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

[David G Dixon]

Sorry, I screwed up!  I meant that I put the zeners over the BP cap (the
first integrator), NOT the LP cap.  Also, the good distortion numbers are
for the LP and/or HP outputs, (which then get some extra filtering), not the
BP output, which is slightly more distorted (but still less than 1%).

When I did this, I was a little concerned about the effect that the zeners
would have on filtered signals, but in fact, it helps there too, because the
SVF has a tendency to go a little bit crazy at high resonance, which can
give some very annoyingly loud output on frequency sweeps.  The zeners keep
it behaving nicely.

> 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.