Here's something I threw together today. It's a pair of very simple square wave oscillators based on two 555 timers and a couple of CD4013 dual flip-flop ICs. The "ring modulator" is actually a single XOR gate. Korg used a variation of this for the ring mod the MS-20 (they used NAND gates in an XOR configuration).
|The prototype on the breadboard|
It's possible to tap the raw 555 signal, as well as the output of the first flipflop. You can either route them straight to the audio output circuit or via the ring mod for a variety of different sounds. I'm going to build a switch matrix that lets me do that without needing to rewire the board. You can use the XOR circuit on its own as an effects processor, but if you're using anything other than square/pulse waves as inputs you'll need a comparator before the XOR gate.
|Schematic (produced in Yenka)|
The next step is to replace the 555s with a PWM circuit with independent frequency and duty cycle control. A sawtooth oscillator and a comparator might work, but I've never done anything like that before, so it should be interesting. I'm definitely going to use the XOR circuit again though, it sounds fantastic.