Elektor Electronics 5/97

The battery-powered preamplifier published in our February 1997 issuemust have made many of our readers long for a matching power ampli-

fier. Odd as it may seem, it is almost two years ago that we last pub-lished a high-end power amplifier. This realization prompted the present

compact power amplifier whose quality and layout match those of thepreamplifier. It offers medium power output, is not overly complex and

has a very high slew rate.

42

Design by T. Giesberts

compactpower amplifier

Fast high-end design

T5

BC550C

T2

BC

T4

BC

T13

BF871

T10

BC550C

T14

BD139

T15

MJE15030

T1

BC

T3

BC

T6

BC560C

T9

BC560C

T12

BF872

T16

MJE15031

T19

BC640

T7

BF245A

T8

BF245A

T11

BF245A

R5

47

0Ω

R6

47

0Ω

R447

Ω

R3

47

Ω

R2

47

k

R10

22

Ω1

R11

22

Ω1

R7

36

Ω5

R42

8k

2

R9

34

0Ω

R15

27

0Ω

R16

22

1Ω

R17

15

0Ω

R12

27

0Ω

R13

22

1Ω

R14

15

0Ω

R18

10

k

R22

27

Ω4

R19

27

Ω4

R25

1k

8

R261k

R28

10

k

R2027

0Ω

R21

4k

7

R23

27

0Ω

R24

4k

7

R1

470Ω

C1

2µ2

C2

1n

C3

100µ 25V

C4

100µ 25V

D1

D2

C9

10n

C8

1n

C11

10n

C10

1n

R8

340Ω

C7

1n

C6

10µ 63V

C5

10µ63V

C12

4µ763V

C13

4µ763V

D3

4V7

0W5

D4

4V7

0W5

C14

2µ2

R27

22Ω

1kP1

C17

1µ

R31

22

Ω

R3522

Ω

R32

39

0Ω

R36

39

0Ω

R29

47

Ω

C15

10µ63V

C16

10µ63V

R30

47Ω

C18

4n7

R37

22Ω

R33

22Ω

T17

GT20D201

T18

GT20D101

R34

0Ω

22

R38

0Ω

22 C20

1000µ 40V

C19

1000µ40V

R39

2Ω2

L1

R432

2Ω R45

27

k R46

1k

8

R40

15

0k

R41

15

0k

R44

27k

C24

220µ25V

D9

D10

D11

C22

2µ2

C21

2µ2

Re1

C23

100µ 40V

OP77

IC32

3

6

7

41

8

D75V6

0W5

D85V6

0W5

Re1 = V23056-A0105-A101

GT20D201

G C E

GT20D101LM317

adj.

BF245

G D

S

BD139

E

C

B

BC550C

CB

E

BC640

CBE

MJE15031MJE15030

C

B E

BC560C

D6

5V1 1W5

BF872

550C

560C

23V2

23V2

23V2

23V2

30V

30V

D5

5V1 1W522V

22V

970043 - 11

LS

LS

LM317

IC1

LM337

IC2

BF871

E

C

B

LM337

adj.

1V

66

0V

10

V1

550C

1V

66

0V

10

V1

560C

1V1

V2

4

4V

6

1V1

V2

4

4V

6

0V037 0V047

32

V

1V

3

0V

35

1V

25

2V

4

0V08

8

0V

22V

0V

35

1V

25

0V08

8

–17V6

+17V6

11V8

1N40023x

0V13

80V

131

2V

3

The introduction gives a good idea ofthe power amplifier described in thisarticle.

The term ‘compact’ refers to boththe power output and the dimensionsof the amplifier. Of course, compact-ness is a relative term, because theamplifier contains no fewer than 19transistors. Over the years, compact-ness has taken on a different meaningthan, say, ten years ago. Then, a com-pact amplifier was understood to con-tain not more than eight to ten tran-sistors; today this figure is nearer totwenty-five. After all, as everybodynow realizes only too well, qualityalways has its price.

As far as output power is con-cerned, the present amplifier is defi-nitely intended for normal domesticuse. It produces some 50 watts into 8 Ωor 85 watts into 4 Ω, which is morethan ample for its intended use. Evenat relatively high volume, a domesticpower amplifier seldom provides more

than 1–2 power output. To high-volume freaks it may sound

odd, but most loudspeakersproduce a sound pressure

of about 90 dB at a distance of 1 metrefor an input of only 1–2 watts—enough to make the neighbours rushfor the telephone.

