The technical demands on microphones used with radio equipment are not stringent in terms of sound quality: a frequency response from around 50 Hz to 5 kHz is entirely adequate for speech. For fixed CB use or for radio amateurs sensitivity is a more important criterion, so that good intelligibility can be achieved without always having to hold the microphone directly under your nose. Good microphones with extra built-in amplifiers can be bought, but, with the addition of a small preamplifier, an existing microphone will do just as well.
The project described here uses only a few discrete components and is very undemanding. With a supply voltage of between 1.5 V and 2 V it draws a current of only about 0.8 mA. If you prefer not to use batteries, the adaptor circuit shown, which uses a 10 kΩ resistor, three seriesconnected diodes and two 10 μF electrolytic smoothing capacitors, will readily generate the required voltage from the 13.8 V supply that is usually available. There is little that need be said about the amplifier itself. Either an ordinary dynamic microphone or a cheaper electret capsule type can be connected to the input. In the latter case a 1 kΩ resistor needs to be connected between the 1.5 V supply volt-age and the positive input connection.
The impedance of the microphone and of the following stage in the radio apparatus are not of any great importance since the available gain of 32 dB (a factor of 40) is so great that only in rare cases does P1 have to be set to its maximum position. With a frequency range from 70 Hz to about 7 kHz, low distortion, and small physical size, the preamplifier is ideal for retrofitting into the enclosure of the radio equipment or into the base of a microphone stand. In case you are concerned about our somewhat cavalier attitude towards distortion: for speech radio the ‘fi’ does not need to be ‘hi’. Quite the reverse, in fact: the harmonics involved in a few percent of distortion can actually improve intelligibility — it’s not a bug, it’s a feature!