Circuit : Andy
Collinson
Email:
anc@mitedu.freeserve.co.uk
Description:
A microphone amplifier that may be used with either Electret Condenser
Microphone (ECM) inserts or dynamic inserts, made with discrete components.
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
necessary.
The frequency response measured across a 10k load resistor is plotted below
simulated using a 12V power source:
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.
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.