2, C5, C15, C17, C26, C28, and C29 – 22 µF, 450 V, electrolytic. Solder in
place.
– Install C3 and C16 – 0.1 µF, 400 V. Solder in place.
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– Install C6 – 0.0033 µF, 600 V. Solder in place.
– Install C7 – 0.01 µF, 400 V. Solder in place.
– Install C8 – 180 pF, 500 V, mica. Solder in place.
– Install C9, C12, C20, C25, C32 – 0.22 µF, 400 V. Solder in place.
– Install C10, C14, C33 – 0.047 µF, 400 V. Solder in place.
– Install C13 – 47 µF, 450 V, electrolytic. Solder in place. Be certain to observe proper
polarity.
– Install C18 and C22 – 0.0022 µF, 600 V. Solder in place.
– Install C19 – 0.022 µF, 400 V. Solder in place.
– Install C21 – 220 pF, 600 V, ceramic. Solder in place.
– Install C23 and C30 – 100 pF, 1000 V disc. Solder in place.
– Install C27 – 0.01 µF disc, 1 kV. Solder in place.
– Find CP – 25 pF, 1000 V ceramic. Set aside for now. This capacitor may be installed
following frequency response tests to pad out the high frequency response of the phono
preamp to more closely match the RIAA transfer curve.
All capacitors have now been installed on the board. Clean the soldering iron tip before
proceeding.
Note that D1 and D2 diodes installed in the steps that follow are sensitive to static discharge.
Take appropriate measures to minimize the possibility of component damage due to static
electricity. Note that the electrostatic protection packaging is not recyclable.
– Install D1 and D2 – zener diode, 200 V, 5W. Position each device about 1/2-inch above
the board. Solder in place. Be certain to observe proper polarity.
– Install the 4-terminal Molex terminal strip. Solder in place.
– Install the 12 pin miniature Molex connector headers (male) at the following positions:
Conn1 and Conn9. Note that the pins are closely spaced. Be careful to not create a solder
bridge between pads. Solder in place.
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Stereo Audio Preamplifier
– Install the 4-pin miniature Molex connector headers (male) at the following positions:
Conn2, Conn3, Conn4, Conn5, Conn6, Conn7, Conn8, and Conn10. Solder in place. Note
that it will be necessary to hold the headers in place from the component side of the board
until the first connection point is soldered. Be careful to not create a solder bridge between
pads.
– Mount the standoffs on the foil side of the board using 4-40 hardware provided. Use a
lock-washer on the screw head (component side) of the board. Standoffs are placed on
each side of the five tube sockets. The screws should be finger-tight for now.
– Find the five tube sockets. On three of the sockets, use a short piece of bare #22 hookup
wire (found in the Tube Socket Hookup Wire package) to connect pins 4 and 5. Crimp in
place. Solder only the pin 5 connections.
– Mount the three sockets with the jumper wires between pins 4 and 5 on the standoffs for
tubes V1, V4, and V5 (the 7025 devices). Pin 1 of the socket can be identified by a number
stamped on the bottom side of the socket. Pin 1 on the PWB is identified by a square pad.
Place the socket in the proper orientation over the PWB so it rests on the standoffs. The
top of the socket (the side the tube plugs into) should face outward. Loosely tighten the
mounting screws. See Figure 5.1a.
– Repeat this procedure for the remaining tube sockets (those with no jumpers between pins
4 and 5), which are V2 and V3.
Note that V3 does not have a square pad on the board denoting pin #1 because for the 6EU7
pin #1 is one of the heater connections (which are wired off the board). Use the same orientation
for V3 as for the other tubes on the board.
In the following steps the tube sockets will be connected. These steps will require a
considerable amount of time and patience. Successful completion is critical to proper operation of
the amplifier.
– Install connecting wires from the PWB pads to the tube socket pins. Solder in place.
Materials for these steps can be found in the “Tube Socket Hookup” package. Note that
the PWB contains pads only for the active pins used on the socket. The heater connections
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WhitakerAudio
are made separately. The recommended procedure for installing the connecting wires is as
follows:
1) Feed a bare wire from the component side through the pad and into the socket
soldering tab.
2) Crimp in place (see Figure 5.1b).
3) From the component side, pull the wire tight and solder in place.
4) Trim the excess lead length.
5) Solder the socket tab.
Table 5.1 shows the pins that should be connected to the PWB.
– Repeat the connection process for each PWB-to-socket connection listed in Table 5.1,
being careful to match up the right pad on the board with the right connection on the
socket. (See Figure 5.1c.) This is best done by starting at pin 1 on one side of the socket,
and on pin 9 on the other side. Check to make sure sufficient clearance is provided for
wires and connecting points on the sockets.
After all of the connecting wires from the PWB to the sockets have been installed, double-
check your work. Make sure all of the appropriate pins have been soldered. Note that no
connections are made to the PWB pad in the center of the tube sockets.
The final step in assembly of the right channel PWB is connection of the heater pins.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V1 tube socket. This will
facilitate access to the pins.
– Open the “Heater Circuit Wire” package and cut two 7-inch pieces of #22 green heater
wire. Place one piece of heat-shrink tubing over one end of each wire.
– Connect one wire to pin 4 of V1. Crimp and then solder (note there is already one wire on
this pin, placed there before the socket was installed). When soldering, make sure the heat-
shrink tubing is pushed back on the wire so it is away from the soldering point. If the heat-
shrink tubing gets hot it will be very difficult to slide into position later.
– Move the heat-shrink tubing along the wire to the base of the pin. Apply a stream of hot air
to shrink the tubing over the wire. Be careful to not apply too much heat.
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Stereo Audio Preamplifier
– Replace the standoff removed from the V1 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
– Crimp the other green wire on pin 9 of V1. Solder in place, keeping in mind the caution
about the heat-shrink tubing.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not apply too much heat.
– Twist the wires together and route to socket V2. The wires should be placed at socket-
level. In this way they will be resting against the chassis when the board is inserted in the
chassis. Do not dress the wires against the PWB as this may increase hum in the output of
the preamplifier.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V2 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V2. As before, slip a piece of heat-
shrink tubing over each wire. Crimp one wire on pin 5, and then crimp the other on pin 4.
Do not solder.
– Cut two 7-inch pieces of #22 green heater wire. Slip one through the heat-shrink tubing on
the lead already connected to pin 5 of V2. Route the wire to the pin and crimp in place.
Solder, keeping the heat-shrink tubing away from the hot wires and the soldering iron.
Slip the other wire through the heat-shrink tubing on the lead already connected to pin 4 of
V2. Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing
away from the hot wires and the soldering iron.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V2 socket. The screws should be finger-tight. The
heater leads should straddle the standoff. Remember to include the lockwasher on the
component side of the board.
– Twist the wires together and route to socket V5. The wires should be placed at socket-
level. Do not dress the wires against the PWB.
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– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V5 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V5. Note that one lead goes to pin 4
and the other to pin 9. As before, slip a piece of heat-shrink tubing over each wire.
– Connect one wire to pin 4 of V5. Crimp in place (note there is already one wire on this pin,
placed there before the socket was installed). Crimp the other green wire to pin 9. Do not
solder.
– Cut two 7-inch pieces of green heater wire. Slip one through the heat-shrink tubing on the
lead already connected to pin 4 of V5. Route the wire to the pin and crimp in place. Solder,
keeping the heat-shrink tubing away from the hot wires and the soldering iron. Slip the
other wire through the heat-shrink tubing on the lead already connected to pin 9 of V5.
Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing away
from the hot wires and the soldering iron.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V5 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
– Twist the wires together and route to the H pads on the PWB. The wires should be placed
at socket-level. Do not dress the wires against the PWB.
– Trim the wire lengths as needed to comfortably reach the pads on the foil side of the
board. Insert one wire in each H position. Do not solder.
– Prepare two 7-inch pieces of #22 green heater wire. Place one piece of heat-shrink tubing
over one end of each wire.
– Connect one wire to pin 1 of V3. Crimp and then solder. When soldering, make sure the
heat-shrink tubing is pushed back on the wire so it is away from the soldering point. If the
heat-shrink tubing gets hot it will be very difficult to slide into position later. Connect the
other green wire to pin 2 of V3. Crimp and then solder.
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Stereo Audio Preamplifier
– Move the heat-shrink tubing along the wire to the base of the pins. Apply a stream of hot
air to shrink the tubing over the wire. Be careful to not apply too much heat.
– Twist the wires together and route to socket V4. The wires should be placed at socket-
level. In this way they will be resting against the chassis when the board is inserted in the
chassis. Do not dress the wires against the PWB as this may increase hum in the output of
the preamplifier.
– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V4 tube socket. This will
facilitate access to the pins.
– Trim the wire lengths as needed to comfortably reach V4. As before, slip a piece of heat-
shrink tubing over each wire. Crimp one wire on pin 4 (note there is already one wire on
this pin, placed there before the socket was installed). Crimp the other wire on pin 9. Do
not solder.
– Cut two 7-inch pieces of #22 green heater wire. Slip one through the heat-shrink tubing on
the lead already connected to pin 4 of V4. Route the wire to the pin and crimp in place.
Solder, keeping the heat-shrink tubing away from the hot wires and the soldering iron.
Slip the other wire through the heat-shrink tubing on the lead already connected to pin 9 of
V4. Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing
away from the hot wires and the soldering iron.
– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot
air to shrink the tubing over the pins. Be careful to not use too much heat.
– Replace the standoff removed from the V4 socket. The screws should be finger-tight.
Remember to include the lockwasher on the component side of the board.
– Twist the wires together and route to the H pads on the PWB. The wires should be placed
at socket-level. Do not dress the wires against the PWB.
– Trim the wire lengths as needed to comfortably reach the pads on the foil side of the
board. Insert one wire in each H position (there will be two green wires in each H pad.
– Solder both H pads from the foil side of the board. Time off any excess lead length.
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– Find tubes V1, V2, V3, V4, and V5. Set them aside in a safe place. The tubes will be
installed during the final checkout process.
– Check the PWB for proper installation of all components. Look for bad solder connections
or a solder splash that might cause a short-circuit.
Assembly of the left channel PWB has now been completed. The finished board is shown in
Figure 5.2. Note that the right channel and left channel PWBs are identical, except that the hum
balance potentiometer is not installed on the left channel PWB. Set the board aside; it will be used
later.
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Stereo Audio Preamplifier
5.3 Chassis and Final Assembly
Unpack the chassis components and position on the work surface. Place a towel or other soft
covering on the work surface to avoid scratching the finish. Note there may be some rough edges
on the drill holes and cutouts. Be careful to avoid cuts or nicks.
– Find the chassis bottom plate and feet hardware. Install using the hardware provided. Note
that four self-tapping #6 screws are provided. Save these for the final step of assembly—
mounting the bottom plate to the chassis.
– Find the side chassis handles and install using the #8 hardware provided. Use the handles
during the following construction steps to ensure a good grip on the chassis.
The estimated time for completion of the following steps is about 6 hours. The chassis and
final assembly process is divided into several sub-sections, allowing the builder to proceed at a
measured pace. Figure 5.4 shows the overall placement of chassis components.
5.3.1 Front Panel Components
Collect the front panel components, which include the following:
• Front panel handles
• Three dual potentiometers (R26, R30, and R32)
• Rotary switch (SW1)
• Pilot lamp (PL1)
• 3.5 mm jack
• Front panel knobs
– Install the front panel handles using the #8 hardware provided. Be careful to not scratch
the finish. Note that the front panel handles are decorative and should not be used to move
the preamplifier after it is has been completed.
– Find the two 1 mΩ dual potentiometers (Bass, Treble). Using a pair of long-nose pliers,
bend the three terminal groups slightly toward the back of the potentiometer. Note that
there is a key tab at the base of the potentiometer. Using pliers, bend this tab down toward
the face of the component.
