[sdiy] 2164-based SVF [was: Re: SEM filter questions]

[David G Dixon]

> Some more 2164 SVF schematics online for you to study:
> 
> http://www.sowa.synth.net/modular/vcf.html

This one is interesting, as he has put the limiting zeners in the feedback
loop of the second (LP) stage.  I wonder if this one would give stable
self-oscillation?

> David Dixon has done something with 2164 filters, but google 
> doesn't find any schematics online.

Yes, I'm a bad boy.  I don't have a website, but happily email stuff to
anyone who asks.  I've converted two of my 2164-based filters into
commercial eurorack modules via Intellijel (with Danjel van Tijn).  I love
2164-based filters, because they don't require expo converters (but still
track decently) and they sound fantastic.

My 4P LPF design has become the "Dr. Octature."  It has two sets of filter
caps for VCF/LFO operation, eight outputs (in "octature"), VCQ, and a
"Quad/Mono" switch for bass boost.  It can also be expanded into a multimode
filter a la Oberheim.

My dual MS-20-inspired VCF design is the "Korgasmatron" which also comes
with a built-in VC Crossfader, a unique "Q-Drive" control for boosting the
filter level relative to resonance, switchable LP-N-HP inputs (like the MOTM
420), and Series/Parallel routing of the two channels.  It can also be
expanded for multiple inputs (each assignable to the various modes) and VCQ,
and has two different styles of limiting diodes: a "modern" setting using
3.3V zeners, and the "classic" setting using the original anti-parallel sets
of three diodes, selectable via jumpers.  I just recently figured out how to
make each channel into a six-mode multimode filter, including 12dB LP, 6dB
LP, BP, 6dB HP, 12dB HP, and a proper deep Notch (not that wimpy one on
there now).  My own unit (which I'm just working on now) will have three
attenuated inputs to each channel, and each input will have its own rotary
switch to select from the six different filter modes.

I also have a 2164-based SVF design with VCQ which I have never built, with
limiting zeners on the first (BP) stage.

I am happy to share schematics and build docs with anyone who asks, as well
as some "tutorial" materials for learning how to analyze the various filter
types, whether they're built with OTAs and buffered caps, or VCAs and
integrators.  I also sell boards (bare or stuffed) and handmade modules.
But, alas, no website (too lazy).