Eurorack is everywhere, there's a billion tutorials and resources for it. There's a million tutorials on 9V stuff but no youtube series like this, so I figured I'd make one.
Personally I just like to work with single supply power - I don't use eurorack or anything like it. It's also pretty easy to run this stuff off a 9v battery or even 6v from some AAs if you feel like having it run slower and lower.
This is a VCO utilizing the CD40106 hex inverter chip, it is a design that is very popular among synth DIYers and has been adapted in numerous ways. Here's what I believe is the source of this design.
Paper Circuit
This is even simpler and more iconic than the VCO. Primordial module, any number you like.
Now we're getting more creative. This module consists of a VCF, square wave oscillator slaved to the input, mixer knob, and 2 LFOs.
Paper Circuit
This is inspired by the ciat-lonbarde tetrazzi. It's a bare-naked relaxation oscillator with a piezo preamp, and a vactrol to attenuate the oscillator.
Paper Circuit