Using ECM's

Article : Andy Collinson
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Overview
This tutorial describes how to use 2 and 3 terminal electret condenser microphones (ECM's). The actual microphone (mic) insert is the small round metal body, which would fit inside a plastic or metal microphone case. A complete microphone would include a windsheild made from foam rubber. The last section shows how to convert a circuit from a dynamic mic input to use an ECM input.


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-cassettes. 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.An ECM operates on battery voltage typically 6V to 15Vdc and draws about 1mA current. A series resistor is required and its value is usually from 1k to 4.7k. This data can be found from the manufacturer or component catalogues.

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
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 left picture. The physical size of all mic inserts is small, typically about 10mm in diamter as shown on the right.


Connections
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 achieved with an output capacitor connected to the power terminal of the ECM, a typical value being 1-10uF.



Conversion from Dynamic Mic to ECM Mic Input
Quite often there is a need to convert a dynamic microphone circuit for use with an ECM. This can be done by adding an appropriate series resistor for the ECM and modifying the circuit as shown below. R3 is the new series resitor to limit current for the ECM.