[sdiy] 2164 Korgasmatron VCF

[Harry Bissell]

LOL...  After reading your description, I happened across an old
electronic workbench file where I had the same circuit (with three antiparallel
diodes btw).  I see how that would work, the big decision is where
to locate the circuit.

I was thinking you put the variable resistance in series with the zeners 
(which would not work correctly as the zener clipping would get progressively
more 'squishy').

I've tried a few different methods with the EFM version, including making the
input resistor into the resonance OTA into two series resistors (like 10K and
50K) and using single antiparallel diodes. The 10K gives a series impedance
that the diodes can work against (without loading the opamp LPF output)

Now how are you getting the notch response ???  (I'm getting a bandpass :^)

I have the two EFM filters set up with a Moog-like coupler, one CV moves both
filters in the same direction, the other sets the difference between the cutoffs.
Its an interesting module.

The neatest thing (imho) about the MS-20 filter is that the resonance takes place
at the zero cross of the input waveform, not at the peaks like most filters...

H^) harry

----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Benno' <benalog77 at gmail.com>, 'Harry Bissell' <harrybissell at wowway.com>
Cc: 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Thu, 12 Aug 2010 13:59:46 -0400 (EDT)
Subject: RE: [sdiy] 2164 Korgasmatron VCF

> 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.


-- 
Harry Bissell & Nora Abdullah 4eva