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– Mount the two 1 mΩ dual potentiometers as follows. Place the lock washers provided on
each shaft. Feed the component through the chassis/front panel. Add a flat washer and
secure with the nut provided. Do not tighten. The terminals of the potentiometers should
face downward toward the chassis bottom plate.
– Find the 250 kΩ dual potentiometer (Volume). Using a pair of long-nose pliers, bend the
three terminals slightly toward the back of the potentiometer. Note that there is a key tab at
the base of the potentiometer. Using pliers, bend this tab down toward the face of the
component.
VolumeInputPowerBassTrebelPhonograph
Microphone
Tuner
Auxiliary
Front Aux
Stereo Preamp J C Whitaker 2012
Front Aux
Right Left Right Left Right Left Right Left Right Left Right Left
Auxiliary Power ConnectorDanger: High Voltages
Do Not Probe
WhitakerAudio Stereo Preamplifier Built in California, USA.Phonograph Microphone Tuner Auxiliary Tape Output Line Output
MicMix
StereoMic
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
TE shell 17
Center
0.15-in
1.12-in
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix NF2D
0.88-in
0.13-in
Center
Neutrix Combo
0.88-in
0.13-in
Center
Neutrix Combo
0.88-in
0.13-in
Center
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
TE shell 17
Cable clamp
Cable clamp
Cable clamp
Cable clamp
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
Neutrix N
F2D
Side view
R30 F26SW1
R32Pilotlamp
a arm b a arm b a arm b
Figure 5.3 Chassis layout (bottom view).
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Stereo Audio Preamplifier
– Mount the 250 kΩ dual potentiometer as follows. Place the lock washers provided on the
shaft. Feed the component through the chassis/front panel. Add the flat washer and secure
with the nut provided. Do not tighten. The terminals of the potentiometer should face
downward toward the chassis bottom plate.
Find rotary switch SW1. Find the front panel knobs. Remove one of the knobs from the
package and attach to the shaft of SW1 (the set screw should work against the flattened side of the
shaft). Turn the switch shaft and observe the number of positions the switch is set for. (Hold the
body of the switch with a rag to avoid the sharp edges of the terminal posts.) If five positions are
found, then the switch is configured correctly. For some number of positions other than five,
remove the knob and remove the mounting nut on the shaft of SW1. You will note there is a keyed
“stop” on the shaft. This stop determines the number of positions that the switch has. Pull the stop
up and move it to a higher position. Replace the nut and knob and recheck.
After the switch position stop has been properly set, remove the knob and observe the body of
the switch. There is mounting tab on one side of the body. Using a pair of wire cutters, remove the
tab.
– Place the lockwasher on the shaft of SW1 and mount the switch using the flat washer and
nut provided. The nut should be finger-tight for now.
– Mount pilot lamp PL1 as follows. Thread the large mounting nut on the pilot lamp and run
down to the base of the threads. Place the lock washer provided on the shaft. Mount the
pilot lamp to the chassis/panel and secure with the decorative nut provided. Run the
decorate nut down to be approximately flush with the mounting shaft. From the underside
of the chassis, spin the large mounting nut up towards the chassis. Do not tighten.
– Find the 3.5 mm Front Panel Aux jack. There is a terminal at the base of the jack near the
mounting shaft. This is the ground terminal. Bend this terminal back slightly toward the
rear of the device. This will make it easier to connect the proper wires to the terminal later
in the assembly process.
– Mount the Front Panel Aux jack to the chassis/panel and secure using the washer and nut
provided. Tighten by carefully rotating the jack body. Position the ground terminal so it
faces one side of the chassis. This will make it easier to connect later. When properly
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mounted, the nut will be flush with the top of the shaft. Do not over-tighten as this may
damage the body of the jack.
– Using caution to not scratch the finish, tighten the three potentiometer mounting nuts. Be
certain that the terminals face downward toward the chassis bottom plate.
– Working from the chassis side, tighten the pilot lamp mounting nut. Position the terminals
of the device so they are parallel with the chassis top. This will make it easier to connect
wires later in the process.
– Move switch SW1 to the extreme counter-clockwise position using pliers while holding
the body of the device. Note the flattened side of the shaft. The switch should be oriented
so the flat side of the shaft points the knob to the first position on the front panel Input
switch (Phonograph). Once in position, tighten the mounting nut. Attach the knob and
check the positioning through the range of inputs. Adjust the switch as needed. Be careful
to not scratch the finish.
– Install the Volume Control knob. Using a small common screwdriver, run the set screw
out so the knob will fit on the shaft. Set the potentiometers to its full counterclockwise
position. Place the knob on the shaft and set to the 7 o’clock position (approximately).
Tighten the set screw.
– Install the Bass and Treble control knobs. Using a small common screwdriver, run the set
screws out so the knobs will fit on the shaft. Set the potentiometers to their approximate
center point. Place the knobs on the shafts and set to the 12 o’clock position
(approximately). Tighten the set screws.
Figure 5.4 Front panel of the stereo preamplifier.
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Stereo Audio Preamplifier
The 12 o’clock position denotes flat response for the Bass and Treble controls. A final
adjustment of these knobs will be made during performance testing.
The front panel components have been installed. The completed front panel is shown in
Figure 5.4.
5.3.2 Back Panel Components
Collect the back panel components, which include the following:
• RCA input jacks (quantity = 10)
• Microphone (1/4-inch) input jacks (quantity = 2)
• Auxiliary power connector
• Microphone mix switch
• Ground lug hardware
• Back panel decals
Examine the back panel cutouts. Note there are 13 large holes. One is for the power connector,
two are for the microphone inputs and ten are for the RCA jacks. The power connector and
microphone input jacks are larger in size than the RCA jacks.
– Mount the ten RCA input jacks using the hardware provided. Orient the components so
the ground lug faces downward toward the chassis bottom plate. Leave the mounting
screws loose for now.
Open the Power Connector Hardware package and remove the chassis-mounted connector
body and associated mounting screws. Reseal the package and set it aside for now.
– Mount the external power supply connector using the hardware provided. The socket
mounting pad rests against the outside of the back panel. Note the pin numbers printed on
the connector side of the socket. Orient the socket so the lettering is upright when viewed
from the rear panel.
– Mount the two microphone jacks using the hardware provided. Leave the mounting
screws loose for now.
Note that the standard connector jack for the microphone inputs is a 1/4-inch device. This
connector may optionally be supplied as an RCA input. For the optional device, mount the
connector in the same manner as the other RCA jacks.
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– Mount the ground lug terminal. Place the three ground lugs on to the 3/4-inch long #6
screw provided. Using a pair of long-nose pliers, bend the ends of the three lug upward
(toward the head of the screw). Insert the screw into the back panel hole so the screw
protrudes out of the back chassis. Install a lock washer on the screw, and then the lock-
tight nut. Spread the ground lugs on the inside of the chassis so they can be used
individually. Tighten the lock-tight nut. Install two flat washers on the screw and then the
wing nut. Tighten the wing nut finger-tight. The finished installation is shown in Figure
5.5.
Find toggle switch SW2. Open the package and remove the mounting hardware. Thread one
mounting nut on the shaft and run it down about 1/4-inch toward the body of the switch. Place the
lock washer on the shaft.
– Install switch SW2 and secure with the available washer and mounting nut. Note there is a
tab on one end of the washer. The tab should face up away from the chassis. Orient the
switch so the switch handle throws from side-to-side.
– Install the 17-inch long back panel decal. The top of the decal is aligned with the top of the
chassis.
– Install the two small decals adjacent to the Mic Mix switch, SW2. The “GND / Mix
Stereo” decal should be placed on the left hand side of the switch. The “Mic Mix (Mono)”
decal goes on the right.
– Tighten the mounting screws on the power connector, RCA connectors, and mic
connectors. Note there is some amount of adjustment range for each connector. To the
extent possible, align the top edge of the connector so it is parallel (or just touches) the top
(a) (b)
Figure 5.5 Installation of the ground lug terminal: (a) inside chassis, (b) outside chassis.
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Stereo Audio Preamplifier
decal. For the best fit against the chassis, tighten alternate screws in stages; i.e., tighten
one side by a small amount, then tighten the other side, etc.
The back panel components have now been installed. The back panel is shown in Figure 5.6.
5.3.3 Cable Clamps and Related Hardware
The cable clamp mounting screws will not be accessible after the PWBs have been installed.
Therefore, they need to be installed first. The screws that hold the cable clamps also serve as
mounting posts for the Plexiglas cover. See the chassis layout drawing in Figure 5.3. Note the
locations of the cable clamps.
– Find the Cable Management Hardware package. There are four sets of mounting
hardware. Install using the hardware provided. Orient the cable clamps as shown in Figure
5.3. The proper order of hardware is as follows (from bottom of chassis to top): screw
head / flat washer / cable clamp / chassis / lock washer / standoff / chrome spacer / flat
washer / screw head. The Plexiglas cover will be installed later.
– Apply the top chassis decal. The decal should be placed adjacent to the front panel in the
center of the chassis.
5.3.4 Install the PWBs
This step requires care and patience. Do not rush installation of the PWBs. Check the foil side of
the boards. Make sure the green heater wires are placed at socket level and are clear of the sockets
so they are not pinched when the boards are mounted to the chassis.
Figure 5.6 Back panel of the stereo preamplifier.
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WhitakerAudio
– Locate the right channel preamp PWB. Remove the screws that secure the sockets to the
standoffs. Set these in a convenient place.
– Carefully place the right channel preamplifier PWB in position from the underside of the
chassis.
– Install the 4-40 x 3/8-inch mounting screws from the top of the chassis to the socket
standoffs on the circuit board. Some adjustment may be needed for the sockets to achieve
proper alignment. Do not tighten the screws until all screws have been threaded and the
sockets properly extend through the chassis cutouts so they are flush with the top of the
chassis. Work carefully to guide the sockets and mounting screws into position. Be careful
to avoid scratching the chassis top finish.
Note that tube shields may optionally be used on the turntable and microphone preamp tubes,
V1 and V2, respectively. Improved noise performance will be realized with the sockets in place.
The performance measurements detailed in Section 7 of this manual assume that shields are not
used. If the user wishes to use the shields, they are installed at this point on the V1 and V2
sockets.
– Repeat this procedure for the left channel PWB.
Sockets properly mated to the chassis are shown in Figure 5.7.
5.3.5 Back Panel Audio Connections
Find the shielded audio cable. There are two types of cables: one has one conductor plus the
shield and the other has three conductors plus the shield. Unless otherwise noted, use the one-
conductor cable. For the single-conductor cable, the white wire is the signal-carrying wire, and
the shield wire is ground.
Figure 5.7 Mounting of the PWB sockets to the chassis.
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Stereo Audio Preamplifier
In the following steps, 4-pin or 12-pin Molex connectors will be used. After stripping the
cable, place a short (approximately 3/4-inch long) piece of heat-shrink tubing on each end of the
audio cable. For the end of the cable inserted into the Molex connector, process (apply heat) after
the wires have been inserted into the connector but before the cable is installed in the amplifier.
The length of the shielded audio cable used in the preamplifier will impact the high frequency
response of the amplifier under certain conditions. For this reason, the cable lengths are kept as
short as possible. Also, to ensure that the frequency response of the channels track closely, the
cable lengths are specified to be identical between channels for similar functions.
Hardware for the following steps can be found in the following packages: 4 Terminal Molex
Housing, 12 Terminal Molex Housing, and Molex Crimp Terminal.
Examine the Molex socket housing. The connector pins insert one way only. As each
connector pin is attached, check for a good crimp. Builders should note that the housing is held
against the PWB-mounted pins by the friction of the connector elements. In order to provide for a
good physical mating, it is recommended that any positions on the header that are not used for
wires (listed as “vacant” in the instructions that follow) be filled with a pin.
