SSI2140-based 3-stage phase shift oscillator

The main outputs A, B, and C are 3x sine-ish waveforms 120° apart.
The MIX output combines them with overdrive/clipping and voltage control over the level of A.
MIX morphs between in and out of phase squarewave versions of A with a sine-ish sweet spot in the middle that is 3x the main frequency.

A toggle switch selects between 3 running rates:
High = approx 7.5Hz - 7.5kHz
Med = approx 0.15Hz - 150Hz
Low = approx 11 minutes per cycle – 1.5Hz
CV will extend the range quite a bit more in each direction.

The panel has coarse and fine frequency knobs, a 1V/octave CV input, an exponential CV input with 'attenuverter' style control, and manual level and CV input for the MIX section.

There is also a jack labelled 'SYNC' which does sort of syncronise the outputs to the input but mostly it merely interferes with/injects the input signal into the oscillator core rather than reset the oscillator like in a traditional sync circuit. I like sending it a squarewave an octave or two lower and sweeping the frequency of the 3phase.

Note: when powering on or switching ranges the oscillations can take a dozen or so cycles to get up to maximum amplitude. Keep in mind when switching to the lowest freq range you might want to crank up the freq knob for a bit to get it going strong before winding down low otherwise you might be waiting a while for anything to happen.

This is the latest (and hopefully last) version of this 3-phase idea I've had several gos at since I started synth DIY.
The first was a vactrol-based trapezoidal LFO, followed by a few failed redesigns that led to a semi-useable audio-rate VCO using 2x LM13700 a few years later.
That version was still a failure. The frequency drifted constantly, it tracked 1V/octave very poorly, and the amplitude of its trapezoidal distorted waveforms varied wildly with frequency.
This time both the voltage controlled elements and exponential control voltage converter are provided by the very fancy SSI2140 IC and I am super happy with the tracking and stability I managed to get from such an otherwise simple circuit.
This VCO design leaves 1x OTA+buffer cell with nothing to do but the Q VCA works perfectly for the MIX output.
(I have more VCO ideas for this chip that I hope I'll get to some time)

8hp +12V: 15mA -12V: 13mA

• 15 mA +12V
• 13 mA -12V
• 0 mA 5V
• 25 mm Depth

Available as an assembled Module and as a DIY project.

This Module is currently available.