[sdiy] Random bits on the Buchla LPG

[Don Tillman]

   > Date: Thu, 18 May 2006 02:35:30 -0400 (EDT)
   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > 
   > 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.

Hey Aaron,

I don't know the answer, but here's my guess.  (Perhaps it will be
entertaining.)

Maybe DB wanted the filter to have a Q of 1.0, and to get that with a
Sallen-Key design where the two resistors are equal and the amp gain
is 1.0, he'd need a capacitor ratio of 4 to 1.  And perhaps he didn't
have those cap values handy, especially given the limited EIA cap
values at the time.

The 220pF/4700pF forms a cap divider, and is effectively a 4920pF
capacitor to 0.955 of the amp output.  The math is a little more
complex when the amp gain is not unity, and with those values the
circuit hits a Q of 1.02.

That's the best I can come up with.

Wouldn't it be better to just parallel up a couple caps to hit the
ratio?  That's what I would have done.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com