Before proceeding, clean the soldering iron tip.
– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 12 terminal Molex
header as follows:
• Shield to pin #1
• White to pin #2
• Position #3 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 9-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #10
• White to pin #11
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• Position #12 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 8-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #4
• White to pin #5
• Position #6 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare an 8-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #7
• White to pin #8
• Position #9 vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 2-inch long yellow wire. Connect one end of the wire to the terminal of SW2
(Mic Mix) closest to the ground lugs. Solder.
Examine the microphone input jacks. Note there are three connections, which are labeled as
follows:
• GR – ground or sleeve. This is the ground connection.
• TP – tip. This is the hot (input) connection.
• RI – ring. This connection is not used.
In the steps that follow, be certain to match the channel inputs with the correct jacks.
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Stereo Audio Preamplifier
– Find the 9-inch audio cable connected to pins 10–12 of the Molex header. This is the input
to the microphone channel. Connect the free end of the cable to the right channel
Microphone input. Connect the wires as follows: shield to GR and white to TP. Solder
the GR connection. Do not solder the TP connection.
– Connect the free end of the 2-inch yellow wire to the TP pin of the right channel
Microphone input. Solder (2 wires).
– Identify the right channel Conn1 connector. Insert the 12 terminal Molex header into the
connector on the board, being careful to orient the header properly. Use the printed legend
on the PWB as a guide.
– Find the audio cable connected to pins 1–2 of Conn1 (Phono). Connect the free end of the
cable to the right channel Phonograph input jack. Solder the white wire to the center
conductor of the RCA jack. Solder the shield wire to the outer conductor. Dress the cable
against the chassis.
– Find the audio cable connected to pins 4–5 of Conn1 (Tuner). Connect the free end of the
cable to the right channel Tuner input jack. Solder the white wire to the center conductor
of the RCA jack. Solder the shield wire to the outer conductor.
– Find the audio cable connected to pins 7–8 of Conn1 (Aux). Connect the free end of the
cable to the right channel Auxiliary input jack. Solder the white wire to the center
conductor of the RCA jack. Solder the shield wire to the outer conductor.
Be certain to dress the wires to the phonograph and microphone inputs close to the chassis and
away from the Conn1 cable bundle. Unless sufficient separation is maintained, it is possible for
crosstalk from the higher-level Tuner and Auxiliary inputs to the lower-level phone and mic input
circuits.
– Prepare a 9-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
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– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn2 (TAPE2) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Connect the free end of the cable to the right channel Tape Output jack.
Solder the white wire to the center conductor of the RCA jack. Solder the shield wire to
the outer conductor. Dress the cable against the chassis.
– Prepare a 25-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn10 (BUF-OUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the back panel right channel Line
Output jack, using the two available cable clamps adjacent to the left channel PWB.
Solder the white wire to the center conductor of the RCA jack. Solder the shield wire to
the outer conductor.
– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 12 terminal Molex
header as follows:
• Shield to pin #1
• White to pin #2
• Position #3 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
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– Prepare a 9-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #10
• White to pin #11
• Position #12 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare an 8-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #4
• White to pin #5
• Position #6 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare an 8-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex
header as follows:
• Shield to pin #7
• White to pin #8
• Position #9 is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 2-inch long yellow wire. Connect one end of the wire to the center terminal of
SW2 (Mic Mix). Solder.
In the steps that follow, be certain to match the channel inputs with the correct jacks.
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– Find the 9-inch audio cable connected to pins 10–12 of the Molex header. This is the input
to the microphone channel. Connect the free end of the cable to the left channel
Microphone input. Connect the shield to the GR pin and the white to the TP pin. Solder
the shield. Do not solder the white wire. Route the wire against the chassis top/back panel
corner.
– Connect the free end of the 2-inch yellow wire to the TP pin of the left channel
Microphone input. Solder. (2 wires).
– Identify the left channel Conn1 connector. Insert the 12 terminal Molex header into the
connector on the board, being careful to orient the header properly. Use the printed legend
on the PWB as a guide.
– Find the audio cable connected to pins 1–2 of Conn1 (Phono). Connect the free end of the
cable to the left channel Phonograph input jack. Solder the white wire to the center
conductor of the RCA jack. Solder the shield wire to the outer conductor. Route the wire
against the chassis.
– Find the audio cable connected to pins 4–5 of Conn1 (Tuner). Connect the free end of the
cable to the left channel Tuner input jack. Solder the white wire to the center conductor of
the RCA jack. Solder the shield wire to the outer conductor.
– Find the audio cable connected to pins 7–8 of Conn1 (Aux). Connect the free end of the
cable to the left channel Auxiliary input jack. Solder the white wire to the center
conductor of the RCA jack. Solder the shield wire to the outer conductor.
– Prepare a 9-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
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Stereo Audio Preamplifier
– Insert the connector/cable just assembled into the Conn2 (TAPE2) pins on the left channel
PWB. Be careful to properly orient the cable, using the legend printed on the board as a
guide. Connect the free end of the cable to the left channel Tape Output jack. Solder the
white wire to the center conductor of the RCA jack. Solder the shield wire to the outer
conductor. Dress the wire against the chassis.
– Prepare a 25-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn10 (BUF-OUT) pins on the left
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the back panel left channel Line
Output jack, using the two available cable clamps adjacent to the left channel PWB.
Solder the white wire to the center conductor of the RCA jack. Solder the shield to the
outer conductor. Dress the excess length of cable between the rear panel and the left
channel PWB.
– Prepare a 24-inch long piece of #18 black hookup wire. Connect one end to one of the
available ground lugs on the back panel. Solder. Route the wire toward the front panel
using the two cable clamps alongside the right channel PWB. Route to the #2 pin on the
right channel PWB barrier strip. It will be connected later.
– Prepare a 32-inch long piece of #18 black hookup wire. Connect one end to one of the
available ground lugs on the back panel. Solder. Route the wire toward the front panel
using the two cable clamps alongside the right channel PWB. Route to the #2 pin on the
left channel PWB barrier strip. It will be connected later.
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– Prepare an 19-inch long piece of #18 black hookup wire. Connect one end to one of the
available ground lugs on the back panel. Solder. Route the wire toward the back panel
power connector. It will be connected later.
Assembly of the back panel audio connectors has now been completed. Use tie-wraps as
appropriate to dress the cabling.
5.3.6 Front Panel Connections
The potentiometers on the front panel are dual-ganged units. In all cases, the front section (the one
nearest the front panel) is used for the right channel and the back section is used for the left
channel. Each potentiometer has three terminals, “a”, “arm”, and “b”. See Figure 5.8.
The length of the shielded audio cable used in the preamplifier will impact the high frequency
response of the amplifier under certain conditions. For this reason, the cable lengths are kept as
short as possible. Also, to ensure that the frequency response of the channels track closely, the
cable lengths are specified to be identical between channels for similar functions.
Before proceeding, clean the soldering iron tip.
– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn7 (TAPE1) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Leave the free end of the cable unconnected for
now.
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– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are is vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn7 (TAPE1) pins on the left channel
PWB. Be careful to properly orient the cable, using the legend printed on the board as a
guide. Route the free end of the cable to the front panel Input switch (SW1), using the
available cable clamp adjacent to the left channel PWB. Clip off the shield wire; it is not
used. Leave the free end of the cable unconnected for now.
In the following steps the Input selector switch, SW1, will be wired. Note there are two
sections to the switch, divided along a centerline down the middle of the device. The section
facing the right channel PWB will be used for the right channel inputs and the section facing the
left channel PWB will be used for the left channel inputs. The wiper arm is the connection post
nearest the front panel. Note also the position of the connection arm when the switch is in the
“Phonograph” position (check with an ohmmeter). This is position #1. Other positions go in
logical order.
Note that in order to gain access to some connection posts it may be necessary to temporarily
remove the Input switch knob and loosen the mounting nut. This will allow the component to be
rotated slightly.
– Find the 12-inch long audio cable from the Conn7 (TAPE1) connector on the right
channel PWB. Connect the clear conductor to the wiper arm of the right channel input
switch (SW1). Do not solder.
– Find the 12-inch long audio cable from the Conn7 (TAPE1) connector on the left channel
PWB. Connect the clear conductor to the wiper arm of the left channel input switch
(SW1). Do not solder.
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– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 position are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn8 (INPUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Connect the white conductor to the wiper arm of
the right channel input switch (SW1). Solder (2 wires).
– Prepare a 16-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn3 (PH-OUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Connect the clear (signal) conductor to the #1
position on the right channel input switch (SW1). Solder.
– Prepare a 15-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
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• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn4 (TUN-OUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Connect the white conductor to the #3 position on
the right channel input switch (SW1). Solder.
– Prepare a 14-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn5 (AUX-OUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Connect the white conductor to the #4 position on
the right channel input switch (SW1). Solder.
– Prepare a 13-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
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– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn6 (MIC-OUT) pins on the right
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1).
Clip off the shield wire; it is not used. Connect the clear (signal) conductor to the #2
position on the right channel input switch (SW1). Solder.
– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 12-inch long piece of #22 black wire. Attach a crimp terminal to one end. Insert
the black wire into the 4 terminal Molex header just assembled as follows:
• Black to pin #3
• Position #4 is vacant
– Insert the connector/cable just assembled into the Conn8 (INPUT) pins on the left channel
PWB. Be careful to properly orient the cable, using the legend printed on the board as a
guide. Route the free end of the cable to the front panel Input switch (SW1). Clip off the
shield wire; it is not used. Connect the white conductor to the wiper arm of the left
channel input switch (SW1). Solder (2 wires). Connect the black wire to the last terminal
position on the right channel side of SW1. (This pin is adjacent to the left channel wiper
arm connection.) This connection is not used by the switch and will serve as a ground
point for the Front Aux connector.
Examine the Front Aux input jack mounted on the front panel. This connector carries three
signals: right, left, and ground. Connections for such jacks are also commonly described as tip,
ring, and sleeve, which correspond to the right, left, and ground connections, respectively.
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– Prepare an 11-inch long piece of 3-conductor audio cable. Connect the black and shield
wires to the ground point just established on the unused terminal on SW1. Solder (3
wires). Connect the red wire to the #5 position on the right channel side of Input switch
SW1. Solder. Connect the white wire to the #5 position on the left channel side of Input
switch SW1. Solder. Note: it may be helpful to temporarily remove the connector to
Conn10 on the left channel PWB in order to better access position #5 of SW1.
– Route the 11-inch 3-conductor audio cable to the Front Aux jack. Connect the black wire
to the common (sleeve) terminal, which is adjacent to the front panel. Solder. Connect the
white wire to the ring (middle) terminal. This terminal can usually be identified visually or
by use of an ohmmeter. Solder. Connect the red wire to the tip terminal. Solder.
– Prepare a 16-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn3 (PH-OUT) pins on the left
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1),
using one of the cable clamps adjacent to the left channel PWB. Clip off the shield wire; it
is not used. Connect the white conductor to the #1 position on the left channel input
switch (SW1). Solder.
– Prepare a 15-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
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• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn4 (TUN-OUT) pins on the left
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1),
using one of the cable clamps adjacent to the left channel PWB. Clip off the shield wire; it
is not used. Connect the white conductor to the #3 position on the left channel input
switch (SW1). Solder.
– Prepare a 14-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn5 (AUX-OUT) pins on the left
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1),
using one of the cable clamps adjacent to the left channel PWB. Clip off the shield wire; it
is not used. Connect the white conductor to the #4 position on the left channel input
switch (SW1). Solder.
