Barber Electronics forum.

One more data point: I plugged the Tone Press into a battery powered amp, and as soon as I plug in the AC adapter I get the hum. That pretty much eliminates the possibility of a ground loop, and I think all I need to take for testing is the pedal and adapter.

Yes, it is going straight into a guitar amp.

I'll report back after I test it with a different supply.

I hooked it up to a Pedal Power 2+, and the hum is gone. A couple of times the Tone Press didn't want to work with the PP2+; after a few moments it shut off (with a slight pop) and the output was dead unless bypassed. This may have been related to plugging in an external supply while the pedal was on, but I'm not sure and haven't been able to repeat the problem. If I get something repeatable I'll post it in the correct forum.

Thanks to all for the help,
Lynn