About

This module is a quad build of this ingenious little circuit, modified by me for an "improved" experience. These are Hz/V oscillators, not V/Oct tracking ones - so if you want some VCOs to play nice pure tonal melodies from your quantized CV generaotr, sorry, these won't work, unless you build/find an exponential converter. Otherwise, these VCOs have huge range for this few parts, have nice buzzy sawtooth outputs, are CVable over the entire range, and - good for some beginner SDIYers - probably work with unipolar power supply. They are good for weird effects, tones, or even as voltage controlled LFO/clocks! If you have an unquantized analog sequencer, you can carefully dial in great melodies despite the Hz/V limitations, too. Overall, it's a basic "strange but does its thing" pack of VCOs, and given that they can be built with a tiny pile of parts, they are great.

UPD: this design kinda sucks! It is fine for pedal builds, but it doesn't make much sense in euro, although it's still fun to mess with. If you want something simple, build the OG thing, or if you're up for something more complex - better take a look at my newer VCO design.

Schematic

The core of the VCO is the rightmost op-amp, which is a very tiny narrow pulse generator. It does so because it charges the capacitor Ctime instantly over the diode, but then discharges way slower. The byproduct of this is a downwards-going sawtooth at the capacitor, which we buffer through T3. Transistor T1 is like a voltage controlled resistor in a sense, as in the bigger the voltage at the base, the more it "helps" the capacitor to discharge, thus, repeating the cycle. Before it is a buffered voltage summator of 2 or 3 inputs - pitch knob, CV in, and (optionally) FM in. CV and FM are both linear in this case, but FM is also DC-decouples the input signal, making it sound more, well, FMish! If nothing is patched into CV or FM in, the attenuators will sorta act like fine tune knobs: if you want to evade that, change the left op amp's configuration to inverting summator with virtual ground, and then invert it once more afterwards: i amimed for the least part count, so i didn't mind.

It's easy to build 4 such VCOs on two TL074s: i'm not sure if the summing stage can be unipolar in the way it currently is, but it also can be omitted altogether and replaced with a passive resistor summator at T1's base. Will be more crosstalky and without gain compensation, but will work for unipolar guitar pedal PSU dronebox people.

Mods, like the FM in, or the sync input and LFO mode, are optional. You can throw them out of the circuit and still get it working. I built two VCOs with FM input mod, and the other two with the Pulse Output mod (same buffer as for sawtooth, but for the op amp output), Sync In (diode to op amp's - in, will reset on short pulses from other VCO pulse outs and stop on longer gates) and the LFO mode switch (the switchable big cTime2 capacitor in parallel to cTime, which increases the overall op amp inverting in to ground capacitance). So first twi are cooler for weird timbres, second two are also usable as LFOs/clocks and are re-syncable. You can build it your way - with all the mods, or without them at all. Have it your way!

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