Condenser microphones were invented in 1916, then commercialized by Neumann in the 1930’s. Today`s studio condenser microphones are all improvements from the original design. An impedance converter is necessary as the voltage produced by the condenser capsule must be sensed with minimal current drain. Historically, the first active electronic element used for this purpose was a vacuum tube. In 1965, Schoeps released a microphone using a Field Effect Transistor (FET). Output drive electronics are needed to provide a low impedance balanced signal to the microphone preamp or recorder input. Tube circuits use a transformer. The original Schoeps circuit uses two PNP transistors as emitter followers after the FET. To this day, most condenser microphones use a variant of the Schoeps circuit or a single FET and a different output topology. The Author has identified a low-noise, low-distortion FET input operational amplifier (op amp), the OPA1642, which also has a low quiescent current. This allows a dual op amp to be supplied by existing P48 phantom power. The design results in significantly lower total harmonic distortion than previously existing circuitry. This paper will explore the history of microphone electronics culminating with the Author’s op amp design.
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