It is clear, therefore, that the presentamplifier has more than ample reserveto reproduce even the highest peaks ina piece of music without any dis-cernible distortion.

A further benefit of the circuit is theuse of such a relatively high quiescentcurrent that with power outputs up to2.5 W the amplifier operates in Class A,which enhances the overall quality ofthe reproduced sound.

There are some other aspects whichwill be reverted to in detail later.Firstly, the power output is producedby two insulated gate bipolar transis-tors (IGBTs). These bipolar MOSFETsor, if you prefer, gate-controlled tran-sistors have been used in power ampli-fiers published by us before.

Secondly, the amplifier uses currentfeedback instead of the more usualvoltage feedback. This has a beneficialeffect on the open-loop bandwidth, thepower bandwidth and the slew rate.

Thirdly, as may be seen from the

technical data, the figures for har-monic distortion, intermodulation dis-tortion and the damping factor arevery good, indeed. They make it clearthat although the amplifier is small insize, it is large in performance.

T H E D E S I G NExperts in audio amplifier technology,as well as many other knowledgeableconstructors, will see from Figure 1that in some respects the design of thepower amplifier is different from theusual arrangement.

To start with, the almost obligatorydifferential input amplifier is not there.In its stead there is a symmetricalinput stage that has some resemblanceto the buffer stage used at the input of

43Elektor Electronics 5/97

1

Figure 1. The mostconspicuous aspectof the circuit diagramis the omission of thetraditional differentialamplifier at the input.The output stage isbased on insulatedgate bipolar transis-tors.

the battery-operated preamplifier.In combination with current feed-

back this results in an amplifier that isappreciably faster than one with thetraditional differential input stage.

The power amplifier has an open-loop bandwidth of some 40 kHz at therelative low amplification of ×2500.This enables the overall feedback,

which many enthusiasts do not like, tobe kept to a quite conservative level.

There is another side to everythingand current feedback is no exception.A well-known but unavoidable draw-back is the poor common-mode andsupply-line suppression. In the presentdesign, the effects of these have beencountered to a large extent, however,

by the use of two regulators in thepower lines to all voltage amplifiers.This, in turn, has the drawback thatthe level of the supply voltage to theseamplifiers is lower than that to the cur-rent amplifiers, although it shouldreally be higher. This limits the maxi-mum drive, but that is compensatedby making the current amplifiers

44 Elektor Electronics 5/97

Technical dataInput sensitivity 1 V r.m.s.Output impedance 47.5 kΩOutput power (0.1% THD) 50 W into 8 Ω; 85 W into 4 ΩPower bandwidth (25 W into 8 Ω) 1.5 Hz–270 kHzSlew rate 37 V µs–1

Signal-to-noise ratio (1 W into 8 Ω) 107 dB (A-weighted)102 dB (B = 22 kHz linear)

Total harmonic distortion (B = 80 kHz)1 W into 8 Ω 0.0015% (1 kHz)25 W into 8 Ω 0.0025% (1 kHz)

0.008% (20 kHz)

Intermodulation distortion (50 Hz:7 kHz = 4:1)1 W into 8 Ω 0.0025%25 W into 8 Ω 0.008%

Dynamic intermodulation distortion (Square wave 3.15 kHz, sine wave 15 kHz)1 W into 8 Ω 0.002%25 W into 8 Ω 0.002%

Damping factor (8 Ω) 700 (1 kHz)450 (20 kHz)

Open-loop parameters (R8 open)Amplification ×2500Bandwidth 40 kHzOutput impedance 0.3 Ω

These data show the excellent performance of the power ampli-fier; exceptionally good are the power bandwidth and the slewrate. Some audio enthusiasts feel that performance data shouldbe taken with a pinch of salt and that the important thing is ahearing test. This is not an opinion shared by this magazine. It istrue that figures do not reveal everything, but it is surely admittedby all that poor data point to properties that cannot have a bene-ficial effect on the sound quality.

