To: analogue@magnus.acs.ohio-state.edu
Subject: VCO tricks for homebrewers
Date: Thu, 14 Jul 1994 16:14:43 -0600
From: Forrest Cook  (cook [at] rdnzl dot eklektix dot com)

Here's an idea for all of you modular builders out there:
There always seems to be a need for more sonic variety from VCO circuits
and I thought I'd share an idea I had about a mod for the PRO-1 and it's
Curtis VCO (I don't have the part number handy, I think it's a 3310?)
The VCO has separate outputs for sine, square, and ramp.  You could
feed each of those outputs into separate op-amp inverter stages then
put a pot across the op-amp output and the VCO output.  Use the pot's
center terminal for the signal output.  You should then be able to
get (+phase->no signal->-phase) out of the vco.  With such a circuit on
all 3 waveform outputs you will have a lot of range for tweaking harmonic
contents.  You should probably put a mixer/buffer op amp stage after all of
the pots to insure a constant impedance for the next stage.  I'd use something
like a 1K or 5K linear for the phase pots.  A long time ago I built an all TTL
computer/sequencer/synth that used a similar technique with 4 divide by 2 square
wave outputs and was able to get a lot of sonic variety out of the beast.
I haven't actually built this on my Pro-1 yet but it seems like a fairly
straightforward mod.  I'd use NE5532 dual op amps myself, your mileage may vary.