Gmi Pro
CB Mic Wiring to Phono Plug?
i’ve got an old shure 100 series cb mic that i want to use for recording and am having trouble.
there are four wires, 2 of them are for the PT and not necessary for use, so i’ve got 2 wires, 1 hot and 1 ground i assume.
i’ve wired it up to a phono plug, but it doesn’t work unless i actually touch the mic element. it’s like it’s using me as a ground. i’m testing out of the case, which is metal.
not sure what’s happening, but, herer’s a pdf. that shows the circuit diagram at the bottom.
http://www.shure.com/stellent/groups/public/@GMS_GMI_WEB_UG/documents/web_resource/us_pro_101c_ug.pdf
any help is greatly appreciated.
You didn’t say if you were recording into a computer, or into audio gear.
According to the pdf, that mike has a carbon granule microphone element, which is commonly used in old telephones and older radio equipment. See:
http://www.aerialpursuits.com/comms/mikes.htm
Now, computers use electret condenser mikes, and pro audio uses condenser or other more advanced mike designs, which will put out very low levels of sound, typically in the few-millivolt range or less. They require a pre-amp for most applications – a computer sound card has the preamp built in. However, the carbon mike requires an external voltage to power it, a little like an electret, and its output voltage is a significant percentage of the input voltage (so, if you put 5 volts through the carbon mike, your output signal will be about 5 volts max). This means that the CB mike you are using needs an external power source, and its signal will be way higher than your average microphone.
Check out this guy’s article. He used a mike from an old telephone, but it’s the same type of mike you have from your CB:
http://www.instructables.com/id/Telephone-Handset-Microphone/
His solution involves adding a single battery (1.5 volts), adding a few resistors to bring that voltage down, and feeding the resulting signal to the line-in jacks rather than the mike jack (since the signal will be about 1 volt or so from the mike, and that’s closer to line levels than mike levels. I think if you added his solution to your mike, it should work.
As for why it works when you hold it… I think your body may have been supplying a few millivolts to the mike, and this was enough to let the mike generate an output of a few millivolts, which is fine for a mike-input jack, but probably less than what you need to get sound from that mike. Maybe you can experiment with the mike-power that comes from the PC sound card, or tap a USB cord to get 5 volts, apply that voltage to the mike, and use resistors to bring it down to line levels. If you have access to phantom power, you can try the same thing. The pdf mentions the current specs that the CB mike takes, so use that as a guide to powering up the mike.
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