VC APF module, 50x200 mm, with three knobs and four jacks

The Analog Output Voltage Controlled All Pass Filter (VC APF) is intended to transform LFO wave shapes.

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 and the number of filter poles.

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.

It produces interesting shape variations on triangle, ramp, and pulse waves. 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.

Git repo