[AH] Re: [sdiy] Is the 904A supposed to....

[Theo]

Yep, the original Elektor Formant filters where designed to have NO
self oscillation too. And that one was from 1977.
Ok Elekor designers have shows them selves retards quite more often,
but the point is the idea that self oscillating filters in a synth is a bad
thing
was not done with even in the 1970s.

Theo



From: Magnus Danielson <cfmd at swipnet.se>

> 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
>