preamplifier

A preamplifier, or preamp, is an electronic amplifier that prepares a small electrical signal for further amplification or processing by increasing its amplitude without significantly altering its content. It sits upstream of a power amplifier or other processing stages in audio, broadcasting, measurement, and instrumentation systems. Its primary function is to raise a low-level signal to a usable level with adequate headroom while preserving fidelity.

There are passive and active preamps. Passive preamps include simple potentiometer-based volume controls and, in some designs, transformer-coupled stages; they do not actively amplify the signal but influence impedance and level. Active preamps use transistors or operational amplifiers to provide gain, improve drive capability, and offer better isolation and control.

Common types include microphone preamps, line preamps, and phono preamps. Microphone preamps increase very small microphone signals to line level and may provide phantom power for condenser mics. Line preamps handle line-level sources and may include additional features such as gain staging or buffering. Phono preamps perform RIAA equalization to compensate for the frequency response of vinyl cartridges and are essential for turntable playback. Instrument preamps exist to boost guitar or bass signals before amplification or processing.

Important specifications include gain, input impedance, noise figure, total harmonic distortion, bandwidth, and slew rate. High input impedance is desirable for sources with high output impedance; low noise and wide dynamic range contribute to fidelity. Preamps may be designed around tubes, transistors, or integrated op-amps, with trade-offs in warmth, headroom, and cost.

In practice, a preamplifier is one stage of a signal chain; the output feeds a power amplifier, an audio interface, or further processing. Some devices combine preamplification with other processing or digital conversion.