[sdiy] So just what *is* the Buchla 291?

[Walker Shurlds]

Hey Aaron,
I think it's almost a biquad state variable... but with the
input injected differently.  It looks like integrator >> amplifier >>
integrator instead of amplifier >> integrator >> integrator.
Typically, the bandpass output is out of the first integrator, so
if you move those around the output here is still true to that.

U3 is still a typical integrator, with the bandwidth control making it
less ideal.  U1 is sort of an integrator, but all the feedback is crazy.
You'd have to resolve the system with it rearranged to see what parts
align to Q and omega and so on, I think.

If you redraw the feedback a bit... it's just the 6k8 + resonance
around the amp stage, 22n around the output integrator, then 220p ||
6k8 around the whole thing.  If we assume that the 470p is filtering
instead of coupling, it gets stupid worse.

Well, that's my attempt at saying random things in hope that one of
them lands on an insight.

Walker Shurlds

On Sat, 8 May 2010 13:19:05 -0400
Aaron Lanterman <lanterma at ece.gatech.edu> wrote:

> I first heard this in person, thanks to Charles Cohen. It sounds
> pretty wicked.
> 
> I had a student build it, but I don't understand it.
> 
> I've heard it called a state variable filter, but when I look at the
> schematic I can't quite map it to any particular state variable
> arrangement I am familiar with. I don't see obvious variable gain
> integrator structures, and the "business capacitors" (i.e. the ones
> that look like the appropriate values for doing audio filtering)
> actually all seem to be in the feedback loops.
> 
> Here's the schematic:
> 
> http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2910_150.jpg
> 
> and here's the schematic redrawn by Marjan Urekar:
> 
> http://urekarm.tripod.com/synth/buchla_vcf_b.pdf
> 
> - Aaron
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