[sdiy] Shruthi 4PM (was re something else...)

[David G Dixon]

> David Dixon and Olivier Gillet were working on the same SVF 
> design at the time, and we shared notes. It was very helpful 
> and productive.
> 
> HTH,
> Tom

My design has since gone into production as the Intellijel uVCF.  It works
very well, gives even resonance over the entire audio range, and
self-oscillates very cleanly and very reliably when the resonance knob is
cranked even down to LFO frequencies.

I put zeners across the LP cap, and did some magic mumbo-jumbo with fixed
resistors in the feedback to sustain clean self-oscillation.  Simulation
suggests sines with THD at around 0.1 to 0.2%, although I haven't measured
it.