Thank you for the suggestions, Dave!

You can check if it's pick up on the lead by shorting out the input jack.
I'm not sure I understand what you mean.
If I short the minijack then it buzzes LOUDLY.
I'm careful not to do that.

there's likely inadequate or failed smoothing in the power supply
Can I fix it?

Other possibilities are hum pickup from something else in
the room - does moving the entire unit make any difference?
No, it hums happily wherever it is.

*I'm not as think as you drunk I am.
But if I come back before I return, please ask me to wait.

-Torben