microphone
I'm trying to understand how mobile phone headsets work. I figured out that the electret mic gets supplied with 2.7V by the phone.
Opening a headset revealed the following PCB:
I looked and measured and came up with the following circuit:
Shouldn't there be at least one cap in series with the mic?
You could do something like this if you're not afraid of op amps. The microphone gets its high impedance, and if it needs higher impedance, increase R1. I have the iPod a 330 Ohm load because I seem to remember reading somewhere that's a good starting point. The output impedance is 10k, and can be increased if necessary. This circuit has a gain of 2, so if you need less, decrease R6, and if you need less than 1, switch it to an inverting configuration. Trying to sum these inputs passively is going to require a matching network.
I think you may be overcomplicating this a little bit - why don't you just connect the electret microphone (observing polarity) to the microphone wires via a 3.3Kohm resistor and see what happens.
Your circuit won't work as it stands because the signal you are trying to put back onto the line gets fed back to the microphone via the 10k and it'll all end up very low gain in my humble opinion.
I don't think there is a substitute for trying it out and if it's not loud enough, putting a 10uF capacitor across the 3k3 resistor I mentioned and if that doesn't sound like the right volume maybe drop the 3k3 to 1kohm.
As for matching the impedance - this won't make a great deal of difference to the kit that recieves the signal. It's likely got an unbalanced input and is probably not differential so I don't think you need to jump thru hoops on this.
If you still believe the mic on its own won't work, try this: -
Best Answer
Electret microphones need a biasing voltage to operate: The 2.7 Volts you measured.
If there is a capacitor in series with the microphone, this would block DC, thus there would be no biasing voltage.
If the preamplifier / buffer inside the phone body needs the bias voltage removed, a capacitor would be placed in that part of the circuit, past the bias voltage provision stage, i.e. on the PCB inside the mobile phone handset.