The unity gain setting simply means that the input signal level will be unchanged;
it will neither be increased or decreased in level. There are two different types
of inputs, balanced and unbalanced, and the position on the control just happens
to be a little different for the two inputs. If you wanted unity gain, you would
just use the mark for the input type you were using.

If your input were a line level signal, you would likely want to use unity gain
which is also referenced as 0db. A line level signal is already at a level that
is appropriate for further processing. So it normally doesn't need any additional
gain.

A mic is very low level though and requires gain to bring it up to the line level
required by the next stages. That is why you need a higher setting on the level
control.

Levels need to be controlled more or less, so that they remain within reasonable
limits. If the level goes too high, distortion results. If it goes too low, one
of the later stages will need additional gain, and adding gain will add some noise.
A lot of gain is necessary with mics, and preamps are very low noise so that the mic
signal can be brought up to line level and remain clean. For that reason it's important
that the gain for a mic occurs in the input amp where it can be done with minimum noise.
So you want to adjust the input level to a point where all of the later levels can remain
at or around their own 0db level.

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