[sdiy] Buchla 295 weird topology

[Donald Tillman]

   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > Date: Sun, 23 Aug 2009 00:03:25 -0400
   > 
   > I'm checking this out:
   > http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2950_200.jpg

This stuff will hurt your brain, man.  :-)

   > The pure lowpass and highpass sections are standard Sallen Keys
   > with an unbuffered first-order passive section slapped onto the
   > front. This turns it into a rather nonstandard Sallen
   > Key. 

Um, not exactly.  It's a third order Sallen Key.  The R and C up front
are part of the circuit and interact with it.  So it's not just not an
additional pole slapped on the front.  (The math on this gets
significantly hairier.)

   > The 8 bandpass filters in the middle, though, are weird. To
   > establish notation, consider the filter that creates output "b,"
   > the 200 Hz filter. If you left out C9, you could redraw it as a
   > multiple feedback filter. But putting the cap in there weirds it
   > up, turning it into something I haven't seen before.
   > 
   > Anyone recognize it?

I'm seeing a mutant Twin-Tee.

  -- Don

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