[sdiy] Is the 904A supposed to....

[Magnus Danielson]

From: "Paul Schreiber" <synth1 at airmail.net>
Subject: [sdiy] Is the 904A supposed to....
Date: Wed, 2 Jan 2002 08:34:48 -0600

Dear Paul,

> >
> > My 904 goes to the brink of self oscillation at the maximum setting of
> > the regen knob, but it will not oscillate. I believe that it was
> > designed that way, thus the need for selecting that resistor to trim the
> > regen control so that it goes almost all the way, but not into
> > oscillation. The minimoog has a trim control to do the same function.
> 
> This brings up a point I need to check with Roger L.: the sservice manual says "Connect a decade
> resistace box across R11 (the 1.8K in most modules) and determine what shunt resistance is
> required to establish the threshold of regeneration."
> 
> What is NOT said is are we trying to *start* regeneration, or *prevent* regeneration!!?!

I've been lurking on this thread until now...

I think you are missing out a historical point. Think way back in
time... then it was assumed bad to have a filter that oscillated. You
therefore spent extra effort to go bloody near the limit (which where
sonically usefull) but safely stay of self-oscillation. So, if you
make this assumption, you will automatically assume that the intent of
this trimming is to avoid self oscillation. Nobody demanded
selfoscillating filters in the mid 60thies.

You just made yourself victim of the change of preferences over
several decades!

I know that the VCS3/Synthi A has filters which intentionally could
selfoscillate (if you don't get the point with a module labeled
"Filter/Oscillator" and has a "response" knob going from "low pass" to
"osc." please let me know).

This makes an interesting subject by its own right, which was the
first synths which had resonating filters by design. We can limit
ourselfs to the 60this in this quest. We allready know about the VCS3,
so it has to be before that one (which came in 1969).

Cheers,
Magnus