I made this rather strange syntheziser around 1972. It consisted of several modules, but the two most important was a rather “clever” VCO with lots of features, and a continuously variable sequencer that can be used as both an envelope shaper or a melody player.
The VCO has all the usual waveforms: sine, ramp+, ramp-, triangle, square, pulse and square/triangle-mixed signals all at the same time. There is also an input-variable DC offset. Frequency modulation can be both linear and logarithmic, and there are control inputs for duty cycle on all waveforms. Everything is made with LM310, and the performance is good and stable. The sine is made with a triangle-shaper of my own design.
The sequencer is made as a large summing amplifier with 50 potentiometer inputs, and rather difficult to make noisefree in these days 🙂 You can see and hear it here: