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.