[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

> I've been wondering the same thing. I've got my old dual EFM module i
> modified so one filter has the 3 anti-parallel diodes, the other is
> without and they both sound very different winding up the resonance. it
> would be good to be able able to control the middle ground between the
> two.

You can do the same thing I told Henry with the anti-parallel triple diode
arrangement.  Just connect one end of the strings of six diodes to the
negative input of the opamp, and connect the other end to the centre tap of
a pot.  Then connect one end top of the pot to the opamp output, and the
other end to ground (possibly through another resistor -- experiment).  You
will get more or less the same thing as variable zeners.  In fact, you will
get even more variation, since the three diodes in series have less voltage
than the zener.