A thought I’d like to share out loud.
I’d consider the “holy grail” of DIY synth module to be a key-tracking (V/Oct) VCO without any specialty chips, a low part count, and an easy build. I’ve never found this - solutions always seem to use specialty chips, like the AS3340 in the LMNC VCO, or require difficult builds, like the TH-555 (which also has a few specialty parts).
So, I was thinking about different approaches. Rather than thinking about VCOs as we normally do - what about KTFs? A high-resonance self-oscillating KTF will essentially act a sine-wave VCO.
Well, it turns out there is already a DIY KTF out there! The Yubi-Synth MiniMoog clone! It does use one specialty part - the now-discontinued CA3046. But, the CA3046 is just a matched transistor array, and DIY transistor matching is already done regularly by the community here, as described in other threads. Other than that, it has no specialty parts other than a reversed potentiometer.
Reports I’ve found online suggest this has four to five octaves of accurate v/oct tracking down to 60Hz with well-matched transistors, and there is advice for boosting the resonance of the filter.
So, I guess my idea is this - modify the Yubi design to remove the audio inputs, circuitry and controls, hard-code the beefed-up resonance all the way to max, replace the CA3046 with a manually matched array - and there you go! A v/oct tracking DIY VCO with no specialty parts! Maybe add a simple ramp/square generator to the sine output for some more variety.
Anyway, this was just an idea that floated into my head, was wondering if the wizards here had any thoughts. Has anybody explored this avenue before, are there any reasons that it wouldn’t work that I’ve not thought about?