Most filter builds reach for a bag of matched silicon or germanium diodes and never look back. This one goes the other way entirely: a galena cat's whisker detector, roughly a hundred years old, sits at the core of a synth filter loosely based on the ladder design from a well known video tutorial.
The mechanical side is where it gets charming. The crystal lives in a transparent glass cylinder clamped between two black fixtures on a wooden block, and the cylinder holds tiny particles inside. An adjustable threaded rod with a knurled brass locking nut sits on the right of the tube mount. Around it, circuit boards and a perforated prototyping board carry potentiometers, integrated circuits, resistors, capacitors and plenty of jumper wiring, all inside a red enclosure.
The standout detail is the interaction: moving the whisker to fresh spots on the crystal creates point contacts with different voltage drops and IV response curves, and each one changes how the filter sounds. The PCB takes six swappable diodes, and the maker keeps four homemade whiskers plus the antique one ready for live gigs. Some music made with the filter is already available online.
It is not all smooth running, though: the cat's whiskers misbehave a lot on stage, and that is the weak point I would flag. I am also left wondering which parts were CNC milled, since that answer went to a comment that has since been deleted. Even with those gaps, turning a crystal radio detector into a filter voice of its own seems like genuinely fun work.