50 W a.f. amplifier
50 W a.f. amplifier
Although audio enthusiastscome inmany sizes andcolours, not many ofthem are prepared tospend a lot of money,time and effort tobuild an a.f. amplifierusing up to 40 transistorsto improve thedistortion by a fractionof a per cent.Therefore, the amplifierdescribed hereshould appeal tothose enthusiasts. Itis compact, presentsno problems and yethas properties thatmake it fully suitablefor all but the mostdemanding audio applications.In short, anamplifier that isgeared to the practicalaudio buff.
The Type TDA7294 IC fromSGS-Thomson is an integrateda.f. amplifier intendedfor use in all sortsof hi-fi application. Its circuitdiagram is shown inFigure 1. Its most prominentfeature is the muchhigher power output thanis usual with this kind ofintegrated amplifier. Accordingto the manufacturer’sdata sheets, thespecial DMOS output stageof the 15-pin chip can deliveroutputs of up to 100 watt. Consideringother properties, such as lownoise, low distortion and reliable shortcircuitand thermal protection circuitsas well, the chip is indeed an interestingone.Having said that, power outputspecifications are often rather optimistic.In this instance, the 100 W appearsto 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 ofspecifyingoutput power.
Moreover,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, withoutany difficulty, 50 W into an 8 load and 80 W into a 4 load. Theseare still very respectable figures, particularlyin view of the reasonable priceof the chip.