ECM microphone preamplifier
Both transistors are low noise types. In the original circuit, I used BC650C which is an ultra low noise device. These are hard to find so I now use BC109C. The circuit is very device tolerant and will set its quiescent point at roughly half the supply voltage at the emitter of the last transistor.
The ECM is a mic insert, designed to operate at around 2 - 10 Volts DC. The 1k resistor limits the current to the mic. 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 the 10k load resistor is shown below:
The noise response of the amplifier measured across the 10k load is shown below. Please note that was measured with the mic insert replaced by a signal generator.
This circuit has an excellant overload margin, and can cope with anything from a whisper to a loud shout, however the amplifier you connect this too can be overloaded, so care should be taken.
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