– Prepare a 13-inch long piece of audio cable. Attach crimp terminals to the white and shield
wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header
as follows:
• Shield to pin #1
• White to pin #2
• Positions #3 and #4 are vacant
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Stereo Audio Preamplifier
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn6 (MIC-OUT) pins on the left
channel PWB. Be careful to properly orient the cable, using the legend printed on the
board as a guide. Route the free end of the cable to the front panel Input switch (SW1),
using one of the cable clamps adjacent to the left channel PWB. Clip off the shield wire; it
is not used. Connect the white conductor to the #2 position on the left channel input
switch (SW1). Solder.
If the input switch (SW1) was moved in order to make connections to the device, reposition
the switch and secure the nut. Reinstall the knob and check positioning.
– Prepare a 12-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into a 12 terminal Molex header as follows:
• White to pin #1 (R26a)
• Red to pin #1 (R26arm)
• Black to pin #3 (R26b)
• Shield to pin #4 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 14-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into the 12 terminal Molex header as follows:
• White to pin #5 (R30a)
• Red to pin #6 (R30arm)
• Black to pin #7 (R30b)
• Shield to pin #8 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
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– Prepare a 13-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into the 12 terminal Molex header as follows:
• White to pin #9 (R32a)
• Red to pin #10 (R32arn)
• Black to pin #11 (R32b)
• Shield to pin #12 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Insert the connector/cable just assembled into the Conn9 pins on the right channel PWB.
Be careful to properly orient the connector, using the legend printed on the board as a
guide. Route the free end of the cables to the front panel. For the cable from the #5 to #8
pins, route through the cable clamp adjacent to the right channel PWB. Clip off the shield
drain wires; they are not used.
– Find the right channel Conn9 R26 cable (R26a, R25arm, R26b). Solder the white wire to
terminal “a” on the front section of R26 (Bass). Solder the red wire to the “arm” terminal.
Solder the black wire to the “b” terminal.
– Find the right channel Conn9 R30 cable (R30a, R30arm, R30b). Solder the white wire to
terminal “a” on the front section of R30 (Treble). Solder the red wire to the “arm”
terminal. Solder the black wire to the “b” terminal.
– Find the right channel Conn9 R32 cable (R32a, R32arm, R32b). Solder the white wire to
terminal “a” on the front section of R32 (Volume). Solder the red wire to the “arm”
terminal. Solder the black wire to the “b” terminal.
Inside chassis view
Potentiometer
Term 1, low side(”b”)
Term 2, wiper(”arm”)
Term 3, high side(”a”)Figure 5.8 Terminal numbering for chassis-mounted potentiometers.
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Dress the cables under the lip of the front chassis edge.
– Prepare a 12-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into a 12 terminal Molex header as follows:
• White to pin #1 (R26a)
• Red to pin #1 (R26arm)
• Black to pin #3 (R26b)
• Shield to pin #4 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 14-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into the 12 terminal Molex header as follows:
• White to pin #5 (R30a)
• Red to pin #6 (R30arm)
• Black to pin #7 (R30b)
• Shield to pin #8 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
– Prepare a 13-inch long piece of 3-conductor audio cable. Attach crimp terminals to the
red, white, black, and shield wires on one end of the cable. Insert the crimped terminals
into the 12 terminal Molex header as follows:
• White to pin #9 (R32a)
• Red to pin #10 (R32arn)
• Black to pin #11 (R32b)
• Shield to pin #12 (GND)
– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the
audio cable. Process, being careful to not use too much heat.
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– Insert the connector/cable just assembled into the Conn9 pins on the left channel PWB.
Be careful to properly orient the cable, using the legend printed on the board as a guide.
Route the free end of the cable to the front panel. Clip off the shield drain wires; they are
not used.
– Find the left channel Conn9 R26 cable (R26a, R26arm, R26b). Solder the white wire to
terminal “a” on the rear section of R26 (Bass). Solder the red wire to the “arm” terminal.
Solder the black wire to the “b” terminal.
– Find the left channel Conn9 R30 cable (R31a, R30arm, R30b). Solder the white wire to
terminal “a” on the rear section of R30 (Treble). Solder the red wire to the “arm” terminal.
Solder the black wire to the “b” terminal.
– Find the left channel Conn9 R32 cable (R32a, R32arm, R32b). Solder the white wire to
terminal “a” on the back section of R32 (Volume). Solder the red wire to the “arm”
terminal. Solder the black wire to the “b” terminal.
Dress the wires under the front chassis lip. Secure with tie wraps as needed.
5.3.7 Install Interconnecting Wiring
The final step in construction of the preamplifier is to install the filament voltage rectifier, board-
to-board wiring and the rear panel power connector wiring. To facilitate cable organization, it is
advisable to install expandable flexible sleeving on these wires. A supply of this material is
provided. To properly perform the following steps, it will be necessary to have a heat gun on
hand. (A common hair dryer may be sufficient.) In addition, the preferred method of cutting the
tubing is to use a so-called “heat knife.” If such a tool is unavailable, scissors may be used.
When completed, the cable bundle will be placed above the circuit boards and audio wiring
and will rest against the chassis bottom plate (when installed). See Figure 5.9 for placement.
The parts needed for the following steps can be found in the Power Connector Hardware,
Chassis Hookup Wire, and Braided Sleeving/Heat-Shrink Tubing packages. The pinout for the
power connector is given in Table 3.2. Before proceeding, clean the soldering iron tip.
– Locate the #18 black wire from the rear panel ground lug that was previously routed to the
power connector. Attach a pin to the wire and insert in the #9 position on the power
connector.
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Two-conductor twisted-pair audio cable will be used to connect the back panel power socket
to the front panel pilot light. Because the pilot lamp is driven by 115 V ac from the power supply
on the amplifier chassis, shielded wire will be used to minimize hum in the low-level audio
circuits of the preamplifier. Note that the audio cable specified is rated for use at up to 300 V.
– Prepare an 18-inch long piece of 2-conductor twisted-pair audio cable. Attach a pin to
each of the clear and black wires.
– Prepare a 9-inch long piece of #22 black wire. Combine one end of this wire with the
shield drain wire of the 2-conductor cable prepared in the previous step. Attach a pin to the
two wires, being careful to securely crimp both in place.
– Insert the pins as follows into the back panel power connector:
• Shield and 9-inch long #22 black wire in the #1 position
• Clear in the #2 position
• Black in the #3 position
– Prepare a 9-inch long piece of #22 red wire. Attach a pin to one end of the wire and insert
in the #4 position on the power connector.
– Prepare a 6-inch long piece of #18 green wire. Attach a pin to one end of the wire and
insert in the #5 position on the power connector.
– Prepare a 6-inch long piece of #18 green wire. Attach a pin to one end of the wire and
insert in the #6 position on the power connector.
– Twist the two #18 green wires just installed, leaving about 1-inch free on the end.
All pins on the power connector have now been installed. Pins 7 and 8 are not used.
– Cut a 5-inch long piece of expandable sleeving. Route the 2-conductor cable, black wire,
and red wire from the power connector through the tubing. Place a short (3/4-inch long)
piece of heat-shrink tubing over each end. Process with the heat gun, being careful to not
use too much heat.
– Prepare a 10-inch long #22 red wire. Combine one end of this wire with the red wire from
the power connector harness. Place both wires in a spade terminal and crimp.
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– Prepare two 5-inch long #18 green wires. Attach spade terminals to one end of each wire
and crimp. Twist the wires, leaving about 1-inch free on each end.
– Prepare two 11-inch long #18 green wires. Attach spade terminals to one end of each wire
and crimp. Twist the wires, leaving about 1-inch free on each end.
– Find the #18 black wire from the back panel ground point routed previously to the left
channel PWB. Combine this wire with the #22 black wire from the back panel power
connector. Attach a spade terminal to the wires (two wires).
– Loosen the four screws on the left channel PWB barrier strip. Insert the spade terminals
just prepared as follows:
• Red wires to position #1. Tighten.
• Black wires to position #2. Tighten.
• One of the 5-inch long green wires and one of the 11-inch long green wires to position
#3. Tighten.
• The remaining 5-inch long green wire and the remaining 11-inch long green wire to
position #4. Tighten.
– Cut a 5-inch long piece of expandable sleeving. Route the wires from the left channel
PWB barrier strip and the pilot lamp cable through the tubing. Place a short (3/4-inch
long) piece of heat-shrink tubing over each end. Process with the heat gun, being careful
to not use too much heat.
– Connect the clear and black wires of the pilot lamp cable to the front panel pilot lamp.
Solder. Clip off the shield drain wire. It will not be used on this end.
– Find the #18 black wire from the back panel ground point routed previously to the right
channel PWB. Attach a spade terminal to the wire.
– Attach a spade terminal to the red wire from the harness.
– Attach spade terminals to the green wires from the harness.
– Loosen the four screws on the right channel PWB barrier strip. Insert the spade terminals
just prepared as follows:
• Red wire to position #1. Tighten.
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• Black wire to position #2. Tighten.
• Green wires to position #3 and position #4. Tighten.
Next, the filament voltage rectifier and filter will be installed. Locate C28 and CR-1. Examine
CR-1 and note the position of the positive and negative terminals.
– Find one of the #18 green wires connected to the left channel PWB barrier strip, position
#3 or #4. Find C28 and note the positive terminal. Add a piece of spaghetti to the bare
lead. (Insulation from a length of #18 wire should suffice.) Combine this wire with the
positive lead of C28 in a single large-gauge (blue) quick-disconnect terminal. Crimp.
– Find the other #18 green wire connected to the left channel PWB barrier strip. Combine
this wire with the negative lead of C28 in a single large-gauge (blue) quick-disconnect
terminal. Add a piece of spaghetti to the bare lead. Crimp.
– Insert the quick-disconnect terminal connected to the positive lead of C28 into the positive
(+) post on CR-1.
– Insert the quick-disconnect terminal connected to the negative lead of C28 into the
negative (–) post on CR-1.
– Double-check the two connections to CR-1.
If heat sink compound is available, place a small amount on the heat sink side of CR-1 before
mounting the component on the chassis.
– Mount CR-1 in the hole provided adjacent to the side handle using the #8 hardware
supplied.
– Find the two #18 green wires from the back panel power connector. Attach a quick-
disconnect terminal to each wire.
– Insert the remaining green wires on the “ac input” posts of CR-1. Use a pair of long-nose
pliers to seat the connectors.
– Dress the green wires from the rear panel power connector against the side of the chassis.
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5.3.8 Final Assembly Check
The stereo preamplifier has now been completed. Make a thorough visual inspection to make sure
everything looks OK. Use the tie-wraps provided to organize and cleanup the wiring. All parts
should be installed, with the exception of the vacuum tubes. If you have any parts left-over,
recheck the procedures documented here and install the part(s) as needed. Figure 5.9 shows the
chassis view of the stereo preamplifier.
– Check for any wiring debris inside the chassis by turning it over to remove any particles.
– Tighten the tube sockets. Use two Phillips screw drivers, one from underside the chassis
and the other from the top of the chassis. Turn in opposite directions to tighten. Caution:
do not use excessive force. Be careful to not scratch the chassis finish.
Before proceeding to Initial Checkout, it is recommended that builders clean up the
workspace and put away all tools. An occasional “workbench reset” is useful for any project.
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Stereo Audio Preamplifier
bFigure 5.9 The stereo audio preamplifier: (a) bottom view, (b) top view.
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6 Initial Checkout
After the preamplifier has been assembled, the following steps are recommended to confirm
proper construction:
1) Remove all power from the unit.
2) Remove all tubes.
3) Using an ohmmeter, confirm that the resistance values listed in Table 6.1 are observed on
the tube socket connections for the right and left channels. All readings are with reference to
ground. Readings within ±5 percent are considered normal. The resistance measurements are
made with the power supply disconnected.
