Feedback


I read this and found it interesting. Thought it might generate some interesting discussion here. 
https://www.audioholics.com/audio-amplifier/amplifier-negative-feedback
128x128mapman
Now some designers have taken a different approach, sculpting the distortion signature to include the 2nd and 3rd harmonics. Its been shown that if these lower orders are present (both treated by the ear as 'richness', 'bloom', etc.) in great enough degree, they can somewhat mask the presence of the higher orders. Nelson Pass takes this approach in his designs, which are some of the best sounding solid state amps out there.
https://www.passlabs.com/press/audio-distortion-and-feedback

This is an article Nelson Pass originally wrote and was published by 6Moons a while back which goes into more detail regarding Ralph's comments on Nelson's use of feedback. It also talks about implementation of feedback, both global and local.

There is another article elsewhere which I cannot find at the moment in which Nelson mentions that small amounts of feedback, the amount of which I cannot remember are fine, as are large amounts in excess of 30dB. So again, there seems to be some consistency in this line of thought that excessive amounts of feedback could be beneficial. I know that the last amplifier designed by Roger Modjeski used greater than 35dB of feedback, and as Nelson himself has mentioned regarding his First Watt designs the application of global negative is determined on whether or not the amplifier sounds better with it or not.

Properly implemented feedback is not a bad thing. I have some amplifiers that use it and some that don't and enjoy then all.


And Roger’s ( RIP ) masterwork RM-9 has switched 3 levels of feedback which aids one in discerning the impact on sonics.

certainly there are amplifiers that sound good with no global feedback as well: Atmasphere, Ayre ( RIP Charlie), Vandersteen, et...

and of course we should credit Dr Matti Otala for his discovery of TIM and relationship to negative F/B. I still have an Audionics CC2 wildly inspired by Otala. It is a very musical SS amp designed and produced in the late 70’s

fatigue free listening.....

@cleo I sent you an email, need bit of help with bringing an RM-4 back to life


Great posts above; thanks to all.

Ralph (@atmasphere), what do you think of Mr. Putzeys’ statement that ...

... the idea that feedback causes TIM is probably most noteworthy for being not just wrong, but also the exact opposite of the truth. TIM happens in the input stage. An increase in global feedback makes the input stage work less hard. That causes a disproportional reduction in TIM.

Thanks. Best regards,
-- Al

@tomic601 the idea of the feedback switch on the RM-9 was not about adjustable feedback, it was about adjustable gain (Nelson Pass talks about the feedback/gain relationship in the article as well), which is why it is referenced as a gain switch. Roger wanted users to have a means to use both passive and active preamps with the RM-9, so he put that feature in to allow the gain to be adjusted accordingly. However, after more questions from users about which position sounded better than he wanted to deal with (Roger's stock answer was, "whatever position sounds best to you") he removed the switch and the feedback/gain was factory set. The RM-10, while not having a switch, has instructions and a chart in the manual for adjusting the gain by swapping out some resistors and capacitors.

Here is another little snippet from Roger. This time on IMD which he felt was very important to limit in circuit designs:
"I intend to write a paper on the more complete story of 2nd harmonic distortion theory. I will say here that it must be minimized (but not at the expense of adding other distortions). What listeners don't seem to realize is that large amounts of 2nd is fine for a single note but not for a full orchestra. As I like to say "Please tell me what is the second harmonic of Beethoven's 9th? The amplifier is having a little trouble figuring it out". Once music becomes a complex signal, the second harmonic argument goes right out the window. The amplifier can only act upon the instantaneous voltage it is given and has no idea what notes are being played. At that point, it's Intermodulation Distortion we had better be taking care of. How many of us have heard our single-ended amps make a mess of a symphony yet play a solo voice beautifully?"

Tony I certainly appreciate the listen and pick which switch position sounds best.... and Lord knows, my thinking on any subject evolves a bit with time but....

this is from the RM-9 owners manual, which includes RM note back to Stereophile in the review of the RM-9

” Not wanting to play “ God” over the situation, I chose to give the user a three position feedback switch so he could change the damping by a factor of approximately 4:1 “... he then mention gain, passive etc

in my system with what I consider to be a very neutral tweeter, I prefer the high gain / low feedback setting.... it sounds glorious...