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20 Watt Class A Power Amplifier Circuit

A single-ended Class-A amplifier is essentially one where there is only one active driven output device. The passive "load" may be a resistor, an inductor (or transformer) or - as in this amplifier - a current sink. Of the three basic options, the current sink offers the highest linearity for the lowest cost, so is the ideal choice.

20W class A power amplifier circuit

Some esoteric (some might say idiosyncratic) designs use inductors or 1:1 transformers, but these are bulky and very expensive. Unless made to the utmost standards of construction, they will invariably have a negative effect on the sound quality, since the losses are frequency dependent and non-linear.

This amp uses the basic circuitry of the 60W power amp (see Index), but modified for true Class-A operation - it should be pretty nice! This amp has been built by several readers, and the reports I have received have been very positive.

With simulations, everything appears to be as expected, but although I have yet to actually build it and test it out thoroughly, no-one has had any problems so far. Using +/-20 Volt supplies - either conventional, regulated or using a capacitance multiplier, it should actually be capable of about 22 W before clipping, but expect to use a big heatsink - this amp will run hot.

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