As usual, the data are accompanied by a number of curvesobtained with an Audio Precision Analyser.Curves A show the total harmonic distortion (THD+N) between20 Hz and 20 kHz. The upper curve refers to 25 W into 8 Ω, andthe lower to 1 W into 8 Ω. Both curves show an exemplary perfor-mance; note that the usual increase in distortion at high frequen-cies is small.Curve B illustrates the distortion at 1 kHz as a function of thedrive power over a bandwidth of 22 Hz to 22 kHz. The slightincrease above about 2.5 W shows that from that point on theamplifier no longer operates in Class A. The clipping point is at50 kHz.Curves C refer to the output power of the amplifier as a functionof frequency with a load of 4 Ω and 8 Ω respectively.Curve D illustrates a Fourier analysis of a 1 kHz signal for an out-put of 1 W into 8 Ω. Although the 2nd and 3rd harmonics areclearly visible, their levels at –105 dB and –122 dB are well belowthat of the fundamental. All other harmonics remain below thenoise floor.Listening tests showed the very good performance of the poweramplifier even better than the technical data. The sound repro-duction was very pleasant with good presence and transparenthigh-frequency performance that never became sharp. Low-fre-quency performance, both with classical and popular music,remained remarkably taut. The amplifier held the loudspeakers inan iron grip as it were and never relinquished control.

vs

0.001

0.010

0.1

1

10THD+N(%)

20 100 1k 10k 20k

FREQ(Hz)

970043 - 13

ELEKTOR DEFAULT3

AUDIO PRECISION THDVSLVL vs measured

.0005

0.001

0.010

0.1

1THD+N(%)

1m 10m 0.1 1 10 50

LEVEL(W)

970043 - 14

AUDIO PRECISION PWR-BAND vs

1

10

100

300LEVEL(W)

20 100 1k 10k 20k

FREQ(Hz)

970043 - 15

ELEKTOR GB3FFT vs

-160.0

-140.0

-120.0

-100.0

-80.00

-60.00

-40.00

-20.00

0.0AMP1(dBr)

0.0 500.0 1.00k 1.50k 2.00k 2.50k 3.00k 3.50k 4.00k

FREQ(Hz)

970043 - 16

A

B

C

D

amplify ×2. Because of this, the outputstage is not arranged as the usual emit-ter follower, but rather as a compoundconfiguration.

Output transistors T17 and T18 are,as mentioned earlier, insulated gatebipolar types. These may be consid-ered as transistors with a MOSFETinput, which has the advantage ofrequiring a smaller current from thedrivers, T15 and T16. As this results inlower loading of the local feedbackloop, it. means that the amplificationof the drivers can be higher, whichenhances the linearity of the currentamplifiers.

I N P U T S T A G EThe input stage of the amplifier con-sists of emitter followers T1 and T2 andsymmetrical voltage amplifiers T5 andT6 (Figure 1). Current feedback isobtained by coupling the output of thepower amplifier to the emitters of T5and T6.

The emitter followers provideimpedance matching and arrange thebase bias of T5 and T6. This arrange-ment relies on the fact that the dropacross R10 and R11 is virtually the sameas that across R3 and R4. The potentialacross the latter two resistors is heldconstant by current sources T3 and T4.Moreover, the reference voltages forthese current sources are provided byD1 and D2, the current through whichin turn is held constant by currentsources T7 and T8.

To prevent the operating levelsbeing upset by temperature effects inspite of these measures, T5 and T6 arethermally coupled to T1 and T2, whileD1 and D2 are thermally coupled tocurrent sources T3 and T4.

For best performance, it is advisableto select transistors T1 and T2 on thebasis of equal base-emitter voltage andcurrent amplification, but make surethat the (offset) potential across R2<50 mV. Since complete symmetry isvirtually unattainable, there willalways be some offset which is whythe circuit based on IC3 has beenadded. This stage compensates theinput offset by placing an identicalpotential at feedback junction R10-R11.This will be reverted to later.

Combinations R14-C9 and R17-C11are compensating networks, while C8and C10 minimize the effects of theparasitic capacitances of T5 and T6.

