Mic Problem

[ampdoc1]

This is not related to amps, but I know there are some knowledgeable electronic techies here.

I work for an AV company (bid estimating these days) and because I've been working with sound products for 40 years, I was called on to solder a bunch of XLR connectors in the stage boxes and the rack terminations, in a large church project our company is finishing up.

After I left, the system was activated, and I was told that there appears to be only one problem. The Pastor's lectern mic input hums, but it works fine with no hum when the mic is connected.

Anyone got an idea what the problem is?

Thanks,
a'doc1

[martin manning]

Gotta be a missing or poor ground that gets established when the mic is plugged in.

[Leo_Gnardo]

The Pastor's lectern mic input hums, but it works fine with no hum when the mic is connected.

1. So... leave the mic plugged in.

2. Possibly, somewhere along the line there's a short from one of the balanced wires to ground. I'm almost discounting this because I'm sure you have checked with an ohm meter at the mic end of the cable & found no shorts from pin 2 to 1, or from pin 3 to 1.

3. Dynamic mic? What's it loaded with? These days many mic inputs are a "bridging" input, that is they present a load much more than the typical 600 or 150 or 50 ohms the mic is rated to "see." Possibly putting a resistor across the input, even if it's not exactly the spec, could drop the hum to almost nothing. Somewhere along the run, the mic cable may be exposed to a very large AC field and that induces the hum until the mic's low impedance is placed across the line.

4. Does this mic require phantom power? If the resistors supplying phantom are not well matched, that can put hum in the line. The better the match, the less hum. 1/10th of 1% if you can.

5. Hum? He has to learn the words to the hymns and sing, not hum along...

[JazzGuitarGimp]

It may also be possible the shield connection for that line is not connected all the way to the mixer's input connector.

[ampdoc1]

Thanks to all of you who responded. I'll pass this along to the installers on the job.

I was just glad there were no more troubles. I'm 63, and the last hour of sitting on the floor and bending over to solder the connectors just about killed my back.