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Electret Microphone Preamplifier






Electret microphones are reasonably priced and perform well, but sometimes they can benefit from a little extra amplification to boost the output and improve the signal to noise ratio. The preamp described here is a well known design that I have been using for years, consisting of just a single low noise transistor in common emitter configuration and a small number of components. It runs quite happily on 5 volts and by using different interface cables it becomes a universal preamp that can be used for both high quality voice recording and ham radio communication




Schematic


The circuit is constructed on a standard 9 x 25 hole strip board with JST XH type PCB connectors for the terminations which is housed in a Hammond 1590BBK (111.5 x 59.5 x 31mm) mottled black diecast box. Audio out and power in is provided by a standard 5 pin DIN socket and the microphone input is via a 3.5mm TRS socket, though the choice of connector is of course yours. A PTT (Push To Talk) switch was added for when used with a radio transmitter. Two types of interface cable were made up, one for a PC and another for my amateur radio transceiver. The PC cable terminates in a 3.5mm TRS plug for microphone audio, and a USB plug to take 5 volts from a USB socket. The transceiver cable terminates in an RJ45 plug which matches the mic socket on my rig (many rigs now use RJ45 connectors for their mic input)




Veroboard layout

CIRCUIT NOTES

Electret microphones have a built in FET (Field Effect Transistor) amplifier that requires a small bias voltage to operate. According to the datasheets I consulted this is normally about 3 volts, achieved here with a 4K7 resistor. This value can be altered to suit other mics

Distortion was greatly reduced by adding a 150 ohm resistor in series with the emitter to introduce some negative feedback. Without it, the upper and lower half cycles of a sine wave appear slightly asymmetrical. The value of this resistor can be altered to set the required gain, which is calculated by dividing the collector resistor by the emitter resistor. So here the gain is 4700 150 ≈ 31

The 5 volts obtained from a computer USB socket is usually swamped by electrical noise. The 330 ohm resistor and 100u capacitor form an RC filter which completely eliminates this dreadful interference. A 1n capacitor was also added across the input to provide some RF filtering as well




                   

3.5mm mic input socket and PTT



                   

5 Pin DIN interface socket



 

Transceiver interface cable (for AnyTone AT-778UV))






PC interface cable




If you fancy making your own microphone to go with this preamp, then take a look at the excellent micbooster.com. The Primo EM272 electret microphone capsule is a good place to start