microphone amplifier with mute switch

elektor July/august 1985 Microphones, unfortunately, produce onlyasmallsignalandthey,therefore, require a special pre-amplifier to boost their output. Because small signals are involved, the signal-to-noise ratio of the pre-amplifier is a very important parameter. In this article, we present two circuits for a pre-amplifier suitable for virtually all occasions: a symmetrical and an asymmetrical version. We have incor- porated a mute switch,which speakers can use when they want to clear their throat. As there is an number of low-noise operational amplifiers available nowadays, the cost of these pre-amplifiers is rela- tively low. The asymmetrical version is shown in figure 1. Switching between high and low impedance matching is possible withswitchS2.OpampAlisarranged as an AC amplifier with a gain of around 27 dB. This stage may also be used as a DC amplifier: R3 and C1are then omitted, and the value of R2is lowered to 22 k. Capacitor C2 limits the bandwidth of the amplifier to ens...