Table 6.1 Typical Ohmmeter Readings with Power Removed and Tubes RemovedTube Pin
No.Typical Reading
Notes
V1, 7025, Phono Preamp
1 > 200 kΩ Varies due to filter capacitor in circuit
2 47 kΩ Measured with no input connected
3 2.7 kΩ
4 ~50 Ω With Hum Balance control at mid-point
5 ~50 Ω With Hum Balance control at mid-point
6 > 200 kΩ Varies due to filter capacitor in circuit
7 680 kΩ
8 2.7 kΩ
9 ~50 Ω With Hum Balance control at mid-point
V2, 5879, Microphone Preamp
1 2.2 mΩ With SW-2 in the Mic Stereo position
2 –
3 1 kΩ
4 ~50 Ω With Hum Balance control at mid-point
5 ~50 Ω With Hum Balance control at mid-point
6 –
7 > 200 kΩ Varies due to filter capacitor in circuit
8 > 200 kΩ Varies due to filter capacitor in circuit
9 1 kΩ
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4) If any deviations from the expected values listed in Table 6.1 are observed, recheck wiring
and components for correct installation. Do not advance to the next step until the resistance
measurements are within tolerance.
5) After the resistance checks have been successfully completed, connect the power supply and
apply power to the unit. Be very careful since high voltages are present. Observe the
preamplifier for any signs of abnormal behavior. When the power supply (power amplifier)
is switched on, the pilot lamp on the preamplifier should light.
V3, 6EU7, Tone Control Preamp
1 ~50 Ω With Hum Balance control at mid-point
2 ~50 Ω With Hum Balance control at mid-point
3 –
4 16.5 kΩ
5 480 kΩ
6 > 200 kΩ Varies due to filter capacitor in circuit
7 > 200 kΩ Varies due to filter capacitor in circuit
8 110 kΩ to 1.1 mΩ
Reading depends on setting of Bass tone control. Values shown are for extreme positions
9 1 kΩ
V4, 7025, Buffer Amplifier
1 > 200 kΩ Varies due to filter capacitor in circuit
2 108 kΩ to 180 kΩ
Varies depending on setting of input volume control
3 1 kΩ
4 ~50 Ω With Hum Balance control at mid-point
5 ~50 Ω With Hum Balance control at mid-point
6 > 200 kΩ Varies due to filter capacitor in circuit
7 > 200 kΩ Varies due to filter capacitor in circuit
8 100 kΩ
9 ~50 Ω With Hum Balance control at mid-point
V5, 7025, Tape Out Amplifier
1 > 200 kΩ Varies due to filter capacitor in circuit
2 108 kΩ to 180 kΩ
Varies depending on setting of Tape Out Level control
3 1 kΩ
4 ~50 Ω With Hum Balance control at mid-point
5 ~50 Ω With Hum Balance control at mid-point
6 > 200 kΩ Varies due to filter capacitor in circuit
7 > 200 kΩ Varies due to filter capacitor in circuit
8 100 kΩ
9 ~50 Ω With Hum Balance control at mid-point
Table 6.1 Typical Ohmmeter Readings with Power Removed and Tubes Removed
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6) Using extreme caution, check for proper voltages at the tube connection points listed in
Table 6.2 for the right and left channels. All voltages are measured with respect to ground
(unless otherwise noted) using a high impedance voltmeter. Variations within ±10 percent
are normal. These measurements can be taken from the top of the chassis by carefully
inserting the voltmeter probe into the socket connection points. This is safer than attempting
to make the measurements from underneath the chassis. Note that the dc heater voltages are
typical when the Hum Balance control is set for mid-point.
Table 6.2 Typical Voltmeter Readings with Tubes Removed and Power Applied(B+ supply of 275 V dc is assumed for these measurements.)
Tube Pin No.
Typical Reading
Notes
V1, 7025, Phono Preamp 1 270 V
2 0 V No input connected
3 0 V
4 3 to 4 V Heater (with Hum Balance set at center).
5 3 to 4 V Heater. There should be 0 V between pins 4 and 5.
6 270 V
7 0 V
8 0 V
9 3 to 4 V Heater. There should be ~7 V dc between pin 9 and pins 4 and 5.
V2, 5878, Microphone Preamp
1 0 V
2 0 V
3 0 V
4 3 to 4 V Heater
5 3 to 4 V Heater. There should be ~7 V dc between pins 4 and 5.
6 0 V
7 270 V
8 275 V
9 0 V
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7) If variations in the expected voltage readings are found, remove all power from the unit and
recheck components and wiring. Do not advance to the next step unless the voltages check
correctly.
8) Remove ac power from the unit.
9) Install all tubes. Install the tube shields on V1 and V2 (if used).
10) Set the tone controls to their center positions. Set the Volume control to the 9 o’clock
position. Set the Input switch to Auxiliary. Remove any connectors from the input jacks.
V3, 6EU7, Tone Control Preamp
1 3 to 4 V Heater
2 3 to 4 V Heater. There should be ~7 V dc between pins 1 and 2.
3 0 V
4 0 V
5 0 V
6 270 V
7 270 V
8 0 V
9 0 V
V4, 7025, Buffer Amplifier
1 270 V
2 0 V
3 0 V
4 3 to 4 V Heater
5 3 to 4 V Heater. There should be 0 V between pins 4 and 5.
6 275 V
7 270 V
8 0 V
9 3 to 4 V Heater. There should be ~7 V dc between pin 9 and pins 4 and 5.
V5, 7025, Tape Out Amplifier
1 270 V
2 0 V
3 0 V
4 3 to 4 V Heater
5 3 to 4 V Heater. There should be 0 V between pins 4 and 5.
6 275 V
7 270 V
8 0 V
9 3 to 4 V Heater. There should be ~7 V dc between pin 9 and pins 4 and 5.
Table 6.2 Typical Voltmeter Readings with Tubes Removed and Power Applied(B+ supply of 275 V dc is assumed for these measurements.)
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11) Power up the unit and very carefully confirm the circuit operating voltages shown on the
schematic diagram given in Figure 1.1. The readings should match the nominal values
within 10 percent. Note that the voltages given in the figure assume an input voltage of +275
V. For other values of input B+ scale appropriately.
12) Check the voltage of the filament supply. With all tubes installed the voltage between
terminals #3 and #4 on either PWB barrier strip should be 6.1 V dc.
Important: Never operate the stereo preamplifier unless all tubes are installed. The voltage of
the dc filament supply is not regulated and if multiple tubes are removed with power applied the
filament voltage can rise to the point that it could damage the remaining tubes.
13) Remove power from the preamplifier. Using a voltmeter, confirm that the power supply
filter capacitors have discharged to 0 V. Observe the circuit boards and components for any
signs of heating or other unexpected behavior. If problems are discovered, proceed no
further. Recheck the circuits for proper construction.
6.1 Functional Tests
The purpose of the following functional tests is to confirm that the preamplifier circuits are
working as expected. Detailed measurements will be taken later.
1) Set the front panel controls as follows:
• Volume control set to approximately 9 o’clock position
• Input switch to Phonograph
• Bass to center position
• Treble to center position
2) Set the gain adjust potentiometers on the PWBs as follows. The right and left channel
controls should be set the same.
• Phono Gain approximately 2/3 clockwise
• Tuner Gain fully clockwise
• Aux Gain fully clockwise
• Mic Gain approximately 2/3 clockwise
• Tape Out Level approximately 1/3 clockwise
• Hum Balance approximately center travel
3) Connect an audio amplifier to the left and right Line Output jacks.
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4) Apply power and observe the tubes for any signs of overheating or other unexpected
behavior. The filaments should be lit. Remove power if any problems are detected, or if
noticeable hum can be heard from the speaker. Recheck wiring and components as needed.
5) Rotate the Input switch through the various positions to confirm that no noise or hum can be
heard from the speakers. Advance the Volume control through each position. It is likely that
some noise and/or hum will be heard on the Phonograph and Microphone inputs, but not an
excessive amount.
6) Move the amplifier input cables from the Line Output jacks to the Tape Output jacks and
repeat Step #5 above.
After these preliminary tests have been successfully completed, operating levels should be set,
as detailed in the next section.
6.2 Setting Operating Levels
In order to achieve the best (lowest) noise performance from the preamplifier, it is necessary to
carefully set the operating points of the preamp stages. There are a number of variables involved.
The following steps are recommended prior to taking performance measurements3. Make these
adjustments on the right channel first, followed by the left channel.
Note that a reference point is needed when setting the operating levels. In the steps that
follow, the Auxiliary input is assumed to be the reference point. This is based on previous
experience with the preamplifier circuits. However, the selection of available source devices is
wide and varied and as such it may be necessary to select a different reference point, depending
on the specifics of a given application. Operational experience is the best guide here.
1) Set the front-panel Volume control for approximately 1/4 clockwise rotation. Set the front
panel Bass and Treble controls for flat response (center positions).
2) Connect an audio voltmeter to the output of the right channel preamplifier.
3) Switch the input to Auxiliary and apply a 1 kHz, 0.30 V rms signal to the right channel
Auxiliary input.
4) Adjust R19 (Auxiliary level control) to the full clockwise position.
3. Be extremely careful while making adjustments below the chassis as dangerous voltages exist. Use an insulated
adjustment tool; do not place hands near the circuit board or components.
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5) Readjust the front panel Volume control to produce 1 V rms output into a 1 MΩ load at the
output jack of the preamplifier. The setting of the Volume control should be within the 8 to
11 o’clock positions.
6) Switch the input to Tuner and apply a 1 kHz, 1.0 V rms signal to the right channel Tuner
input.
7) Adjust R18 (Tuner level control) to produce 1 V rms into 1 MΩ at the output terminal of the
preamplifier. The proper position for R18 should be approximately 2/3 clockwise. Do not
readjust the front panel Volume control.
8) Switch the input to Phonograph and apply a 1 kHz, 0.0072 V rms signal to the right channel
Phono input.
9) Adjust R16 (Phono level control) to produce 1 V rms output into a 1 MΩ load at the output
terminal of the preamplifier. Do not readjust the Volume control. The proper position for
R16 should be approximately 2/3 clockwise
10) Switch the input to Microphone and apply a 1 kHz, 0.0145 V rms signal to the right channel
Microphone input. Check to make sure the Microphone Mix switch on the back panel is set
to Stereo.
11) Adjust R17 (Microphone level control) to produce 1 V rms output into a 1 MΩ load at the
output terminal of the preamplifier. Do not readjust the Volume control. The proper position
for R17 should be approximately 2/3 clockwise.
Note that the output levels of different types of input devices varies widely. The setting
described here is a good starting point. Further adjustments may be needed depending on the type
of source material used.
12) Switch the preamplifier to Phonograph and apply a 1 kHz, 0.0072 V rms signal to the right
channel Phono input. Set the ac voltmeter to read decibels and adjust for a convenient
reference point.
13) Remove the input signal and connect a shorting connector to the Phono input.
14) While observing the residual noise level on the voltmeter, adjust R42 (Hum Balance control)
for a minimum reading. This setting can have a significant effect on the noise reading (up to
10 dB), so adjust carefully. At this point, a reading of –70 dB or so should be achieved.
15) Repeat steps #2 through #14 for the left channel of the preamplifier, with exception of step
#5. Do not readjust the front panel Volume control.
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Note that during adjustment of R42, the Hum Balance control, the optimal setting for one
channel may not coincide with the optimal setting for the other channel. The impact of this
adjustment can be significant. In the event that the minimum noise settings do not coincide, split
the difference between the two channels.
During adjustment of the input levels, the proper positions for the on-board potentiometers
should be nearly identical between channels. Minor gain variation between channels is normal.
For proper balance between channels it may be necessary to repeat the procedure outlined above
if the left channel output is noticeably less than the right channel, in which case the left channel
gain would be used as the reference point for setting the front panel Volume control.
16) Connect an audio voltmeter to the right channel Tape Output jack. Connect the signal
generator to the right channel Auxiliary input and switch the front panel Input control to
Auxiliary. Apply a 1 kHz, 0.30 V rms signal to the right channel Auxiliary input and adjust
the Tape Out Level control (R40) on the right channel PWB to produce 1 V rms at the Tape
Output jack. The proper position for R40 is about 1/3 clockwise rotation. Repeat this
procedure for the left channel.
The foregoing settings should provide a good starting point for use of the preamplifier. Touch-
up adjustments using program material can be made later.
After these adjustments have been made, remove power and install the bottom chassis plate.
After installing the bottom of the chassis, the measured noise floor should decrease by several
decibels.
6.3 Important Operational Note
The back panel power connector to the amplifier/power supply includes two ground connections
to make sure that a solid ground is always maintained between the preamplifier and the power
amplifier. Maintenance of the ground is essential to safe operation of the preamplifier. As a
measure of additional safety, a separate ground wire, connected to the ground posts on the back
panel of the amplifier and the preamplifier, must always be in place and secured. Do not operate
the preamplifier without all covers in place and the separate ground connection installed.
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7 Performance Measurement
While not strictly necessary, measurements should be taken on each channel of the stereo
preamplifier upon completion. It is acknowledged that the only performance measurements for
audio gear that really matter are how it sounds. Still, if the test equipment is available,
measurements are useful in confirming proper construction and serve as a baseline for any future
maintenance tasks.
Performance measurements should be taken at an output level of 1 V rms into a load of 1 MΩ.
Typical readings are given in Table 7.1. Measurements should be taken at the following input
positions:
• Phonograph.
• Microphone.
• Tuner/Auxiliary/Front Aux. The performance of these three inputs should be identical.
Set the tone-control potentiometers for their center (flat) response. The flat setting points for
the Bass and Treble controls are best found by using a sweep generator while monitoring the
output of the preamplifier with an ac voltmeter. Small variations from the optimal flat response
are not unusual between channels. Any deviation of more than about 1 dB may be cause for
further investigation. The following procedure is recommended.
1) Connect a sweep generator to the right channel Auxiliary input. Switch the Input to
Auxiliary. Connect an audio voltmeter to the right channel Line Output jack.
2) Set the sweep generator to produce a 1 kHz signal at 0.3 v rms. Adjust the Volume control to
produce 1 V rms at the Line Output jack. Adjust the audio voltmeter to read 0 dB.
3) Set the generator begin the sweep at about 15 Hz and end the sweep at about 1 kHz. Begin a
repetitive linear sweep of about 1 second duration.
4) Slowly adjust the Bass control for the flattest response, which should be in the range of +/–1
dB.
5) Move the generator input to the left channel Auxiliary input and move the audio voltmeter
input to the left channel Line Output. The left channel response should closely match the
right channel response.
6) Loosen the Bass control knob set screw and reposition the knob so the white line faces
straight up. Tighten the set screw.
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7) Set the generator begin a sweep at about 1 kHz and end the sweep at about 20 kHz. Begin a
repetitive linear sweep of about 1 second duration.
8) Slowly adjust the Treble control for the flattest response, which should be in the range of +/
–1 dB.
9) Move the generator input to the right channel Auxiliary input and move the audio voltmeter
input to the right channel Line Output. The right channel response should closely match the
left channel response.
6) Loosen the Treble control knob set screw and reposition the knob so the white line faces
straight up. Tighten the set screw.
Measurements should next be taken on the Tape Out buffer amplifier circuit. The following
procedure may be used.
7) Set the front panel Input switch to Auxiliary.
8) Connect the signal generator to the Tuner RCA jack on the back panel.
9 Connect the audio analyzer to the appropriate Tape Out RCA jack on the back panel.
10) Performance of the Tape Out circuit should meet or exceed the targets listed in Table 7.1 for
the Auxiliary input. With an input of 0.3 V rms, the output level at the Tape Out jack should
be approximately 1 V rms into 1 MΩ.
For the measurements listed in Table 7.1, with the input signal levels as given, a 1 V rms
output at the Line Out jacks should be observed with the front panel Volume control in the 8 to 11
o’clock position. For measurements of the phonograph and microphone circuits, use the Tape
Output jack since that port bypasses the tone-control stage and thus provides for more accurate
measurements.
Table 7.1 Performance Targets for the Stereo PreamplifierTest Parameter Target Value Notes
Tuner/Auxiliary/Front Aux Inputs
Input level 0.3 V rms1.0 V rms
AuxTuner
Frequency response ±2 dB, 15 Hz to 20 kHz
THD 0.6% Note 1
IMD 0.5% 4:1 mix ratio
Noise, unweighted, (shorted input terminals)
–70 dB
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Measurement of the frequency response of the phonograph circuit may indicate performance
that is not within the specified tolerance of the RIAA curve at the high end of the frequency range.
If, upon test, excessive falloff at high frequencies is observed, add capacitor CP (20 pf) to the
phono preamp circuit. This component can be soldered in place from the foil side of the board. It
is not necessary to remove the PWBs to add this part. Remove all power from the unit before
removing the bottom chassis plate. Using a voltmeter, check to be certain that all high voltage
capacitors have fully discharged. Replace the chassis bottom plate before restoring power to the
unit. Repeat the phono circuit frequency response tests after capacitor CP has been added.
It is important to remember that the performance of any circuit is determined in large measure
by the performance of the active devices. In the case of tubes used for low-level audio circuits,
some suppliers offer specialized services for certain types as a way of optimizing the application.
Tone Control Treble control maximum boost +16 dB At 20 kHz
Treble control, maximum cut –16 dB At 20 kHz
Bass control maximum boost +16 dB At 20 Hz
Bass control maximum cut –16 dB At 20 Hz
Phonograph Input Input level 0.0072 V rms
Frequency response ±1 dB, 20 Hz to 20 kHz
Note 2
THD 0.6% Note 1
IMD – Note 3
Noise, unweighted, (shorted input terminals)
–70 dB
Microphone Input Input level 0.0145 V rms
Frequency response ±2 dB, 30 Hz to 15 kHz
Note 4
THD 0.5% Note 1
IMD 0.8% 4:1 mix ratio
Noise, unweighted, (shorted input terminals)
–70 dB
Notes:
1 THD measurements taken at 20 Hz, 100 Hz, 1 kHz, 10 kHz, ands 20 kHz. The highest reading is recorded.
2 The frequency response of the phono preamp is relative to the RIAA equalization curve.
3 No IMD measurement is taken on the phono preamp because of the equalization curve of the circuit.
4 If an external impedance matching transformer (low-to-high) is used, performance of the microphone preamp will be largely determined by the characteristics of the transformer.
Table 7.1 Performance Targets for the Stereo Preamplifier
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For example, the 7025 (12AX7) can be ordered from at least one online vendor4 as: 1) standard
functional test, 2) high gain, 3) low noise and microphonics, or 4) matched set. For the
preamplifier, it would probably be a good idea to consider the low noise selection service, since
any reduction in noise—particularly for the phono and mic stages—will be beneficial to overall
performance.
7.1 RIAA Equalization
The purpose of pre-emphasis in vinyl disk recording and the complementary de-emphasis in
playback is to boost the amplitude of the recorded waveform at higher frequencies to improve the
signal-to-noise ratio (S/N). The emphasis, adopted as a standard by the Recording Industry
Association of America (RIAA) is known as the RIAA characteristic. The complementary
recording and reproduction curves are illustrated in Figure 7.1. Note the response is not a simple 6
dB per octave filter, but rather a shelved response, which was intended to compensate for certain
constraints relating to fine-groove disk record production and the legacy practices that existed at
the time the curve was adopted by the RIAA.
4. www.tubedeopt.com; there may be other vendors offering this service as well. The cost difference for the various
selection options is typically small.
Frequency, Hz20 50 100 200 500 1000 2000 5000 10,000 20,000
20
15
10
5
0
–5
–10
–15
–20
Rel
ativ
e ve
loci
ty o
f vol
tage
leve
l, dB
Reproducing characteristics
Recording characteristics
75 μs318 μs3180 μs
Figure 7.1 Characteristics for fine-groove records. (RIAA Dimensional Characteristics for 33-1/3 rpm Records.)
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Table 7.2 provides tabular values for the characteristic curve., referenced to 1 kHz. The data
range from 20 Hz on the low end to 20 kHz on the high end.
7.2 Final Touches
The stereo preamplifier has now been completed. Remove power from the unit and carefully
place the Plexiglas cover over the top. Secure with the four #6 screws and flat washers provided.
Use of the cover is recommended as it serves to protect the tubes from users (and users from the
tubes). It also aids cooling somewhat by creating a “chimney effect” that collects cool air from the
edges of the chassis and exhausts heated air from around the tubes and transformers. Do not
overtighten the mounting screws.
If the bottom chassis plate has not already been screwed on, do so now.
Table 7.2 Frequency Response as a Function of the RIAA Target ValuesFrequency, Hz RIAA Curve Target
Level, dBFrequency, Hz RIAA Curve Target
Level, dB
20 +19.3 1,000 0
30 +18.6 1,500 –1.4
40 +17.8 2,000 –2.6
50 +17.0 3,000 –4.7
60 + 16.1 4,000 –6.6
70 + 15.3 5,000 –8.2
80 +14.5 6,000 –9.6
100 +13.1 7,000 –10.7
110 + 12.4 8,000 –11.9
115 +11.6 9,000 –12.9
150 +10.2 10,000 –13.7
200 +8.3 12,000 –15.3
250 +6.7 14,000 –16.6
300 + 5.5 15,000 –17.2
400 +3.8 16,000 –17.7
500 + 2.6 18,000 –18.7
600 + 1.9 20,000 –19.6
700 + 1.2
800 + 0.7
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Stereo Audio Preamplifier
It is recommended that the performance of each channel be recorded for future reference.
Table 7.3 is provided for that purpose.
Figure 7.2 shows the completed stereo preamplifier with the Plexiglas cover installed.
Since the shelf life of a receiving tube is essentially unlimited, it is good practice to purchase
at least one spare tube for each type used.
Important: Never operate the stereo preamplifier unless all tubes are installed. The voltage of
the dc filament supply is not regulated and if multiple tubes are removed with power applied the
filament voltage can rise to the point that it could damage the remaining tubes.
Table 7.3 Measured Performance of the Stereo PreamplifierTest Parameter Right Channel Left Channel
Tuner/Auxiliary/Front Aux Inputs
Input level, AuxInput level, Tuner
Frequency response
THD
IMD
Noise, unweighted, (shorted input terminals)
Tone Control Treble control maximum boost
Treble control, maximum cut
Bass control maximum boost
Bass control maximum cut
Phonograph Input Input level
Frequency response
THD
IMD
Noise, unweighted, (shorted input terminals)
Microphone Input Input level
Frequency response
THD
IMD
Noise, unweighted, (shorted input terminals)
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WhitakerAudio
Figure 7.2 Stereo preamplifier with the decorative cover in place.
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Stereo Audio Preamplifier
8 Troubleshooting Guidelines
Even an audio device that has been carefully built can experience problems. When presented with
a problem, proceed in an orderly manner to trace it down. Many failures are simple to repair if
you stop and think about what’s happening. Looking over the schematic diagram for 10 to 15
minutes to consider the possible causes can save hours of trial-and-error troubleshooting. Never
rush through a troubleshooting job.
Stereo audio products offer the technician an added benefit in that the same problem rarely
manifests itself in both channels. As such, the technician has a “known good” circuit as a
reference.
When checking inside the unit, look for changes in the physical appearance of components.
An overheated resistor or leaky electrolytic capacitor may be the cause of the problem, or point to
the cause. Devices never fail without a reason. Try and piece together the sequence of events that
led to the problem. Then, the cause of the failure—not just the more obvious symptoms—will be
corrected. In high voltage power supplies, look for signs of loose connections and overheating.