V O L T A G E A M P L I F I E R SA N D O U T P U T S T A G EVoltage amplifiers T5 and T6 drive apush-pull amplifier based on T9–T13,which is arranged as a cascode stagefor the following reasons. Firstly, suchan arrangement limits the potentialacross, and the dissipation of, theactual voltage amplifier T9-T10. Sec-ondly, a cascode configuration is ideal

for obtaining high amplification cou-pled with a large bandwidth.

The d.c. operating point of the volt-age amplifier is held stable by zenerdiodes D3 and D4, the current throughwhich is in turn held constant by cur-rent source T11.

The voltage amplifiers are loadedby the input impedance of T15-T16.Since this varies in accord with theoutput current of the power amplifier,it is linearized by resistor R28. Capaci-tor C17 prevents any direct voltagefrom reaching the base of T16.

The circuit based on T14 forms atemperature-independent transistorzener which has become a familiaraspect in many power amplifiers. Thistransistor is mounted on the heat sinkadjacent to output transistors T17 andT18 and stabilizes the quiescent currentthrough these devices. The level of thiscurrent is set with P1.

Resistor R27 provides the additionalnegative temperature coefficientneeded to compensate the time delayand thermal decay in the heat sink.

The output of the voltage ampli-fiers is applied to current amplifierT15–T18. Strictly speaking, this is acompound rather than a currentamplifier, since, apart from currentamplification, it also provides voltageamplification. The voltage gain (×2) isdetermined by resistors R29 and R30;capacitor C18 serves to improve theresponse of the stage.

Since in a compound amplifier thecollectors of the driver transistors formthe output of the current amplifier, thegate-emitter potential does not influ-ence the maximum drive from thevoltage amplifier. This is an importantbenefit, because the gate-emitterpotential may be several volts. Theonly limiting factor in the presentamplifier, is, therefore, the knee volt-age of T17 and T18.

Emitter resistors R34 and R38 arelow-inductance types to prevent anytendency to oscillation or other spuri-ous effects.

Inductance L1 wound on R39enhances the performance of thepower amplifier when the load iscapacitive.

Zener diodes D5 and D6 protect thegates of T17 and T18 against overdrive.

In operation, the temperature ofdrivers T15 and T16 strongly affectsthe setting of the quiescent current.So as to make the power amplifier asstable with temperature variations asfeasible, these transistors are, there-fore, also mounted on the same heatsink as T14, T17 and T18. While it istrue that the quiescent current willalways vary to some (small) degreewith temperature, the thermal cou-pling of the various transistorsensures that any drift is well withinacceptable limits.

M I S C E L L A N E O U SA S P E C T SSo as to keep the power amplifier ascompact as possible, it does not containelaborate protection circuits.There is,however, a power-on delay to suppressannoying clicks and plops at on and offswitching. Delayed on/off switching ofthe power lines is effected by relay Re1.To avoid any noise on the signal pathsvia the supply lines, the relay has inde-pendent power lines. For this purpose,D10 and D11 rectify the secondary volt-age of the mains transformer. Theresulting direct voltage is buffered byC23 which ensures that the energizingvoltage for the relay is 22–24 V.

After the power has been switchedon, T19 begins to conduct slowly viaR44-R45-C24. It takes a few secondsbefore this transistor is fully on: onlythen will the relay be energized. Whenthe power is switched off, C24 israpidly discharged via R46 so that therelay is deenergized without any dis-cernible delay.

To aid the offset setting, IC3 com-pares the output of low-pass filterR40-C21 with the direct voltage at theoutput of the power amplifier. If thereis a difference, the operating point ofT5-T6 is adjusted via R42 in such amanner that the average output of thepower amplifier remains at earthpotential. This arrangement ensuresthat the offset voltage at the outputcannot exceed that of IC3 (≤ 100 µV at25 °C). It is a fact, however, that tinyvariations caused by temperaturechanges cannot be eliminated entirely.

Zener diodes D7 and D8 reduce thesupply voltage of ±23.2 V to the req-uisite level for IC3.

The reasons for regulation of thesupply lines to the input and voltageamplifiers has already been men-tioned. The regulation is provided byIC1 and IC2. Type LM317 and LM337respectively have been used for theseregulators because they provide verygood ripple suppression and tolerate ahigh peak input voltage. A furtheradvantage of them is that their outputpotential can be adjusted very accu-rately by means of resistors R20-R21and R23-R24 respectively. CapacitorsC15 and C16 increase the ripple sup-pression to 70–80 dB.

