This VCF (Voltage Controlled Filter) is made from a tl071 op amp chip, 6 resistors, 3 capacitors, 2 pots and a diode. The schematic is very simple, and takes about an hour to build (less if you’re good at this type of stuff). In this video I am generating a square-like wave using my 4093 based VCO (the round black module on the right), which is fed into the VCF (the middle one with the white and red striped-base). Midway through the video I turn on my 4017 Sequencer, whose output goes into the CV input of the VCF. This turns the solid note into a sort of harmonic/overtone melody, which can further be manipulated by adjusting the filter/sequencer. The resonance pot at full resistance turns the wave back into it’s original form. At little or no resistance you can see the waveform is more strongly influenced by the cutoff pot. The cutoff pot adds (or takes away) harmonics/overtones depending on how you manipulate it. I highly recommend this circuit. It is very easy to build, has very common component values, and makes every instrument sound crazy. I can plug my guitar into it and get a good Frank Zappa “Black Napkins” effect when my sequencer controls it.