[sdiy] Nonlinearities in IR3109 filters

[Magnus Danielson]

From: "Sean Costello" <seancostello2003 at comcast.net>
Subject: Re: [sdiy] Nonlinearities in IR3109 filters
Date: Wed, 2 Aug 2006 13:11:56 -0700
Message-ID: <003601c6b66f$e7fb0290$0400a8c0 at analognuf38wu4>

> I had just assumed that most OTA filters relied upon the nonlinearities in
> the OTAs themselves to keep the self oscillation amplitude within reasonable
> areas. Antti's analysis of a hypothetical SSM 2040 seems to indicate that an
> OTA based cascaded 4-pole with feedback will have higher self-oscillation
> amplitude than a Moog ladder, unless some sort of feedback limiting is
> included. Very interesting.

For me, using diodes in the feedback (such as in the Obie SEM as KD pointed
out) is not as such associated with OTAs from my point of view, but rather with
State Variable Filters where the feedback balance at or near self resonance is
a delicate balance. Wiring up like a SSM 2040 or SSM 2044 and feedback (just
like the Moog ladder) has a much more stable balance at that point.

So, I have never viewed OTAs but rather the filter topology as being what
requires more or less control of the feedback. In the EMS VCS-3/Synthi-A there
is no real limitation, rather you vary the amount of overdrive and you do hear
the shift in character as you turn it deeper into the self-oscillation range of
the resonance knob. The point where it goes into self resonance isn't very
clearly defined, but you know more or less where it is for a particular Synthi
so there is no real problem.

Cheers,
Magnus