3phaseVCO 12hp

3phaseVCO

3phaseVCO

The core of this module is three distorted sine-ish waveforms 120° apart.
These outputs are labelled A, B, and C.
A, B, and C are summed together with overdrive to make the MIX output.
The MIX knob and CV applied to the MIXCV jack modulate the level of A in this mix, morphing between two flavours of rounded square wave at each end, with a sweet spot near the middle that mimics 3X the main frequency (octave + perfect fifth).
The MIXCV knob attenuates whatever is plugged into the MIXCV jack.

FREQ and FINE adjust the frequency between 0.08Hz (~12 seconds per cycle) and ~1.7kHz. Adjust C1, C2, and C3 to modify this range.
A notable drawback feature of this simple design is that the amplitude of A, B, and C is proportional to frequency. Expect approx. 5Vpk-pk at the lowest frequency and 12Vpk-pk at the top end of the range.

LIN - linear frequency modulation input (with attenuator)
EXP - 1V/octave control voltage input.
1V/octave tracking is calibrated with the trimmer accessible from the front panel between the FREQ and FINE knobs, expect at least 2 octaves of good tracking in the middle of the range.

The 12HP version has pulse width modulated square/pulse outputs for each phase and two 'XOR' outputs.

With nothing plugged into a phase's PWM input the PWM knob varies the pulse width from 50% fully CCW down to 0%.
Where you reach 0% varies with frequency and the knob has a lot of 'dead' space at the end unless the VCO is running at the upper end of it's range. This bug feature doesn't bother me too much as it's useful to turn one or more phases 'off' to get different sounds from the PWM XOR output.
Plugging a signal into a PWM input converts the corresponding PWM knob into an attenuator for that signal. -ve control voltage will result in +ve duty cycle and vice versa.
'XOR' is the logical exclusive-OR of the main outputs and makes a square wave approx 3x the main frequency (you can still hear a little of the fundamental because the core is simple and imperfect)
'PWM XOR' is pretty much the same but derived from the pulse outputs instead of the main outputs. Varying the pulse width of the individual phases changes the timbre of PWM XOR. The most dramatic changes are noticed when one or more of the pulses it is derived from reaches 0 or 100% duty cycle.

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