Historical look at amps

I don't know of any designer that looks to feedback as a solution to bias stabilisation in a SS amp. Lowering noise, distortion and output impedance, yes. Increasing bandwidth and input impedance, ditto.

Maybe the guys who design tube gear do, but I don't hang around with any.

But since this was supposed to be about a "historical look at amps", and feedback...........let me ramble on some about the stuff that I have done.

Mid 80s.......overall feedback, but much less than usual. Maybe 10-20 dB.

Late 80s.....no loop feedback in the voltage gain stage, loop feedback for the outputs. (Similar to Stasis.)

Mid 90s.......no loop feedback anywhere.

Early 00s........"digital" amps. Tons of loop feedback.

Looks like we have come full circle.

Bombaywalla, perhaps the point was missed by me. As far as realization, others may indeed come to the same conclusion and others may not. How ever I think it important to realize that our systems are complex and some attributes of sound that may be particularly pertinent to an individual may be better served due to technical or budgetery needs to use certain components that that may be better served by alternate means of amplification. Just so we undertand each other, I don't take any personal offense by your opinion. I just thought that another perspective was in order. Best Regards.

We may have come full circle but that circle has some waves. For example, one of the more respected SS amps that has received great reviews is the Parasound JC-1's. Well, guess what, it has 30db of GLOBAL feedback! (and to me, it sounds like it---it's soundstaging is relatively 2 deminsional) Global feedback is being used more than you think. Ayre, Theta, some Pass and maybe another or so don't use global feedback. It's still fairly uncommon. These amps have higher distortion values and are generally lighter in the bass than higher FB amps. They act somewhat like a tube amp and clip similarly.
I agree completely with Bombaywalla. Marketers try to influence you with BS but it is up to the individual to educate themselves. All designs have tradeoffs. As one remains in the hobby, your tastes with change---usually with age. There is no doubt in this "Old fart's" mind that no global feedback amps offer a higher level of sonic purity (and notice here I didn't say anything about sounding better---that's for you to decide.) If you listen to voices and instruments, you'll find a higher level of natural sound.
I played sax for 30 years and I can tell you without hesitation that the low feedback (0 GFB) amps sound more realistic (they may not meet the dynamic criteria or impress with a "Balls to the wall" sound some favor.) They are certainly not for everyone. I can also say GOOD tube amps are even closer. They offer that special bloom but not all. Some are noticably soft and bloated. I think the perfect amp would be tube mids and highs with SS bass.
I always thought the "High end" was about accurate portrayal of a source. If it's not and it's about "Good sound" then I need to get out because it all becomes a moot point. There's a huge difference between what offers good sound and what is an accurate portrayal of the recorded venue. Some stuff just doesn't sound good and shouldn't sound good.
So much equipment is offered because of the tastes of the individual. However, some of it is wrong and a little of it is right. It does take years to figure a lot of this out. I've been at it 35+ years and it is amazing how my tastes have changed since the beginning.
BUT---to each his own! Everyone knows what they like.
With all that said, I will add one last thing. It is my held belief that some of the differences in sound of no feedback and feedback amps is do to time domain distortions created by out of phase global feedback---but this is a discussion for another day.

I don't know of any designer that looks to feedback as a solution to bias stabilisation in a SS amp. Lowering noise, distortion and output impedance, yes. Increasing bandwidth and input impedance, ditto.

I agree. Feedback is not used for bias stability in transistor or for that matter tube amps. There are numerous stable bias schemes for amps and they don't involve feedback.

Some amps use DC servo control to maintain zero potential at the output of a push pull amp to avoid using coupling caps. This might be interpreted as feedback since the output potential is monitored, and a correction voltage fed back to a prior stage, but this is usually not considered as feedback since it works well outside the audio band, and is used for an entirely different reason than those listed above.

Still no response from Ayre on my inquiry about their claim of zero feedback.

THis is fun. Bombaywalla you need to quit SHOUTING. I was not DEAD WRONG. I was relating what Ayre says. I hold no religious conviction. I live 50 miles from the Ayre factory in Boulder CO and my comments were taken from my discussions with engineering and technical staff members at their factory, not just their marketing hype. I'm not saying your wrong in your opinions which if you read my original post showed the problems of negative feedback. Some reading this post might think this is all a mine-is- bigger-then-yours arguement so I am going to explain the non-circuit design reason for feedback being good and bad..
Feedback is almost always NEGATIVE feedback. Positive feedback is used for oscilators. YOu don't want a amplifier to be an oscillator...its hard on speakers and your ears. Negative feedback takes a bit of the amps output and applies it to the input but inverted (negative sign) This tends to damp oscilations and cancel output errors.
The real problem with feedback is the amp or even the transistor have a non zero propagation time delay. Given the perfect amp a wire with gain the wire itself delays the signal. (Signals do not travel at the speed of light in electronics. The signal travels at just a small fraction of the speed of light, the drift current speed.) All circuitry including the bare wire, take a finit amount of time for a signal presented at its input to reach its output. Feedback circuits regardless of their design therefore are adding an "old" signal that has propagated through the device to the new signal at the device input. For a steady state signal that's not a problem but for music even a few miliseconds of delay mean the feedback signal applied doesnt match the new input signal. Global feedback has this problem in a major way because there is significant delay accumulated through all the amps circuitry. Local feedback would just have to cope with the delay in one FET or transistor. TIM as I mentioned in the first message of this thread is sensitive to this time delay distortion.
By the way there are lots of electronics without feedback. But we are talking about amplifier circuits here so the arguement is whether all or most use negative feedback which is good for stability and bad for music.