A mic preamp must boost mic-level signals (–70 to –50 dBu) to line level without adding audible noise

Microphones output extremely low signal levels, typically –70 to –50 dBu at nominal voice levels. A mic preamp provides 40–70 dB of gain to bring the signal to line level (approximately –20 to 0 dBu nominal). The preamp is the first and most noise-critical stage in the signal chain; any noise it adds is amplified along with the signal through all subsequent stages. Equivalent Input Noise (EIN), usually specified in dBu(A), quantifies how quiet a preamp’s own noise is when referred back to its input. A good professional mic preamp has EIN better than –128 dBu(A). The input impedance of the preamp (typically 1–10 kΩ for mic inputs) must be significantly higher than the mic’s output impedance (50–200 Ω) to avoid frequency response degradation and level loss.

Examples

A condenser mic at –60 dBu requires about 60 dB of gain from the preamp to reach 0 dBu nominal. If the preamp has an EIN of –128 dBu(A), the residual noise at the output is approximately –68 dBu(A) — well below the nominal level.

Assessment

Why is it critical to minimize noise in the mic preamp stage rather than adding a noise-reduction plugin later in the signal chain?