ecm mic preamp.html


A microphone amplifier that may be used with either Electret
Condenser Microphone (ECM) inserts or dynamic inserts, made with
discrete components.

Ecm mic preamphtml


Both transistors should be low noise types. In the original
circuit, I used BC650C which is an ultra low noise device. These
transistors are now hard to find but BC549C or BC109C are a good
replacement. The circuit is self biasing and will set its
quiescent point at roughly half the supply voltage at the emitter
of the last transistor.

The electret condenser microphone (ECM) contains a very
sensitive microphone element and an internal FET preamp, a power
supply in the range 2 to 10 volts DC is therefore necessary.
Suitable ECM`s may be obtained from Maplin Electronics. The 1k
resistor limits the current to the mic. This resistor should be
increased to 2k2 if a supply voltage above 12 Volts DC is
used and is not needed if the Mic insert is dynamic. The output
impedance is very low and well suited to driving cables over
distances up to 50 meters. Screened cable therefore is not

The frequency response measured across a 10k load resistor is plotted below simulated using a 12V power source:

Ecm mic preamphtml

The noise response of the amplifier measured across the 10k
load is shown below. Please note that this plot was made with the
mic insert replaced by a signal generator.

Ecm mic preamphtml

This preamplifier has excellent dynamic range and can cope
with anything from a whisper to a loud shout, however care should
be taken to make sure that the auxiliary equipment i.e.
amplifier or tape deck does not overload.

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