[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

> Hopefully it will sound good!

Thanks!

> Are the 3.3V zeners still in the feedback loop of the resonance amps?
>  From my understanding once the threshold voltage of the diodes is
> reached the amp becomes unity gain, not clamping the signal as it would
> if the diodes were going to ground or in the feedback loop of an
> inverting op amp stage.

I'm not sure.  However, the signal through the resonance amp never seems to
clip, and the sine amplitude does not affect the degree of resonance (or the
self-oscillation point), so you are probably right.  However, I've rigged
mine so that the amp reaches unity gain at a resonance pot setting of 30%.
Both the HP and LP sections go into self-oscillation at a resonance amp gain
of 2 (60% pot setting), but if the HP section is already self-oscillating,
then the LP section goes into self-oscillation at a gain of 1 (30% pot
setting).  The behaviour of this filter is really quite complex!

Now that I've got it all laid out in the original topology (fixed HP and LP
stages), I think I'll redo the whole thing and put in switches to allow each
section to be HP, LP or N (a shameless copy of the MOTM 420).  I'm presuming
that N is obtained simply by feeding the input to both the HP and LP
destinations (?).  I'll find out soon enough in simulation, but it makes
sense.

By the way, looking again at the MOTM 420 BOM document on the web, I see
that SPDT On-Off-On switches are used to switch HP-N-LP.  If N requires both
HP and LP connections simultaneously, I'd love to know how this is pulled
off with such a simple switch.  Does the switch simply shunt one or the
other input to ground?  Add an equal but opposite signal?  Any hints would
be most appreciated!  I'd like to avoid having to use DPDT On-On-On switches
for this function, if possible.