Simple microphone pre-amp with tone correction
The purpose of this simple pre amplifier is to increase the output of a typical electret microphone or insert and also perform some tone correction in the form of LF cut to permit connection to any amateur transmitter or transceiver.
The unit is housed in an eddystone die cast box and so is fully screened. It is powered by a standard PP3 battery mounted internally, the current drawn is below 2mA so the battery will last a very long time.
For a microphone producing in the order of 40mV peak to peak the unit will provide an output of about 100mV to 1.5V peak to peak adjustable by a small pre set pot which is accessible through a small hole in the box.
Two levels of bass "roll off" are available in the form of a small 3 position toggle switch which will also switch the unit off.
The input and output sockets are standard 3.5mm jack sockets located on either side of the unit.
The purpose of this simple pre amplifier is to increase the output of a typical electret microphone or insert and also perform some tone correction in the form of LF cut to permit connection to any amateur transmitter or transceiver.
The unit is housed in an eddystone die cast box and so is fully screened. It is powered by a standard PP3 battery mounted internally, the current drawn is below 2mA so the battery will last a very long time.
For a microphone producing in the order of 40mV peak to peak the unit will provide an output of about 100mV to 1.5V peak to peak adjustable by a small pre set pot which is accessible through a small hole in the box.
Two levels of bass "roll off" are available in the form of a small 3 position toggle switch which will also switch the unit off.
The input and output sockets are standard 3.5mm jack sockets located on either side of the unit.
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Parts List
R1 * 10k ohm (see below)
R2 820k ohm
R3 2.2k ohm
VR1 1 k ohm min pre set
C1 47 mfd
C2 220pF
C3 4.7 mfd
C4 0.01mfd
C5 0.01mfd
TR1 BC107 etc
SW1 min 3 pos toggle
SKT1 3.5 mm jack socket
SKT2 3.5mm jack socket
PP3 battery connector and battery.
Small Eddystone or similar metal box.
NB, R1 is only needed if the microphone is not self powered.
