Preamplifier Circuits

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A preamplifier (preamp) circuit is an electronic amplifier that prepares a small electrical signal for further amplification. A preamplifier, or control amplifier, functions as a stage between an amplifier and a source component in an audio system, raising the level of the signal.

The noise performance of a preamplifier is critical according to Friis's formula, when the gain of the preamplifier is high, the SNR of the final signal is determined by the SNR of the input signal and the noise figure of the preamplifier. The preamplifier provides voltage gain from 10 millivolts to 1 volt but no significant current gain.

The power amplifier provides the higher current necessary to drive loudspeakers. Most moving coil preamplifiers can provide up to 62 dB of gain, while moving magnet designs provide up to 40 dB of gain. Microphone preamplifiers have XLR style inputs on them, and can be either tube or transistor-based designs. Microphone preamplifiers also have to be able to provide 48 volts of phantom power that is needed for many microphones to work.

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