C O N S T R U C T I O NThe power amplifier is best built onthe double-sided printed-circuit boardshown in Figure 2. Making the boarddouble-sided meant that it could bemade very compact without the neces-sity of (too) long copper tracks atessential locations. The layout is unam-biguous so that population of theboard is straightforward.

Transistors T14–T18 are locatedneatly in a row at one side of theboard to facilitate their fixing on to the

45Elektor Electronics 5/97

46 Elektor Electronics 5/97

C1

C2

C3 C4

C5

C6

C7

C8

C9

C10 C11

C12

C13

C14

C15

C16

C17 C18

C19

C20

C21

C22 C23

C24D1 D2

D3 D4

D5D6

D7D8

D9

D10

D11

H1

H2H3

H4

IC1

IC2

IC3

L1

P1

R1R2

R3

R4R

5

R6

R7

R8 R9R10R11

R12

R13

R14

R15 R16

R17

R18R19

R20R21

R22

R23R24

R25

R26

R27

R28

R29

R30

R31

R32

R33

R34R

35 R36

R37

R38

R39

R40

R41

R42

R43

R44

R45

R46

RE1T1 T2

T3 T4T5 T6

T7

T8

T9 T10

T11T12 T13

T14

T15T

16T17 T18

T19

0

-

+

LS- LS+

~~0

T

970043-1

970043-1

Parts list

Resistors:R1, R5, R6 = 470 ΩR2 = 47 kΩR3, R4 = 47 ΩR7 = 36.5 Ω, 1%R8, R9 = 340 Ω, 1%R10, R11 = 22.1 Ω, 1%R12, R15, R20, R23 = 270 ΩR13, R16 = 221 Ω, 1%R14, R17 = 150 ΩR18, R28 = 10 kΩR19, R22 = 27.4 Ω, 1%R21, R24 = 4.7 kΩR25, R46 = 1.8 kΩR26 = 1 kΩR27, R31, R33, R35, R37, R43 = 22 ΩR29, R30 = 47 Ω, 5 WR32, R36 = 390 ΩR34, R38 = 0.22 Ω, 5 W, low inductanceR39 = 2.2 Ω, 5 WR40, R41 = 150 kΩR42 = 8.2 kΩR44, R45 = 27 kΩP1 = 1 kΩ preset

Capacitors:C1, C14, C21, C22 = 2.2 µF metal-

lized polyester, pitch 5 or 7.5 mmC2, C7, C8, C10 = 1 nFC3, C4 = 100 µF, 25 VC5, C6, C15, C16 = 10 µF, 16 V,

radialC9, C11 = 10 nFC17 = 1 µF, metallized polyester,

pitch 5 or 7.5 mmC18 = 4.7 nFC19, C20 = 1000 µF, 40 V, radialC23 = 100 µF, 40 V, radialC24 = 220 µF, 25 V, radial

Semiconductors:D1, D2 = LED, 5 mm, flatD3, D4 = zener diode, 4.7 V, 500 mWD5, D6 = zener diode, 5.1 V, 1.5 WD7, D8 = zener diode, 5.6 V, 500 mWD9–D11 = 1N4002T1, T3, T6, T9, =BC560CT2, T4, T5, T10 = BC550CT7, T8, T11 = BF245AT12 = BF872T13 = BF871T14 = BD139T15 = MJE15030 (Motorola)T16 = MJE15031 (Motorola)T17 = GT20D201 (Toshiba)T18 = GT20D101 (Toshiba)T19 = BC640

Integrated circuits:IC1 = LM317TIC2 = LM337TIC3 = OP77 (Analog Devices)

Miscellaneous:L1 = see textRe1 = relay, 24 V, 875 Ω with single

make contact 16 A, 250 V, e.g.Siemens Type V23056-A0105-A101

5 off PCB mounting flat 3-way socketsHeat sink 1.2 K W–1 or smaller – see

text; e.g. Fischer SK85SA/75 mmfrom Dau (telephone 01243 553 031)

Insulating washer and insulatingbushes for T14–T18

PCB Order No. 970043 – see Read-ers Services towards the end ofthis issue

Figure 2. The double-sidedprinted-circuit board for thepower amplifier. Large currentsflow over only a few tracks.

