[sdiy] Nonlinearities in IR3109 filters

[René Schmitz]

Magnus Danielson wrote:

> 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.

The late MS20 and MS50 also fall in that category. Unless the gain is 
rolled off with increasing amplitude (its not a real clipper as its 
still unity gain for high amplitudes), the oscillation builds up, until 
it finds the next delimiting nonlinearity, which is the opamp rails... :-P

> 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

Good way of thinking. The MS50 is "basically" the same thing topology 
wise than the MS20, while the 50 doesn't use OTAs, and they share this 
aspect.
(And a more generalised remark: Keep clear of generalisations in 
general. :-))

Cheers,
  René

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uzs159 at uni-bonn.de
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