Compensating multi-stage feedback (was: RE: all tranny vca+ )

[Grant Richter]

The primary reason I have heard is failure of the negative feedback loop 
during transients. For instance if you look at a virtual ground node in an
op-amp you will notice that it will bounce around a bit.

During these periods the loop is actively servoing and the
loop is not entirely stable. These effects are mostly caused
by parasitic capacitors at device pins forming unintended
phase lead and lag networks.

Since op-amps are designed as computing elements, their behavior
for steady state DC is the most defined (when the loop has settled).
The audible effect of the failure of negative feedback during transients is
under debate. It is know to happen, and the brute force approach is to
eliminate feedback in the circuit and therefore eliminate the problem.

This is easy to do if you can get active unity gain stages that are stable
over temperature. But it is quite difficult to do that, which is why op-amps
were developed to begin with.

----------
>From: "J. Larry Hendry" <jlarryh at iquest.net>
>To: <synth-diy at node12b53.a2000.nl>
>Subject: Re: Compensating multi-stage feedback (was: RE: all tranny vca+ )
>Date: Sat, Jul 1, 2000, 11:49 AM
>

> OK, I am new to this game and have read this discussion with interest.  I
> will have to claim that I was totally ignorant to the use of op amps until a
> few months ago.  Now, I know just enough to make some op amp circuits work.
> I have a couple  of references that I am using on the subject.  1 - the
> simple Radio Shack booklet and 2 - the very in depth "Art of Electronics"  I
> have built a few of these circuits for summing and inverting and have the
> basic hang of it.
>
> What is escaping me here is why the use of an op amp and negative feedback
> in the audio path is perceived by some to be a bad thing.  For example, if I
> wanted to boost the level of a line level audio signal to use in my higher
> voltage modular audio path, seems like a simple op amp (one with the right
> characteristics for audio) would be easy choice.
>
> Without starting a discrete vs. op amp vs. tube all out onslaught, I would
> appreciate hearing individual reasons why one would choose to build a
> discrete circuit in the audio path as opposed to a good op amp.  Subjective
> reasoning is really what I am after.
>
> As always, thanks.  I don't post much here as my knowledge level is so far
> below the qualified ones on this list.  But, I read almost all of it and
> appreciate the great spirit of sharing.
>
> Larry Hendry
>
>
> ----- Original Message -----
>>100% agreed. I'd even go one step further: The complete audio path can be
>>done without negative feedback. (And hence no compensation.)
>
> From: jhaible <jhaible at debitel.net>
> That's what I intended to say. Been there, done that - the audio path of my
> JH-4 synth is completely built that way. The only multi stage feedback loop
> is in the VCFs, to get resonance.
>
>
>