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. For a long time transformer microphone inputs were the only real option. The cost and weight of a mic 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 pads, or attenuators, are used when the output is too high for the mixing console input to cope with; this typically happens when users put a microphone inside a kick-drum. A capacitor capsule has an extremely high output impedance, equivalent to a very small capacitor of a few picoFarads.
