ABSTRACT
A microphone preamplifier is a serious design challenge. It must provide a gain variable from 0 to 70 dB or more to amplify deafening drum-kits or discreet dulcimers, generate minimal internal noise, and have a high-common-mode rejection ratio balanced input to cancel noise pickup in long cables. It is rare to use input transformers to match a low-impedance microphone to the preamplifier, since the cost and weight penalty is serious, especially when linearity at low frequencies and high levels is important. For a long time transformer microphone inputs were the only real option. The cost and weight of a microphone input transformer for every channel is considerable, especially at the quality end of the market, because a large transformer core is needed to handle high levels at low frequency without distortion. Microphone transformer technology had its own advantages. Microphone pads, or attenuators, are used when the output is too high for the mixing console input to cope with.
