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RE: This is interesting

Posted by andy_c (A) on August 23, 2007 at 14:41:19

In Reply to: RE: This is interesting posted by Dan Banquer on August 23, 2007 at 13:04:20:

This does not surprise me at all. What most audio gurus don't understand is that when feedback is applied around different topologies with different open loop characteristics, the open loop characteristics will define how much or how little feedback is used and also define what amount of frequency compensation needs to be used.
Duh!

I was actually expecting to see in all cases (in the simulations) results similar to what Baxandall saw - namely, some increase in higher-order harmonics as the feedback was increased from zero. When Baxandall's circuits were duplicated in simulation, we did indeed see that. But with other circuits, such as a class AB BJT output stage with optimum bias, it did not show up at all. For that circuit, all harmonics were reduced in level, even for low levels of feedback. Of course, this class AB circuit did not have a whole lot of distortion without feedback, due to its optimum bias.

But there's a common attribute of the scenarios in which the higher-order harmonics increase as you increase the feedback from zero. That common factor is that the open-loop amplifier has gross levels of distortion. In Baxandall's FET circuit, it's a bit less than 10 percent second harmonic. In his BJT circuit, it's more than 10 percent second harmonic. The time domain waveforms were badly asymmetrical, with a large DC shift due to the even-order distortion. This just gets back to common sense - don't use feedback to "clean up" a grossly nonlinear design - use it to improve an already good one.

Also, a point about the article referenced by Brad. The author concludes that for the same level of distortion, a SET amp sounds much better than a solid state amp. That does not surprise me either. For a solid state amp to exhibit the gross levels of distortion that a SET amp does in normal operation, the solid-state amp must be driven into clipping. Of course it will sound bad. A more valid comparison is how they sound in normal operation, putting out, say, a couple of Watts.

Glad your music server is going well for you. My Squeezebox display just went bad, so I have to decide whether to have it fixed or get something else :-(.

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