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Build The Ultra-LD 100W Amplifier Module: Part One

This 100W class-AB amplifier module has ultra low distortion.

By Leo Simpson

This amplifier module has been under development, on and off, since late 1998. It was in July and August 1998 that we featured the ultra-low distortion 15W class-A amplifier. Since then, that amplifier has become our benchmark. Its distortion is so low that we had to resort to new procedures to be able to measure it.

Inevitably, soon after the 15W class-A amplifier had been published, we wondered about producing a higher power version. As good as the 15W amplifier is, it is still only 15W and on many types of music, particularly opera and classical piano, it simply does not have enough power. So we thought a 100W version would be really good.

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The final version differs slightly from this prototype module. It delivers 100W into 8Ω with very low distortion.

However, we shrank back from the idea of producing a 100W per channel class-A amplifier. After all, the stereo version of the 15W class-A amplifier dissipates about 100 watts at all times. If we produced a 100W stereo version, it would dissipate around 600 to 700 watts at all times. In other words, it would make a good heater for small rooms.

So we wondered whether, with the lessons we had learned in the development of the class-A amplifier, we could apply them to a class-AB amplifier and get similarly dramatic results. That was the hope anyway, as we set out in late 1998 to produce this new amplifier.

That we are publishing the results only now is a reflection on how difficult the process has been. Is this new amplifier as good as the 15W class-A amplifier? Alas, no. As far as we can tell, using currently available semiconductors and circuit techniques, it will never be possible to produce a class-AB amplifier as good as our 15W class-A module. However, all the development has produced an amplifier that is a major improvement on the 125W Plastic Power module published in April 1996.

This new module has much lower distortion at the higher frequencies from 5kHz to 20kHz and it is quieter although not dramatically so, since the Plastic Power module was very quiet anyway.

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