The electret technology has already been described in several papers. This paper will show how an electronic symmetrical line output level electret microphone can be designed for public-address use. Because of the small dimensions of an electret unidirectional microphone insert the proper acoustical parameters, such as suitable frequency-response and constant front-to-random index throughout the useful frequency band, can easily be obtained. Application of recently available electronic circuitry and components in the built-in line-output level amplifier and limiter enable transmission over unscreened cable up to 1 km length. This microphone is compatible with any moving-coil or other electret microphone. Because of the integration of transducer and electronics, an excellent S/N ratio can be obtained. Th'power for the electronics being supplied over separate wires by the power amplifier the microphone requires no built-in batteries. Its line amplifier can drive either a power amplifier up to rated output or a 600 Ohm line. The philosophy of this design will be explained in this paper.
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