Low-Noise Microphone Preamplifier

EE 36 October 1987 LOW-NOISE MICROPHONE PREAMPLIFIER A quality preamplifier that can be configured for low and high impedance, as well as balanced and unbalanced, microphones. Most dynamic microphones of European make (AKG, Beyer, Sennheiser) have a character- istic impedance of 200 Q, while those of Far Eastern origin are usually terminated in 500 to 600 Q. The impedance of elec- tret condenser microphones is type specific, and usually in the range from 600 to 1000 Q. Most microphones have a sensitivity of 2 . . . 3 mV/Pa. The termina- tion impedance, i.e., the input impedance of the preamplifier or the microphone transformer, should be equal to or higher than the impedance of the microphone. The transformer is often precisely matched to the microphone impedance (power matching), while pre- amplifiers give optimum per- formance when the termination impedance is slightly higher than the microphone im- pedance (voltage matching). The use of a transformer with a an impedance ratio o...