Electrical loading of microphones by preamplifier circuits has not been of great concern to the audio engineer, heretofore. In the past, the input impedance of preamplifiers has been so much higher than the output impedance of microphones that the effect upon the performance of microphones has been indeed negligible. The recent appearance of transistorized preamplifiers whose input impedance is sometimes quite low, and the growing use of high-sensitivity ribbon microphones which have a highly frequency-variant output impedance has stimulated new interest in this loading problem. A study of the Thévenin equivalent circuit of certain common types of broadcast microphones discloses that the input impedance of a preamplifier must be maintained at a value at least five times the nominal impedance of the microphones with which the preamplifier may be used-in order to avoid undesirable alteration of the microphone's frequency-response characteristic. This applies unless the preamplifier has been designed for a particular microphone.
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