This is a very interesting point. As I see this, there are, generally speaking, two types of oscillators. The “noisemaker” kind, simple and crude that make some “noise” by turning potentiometer and the like, and VCOs that respond to a V/octave (or similar) standard and can be tuned and played in musical instruments. I think that, if anyone’s confused, it’s because the SSO is a noisemaker type of oscillator that tries to pass as a VCO.
I haven’t build a VCO myself yet, as I am working my way through easier circuits, but as far as I’ve read, the options for a decent DIY VCO are the following:
- A design around the 3340 IC, like the Hero/Sidekick VCO by @analogoutput, the 3340 breakout board by @Sonosus, the “Curtis VCO” by Gerbrand Sterrenburg, or even this design. There are also many commercial options, like the 1222 Performance VCO by LMNC in Kosmo format, and many more options for Eurorack that can be adapted to Kosmo (Kassutronics, etc). If one feels confident enough, they can even try a stripboard layout, or even clone a 3340-based VCO off a vintage synth off a schematic.
- Adapting another IC to use as a 1V/octave VCO. Thomas Henry has designed VCOs around the XR2260 and the LM566. If you can find these obsolete ICs, there are both commercial, open source and stripboard designs around…
- Designs based on CMOS ICs. @moritzklein has a design that uses the 40106 Schmitt Trigger, René Schmitz one that uses the 4069 inverters, Thomas Henry has one that uses the CMOS version of the 555 timer, and another one that uses the 4046 PLL.
- Discrete approaches using op amps, like the David Haillant VCO mentioned above, and variants, including this one by Louis-Pierre Geerinckx), or the Polivoks VCO by Erika Synths (available in github).
The catch with all the designs that do not use the 3340 or one of the obsolete ICs is that they require other fancy parts like thermistors and/or matched transistors and/or OTAs…
Pick your poison, so to speak!