he mic preamp (preamplifer) is where the input signal is conditioned so that
it is suitable for further processing in the console.
The phase button inverts the signal polarity so that positive becomes negative
and negative becomes positive.
This is useful in two ways:
It is not uncommon for a mic cable to be wired incorrectly in inverted polarity.
With a single mic this is generally inaudible. When two mics are used for
stereo, and one is inverted, then the resulting out-of-phase sound is definitely
noticeable. In many situations it is better to use the 'quick fix' of the
phase button, then repair the cable later.
When many microphones are used close together, as on a drum kit, the spill
(leakage) between the mics can cause unpredictable interference effects. It
is good to experiment with the phase buttons. Often, pressing the phase button
on one of the channels - you have to experiment to find out which - will improve
the overall sound.
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