[sdiy] Random bits on the Buchla LPG

[Aaron Lanterman]

I've been going over the different versions of the LPG - the 292B and 
292C, and the Music Easel, and pondering the various differences.

I see the basic Sallen-Key structure, and the influence of switching in 
the VCA mode in addition to the VCF mode. I'm wondering about the purpose 
of random odds and ends, capacitors and resistors here and there, that 
fall outside that structure.

Let's start with the 292C.

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2920C_200.jpg

For instance, the feedback cap is a - looks like 4.7 nF cap. (Interesting, 
that's what he uses in his oscillator cores. Must have had a bunch handy?)

Anyway, between the feedback cap and where it ties between the vactrol 
resistors, there's this 220... uh, I can't really read the units! cap to 
ground, and I can't figure out its purpose.

Then, the second Sallen-Key cap is 910 pF. If the VCA mode is switched in, 
a 10K resistor to ground is dropped in in parallel to it. But, regardless 
of what mode, there's also a massive 4.7 meg resistor there. I can't 
figure out it's purpose either.

In the 292B version, the 4.7 meg resistor is 2.2 meg instead, the resistor 
that gets dropped in is 15K instead of 15K, and the weird 220 something 
resistor is just - well, it's 220 with no units, does that really mean 220 
microfarads???? Can't be that would make no sense...

Let's check the Easel...

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_10_200.jpg

Ah, yes, 220 pF. That makes sense. Looking at the values - interesting, 
the values of caps and resistors matches the 292B, but there's electronic 
switches like in the 292C. I suspect the Easel was an intermediate stage 
between 292B and 292C, like it was between the 258 and 259 oscillators.

Anyway, this is something I run into often - I see the overall circuit 
structure, but there's odds and ends that I can't really explain why 
they're there or how one would come up with putting them in.

Any clues what the big resistor and the 220 pF are meant to do, or how one 
would come up with putting them in? What would happen if one left them out 
(of course the thing to do would be breadboard it up and try it, but 
initial thought experiments are always good)

I want to flesh out my Sallen-Key lecture for the Fall, and it would be 
good if I could explain more of the circuit. (When I analyzed the LPG in 
class, I left out a lot of stuff.)

Thanks!

  - Aaorn

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
School of Electrical and Comp. Eng.    Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
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