2

soldeerzijde

componentenzijde

heat sink. They must, of course, beelectrically isolated from each otherwith the aid of insulating washers andbushes for the fixing screws. In view ofthe required temperature stability, it isadvisable to keep the terminals of T14as long as possible to ensure that thedevice, when mounted, is at about thesame height as the centre of T17. Thismakes for good thermal coupling.

The other semiconductors and inte-grated circuits do not need extra cool-ing.

As mentioned earlier, good thermalcoupling of a number of components(T1-T5; T2-T6; D1-T3; and D2-T4) in theinput stages is also imperative for goodperformance. The easiest way ofensuring this is to secure the variouspairs of components together withcable ties. In the case of D1-T3 andD2-T4 this is possible only if flat diodesare used. Tie the pairs together beforefitting and soldering them on to theboard. Mind the orientation of the

diodes. It is advisable to mark the tran-sistors with n-p-n or p-n-p, as the casemay be, so that it is clear at a laterstage which pair is which.

Inductor L1 consists of eight turnsof 1.5 mm dia. enamelled copper wirewound around a 9 mm drill bit. Whenit is wound, withdraw the drill bit,insert R39 into the coil and fit and sol-der the assembly on to the board at aheight of a few millimetres above thesurface.

All connections that carry large cur-rents are output via PCB mounting flatsockets. The power connections are atthe middle of one side of the boardmarked LS+ and LS–. The power linesto the relay are connected via ordinary

47Elektor Electronics 5/97

Suggested power supply (for monoamplifier):

Toroidal mains transformer, sec-ondary 2×22 V, 160 VA

Bridge rectifier 200 V, 35 A4 off electrolytic capacitor 10,000 µF,

50 VFuse holder and fuse 800 mA, slow

3

Figure 3. Photographof the completed pro-totype board with heatsink. Two- and three-pin flat PCB mountingconnectors are usedfor the loudspeakerand power line con-nections.

200V/35A

2x 22V160VA

0A8 T

50V30V

30V

970043 - 1222V 22V

power-ondelay

4x 10,000µ

4

Figure 4. The powersupply may be keptsimple, but shouldhave ample reservepower. Note that thediagram shows adesign for a monopower amplifier.

Most readers of this magazine knowthat it is published in identical form(at least as far as the articles are con-cerned) in Dutch, French and Ger-man, as well as in Greek, Polish, Por-tuguese, Spanish and Swedish in non-identical form. The Dutch, English,French and German issues are pro-duced in the Netherlands, but pub-lished in the relevant country. Theseissues are the responsibility of aninternational team of writers/transla-tors and technical editors.

On the face of it, there should beno problem in producing a suprana-tional electronics magazine, sinceelectronics is more international thanany other branch of engineering. Theelectronics market is a world market:there is not one semiconductor or ICmanufacturer who does not operateinternationally. Also, norms and spec-ifications tend to be internationallyagreed.

Working in a multi-language teampresents no real difficulties: most ofthe editorial staff have workedtogether for almost twenty years. Ofcourse, technical translating is quitedifferent from literary translating: forone, the technical translator has toknow and understand the subject!

Most difficulties arise from the dif-ferences in standards laid down manyyears ago. For instance, an article ontelevision will have to take intoaccount that France (and easternGermany) uses Secam, England PAL-I (but many other English-speakingcountries NTSC), and Germany andthe Netherlands PAL-B/G. The sameapplies to the different mains voltageoutlets in the four countries. Astratransmissions are popular in Britainand in Germany, but not much in theNetherlands. Similarly, GSM mobiletelephones for the E-band(1800 MHz) are in use in Britain andin Germany, but not in France andHolland.

Surprises still occur as well: chip-board or PVC tubing of the densityspecified in a German design are notavailable in the Netherlands. AndSiemens products are not not easilyavailable from retail outlets even inGermany. Strangest of all: try to finda bicycle rear light that is standard inall four countries!

[975043]

In passing …board terminals. Note that these linesconsist of three wires: this means thatthe earth line must NOT linked to theamplifier earth.

