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[Russell Ault]

I've run into a problem where every new data point seems to make me scratch my head even harder, so I thought I'd see if anyone here had any ideas:

The backstage paging mic in the theatre I'm working in right now sends an audible (although, mercifully, not note-worthy) cut sine-wave buzz into the paging system, but only when a particular light is on (a light that is, incidentally, not hung very close to the mic) and only when the mic is switched off; when the mic is live or when the circuit is left open the buzz goes away. The typical setup involves an SM58 running into a project box with a momentary normally closed button that shorts pins 2 & 3, but replacing the box and mic with an SM58S produced the same result.

While I suspect there might be something wrong with the wiring of this particular light (since only the one light causes the buzz), the traditional division of departments makes me want to have more than a mere suspicion before I start making more work for the LX. And regardless of the source of the buzz, the problem is so specific and bizarre it makes me think that something is almost certainly wrong on the audio side of things anyway.

TL;DR: what kind of mis-wiring would allow a buzz into a microphone signal path only when hot and cold are shorted together?

Thanks!

-Russ

[Jean-Pierre Coetzee]

I've run into a problem where every new data point seems to make me scratch my head even harder, so I thought I'd see if anyone here had any ideas:

The backstage paging mic in the theatre I'm working in right now sends an audible (although, mercifully, not note-worthy) cut sine-wave buzz into the paging system, but only when a particular light is on (a light that is, incidentally, not hung very close to the mic) and only when the mic is switched off; when the mic is live or when the circuit is left open the buzz goes away. The typical setup involves an SM58 running into a project box with a momentary normally closed button that shorts pins 2 & 3, but replacing the box and mic with an SM58S produced the same result.

While I suspect there might be something wrong with the wiring of this particular light (since only the one light causes the buzz), the traditional division of departments makes me want to have more than a mere suspicion before I start making more work for the LX. And regardless of the source of the buzz, the problem is so specific and bizarre it makes me think that something is almost certainly wrong on the audio side of things anyway.

TL;DR: what kind of mis-wiring would allow a buzz into a microphone signal path only when hot and cold are shorted together?

Thanks!

-Russ

I would really like to have someone with some Electrical Engineering knowledge chime in but I think this might be related to the CMRR being less or non-existent when the two lines are shorted together.

Have you tried using the mic in a different place such as going into the FOH console and out of the mains but in the same physical location to try to eliminate other things in the signal chain.

[Mac Kerr]

I've run into a problem where every new data point seems to make me scratch my head even harder, so I thought I'd see if anyone here had any ideas:

The backstage paging mic in the theatre I'm working in right now sends an audible (although, mercifully, not note-worthy) cut sine-wave buzz into the paging system, but only when a particular light is on (a light that is, incidentally, not hung very close to the mic) and only when the mic is switched off; when the mic is live or when the circuit is left open the buzz goes away. The typical setup involves an SM58 running into a project box with a momentary normally closed button that shorts pins 2 & 3, but replacing the box and mic with an SM58S produced the same result.

While I suspect there might be something wrong with the wiring of this particular light (since only the one light causes the buzz), the traditional division of departments makes me want to have more than a mere suspicion before I start making more work for the LX. And regardless of the source of the buzz, the problem is so specific and bizarre it makes me think that something is almost certainly wrong on the audio side of things anyway.

TL;DR: what kind of mis-wiring would allow a buzz into a microphone signal path only when hot and cold are shorted together?

Thanks!

-Russ

Are you sure the switch actually shorts pins 2&3 together? If it just opens the line that will make the mic cable into an antenna. Very often those mic switches just open the line rather than shorting it.

Mac

[John Roberts {JR}]

maybe swap out the mic cable as a test?

JR

[Kevin Maxwell]

Why do we always think that is audios problem? It may very well be, but it isn’t always. And it may be a combination of both and it won’t go away till you fix the audio and the lighting problems. I built a lighting dimmer system a while ago under an electrical engineers directions, as a side project for him, while working for an electronics company. I built prototypes of stuff and test gear to test the production devices. I learned a lot about how lighting dimmers work and how they can interfere with audio gear. 

I was doing sound for a video shoot and we had a Buzz. No dimmers in use at all. It turned out to be a bad lamp socket, burned and arching. On another video shoot (many years ago) of a few nights of a live concert we were getting interference on the audio lines to the audio truck. Long story short, I used a Shure FP11 headphone amp with the sensitivity cranked up all the way, with only the headphones plugged into it no input used. I waved it over the cabling (like a metal detector) and found that everywhere there was a control cable connector for the Vari-Lite fixtures (120 of them) there was the same noise. And they were lying on top of mic cables and snakes. More information available on request.

I just wanted to mention other posable causes.

[Bob Leonard]

Are you sure the switch actually shorts pins 2&3 together? If it just opens the line that will make the mic cable into an antenna. Very often those mic switches just open the line rather than shorting it.

Mac

Mac has given you the answer above. You might explore the possibility of wiring a .001 non polarized (ceramic) cap from pin one (1) to ground inside the mic. That may stop the hum.

[Mike Caldwell]

check that pin 1 is not tied to the shell any where along the line.

[Kevin Graf]

The only time a pin #1 to shell connection is a problem is when the shell comes in contact with a metal object that has a voltage/ground potential.

[Don T. Williams]

Sounds like it could be a mic cable or wiring problem.  Are you certain Pin 1 is wired to the shield from the mic all the way to the mixer input? Pin 1 to pin 3 swaps happen during installation or repairs.  It's worth testing even if it takes  200' of mic cable to plug into a tester or to use a multimeter to confirm 1 to 1, 2 to 2, 3 to 3. 

[Steve M Smith]

Is the input balanced?


Steve.

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