Do not overlook the possibility of tube failure. Tubes can fail in unpredictable ways, and
substitution may be the only practical test.
Troubleshooting through the process of elimination can also be useful. This technique
involves isolating various portions of the circuit—one section at a time—until the defective
component is found.
Never touch anything inside the unit without first removing all ac power and discharging all
filter capacitors with a test lead to ground. Most plate power supplies include bleeder resistors to
drain-off the charge on the capacitor(s) in the circuit. However, some designs do not include this
safety feature. Be careful.
Analyze each planned test before it is conducted. Every test in the troubleshooting process
requires time, and so steps should be arranged to provide the greatest amount of information about
the problem.
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9 Safety Considerations
Electrical safety is important when working with any type of electronic hardware. Because
vacuum tubes operate at high voltages and currents, safety is doubly important. The primary areas
of concern, from a safety standpoint, include electric shock and hot surfaces of vacuum tube
devices. Table 9.1 lists required precautions for persons working around high voltages.
All persons working around high-voltage equipment should be familiar with first aid
treatment for electric shock and burns. Always keep a first aid kit on hand. Obtain detailed
information from the local heart association or Red Cross chapter. Personalized instruction on
first aid usually is available locally.
It is advisable to wear glasses or other protective eye wear when building or working on
electronic equipment. Injuries can result from solder splashes, cleaning chemicals, and other
unexpected events. When trimming the leads of a component, be careful to hold the excess length
so that when cut the lead does not fly towards the face.
Table 9.1 Safety Practices for Working Around High Voltage Equipment Remove all ac power from the equipment. Do not rely on internal switches to remove dangerous ac.
Discharge all capacitors using a grounded probe.
Do not remove, short-circuit, or tamper with interlock switches on access covers, doors, or enclosures.
Keep away from live circuits.
Allow any component to completely cool-down before attempting to replace it.
If a leak or bulge is found on the case of an electrolytic capacitor, do not attempt to service the part until it has completely cooled.
Avoid contact with hot surfaces within the system.
Know your equipment and do not take chances
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Stereo Audio Preamplifier
10 Notes and Tube Data
The basic design of the stereo preamplifier is adapted from a circuit contained in the RCA
Receiving Tube Manual, dated 1975. Circuit descriptions are based on the text contained in that
publication.
10.1 Tube Characteristics
The data given here have been abstracted from the RCA Receiving Tube Manual (1975) to reflect
usage in this product. The illustrations are adapted from the RCA manual as well.
10.1.1 6EU7
The 6EU7 is a miniature type used in high-gain resistance-coupled low-level audio amplifier
applications where low hum and non-microphonic characteristics are important. A 6EU7 is used
in the tone-control stage of the stereo preamplifier.
The 9-contact pinout is given in Figure 10.1. Device characteristic curves are shown in Figure
10.2.
Selected tube parameters are listed in Table 10.1.
H
H
NC
Kt2Gt2
Pt2Pt1
Gt1
Kt21
2
3
45
6
7
8
9
Legend:H = heaterK = cathode, triode (t) 1 or 2G = grid, triode (t) 1 or 2P = plate, triode (t) 1 or 2NC = not connectedFigure 10.1 Terminal pinout of the 6EU7 tube.
Type 6EU7each unit
Pla
te m
illia
mpe
res
Plate volts0 100 200 300 400
4
3
2
1
5
Figure 10.2 Characteristic curves for the 6EU7 tube.
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10.1.2 5879
The 5879 is a miniature type used as the input stage of a high-quality audio amplifier. A 5879 is
used in the microphone preamplifier circuit.
The 9-contact pinout is given in Figure 10.3. Device characteristic curves are shown in Figure
10.4.
Table 10.1 Key Parameters of the 6EU7.General Characteristics
Heater (ac/dc) 6.3 V, 0.3 A
Heater-cathode voltage
Peak value ±200 V maximum
Average value ±100 V maximum
Direct interelectrode capacitance (approx.), each unit
Grid to plate 0.15 pF
Grid to cathode and heater 1.6 pF
Plate to cathode and heater 0.2 pF
Equivalent noise and hum voltage (referenced to grid, each unit), average value1
1.8 μV rms
Class A1 Amplifier, Maximum Ratings (Design-Maximum Values), Each Unit
Plate voltage 330 V
Plate dissipation 1.2 W
Grid voltage
Negative bias value 55 V
Positive bias value 0 V
Characteristics
Plate voltage 100 V 250 V
Grid voltage –1 V –2 V
Amplification factor 100 100
Plate resistance (approx.) 80 kΩ 62.5 kΩTransconductance 1250 μmhos 1600 μmhos
Plate current 0.5 mA 1.2 mA
Notes:1 Measured in “true rms” units under the following conditions: heater volts (ac) = 6.3 V; center tap of heater trans-
former connected to ground; plate supply = 250 V; plate load resistor = 100 kΩ; cathode resistor = 2700 Ω, cath-ode bypass capacitor = 100 μF; grid resistor = 0 Ω; and amplifier covering frequency range 25 Hz to 10 kHz.
G1
G2
G3
NC
K
HH
NC
P1
2
3
45
6
7
8
9
Legend:H = heaterK = cathodeG = grid #1, 2, P = plateFigure 10.3 Terminal pinout of the 5879 tube
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Stereo Audio Preamplifier
Selected tube parameters are listed in Table 10.2.
Table 10.2 Key Parameters of the 5879.General Characteristics
Heater (ac/dc) 6.3 V, 0.15 A
Peak heater-cathode voltage ±100 V maximum
Direct interelectrode capacitance (approx.)
Grid #1 to plate 0.11 pF maximum
Grid #1 to cathode, heater, grid #2, and grid #3
2.7 pF
Plate to cathode, heater, grid #2, and grid #3
2.4 pF
Class A1 Amplifier, Maximum Ratings (Design-Maximum Values)
Plate voltage 330 V
Plate dissipation 1.25 W
Grid #1 (control grid) voltage
Negative bias value 55 V
Positive bias value 0 V
Grid #2 (screen grid) voltage See Figure 10.5
Grid #2 (screen grid) supply voltage 330 V
Grid #2 (screen grid) input
For grid #2 voltages up to 165 V 0.24 W
For grid #2 voltages between 165 V and 300 V
See Figure 10.5
Characteristics
Plate voltage 250 V
Grid #1 (control grid) voltage –3 V
Grid #2 (screen grid) voltage 100 V
0 100 299 300 400
Type 5879 triode sectionType 5879pentode connection
Grid #2 = 100 Vgrid #3 = 0 V
Plate volts Plate volts0 100 200 300 400
Pla
te m
illia
mpe
res
Pla
te (I
) o
r grid
#2
(I )
mill
iam
pere
sb
c2
14
12
10
8
6
4
2
6
5
4
3
2
1
0 100 200 300 400
Grids #2 and #3connected to plate
(a) (b)
Figure 10.4 Characteristic curves for the 5879 tube: (a) triode section, (b) pentode section.
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For certain voltage amplifier types, this device included, the maximum permissible screen
grid input varies with the screen grid voltage, as shown in Figure 10.5. Full rated screen grid input
is permissible at screen grid voltages up to 50 percent of the maximum rated screen grid supply
voltage. From the 50 percent point to the full rated value of supply voltage, the screen grid input
must be decreased. The decrease in allowable screen grid input follows a curve of the parabolic
form. The rating chart is useful for applications utilizing either a fixed screen grid voltage or a
series screen grid voltage-dropping resistor; specifically:• When a fixed voltage is used, it is necessary only to determine that the screen grid input is
within the boundary of the operating area on the chart at the selected value of screen grid volt-
age to be used.
· When a voltage dropping resistor is used, the minimum value of resistor that will assure tube
operation within the boundary of the curve can be determined by the relation
Where:
Rg2 = minimum value for the voltage-dropping resistor in ohms
Ec2 = selected screen grid voltage in volts
Ecc2 = screen grid supply voltage in volts
Pc3 = screen grid input in watts corresponding to Ec2
Grid #3 (suppressor grid) Connected to cathode at socket
Plate resistance (approx.)
2 mΩ
Transconductance 1000 μmhos
Plate current 1.8 mA
Grid #2 (screen grid) current 0.4 mA
Grid #1 (control grid) voltage (approx.) for plate current of 10 μA
–8 V
Maximum Circuit Value
Grid #1 (control grid) circuit resistance 2.2 mW
Table 10.2 Key Parameters of the 5879.
( )2 2 22
2
c cc cg
c
E E ER
P−
≥
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Stereo Audio Preamplifier
10.1.3 7025 (12AX7)
The 7025 is a miniature type used as a phase inverter or resistance-coupled amplifier in high-
fidelity audio amplifiers. The 7025 is used in the phonograph preamp and line-out buffer
amplifiers in the stereo preamplifier. This type is identical with the 12AX7A (ECC83) tube except
that it has a controlled equivalent noise and hum characteristic. Each triode is independent of the
other except for the common heater.
The 9-contact pinout is given in Figure 10.6. Device characteristic curves are shown in Figure
10.7.
This curve also applies to typesin which grids #2 and #4 are connected
together within the tube
Grid
#2
inpu
t exp
ress
ed a
s a
perc
enta
ge o
f max
imum
grid
#2
inpu
t rat
ing
100
80
50
40
20
0 20 40 60 80 100 120Grid #2 voltage expressed as a percentage of maximum
grid #2 supply voltage rating
Area of permissible operation
Figure 10.5 Grid #2 (screen grid) input rating curve.
Pt2
Pt1Kt2
Ht2
Gt2
Ht1
Gt1
Kt1H
1
2
3
45
6
7
8
9
Legend:H = heaterK = cathode, triode (t) 1 or 2G = grid, triode (t) 1 or 2P = plate, triode (t) 1 or 2Figure 10.6 Terminal pinout of the 7025 tube.
Plate volts0 100 200 300 400
Type 7025
Pla
te m
illia
mpe
res
3
2
1
Figure 10.7 Characteristic curves for the 7025 tube.
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WhitakerAudio
Selected tube parameters are listed in Table 10.3.
Table 10.3 Key Parameters of the 7025.General Characteristics
Heater (ac/dc)
Parallel connection 6.3 V, 0.45 A
Series connection 12.6 V, 0.15 A
Heater-cathode voltage
Peak value ±200 V max.
Average value 100 V max.
Direct interelectrode capacitance (approx.) Unit #1 Unit #2
Grid to plate 1.7 pF 1.7 pF
Grid to cathode and heater 1.6 pF 1.6 pF
Plate to cathode and heater 0.46 pF 0.34 pF
Class A1 Amplifier, Each Unit, Maximum Ratings (Design-Maximum Values)
Plate voltage 330 V
Plate dissipation 1.2 W
Grid voltage
Negative bias value 55 V
Positive bias value 0 V
Plate dissipation 1.2 W
Equivalent Noise and Hum Voltage Referenced to Each Grid (each unit)
Average value (rms)1 1.8 μV
Maximum value (rms)2 7 μV
Notes:
1 Measured in “true rms” units under the following conditions: heater volts (ac) = 6.3 V (parallel connection); center tap of heater transformer connected to ground; plate supply = 250 V; plate load resistor = 2700 Ω; cathode bypass capacitor = 100 μF; grid resistor = 0 Ω; and amplifier covering frequency range 25 Hz to 10 kHz.
2 Same conditions as for “average value” except the cathode resistor is unbypassed and the grid resistor = 50 kΩ.
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Stereo Audio Preamplifier
Stereo Preamplifier
Addendum
The Stereo Preamplifier User and Assembly Manual is written with the assumption that the user
has purchased the product from WhitakerAudio as a ready-to-assemble package. For builders that
have acquired the components on their own and are using the manual as an assembly guide, it is
recommended that the parts be divided into the packaging referred to in the manual.
