Choosing a replacement electret microphone

Question

I'm trying to select a suitable replacement electret microphone for an analog (POTS) phone. The existing electret is completely unmarked so I don't know its part number or parameters. I'm not well versed with these parts so in regards to choosing a replacement I'm not sure which parameters are important and whether modifications need to be made to the existing circuit. I've reverse-engineered the schematic for the phone; the audio input and virtual ground (labeled V_AUDIO) sections are shown below.

Here's my thinking:

• My understanding is that with electrets the value of the load resistor (R29, 4.7kΩ) should match the electret's impedance (see reference design below). Thus I should thus choose a 4.7kΩ electret. These, however, don't appear to be available.

• When measuring the voltage drop across R29 I read 0.8384V which implies that the current across it is about 180µA. Is this instead is parameter I should be matching? (i.e. choose a 200µA rated part)

• The most common impedance for electrets of the appropriate physical size seems to be 2.2kΩ. Could one of these (with an appropriate voltage range) be used, perhaps with the load resistor changed to a different value?

Any guidance would be appreciated.

Existing circuit:

Measured DC voltages shown in purple, analog signal from the electret (measured at MIC+) is ~20mV peak-to-peak.

Reference design (for a 2.2kΩ electret):

_{Source: PUI Audio}

Answer 1

2.2kΩ is a common impedance value used with electret microphone capsules but it is sort of arbitrary: the FET amplifier in such a capsule essentially works as current source over a wide range of voltages. The operating current (and the minimum voltage where output current is independent of the voltage, usually between 1V and 2V) is usually what is actually in the data sheets for individual capsules.

Phone capsules tend to get along with comparatively little current, like 0.3mA. 180µA is very low, so I suspect that the capsule is operated below its specified minimum "linear operation" voltage. You will likely want to look for a capsule that has rather low ratings for operating current and voltage.