One Transistor Electret Mic Preamp

Electret microphones are reasonably priced and perform well, but adding a little extra amplification dramatically improves the signal to noise ratio. The preamp described here is a well known design that I have been using for years, which runs quite happily on 5 volts and consists of just a single low noise transistor in common emitter configuration and a small number of components. It sounds great, particularly when fed into the line input of a PC


Veroboard layout


Electret microphones usually have a built in FET (Field Effect Transistor) amplifier that requires a small voltage to operate. Most datasheets I consulted state about 3 volts, including the one for the mic I use, which is provided here by a 3K3 feed 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 audio waveform appears slightly asymmetrical, giving a harsh sound. 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. To reduce the gain, simply increase the emitter resistor value

It was important that the circuit could be powered from a standard computer USB socket, but the problem with this is that the 5 volts obtained is usually swamped by electrical noise. Here though, this noise is completely eliminated by a simple RC filter formed by the 220 ohm resistor and 100u capacitor. A 1n capacitor was also added across the input to provide some RF filtering

Beware if powering from a USB phone charger. All the ones I tested with this circuit were not clean and exhibited dreadful noise

The circuit is constructed on a standard 9 x 25 hole strip board with JST XH type PCB connectors for the terminations

If housing the board inside a painted enclosure, you may have to file or scrape some paint away where the front and rear panels touch the case, otherwise electrical continuity could be lost resulting in the circuit not being fully screened

If you want to build a microphone to go with this preamp, take a look at micbooster.com. I got great results by housing a Primo EM272Z1 electret mic capsule inside a Behringer XM8500 body. The XM8500 is inexpensive, metal and the windshield unscrews making it ideal for DIY mic projects

Internal view of exruded alumimium enclosure

Rear panel showing C type USB power socket and 3.5mm audio output jack

Front panel showing microphone input jack