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

[tomg]

> [Mixers...]

This is something I noticed and solved to my satisfaction a while back
I wanted to know why some integrated circuit mixers sounded so warm
and others so harsh. It has very little or nothing to do with opamps
or using them as amplifiers.

Most if not all mixers these days use dual fet-input opamps. Usually
configured non-inverting,  X10 for amps then scattered liberally throughout
the mixer being used for everything else. The opamps all sound  about  the
same. There are some premium opamps out there that do sound different
( described as better by some ) but to me it's just different. I actually
prefer coaxing "the sound" out of run of  the mill opamps.

>    I recall the issue about having capacitors in the signal path.
>
> I hear a more obvious effect than I'd expect from caps, but yeah,
> it's entirely possible.

Getting good sound out of opamps has everything to do with coupling
and EQ. The same "bad" sounding mixer (or guitar/bass amp) can be
built with your choice of tubes or transistors.

That first generation or two of  IC stuff sounded so bad! I think the
first stuff to get it right was the Gibson SG line. (They used Maestro
phasers and I think there may have been a Moog connection. That
last part is pure speculation) The first stuff to get it really wrong was
that line of astrology amps by Fender. Other opinions may vary....

[..snip...]

> I was also thinking of one particular effect here.  In a tube guitar
> amp, a sudden transient will cause the grid to conduct on the positive
> peak, charging the coupling capacitor, and changing the bias until the
> capacitor has had a chance to discharge.  Eric Pritchard claims in
> his tube emulation patents that this is an important part of the
> characteristic tube guitar amp sound.  I don't know if I believe that
> or not.  I know from personal experience that an extreme case of
> coupling cap charge sounds awful, the signal can actually cut off
> after a transient, but I can't say I've studied more subtle cases.

I think this makes sense and it sounds like a good way to describe
"tube-compression" (the sweet sound that happens when a tube runs
out of head-room gently distorts and then even more gently recovers
as the signal is reduced) but I have my doubts too. OTOH I can't
think of a single circuit that would dispute it. He might be right.

> I'm sure I've gone way off topic at some point here.  :-)
>
>   -- Don

We are in full tilt audio territory.....;-)

Tom