[sdiy] Buchla 158 sin-sawtooth oscillator

[Donald Tillman]

   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   > Date: Sun, 27 Sep 2009 23:06:57 -0400
   >
   > Anyone have any clue how this works?
   > http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_1580_1_200.jpg

Hokay...

IC1 provides some voltage for the exponential converter.

IC2 and IC3 make up the exponential converter, with the left NPN
running at a constant current and the exponential current coming out
of the right NPN.

C3 (or C5?  It's smudged.) is the integrating cap, it hosts a
negatively traveling sawtooth.

IC4 is a voltage follower that buffers the sawtooth.  This will be
used later...

IC5 inverts that.

Q2, Q3 make up a diff amp, and with Q4 providing positive feedback, it
becomes a Schmitt Trigger.   And that drives Q1 which performs the
postive reset on the cap.

The sawtooth and inverted sawtooth are applied to Q5 and Q6, they
function as a full wave rectifier, and that give you a triangle wave.

Q8, with diodes CR2 and CR3 shape the triangle into a sine.

These Buchla VCO's have a waveshape control input voltage that pans
between sawtooth and sine.  This was an important feature as the
Buchla filters were not terribly exciting, so you need another way to
get that envelope control of harmonic content.

So the buffered sawtooth from 6 steps above is applied to Q7, which is
operated as a voltage controlled resistor.  That resistor-mixes with
the sine output.

IC6 is the final buffer.

There ya go!

  -- Don

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