"Sound Mosfet Design" by Douglas Self, pub Sep 1990, p760

Comments in course of preparation

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"Class A to 300W" by Colin Wonfor, pub Mar 1999, p188

This is the letter I sent to EW:

"I read Colin Wonfor's article on Class-A with interest. Having spent some time myself trying to make Class-A amplifiers both tractable and as linear as they should be, I was surprised he did not adopt any of the technology introduced in the Class-A (March 1994) and Trimodal (June, July 95) designs. No doubt there were good reasons, but it would be nice to know what they were. Likewise it is a great pity that no performance details were given, to confirm that the great cost and heat are really worthwhile.

My investigations showed that the use of power FETs in the Class-A output stages makes linearity worse. The Class-B problem of a horribly jagged crossover region is no longer relevant, but the low device transconductance still causes high distortion.

An automatic bias controller would have cost a few pence, and removed the need for a perilous quiescent-adjust preset that allows infinite current to flow at one end of its travel.

The instruction to trim the DC offset to less than 10mv with a preset seems rather strange when it is straightforward to make an amplifier that gives an offset of less than 25 mV with no adjustments. It concerns me that there is no effective DC-offset protection given the large amount of power available. I note the output fuse (no value shown) but I would be interested to know how the constructor is going to select its value so it can protect loudspeakers without nuisance-blowing or introducing distortion. A 1 Amp slo-blo output fuse carrying 20W/8 Ohm generates 0.01% THD (third harmonic) at low frequencies, so fuse distortion could easily make the finer nuances of Class A linearity somewhat irrelevant.

It seems astonishing to use the constant-current (single- ended) mode for a large Class-A power amplifier as this doubles the already enormous heat dissipation. The largest version described releases 3 KW of heat, which will not be significantly reduced by playing music at full volume. Surely this will be an uneasy companion in summertime? The quiescent current for the so-called 300W/4R version is prescribed as 10.2A, so negative peaks cannot go beyond 4 x 10.2 or 40.8V below ground. This corresponds to a maximum sine output of only 208 W, so using supply rails as high as +/-75V appears to do nothing but increase the power dissipation. It allows no safety margin for loudspeakers that fall below a nominal 4 Ohm impedance. Perhaps there is a typo in the article for this huge emission of heat makes little sense to me.

At the risk of seeming discouraging, no information is given that makes me see this design as a significant advance in Class-A amplification."