The completed board with heatsink is shown in Figure 3. The heatsink must have a thermal resistance of1.2 K W–1 or smaller.

P O W E R S U P P L Y A N DQ U I E S C E N T C U R R E N TWhen the board has been completedand thoroughly inspected for anyfaulty workmanship (including a one-by-one check of the componentsagainst the part list), a few operationaltests need to be carried out. Naturally,a power supply is needed and a suit-able arrangement for this is shown inFigure 4. This is a traditional setup ofpower-on delay, mains transformer,with a 2×22 V, 160 VA secondary, a35 A bridge rectifier and four 10,000 µFelectrolytic reservoir capacitors. Thissupply provides sufficient currenteven when 4 Ω loudspeakers are used.Note that the diagram is for a supplyfor a mono amplifier: two are neededfor a stereo amplifier.

The diagram also shows how thepower lines for the relay are obtained.

The power-on section is advisablebut not obligatory. It serves to preventlarge current peaks during switch-on.

As in all power supplies, good,firm, well-soldered connections are ofprime importance. They ensure thatthere is no unnecessary resistance inthe path of large currents.

Before the power is switched on, itis vital that preset P1 on the poweramplifier is turned completely anti-clockwise. If this is not done, there is arisk that the quiescent current throughthe output transistors quickly reachesa very high level and the circuit is notdesigned to cope with this.

After the power has been switchedon, check with a multimeter whetherthe potentials at the output of IC1 andIC2 are as indicated on the circuit dia-gram in Figure 1 (±23.2 V respec-tively). Then, measure the output volt-age of the amplifier, which must be 0 Vor very nearly so. If it is not, recheckthe entire construction, particularly theinput stages.

When up to this point all is welland the LEDs light, it may be assumedthat the entire power amplifier is ingood working order. It is then time tocheck all the potentials at the testpoints indicated in Figure 1.

Next, set the quiescent current withP1. This is fairly high in this poweramplifier: about 400 mA when a heatsink as specified is used; if a heat sinkof ≤ 0.6 K W–1 is used, the current mayeven be as high as 500 mA. Connect amillivoltmeter set to its 200 or 250 mVrange across R34 or R38 and turn P1slowly clockwise until the meter reads

88 mV (current = 400 mA) or 110 mV(current = 500 mA).

Leave the amplifier to warm upthoroughly and then readjust P1 asrequired.

A S S E M B L YThe design of the printed-circuit boardis intended for its use as a monoamplifier. From a quality point of view,this is the best way of building anamplifier. Use a compact enclosure,perhaps with integral heat sink, andassemble board and power supply inthis to make a stand-alone monoamplifier. A stereo amplifier is obtainedby building two of these mono unitsand simply stacking them.

Wiring up the assembly is straight-forward. Use heavy-duty wire for the+ve, –ve and earth supply lines andlink them to the appropriate terminalson the board. Use heavy-duty wirealso for linking the loudspeaker termi-nals with the LS+ and LS– PCBmounting 4 mm sockets on the board.

Link the phono input socket at therear of the enclosure to the two inputterminals on the board via a shortlength of screened audio cable.

Next, link the two 22 V~ and 0lines to the relay – remember that the0 line must not be connected to theamplifier earth – see below.

The mains inlet should be of goodquality, preferably with an integralfuse holder. The fuse rating is shownon the serial number label that shouldbe affixed to the rear of the cabinet.

It is imperative that earth loops areavoided. This is best done by ensuringthat there is link between the mainsearth, the 0 supply line and the cabi-net earth at only one point. Normally,and most conveniently this is at thesignal input socket. If a non-insulatedphono socket is used, this is automat-ically connected to the cabinet earth,so that no further action is needed.

It is, of course, possible to assembletwo mono amplifiers using a singlepower supply in one cabinet, but thisis not advisable. Even if it is desired touse only one cabinet, each of the monoamplifiers should have its own discretepower supply. The introductory pho-tograph shows that this is how theprototype was constructed. The cabi-net used for the prototype was pro-vided with an integral heat sink andmeasured 445×75×305 mm(W×H×D).

[974003]

49Elektor Electronics 5/97

50Hz

No. 970043

240V

F = 0A8 T (2x)

ELEKTOR

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