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Portable Microphone Preamplifier

Portable Microphone Preamplifier

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise- in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply.
Circuit operation:

The circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Notes:

Harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).

Maximum input voltage (level control cursor set at maximum) = 25mV RMS

Maximum input voltage (level control cursor set at center position) = 200mV RMS

Enclosing the circuit in a metal case is highly recommended.

Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.

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