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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: enter image description here

I looked and measured and came up with the following circuit: enter image description here

Shouldn't there be at least one cap in series with the mic?

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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.

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The diode is there to clamp the voltage to 2.7V, it's a zener. The two caps are bypassing any AC on the line, one at low freq, the other at high freq.

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You want the AC on the line - it's the microphone signal! – Connor Wolf Apr 27 '13 at 22:54
@ConnorWolf - The caps could be removing non-audio frequencies. – Joel B Mar 31 '14 at 19:59

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