Voltage controlled all pass filter

This is an all pass filter (APF) intended to transform LFO wave shapes. A more detailed writeup is at Voltage controlled all pass filter – Analog Output .

An all pass filter, as the name suggests, passes sine waves of any frequency with no change in amplitude. What does change is the phase angle, the relationship between the zero crossings (or peaks) of the input signal and those of the output. The phase shift depends on the frequency. A single pole APF produces a phase shift varying from 0° at low frequency to -90º at the corner frequency to -180° at high frequency. A two pole APF gives a phase shift from 0° at low frequency to -180º at the corner frequency to -360° at high frequency — the latter half of which is the same as +180° to 0°.

Putting a non sine periodic wave through an APF changes the wave’s shape, since the different harmonics undergo different phase changes; the peak to peak amplitude can change as well, but usually not by a lot. Since the human ear isn’t very sensitive to harmonic phases, this doesn’t do much of anything interesting to audio waves. But it can produce interesting variants of low frequency CV waveshapes.

This module provides single pole and two pole APF outputs. It has input jacks for an LFO signal and a ±5 V corner frequency control voltage. There are attenuator pots for both inputs, and a knob to set the initial corner frequency.

Here you can see what it does to a sawtooth input wave (top). Middle and bottom traces are the one and two pole APF outputs. A slow LFO sine wave is going into the CV input.


A sin wave will not change shape, but its phase shift may be useful, if you want to vary two patch parameters with out of phase sin waves.

Schematics, KiCad design files, Gerbers, and documentation in the GitHub repo: GitHub - holmesrichards/vcapf: Low frequency voltage controlled all pass filter synth module in Kosmo format.


If you take a sine wave and its quadrature pair i.e orthogonal or 90º out of phase, you can do some interesting modulation with those. Take a signal, modulate it with both the quadrature phases and then mix the resulting outputs - you can probably get some interesting phase or frequency shifted results because some of the side bands should be supressed leading to more harmonic results.

I did this digitally here - but maybe you can make an analog patch that does something similar?



It’d probably be better to use a quadrature oscillator if you’re going to get into something like that but maybe this could give you a taste of it first?


Indeed, you’re spot on. I thought about the patch with 90º offset and realised that you’d have to adjust the corner frequency if you ever changed the incoming lfo frequency to keep the phase aligned, which is less than ideal - the quadrature oscillator looks like it’d do a much better job of it.

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