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Thanks Jerry and Scott! -- when I tie the unused wire from the plug to ground as suggested, the mic goes silent - meaning totally dead with no signal at all being recorded.

So, currently the live channel is going to tip, and ring is disconnected, resulting in hum, but joining ring to ground it goes dead, so I assume a mono plug won't help.

I have crudely tried joining signal to ring (in addition to tip) but it didn't seem to make a difference. I also crudely tried a 10k resistor between ring and ground (with no signal connection to ring) and it also made little difference (maybe like 2dB less hum, but didn't really help.) But I did both of these things in a hack way; just holding the exposed wires (using a plastic-handled tool so my hands weren't pinching the joint directly). I can try a more "real" version if it would seem useful.

Any ideas on a next move?

Some other notes in case they reveal that I am doing something dumb:

- mic cable is two conductors inside a braided copper shield; I use both of them joined to carry the signal from the mic (since I don't need both)

- mic is not shielded besides its factory metal enclosure, which AFAIK is adequate for PiP mics

- joint between mic cable and the pigtail leading to the plug (maybe 10mm) is not totally shielded: maybe this is my issue? Similarly, where the cable joins the capsule is not fully shielded (maybe 6mm). I had assumed these spots (which are small) might be irrelevant, but maybe I was naive.

- I mention again in case it's a clue: a different stereo PiP mic (one mic wired to tip, one to ring) works without hum, though the signal level seemed poor

- no idea what voltage the PiP supply is but I could measure if useful

Thanks for any further thoughts!

-c