The sub oscillator was one of the first synthesizer modules I ever built, back around 1980. The first version was designed with the ETI 4600 in mind, and was published as a circuit idea in the Australian edition of ETI.

Another circuit I used at the time to "fatten up" the sound of my single oscillator synth was a "harmony generator", achieved by running a 4017 decade counter chip wired to divide by three or six.

This design combines both of these circuits, giving a two channel sub oscillator, which allows each channel to be used independantly, or driven from the same oscillator, but set to different intervals. As a bonus, both channels can be multiplied or "digitally ring modulated" giving even more effects.

Some ideas on how to use this module:

Fed by two VCO's it can operate as two independant sub-oscillators, with optional ring modulated outputs.

Running both sub-oscillators from the same VCO, tuned to a fifth over the base note you require, and with the prescaller set to divide by 3, it is possible to create harmonies. An unusual effect here is that the ring modulated outputs give a fatter chord-like sound that remains indepenant of minor or major scales, allowing "one finger chords" which can be handy when used with a related sequencer driven bass line.

Careful mixing of the ring modulated output with one channel's divided output results in some interesting sounds, especially if one of the channels is being driven from a low frequency oscillator. Running like this, it could be considered to be a "harmonic sequencer"

Running both inputs from the same LFO/clock signal, and feeding the mixed output into the 1V/oct input of a VCO gives you a pattern based sequencer, where the prescaler, ring modulated outputs, etc., all have a bearing on the pattern. Quite interesting patterns can be created this way. Each knob of course varies part of the overall structure, instead of a single note as per a regular step sequencer.

A little on how it works:

In a way, this design can be considered as a sub oscillator construction kit, as there are many ways it can be configured.

Input stage of the sub-oscillator. Two are used, one per channel.

Prescale stage of the sub-oscillator. This is used only in the "B" or second channel.