Using Electret Condenser Microphones
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Description:
This page describes how to use or convert a circuit using a three terminal electrey condenser
microphone (ECM) to a two terminal type.
ECM inserts can be bought quite cheaply from many electronics outlets, and offer high quality
sound output. They can also be salvaged from old cassette players and radio-casettes. An
ECM contains a very sensitive electret type microphone (high output impedance) and an.
integral FET amplifier. The amplifier stage buffers the high output impedance of the mic and
boosts an average speech signal to around 1 to 2mV when spoken about one metre away from the
mic insert.
Mic Inserts
Viewed from above all mic inserts look as below:
Two Terminal Type ECM
The ground or common connection of a two terminal ECM insert can be identified as the solder
connection that is touching the case or body of the mic, see below.
Three Terminal Type ECM
With a three terminal ECM, the ground or common connection will be touching the case or body,
the other two contacts will be the audio output and power pins, see below.
Conversion from 3 to 2 wire ECM's
The symbol for a 3 terminal ECM insert is drawn below showing, common, power and signal outputs.
To use a 2 terminal ECM, the signal output is connected to the power terminal (fed as always
via a current limiting resistor, typical value 1k or 2k2). The signal output therefore has
a DC component which is removed via the output capacitor, a typical value being 1 - 10uF.
