Monday, August 10, 2009

Using Electret Condenser Microphones

This page describes how to use or connect and use 2 and 3 terminal electret condenser microphones (ECM's). The bottom section shows the connections and how to substitute from one type to another.

Mic Inserts
Viewed from above all mic inserts look similar to the left image. 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.

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 right.

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.

Connecting 2 and 3 terminal ECM's

The schematic symbol for a 3 terminal ECM insert is shown on the left diagram. It has separate power, common, and signal outputs. The schematic symbol for a 2 terminal ECM insert is shown on the right diagram. To use a 2 terminal ECM, the signal output is connected to the power terminal, fed via a current limiting resistor, (typical value 1k or 2k2). The signal output therefore has a DC component which must be removed before connecting to an amplifier. This is acheived with an output capacitor connected to the power terminal of the ECM, a typical value being 1-10uF.
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