Headset Mic Wiring - Any Electrical Gurus Here?

[AaronDC8402]

I've been using an in-the-ear style headset that I made myself for about 4 months now. My Lightspeed Sierra has only made it out of its case a few times since I got the homemade one working. The mic that I originally used is a cheap $30 mic from Aircraft Spruce. It's designed to be a replacement for a typical lower end headset. It is a noise cancelling elecret. The problem is that it's mounted on the end of a rigid piece of metal.

I recently bought a flexible boom mic from a Bose Aviation X headset. It's an Intricon Model 755. This mic comes with 3 wires, and I'm a little unsure exactly how it's supposed to be wired in. Intricon has great documentation. The schematic and specs are listed below. It's been a while since I had a circuits class, so I might be getting my old text books back out to catch up.

The mic plug will get a ground connection and an output connection. I'm assuming the ground will go to the ground on the schematic. I'm expecting that output on the plug will go to output on the schematic. What confuses me a little is that the specs say the V+ and "output" wire can be used interchangeably. Any electrical gurus have any other strong suggestions?

 

[HRM]

I think you have it, besides, there are lots of electret circuit designs out on the web.

The reason that the red and green are interchangeable is because the internal amp doesn't care which side it get's voltage from and which side it puts voltage out (it's a bias, not a direct power connect).

Imagine the battery is a pump, there's a pressure side and a sump, or return side. Now hook a loop of tubing to the pump, fill with water and switch it on. The pump circulates the water and everything is happy. If you switch the pump connections nothing changes (except the direction of the flow).

Now pinch the tubing rhythmically with your voice. The tubing elsewhere will pulsate accordingly--that is what the electret element is doing. To get the effect the pump has to be on and keeping the tubing pressurized. Again, the effect will still work if you switch pump connections.

Wait! What about the little amp in there? Doesn't it care about polarity? Well, it would if it was a BJT, but being a FET it doesn't care either.

Extra credit for knowing what the acronyms mean...

[AaronDC8402]

Mystery solved!

I was wrong about "common" on the schematic connecting to the base of the mic plug. Turns out "common" isn't used at all. For those that are curious, the base of the mic plug connects to the green wire and the middle ring of the mic plug goes to the red wire. 

Looking forward to getting the new mic buttoned up, and try it out in the air. On the ground it seems to be noticeably clearer. And the PMA7000B auto-squelch seems to pick it up a little better too.

 

[AaronDC8402]

@HRM Thanks for the info. I had figured it out by the time you posted, but I actually understand why now. Never heard the analogy of the electret "pinching the tubing".

[Piloto]

The common may be the green wire you mention. COM radios provide the bias and audio connection on the same wire. I would  bypass the 25uf capacitor and connect the Output line to the ring and the Common to the sleeve or plug ground. Use shielded wire to avoid RF feedback. Tie the green wire to the gray wire.