3phaseVCO

SSI2140-based 3-stage phase shift oscillator

This module uses the SSI2140 filter IC as the building blocks of a 3-stage phase shift oscillator. Three voltage controlled integrators are connected in a loop with some negative feedback to keep the amplitude stable.

A, B, and C are three sinusoidal waveforms 120° apart.
They have a little distortion/character to them at audio rates but are much more sinusoidal in LFO modes.

The MIX output combines overdriven versions of the three main outputs, with voltage control over the level of A.
Summing together three-phase sinusoidal waveforms would normally result in no output at all, so the MIX output is really summing the distortion added to the three signals.

The MIX knob and MIXCV morph 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. Setting up this sweet spot with the MIX knob then feeding a nice and slow sine LFO to MIXCV makes an interesting phaser-like sound.

FREQ and FINE knobs are for manually adjusting the frequency.

The RANGE 3-way toggle switch selects 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.

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.

1V/O and CV inputs provide exponential frequency control. CV has an 'attenuverter' style control for attenuating and/or inverting the control voltage.
The brass screw poking through the panel between the FREQ and FINE knobs is for calibrating the V/octave tracking.

SYNC sort of synchronises the outputs to whatever you patch in here 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 but you can experiment sending different modulated, boosted, or attenuated signals here.

This is the latest (and hopefully last) version of this 3-phase idea I've had several goes 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)

https://isaacbeers.square.site/3phasevco