[sdiy] [OT] Analog synths with 2 pole filters

[David G Dixon]

> > This is absolutely what I found with the SSM2164 SVF. It 
> squeals like hell at the top, but getting a reliable 
> oscillation down to the lowest frequencies requires a lot more gain.
> >
> > Dave Dixon and Olivier Gillet and I all shared notes on 
> this filter, so perhaps they have something to add on this 
> point? -Guys?

I can corroborate this observation.  Use high quality, low-leakage caps
(polystyrene, high quality polypropylene) for a start.  I prefer
double-ranging my oscillating filters, using 100x larger caps for LFO-range
oscillation.

Also, concerning using zeners in the BP feedback, I was originally concerned
because this also limits the amplitude of the filter even when it isn't
oscillating.  However, given the propensity for the 2164 SVF to give very
high amplitudes at high (but sub-oscillatory) resonance, I found the zeners
to be generally desirable.  We've certainly had no complaints with our
eurorack SVF filter in this regard.