44 Elektor Electronics 11/96

The Type TDA7294 IC fromSGS-Thomson is an inte-grated a.f. amplifier in-tended for use in all sortsof hi-fi application. Its cir-cuit diagram is shown inFigure 1. Its most promi-nent feature is the muchhigher power output thanis usual with this kind ofintegrated amplifier. Ac-cording to the manufac-turers data sheets, thespecial DMOS output stageof the 15-pin chip can de-liver outputs of up to 100 watt. Con-sidering other properties, such as lownoise, low distortion and reliable short-circuit and thermal protection circuitsas well, the chip is indeed an interest-ing one.

Having said that, power outputspecifications are often rather opti-mistic. In this instance, the 100 W ap-pears to refer to the IEC norm for musicpower with 10 per cent distortion,which, as far as hi-fi applications areconcerned, is not the correct way of

specifyingoutput power. More-

over, with peak supply voltagesof 40 V and a load impedance of 4 ,the maximum dissipation of the IC willeasily be exceeded. For these reasons,the supply in the present amplifier hasbeen kept down to a safe 30 V. Atthese voltages, the chip delivers, with-out any difficulty, 50 W into an 8 load and 80 W into a 4 load. Theseare still very respectable figures, par-ticularly in view of the reasonable priceof the chip.

C I R C U I T D E S C R I P T I O NThe circuit diagram of the amplifier inFigure 2 shows that the IC needs only

50 W a.f. amplifier

Specificat

ion

Input sensitiv

ity:

1.3 V (50 W i

nto 8 )

Input impeda

nce:

10 k

Bandwidth:

16 Hz 100

kHz

Slew rate:

10 V s1

Output powe

r:

50 W into 8

(0.1% THD)

82 W into 4

(0.1% THD)

Signal-to-noi

se ratio:

105 dBa (1 W

/8 )

THD+N with

40 W into 8

:0.002%

(1 kHz)

< 0.04% (20

Hz 20 kH

z)

Although audio en-thusiasts come in

many sizes andcolours, not many of

them are prepared tospend a lot of money,

time and effort tobuild an a.f. amplifierusing up to 40 tran-

sistors to improve thedistortion by a frac-

tion of a per cent.Therefore, the ampli-

fier described hereshould appeal to

those enthusiasts. Itis compact, presentsno problems and yet

has properties thatmake it fully suitable

for all but the mostdemanding audio ap-

plications. In short, anamplifier that is

geared to the practi-cal audio buff.

uses onlyone IC

uses onlyone IC

Source: SGS Thomson

a small number of external compo-nents. To keep the harmonic distortionlow, the amplifier has a large feedbackratio and its closed-loop gain has beenrestricted to only 24 dB.

The input signal is applied to pin 3via capacitor C1 and low-pass filterR6-C10. The filter improves the pulseresponse and flattens the frequency re-sponse. For minimum output offset,the values of R1 and R3 should beequal, so that the input impedance is10 k. The roll-off frequencies of R1-C1and R2-C2 determine the lower band-width limit of the amplifier: with val-ues a specified, this is about 16 Hz. Theupper 3 dB point is at about 100 kHz.

The amplifier is muted by a rele-vant input to pin 10 and placed in thestand-by mode by a relevant signal atpin 9. Muting should always take placebefore the stand-by mode is selected.Connecting the mute and standbypins permanently to the supplyline ensures that the amplifiercomes on immediately thepower is switched on. Anyswitch-on clicks may beeliminated by increasingtime constants R3-C4and R5-C5.

If large-value elec-trolytic capacitors areused in the power sup-ply, switching off will berather slow. If that is con-sidered a nuisance, an ex-ternal mains detectionnetwork may be added.This can consist of, say,two diodes and two smallsmoothing capacitors for rec-tifying the secondary voltage of themains transformer. The board has pro-vision for this in the form of additionalsoldering pins adjacent to the muteand stand-by inputs: an earth pin incase use is made of an external pro-tection circuit and a plus pin if suchprotection is not foreseen.

C O N S T R U C T I O NIt is best to build the amplifier on theprinted-circuit board shown in Fig-ure 3. The illustration proves what acompact unit this amplifier is. In viewof the scarcity of components, popu-lating the board is very simple.

