[sdiy] Simple discrete Unity-Gain Follower ?

[Grant Richter]

>> 
>> Most of the "audiophile" distaste for feedback loops refers to
>> the failure of the feedback loop, not when it's working properly.
> 
> Grant, you didn't really mean to say that, did you?

Yes, I did, I think we are both saying the same thing in different words.

I am guilty of using the word "audiophile", which is practically undefined,
as I have never found two audiophiles who agree the same things sound good
(to them).
> 
> There are a number of serious problems with feedback loops, even when
> they're working properly.  The most blatent is that the output is the
> amplifier gain times the error.  That's just philosophically wrong at
> a deep fundamental level.
> 

This is inherent to the design of a responding servo-mechanism. The "error
amplifier" is a simple differential input stage, and the action of negative
feedback is to cause the output to track the input signal as closely as the
servo-mechanism can. If this works perfectly, the output will be an exact
replica of the input signal. And for DC, it does work pretty darn well.

However, you make an EXCELLENT point. In the case where the feedback fails
(the servo-mechanism is not able to follow the input signal), that error is
amplified by the very high gain of the op-amp stage. The error could be
caused by slew rate limitations, inadvertent phase shifts, non-monotonic
linearity or other errors that the loop can't completely keep up with or
compensate for.

It makes sense to me now, why the audiophile designs use a minimum of gain
and a minimum of feedback. The less internal gain, the less amplification of
feedback errors.

Thank you for the enlightening comment. I definitely learned something
useful.