Microphone Preamplifiers
Douglas Self · Focal Press eBooks · 2020
A microphone preamplifier is a serious design challenge. It must provide a gain variable from 0 to 70 dB or more to amplify deafening drumkits or discreet dulcimers, generate minimal internal noise, and have a high-common-mode rejection ratio (CMRR) balanced input to cancel noise pickup in long cables. Microphone transformer technology had and still has its own advantages. A balanced input with a good and constant CMRR was inherent. The cost incentive to develop an effective low-noise transformerless microphone input was considerable, and after much experimentation, the arrangement became pretty much standard for some years, being extensively used in mixers around the period 1978–1984. The microphone preamp architecture has the merit of simplicity, but because there is no global feedback loop around the input transistors, its distortion performance falls far short of the rest of a mixer, which typically consists of pure opamp circuitry with very low distortion.