Could it be because your mic plus isn't wired like your PC sound card (don't laugh gang, I see this a lot in the Commercial Broadcast arena).
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Point...
I have a cheapie Multimedia headset. On the Dell SAM Computers it is quiet (doesn't seem to have enough sound). So I built a mic preamp like the one shown here:::
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http://www.phoenixcomputerlabs.com/All- ... -Amps.html...
Worked fine after that..
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Then I plugged it into a Gateway that we set up with audacity for a studio computer... Boom, really loud (in fact too loud). So I took the preanp off and the headset worked just fine without it on the gateway...
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Just for giggles I loaded one of the gateway computers up with SAM... Mic worked just fine..
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Dug into the dell motherboards.... The Mic power lead is the same lead as the mic (microphones are 3-wirejacks, Sleve- common, Ring - Mic Power, Tip - Mic Audio). So the ring mic power isn't supported in the Dell Audio scheme.
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Point: Cheapie headsets (and I'm not knocking cheapie headsets) can be wired both ways. Some have the plug wired with power to the electrete mic from the ring, some are wired for the power to come from the tip along with the audio. Some are even wired with both the tip and ring tied together. Since the mic audio signal path is tip active audio and sleeve being common - these headsets are not affected by which sound card audio scheme you are using.
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If you are really curious you can wire an in line stereo audio cord with the wires stripped back. Put a 1.5v battery in line with the ring connections. If your mic on the headset is loud - you have figured out the problem. If you don't want o use the battery, tie the ring and tip leads together.
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Me I just cobble a simple mic amp and power the mic amp with a 9V battery. So far my pre amp has worked for over a seven months.
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Hope this helps..
Dave
dataman19