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

[Don Tillman]

   From: "J. Larry Hendry" <jlarryh at iquest.net>
   Date: Sat, 1 Jul 2000 11:49:17 -0500

   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.

My opinion:

The problem with opamps is that they operate by remote control.  In a
discrete circuit you can follow the the audio signal goes through a
series of stages, but in an opamp we have a servo arrangement that
tries to do what it can to make the voltages at the + and - terminals
the same, and the rest of the circuit sets things up so that there
will be some appropriate signal gain if the + and - terminals are the
same.

Being a servo system, pole compensation is necessary for stability.
And when something breaks down, like during clipping, it does so in a
nonmusical way until the servo mechanism starts going again.

The opamp is also in the position of amplifying the error.  The
difference between the + and - inputs is the "error", or the
difference between what the output is and what it ought to be.  That
error is amplified by the opamp itself and presented to the output.
So at a very fundamental level, an opamp's output is an error signal.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com