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Electret Microphone & Preamplifier




The microphone input of the average PC can be quite noisy unless its gain is reduced, but reducing the gain makes it useless as a microphone input! Enter the Mic Preamp. This simple project uses a single high gain transistor to boost the output from a low cost electret microphone insert, up to line level at good quality. I first used this circuit many years ago in a ham radio transmitter where it proved to be stable and reliable. Although the circuit runs happily on a 9 volt PP3 battery, in this application its voltage supply is derived from the 5 volts available on a standard USB socket. The schematic and veroboard layout are shown below



Note: The 10K resistor supplies phantom power to the electret mic insert. The 2u2 capacitor is a non-polarised electrolytic





The layout isn't critical and it can be constructed pretty much any way you want. Here I've used a small aluminium enclosure with a 3.5mm mono jack socket for the input and an 'old school' 5 pin din socket for the output and voltage supply. The connecting lead to the computer was made from an unused USB cable. The socket end was removed and the +5V and ground connections verified with a volt meter, though generally these will be colour coded red and black respectively, with the two unused data lines being green and white. These unused data lines should be terminated safely by folding them back and sealing them with Araldite within the 5 pin din connecting plug. A screened cable carries the audio out of the 5 pin din to the computer where it terminates in a 3.5mm stereo jack plug with the left and right tags joined together for mono




The microphone body itself (see photo at top of page) is easy to build, consisting of a length of wire coat hanger fed through the outer insulation of a piece of coaxial TV cable. It's easier to first feed the screened microphone cable through, followed by the coat hanger which should be straightened out to start with, and then bent to the required shape afterwards. The business end of the mic is made from the body of a coaxial TV plug (nickel plated types look best) which is also the perfect size to take a Maplin electret mic insert (pt.no. FS43). Use electrical tape to pad it out so that the insert is located towards the top and doesn't wobble. Finally, connect a 3.5mm mono jack plug on the other end of the cable and it's finished. Alternatively, a cheap lapel microphone could also be used instead