The famous VCO module by Thomas Henry based on the 555 timer IC. This was the first module I ever designed a PCB for, which, in hindsight, was not the smartest move. It took me quit a while to get it bug-free, but now that it works, I love it! Over the years I read a lot of articles and booklets by Thomas Henry and if you’re into this synth diy stuff, I highly recommend checking out him and his work. If you want to build/buy one yourself, there is also an excellent version out there by a guy named Fonitronik, there is a link below.
The audio samples are straight from the 555 VCO, CV signal is coming from a S&H module.