The back surface of the IC is linkedinternally to the negative supply rail.Consequently, to preclude electricalcontact between the heat sink and theenclosure, the heat sinkis mounted on theboard. Insulating mate-rial between the heatsink and the IC is, there-fore, not needed, al-though the use of someheat conducting pasteis advisable.

In the selection of a

suitable heat sink, acontinuous output of50 W into 8 was as-sumed. The selectedheat sink is also allright for music outputs

of 80 W into 4. Problems caused byhigh temperatures are very unlikely,since the IC has internal thermalprotection that causes the mute tocome into operation at 145 C andswitches the amplifier to stand by at150 C.Provision for connecting the power

lines to the board is by three PCB ter-minal blocks (clamping-screw type).These ensure loss-free passage of thesupply current.

The symmetrical power supply is

45Elektor Electronics 11/96

1

Figure 1. The TDA7294has standard thermaland short-circuit pro-tection circuits. Themute function pre-cludes annoying onand off switchingnoises.

C3

2263V

C9

1000 40V

C7

1000 40V

C6

100n

C8

100n

C10

2n7

C5

1063V

C4

1063V

C2

2263V

R1

10k

R6150

R310k

R410k

R522k

R2

680

BOOTSTRAPTDA7294

STBYGND

IN+MUTE

PWVS

+PWVS

IC1

MUTE

STBY

OUT

VS

ININ+

+VS

14

15

10

13

6

81

23

4

9

7C1

15

LS1

TDA7294

151

30V

30V

mute

standby

954044 - 11

2

Figure 2. In the finaldesign of the ampli-fier, supply voltages of30 V were decidedupon; these are morethan sufficient for apower output of 50 Winto 8 .

47Elektor Electronics 11/96

best constructed from atoroidal mains trans-former, a 25 A bridgerectifier and two10,000 F, 50 V elec-trolytic capacitors.

F I N A L L YAs mentioned before, thanks to itsgood performance and high power

output, the amplifier isin principle usable invirtually any hi-fi set-up. Owing to its com-pactness, it is particu-

larly suitable for use in combinationwith a preamplifier as an integratedamplifier or as part of an active loud-speaker system where space is almostalways at a premium.

For those who would like someproof of the figures given in the spec-ification table, Figure 4 shows the dis-tortion characteristic of the amplifierobtained with a spec-

trum analyser. The measurementswere carried out at an output powerof 40 W into 8 and a bandwidth of80 kHz. As usual, the characteristicslopes upward at higher frequencies,but the distortion does not exceed 0.04per cent. In a large part of the a.f.range (up to about 1 kHz), the total-harmonic-distortion-plus-noise (THD+N)does not even rise above 0.02 per cent.This sort of performance is excellentfor all but the most demanding appli-cations. [960079]

960079-1C1

C2

C3

C4C5

C6

C7

C8

C9

C10IC1

R1

R2

R3

R4R5R6

T

960079-1mute

T

+-

T

standby

+

+

-

3

Figure 3. The printed-circuit board is verycompact and evenhouses the requisiteheat sink.

Figure 4. The distor-tion characteristic,measured with an out-put power of 40 W into8 , is excellent forthis type of amplifier.

Elektor DEFAULT vs

.0005

0.001

0.010

0.1

1THD+N(%)

20 100 1k 10k 20k

FREQ(Hz)4

960079-1

Parts list

Resistors:R1, R3, R4 = 10 kR2 = 680 R5 = 22 kR6 = 150

Capacitors:C1 = 1.5 F, 63 V*C2, C3 = 22 F, 63 V, radialC4, C5 = 10 F, 63 V, radialC6, C8 = 100 nFC7, C9 = 1000 F, 40 V, radialC10 = 2.7 nF*, pitch 5 mm* metallized polyester

Integrated circuits:IC1 = TDA7294V

Miscellaneous:3 off PCB terminal block with clamp-

ing screws1 off heat sink, 2.5 K W1 (e.g. Fis-

cher Type SK100, available fromDau telephone 01243 553031))

for power supply:1 off mains transformer, 222 V,

80 VA2 off electrolytic capacitor, 10,000 F,

50 V1 off 25 A bridge rectifierPCB Order no 960079-1 (see Read-

ers Services towards the end ofthis issue)

Contents: Contents