In between a lot of other things, I am back to making piezo preamps based on Alex Rice’s design. They impedance match piezo discs (notoriously tricky things), giving a much better response at low frequencies than when they are just plugged into a mixer/recorder.
The preamp requires a matched pair of JFETs. I used to do this by ear, testing pairs in the circuit through a mixer, but I finally got my head around the matching process a couple of years ago thanks to SmudgerD’s Stompville blog. In a fit of madness I even sat down at the time and built a (point-to-point soldered) fet matcher based on SmudgerD’s PCB overlay.
I’m so proud that this thing still works, given it looks more like a sculptural work than a proper circuit. I am, however, going to treat myself to a new tester to be more future proof.