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

Electret microphone capsules 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 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 to me that this circuit could be powered from a standard computer USB socket. The problem though, is that the 5 volts obtained is usually swamped by electrical noise. The 330 ohm resistor and 100u capacitor form an RC filter which completely eliminates this interference. 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 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