Adventures In Audio
Gain control

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Friday May 23, 2003

The mic preamp (preamplifer) is where the input signal is conditioned so that
it is suitable for further processing in the console.

Each channel of the console has two input sockets; mic input and line input.

The mic input can accept the output of a microphone (naturally) or a DI (direct
inject) box. Nothing else should be connected to a mic input.

Anything else will be connected to the line input.

A microphone has a low output level. A typical figure would be 10 mV (one hundredth
of a volt). The console likes to work on a signal level of around one volt to
keep well above the inevitable noise signals that will be present.

This means that the signal has to be boosted by 100 times, or 40 dB (40 decibels).

Of course, the level that comes from the microphone depends on the level of
the sound source, and its distance. In practice a mic preamp needs a range of
gain of 20 dB up to 60 dB. Some preamps go beyond this range, from
0 dB to 80 dB. 0 dB means x1 or unity gain - in other words no gain
at all. 80 dB means x10,000. This would cover all situations from a mic
being placed 10 mm from the point where the kick drum pedal hits, to a
watch ticking at twenty paces.

There would be no advantage in providing more than 80 dB of gain because
the noise produced by the mic would be amplified above the console's noise level.

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