I’ve been on a deep dive on biasing JFET gain stages for guitar pedals over the past couple of weeks. It’s been a lot of time in the weeds, but I’m emerging with a quicker and more consistent method of biasing for the results I want.
Setting aside the details aside (links here, here, here, and here if you want those), within a batch of JFETs of the same part number individual characteristics will vary quite a bit. To get reliable performance, JFETs must be measured and either 1) sorted into a group that will work in a given circuit and a group that won’t, or 2) individually biased to work in a circuit by adjusting the resistors surrounding the JFET (Rd and Rs in the diagram above).
I’ve been running controlled experiments, recording results, and generally doing low-level basement science and engineering with the goals of learning what matters and what doesn’t, and figuring out efficient ways of achieving the results I need for pedals.
There are a couple more things to check, but I’ll present conclusions and my biasing method (nothing novel, just a straightforward step-by-step process) here or at mrfuriousaudio.com when I have them.