开云体育

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

On 2/23/25 19:22, Jerry Lee Marcel via groups.io wrote:

What about grounding the unused channel?

Le 24/02/2025 à 01:01, Casey via groups.io a écrit?:

Hey all -- I have a project I'm working on that involves a raspberry pi, a cheap USB audio interface <>, and a small electret capsule (JLI-61A <>) powered from that interface via PiP.

Everything works, but there is 60Hz mains hum.

I believe the the audio interface is providing a stereo 1/8" jack, but it mixes to mono internally - the computer is only provided a mono capture by the audio interface. (If I use a different set of stereo PiP mics that I made which are wired to a stereo plug, there is no hum, and both capsules hear audio in the result, but the overall signal is weak.)

The mic capsule is wired to a stereo plug (using shielded lav cable), but the signal only goes to one channel of that plug, so I presume the disconnected channel is just feeding in hum, is mixed with the signal in the interface, and that explains the hum in the result.

Does my diagnosis sound correct? If so, what would you recommend? Should I just wire the signal from the mic to both the tip and sleeve of the plug, or will that mess up the interaction with the capsule somehow? Or would it be smarter to get a mono plug?

Thanks!

-Casey