I got the idea for this circuit from a Synth-DIY thread. This suboctave unit accepts a +/-5v sawtooth and generates the following waveforms within the circuit:
The sine suboctaves have the advantage of adding a big bass boost without adding a lot of buzzy harmonics.
The output mixer has pots to mix in the original saw with the other waveforms. This implementation mixes in the square and sine waveforms only, but it would be a pretty simple matter to mix in the suboctave sawtooths (sawteeth?) and triangle waveforms also.
The circuit is designed to handle a +/-5v sawtooth at the input. I added the INV switch to be able to handle either up-ramp or down-ramp saws. You can also get some very interesting waveforms by purposely having this switch in the "wrong" position.
There are two trimmers per stage. Looking at the output of U1:8 on a scope, adjust RV3 to get the best looking
triangle. Then look at the output of U1:14,, and adjust RV1 for the best looking sine. Perform the same
operation on the second stage by looking at U5:8 and adjusting RV4 for best triangle, and at U5:14 and adjusting
RV2 for best sine.