In the process of acquiring parts for this amplifier, a number of vendors will be used. As such,
parts will arrive lumped together and will also arrive at various points in time. The process used
by WhitakerAudio involves organizing the components into three major groups:
• Right channel printed wiring board (PWB) components
• Left channel PWB components
• Chassis components
Parts are organized by component number and device type. For example, all 1 kΩ 1/2-watt
resistors for the right channel PWB are packaged together. Each group of parts is placed in a
resealable (clasp-type) envelope with a self-adhesive label applied. Labels such as Avery #5260
are useful for this purpose. Recommended envelopes include the following:
• 2.25-inch by 3.5-inch envelope (Staples no. 1 coin envelope)
• 3-3/8-inch by 6-inch envelope (Staples no. 6 coin envelope)
• 6-inch by 9-inch clasp envelope (Staples)
While this sorting requires some up-front organizational time, it serves as a check that all parts
are accounted for and speeds assembly of the PWBs and chassis.
The following tables list the labels used for the three component groups.
Table 1 Right Channel PWB Component Package LabelsC1, C4, C11, C24, C3125 μF, 50 V, electrolytic,
right channel
R147 kΩ, 0.5 W,right channel
V1, V4, V57025/12AX7 tube,
right channel
C2, C5, C15, C17, C26, C28 C29
22 μF, 450 V, electrolytic,right channel
R2, R72.7 kΩ, 0.5 W,right channel
V25879 tube,
right channel
C3, C160.1 μF, 400 V,right channel
R3, R5, R12, R25, R28, R31, R35, R37, R39, R45
100 kΩ, 0.5 W,right channel
V36EU7 tube,
right channel
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WhitakerAudio
C60.0033 μF, 600 V,
right channel
R439 kΩ, 0.5 W,right channel
D1, D2Zener, 200 V, 5W,
right channel
C70.01 μF, 400 V,
right channel
R6, R14, R20470 kΩ, 0.5 W,right channel
Heater circuit wire6 ft., #22, green, solid,
right channel
C8180 pF, 500 V, mica,
right channel
R8680 kΩ, 0.5 W,right channel
4-terminal Molex barrier strip,right channel
C9, C12, C20, C25, C320.22 μF, 400 V,
right channel
R9, R15, R29, R38, R4115 kΩ, 2 W,right channel
¾-inch #4 threaded standoff(quantity = 10),right channel
C10, C14, C330.047 μF, 400 V
right channel
R10, R2322 kΩ, 0.5 W,right channel
12 terminal Molex header(quantity = 2),right channel
C1347 μF, 450 V, electrolytic,
right channel
R112.2 mΩ, 0.5 W,right channel
4 terminal Molex header(quantity = 8),right channel
C18, C220.0022 μF, 600 V,
right channel
R13, R24, R36, R461 kΩ, 0.5 W,right channel
9-pin tube sockets(quantity = 5),right channel
C190.022 μF, 400 V,
right channel
R16, R17, R40250 kΩ potentiometer, linear,
right channel
Printed Wiring Board,right channel
C21220 pF, 600 V, ceramic,
right channel
R211.5 kΩ, 0.5 W,right channel
Tube socket hookupbare wire, heat shrink, tie wrap
right channel
C23, C30100 pF, 1000 V disc,
right channel
R2215 kΩ, 0.5 W,right channel
CP25 pF, 1000 V ceramic,
right channel
C270.01 μF disc, 1 kV,
right channel
R2710 kΩ, 0.5 W,right channel
R42100 Ω, 1 W, potentiometer,
right channel
R47100 kΩ, 2W,right channel
R33, R34, R43, R44220 kΩ, 0.5 W,right channel
R18, R19100 kΩ potentiometer, linear,
right channel
Table 2 Left Channel PWB Component Package LabelsC1, C4, C11, C24, C3125 μF, 50 V, electrolytic,
left channel
R147 kΩ, 0.5 W,left channel
V1, V4, V57025/12AX7 tube,
left channel
C2, C5, C15, C17, C26, C28 C29
22 μF, 450 V, electrolytic,left channel
R2, R72.7 kΩ, 0.5 W,
left channel
V25879 tube,left channel
Table 1 Right Channel PWB Component Package Labels
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Stereo Preamplifier
C3, C160.1 μF, 400 V,
left channel
R3, R5, R12, R25, R28, R31, R35, R37, R39, R45
100 kΩ, 0.5 W,left channel
V36EU7 tube,left channel
C60.0033 μF, 600 V,
left channel
R439 kΩ, 0.5 W,left channel
D1, D2Zener, 200 V, 5W,
left channel
C70.01 μF, 400 V,
left channel
R6, R14, R20470 kΩ, 0.5 W,
left channel
Heater circuit wire6 ft., #22, green, solid,
left channel
C8180 pF, 500 V, mica,
left channel
R8680 kΩ, 0.5 W,
left channel
4-terminal Molex barrier strip,left channel
C9, C12, C20, C25, C320.22 μF, 400 V,
left channel
R9, R15, R29, R38, R4115 kΩ, 2 W,left channel
¾-inch #4 threaded standoff(quantity = 10),
left channel
C10, C14, C330.047 μF, 400 V
left channel
R10, R2322 kΩ, 0.5 W,left channel
12 terminal Molex header(quantity = 2),left channel
C1347 μF, 450 V, electrolytic,
left channel
R112.2 mΩ, 0.5 W,
left channel
4 terminal Molex header(quantity = 8),left channel
C18, C220.0022 μF, 600 V,
left channel
R13, R24, R36, R461 kΩ, 0.5 W,left channel
9-pin tube sockets(quantity = 5) ,
left channel
C190.022 μF, 400 V,
left channel
R16, R17, R40250 kΩ potentiometer, linear,
left channel
Printed Wiring Board,left channel
C21220 pF, 600 V, ceramic,
left channel
R211.5 kΩ, 0.5 W,
left channel
Tube socket hookupbare wire, heat shrink, tie wrap
left channel
C23, C30100 pF, 1000 V disc,
left channel
R2215 kΩ, 0.5 W,left channel
CP25 pF, 1000 V ceramic,
left channel
C270.01 μF disc, 1 kV,
left channel
R2710 kΩ, 0.5 W,left channel
R18, R19100 kΩ potentiometer, linear,
left channel
R47100 kΩ, 2W,left channel
R33, R34, R43, R44220 kΩ, 0.5 W,
left channel
Table 3 Chassis Component Package Labels
Plexiglas trim,chassis
Large knob(quantity = 4),
chassis
Top chassis decal,chassis
Table 2 Left Channel PWB Component Package Labels
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WhitakerAudio
R26, R301 mΩ potentiometer, audio
taper, dual gang,chassis
Handle, side(quantity = 2),
chassis
Braided sleeving andheat-shrink tubing
chassis
SW1Rotary switch,
chassis
RCA connector(quantity = 10),
chassis
Chassis hookup wirechassis
SW2Toggle switch,
chassis
Mic connector(quantity = 2),
chassis
Ground lug hardware,chassis
Main chassis, 10-in x 17-in x 3-in, steel
chassis
3.5 mm connectorchassis
Cable management hardwarechassis
Main chassis baseplate, 10-in x 17 in, steel,
chassis
Power connector hardwarechassis
Molex crimp terminal(quantity = 128),
left channel
Front panel, 19-in x 3.5-in,chassis
12 terminal Molex housing(quantity = 4)
chassis
Pilot Lamp,chassis
Rear panel overlay(quantity = 3),
chassis
4 terminal Molex housing(quantity = 16)
chassis
Chassis feet(quantity = 4),
chassis
Front panel handles(quantity = 2),
chassis
Molex crimp terminal(quantity = 64),
chassis
R32250 kΩ potentiometer, audio
taper, dual gang,chassis
C284700 μF, 50 V, electrolytic
chassis
CR-1bridge rectifier, 25 W, 50 V
chassis
9-pin tube socket shield(quantity = 4)
chassis
Table 3 Chassis Component Package Labels
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Cable Assembly Procedure
Cable Assembly Procedure
The stereo preamplifier relies on the power supply contained in the audio amplifiers of the
WhitakerAudio line to function. Power from the amplifier is coupled to the preamp and/or tuner
through a connecting cable. This document describes the procedures for assembling the necessary
cable.
The pinout for the power connector is given in Table 1.
1 Procedure
Expandable flexible sleeving is used to bundle the wires of the cable assembly. To properly
perform the following steps, it will be necessary to have a heat gun on hand. (A common hair
dryer may be sufficient.) In addition, the preferred method of cutting the tubing is to use a so-
called “heat knife.” If such a tool is unavailable, scissors may be used.
Examine the connector body. The pin numbers are stamped on both sides of the device. Be
certain to place each pin in the correct position.
The following procedure is recommended for assembling the connecting cable.
1.1 Step-by-Step Instructions
The standard length for this cable is 3 ft. Users requiring a longer (or shorter) cable should adjust
the following steps accordingly.
– Prepare two 3-ft pieces of #16 black stranded wire. Attach male connectors on one end
and female connectors on the other end.
Table 1 Power Connector PinoutPin No. Function Color Code
1 Ground Black
2 Pilot lamp White
3 Pilot lamp White
4 Auxiliary B+ Red
5 Filament Green
6 Filament Green
7 Not used
8 Not used
9 Ground (Black
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WhitakerAudio
– Prepare a 3-ft piece of #16 red stranded wire. Attach a male connector on one end and a
female connector on the other end.
– Prepare two 3-ft pieces of #16 white stranded wire. Attach male connectors on one end
and female connectors on the other end.
– Prepare two 3-ft pieces of #16 green stranded wire. Attach male connectors on one end
and female connectors on the other end.
– Insert the male connectors into a matching housing as follows:
• Black wire to pin #1
• White wire to pin #2
• White wire to pin #3
• Red wire to pin #4
• Green wire to pin #5
• Green wire to pin #6
• Black wire to pin #9
Note that pin #7 and pin #8 are not used.
– Loosely twist the two white wires together.
– Loosely twist the two green wires together.
– Cut a 3-ft length of expandable sleeving and place over the cable bundle. Cut two 3/4-inch
long pieces of heat-shrink tubing and place on each end of the sleeving. Process (heat) the
tubing closest to the connector socket. Caution: do not use too much heat as it can melt the
sleeving.
– Assemble the connector by screwing the housing onto the connector body. Install and
secure the cable clamp.
– Slide the housing for the female connector on to the cable. Be sure to properly orient the
housing with respect to the connector.
Using an ohmmeter, identify the black wire inserted in the pin #1 position on the male
connector. This wire should be connected to pin #1 on the female connector. For the white wire
pair and the green wire pair, an exact match between the connectors is not necessary.
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Cable Assembly Procedure
– Insert the female connectors into a matching housing as follows:
• Black wire to pin #1
• White wire to pin #2
• White wire to pin #3
• Red wire to pin #4
• Green wire to pin #5
• Green wire to pin #6
• Black wire to pin #9
Note that pin #7 and pin #8 are not used.
– Process (heat) the tubing closest to the connector socket. Caution: do not use too much
heat as it can melt the sleeving.
– Assemble the connector by screwing the housing onto the connector body. Install and
secure the cable clamp.
The connecting cable has been completed and is ready for use.
2 Ground Connection Cable
The back panel power connector to the amplifier/power supply includes two ground
connections to make sure that a solid ground is always maintained between the preamplifier and
the power amplifier. Maintenance of the ground is essential to safe operation of the preamplifier.
As a measure of additional safety, a separate ground wire, connected to the ground posts on the
back panel of the amplifier and the preamplifier, must always be in place and secured. Do not
operate the preamplifier without all covers in place and the separate ground connection installed.
The ground connection cable is assembled as described in the following steps:
– Prepare a 3-foot length of green #16 wire.
– Attach a ring terminal to each end of the cable. Crimp.
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