Made another breadboard version and then used that same 2N5458 in a second PCB build and got the same result, so I think I can be confident that there's something wrong with my circuit or PCB. So, back to the drawing board with this one, I think.

Thanks, everyone!

 

A 2N5458 Jfet has a range of Vgs(off) spec of from -1V to -7V then some will be saturated and some will be cutoff. Did you measure the DC voltage at the drain pin to see?

But when the Jfet does nothing then there should be no 60Hz noise. Unshielded wiring problem?

 

I did not measure the DC voltage at the drain pin. Unfortunately I'd already decided this project was a failure and have salvaged some components from it, so I can't check this without rebuilding another one. I probably need to spend some more time experimenting with JFETs before I try again.

Thanks again for your suggestions!