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Sure?

Posted by 13th Duke of Wymbourne on November 21, 2024 at 11:52:03:
I would say that almost all solid state amplifiers follow the Lin topology (input differential Gm stage - transimpedance stage - output buffer/follower) which is, basically, the same as used by op amps. So, the feedback signal will connect to the input gate/base of the input diff pair just the same in audio power amps as in op amps.

I think this idea that feedback intermodulates with the signal in the amplifier is specious. As we know, the closed loop performance is given by A/(1+AB) and as A becomes larger and larger A effectively drops out of the equation and performance is defined by the feedback network, usually resistors. A portion of the output is subtracted from the input signal to create the error signal - this is what is amplified by A. The error signal is pre-distorted and amplified so that distortion at the output is minimized. IMO, it is a red herring to think of what is passing through the amplifier - yes, it is not 'pure' but it is exactly what is required to make the output 'pure'. The only thing we care about what goes on inside A is that the small signal gain remains high and there is no saturation in either the time domain or amplitude domain.

I don't know Norman Crowhurst's writings but I do know Baxandall's analysis of applying feedback to a pure second-order device. The way to think about what is happening is that the feedback is straightening the very curved transfer curve. Even if you have a transfer characteristic that looks straight visually it will still have some curvature and wiggles that equate to high order terms in the power series expansion of that characteristic and they create high(er) order distortion. You can play with this in MS Excel by plotting a straight line and doing a power series fit that will have only the linear term. Then add points to make subtle deviations to that line and the power series fit will then calculate the added high order terms. Only a perfectly straight line produces no distortion and there is no perfectly straight transfer characteristic. Just as there is no perfectly square law characteristic to start with.
In Baxandall's think-piece, second-order and total distortion decreases as feedback is increased but all other terms increase until you reach an inflection point (approx 20dB loop gain) and increasing feedback above that point then reduces all distortion components. This is explained in Bruno Putzey's 'F-Word' article. The worst thing would be to apply only 'a little' feedback because of the widely held view that 'a lot' is bad but puts you right at that inflection point where the higher harmonics are their worst! Nelson Pass has written of the Baxandall example that if the purely second harmonic is not objectionable then why apply any feedback if you need to apply a lot to lower the high order harmonics back to where they are with no feedback - though he also writes about using 'a little' feedback in his amplifiers? Maybe hedging his bets.

I'll leave the subjective impacts of distortion spectra aside because it's, er, subjective and I see that amplifiers of all different topologies are highly regarded so, maybe, non of this really matters?