Starting a voltage controlled filter from a handful of passive parts is a classic trap, and this build tackles it head on. The passive four-component filter simply is not ideal if you want a high-quality VCF with resonance, and chaining two passive stages causes interference between them. The maker's answer is to go active: buffers are added after each filter stage so the stages work in complete isolation, which turns the design into an active two-pole low pass and opens the door to resonance.
The teaching approach is what stands out to me. Op-amps, buffers and negative feedback get explained before the active filter is built, including a worked example where an op-amp multiplies a 3V difference between 8V and 5V inputs. The hand-drawn circuit diagrams on grid paper, with the schematic titled FULLY RESONANT LPF, give it a warm, approachable feel. The circuit itself was assembled on a breadboard and tested with an oscilloscope.
The parts list is nicely modest: one TL074 quad op amp, a 100k linear stereo potentiometer with 500k suggested for more range, a 100k linear mono pot, a few resistors, two 1uF and two 10nF capacitors, and six 14148 diodes. The maker also built a dual power supply on perfboard providing plus and minus 12V, which matters since the op-amp output is limited by those rails.
The results look good on the scope: a square wave smoothed into a rounded wave that sounds cleaner than the passive version, and turning the PEAK knob adds a ringing sound as resonance amplifies frequencies around the cutoff. One open question for me is how it behaves as a full module; the demo used a module labeled MS20VCF, and a spin-off running the finished filter on a single 9V battery is only a maybe at this point.
Project by Moritz Klein. Watch it on their YouTube channel: