I like to think of signal as water running through pipes. In order to avoid pressure buildups in our plumbing system, we need to make sure that all the valves along the way are open accordingly.
There is a real signal flow direction in sound; You don't plug the mixer into the mic, but you plug the mic into the mixer. On a mixer, signal flows between a channel input and various possible outputs encountering several different processes.

Signal Flow and Gain Structure are important notions to understand. This applies to everything between your mic and your speakers or your guitar and the recording tape . Signal flow integrity is essential to sound quality. We are going to narrow the topic on the mixer component, which is often the common denominator in a sound system.

Calibrating Gain Structure:

Wether you input a mic or a line level instrument into one
of the channel of your mixer, the first knob you'll probably find is a Trim or input gain level control.
Think of this as the first stage or the "the main pressure valve". To "calibrate" the gain structure, you want to achieve unity. Unity , in simple terms , means that the signal flows evenly thru the different stages.
To do this, you need to set the fader/knob control of your channel strip and the main/master to 0db.
While playing or singing into your mic, adjust the Trim
or input gain knob until the light indicator or the needle
of the VU-meter reaches 0db. (On a typical small scale mixer, activating the main level led indicator is done by pressing the solo button of the channel)
You will probably find that each channel input may
require different input gain setting. For example
you'll find that a mic requires more gain than a
keyboard.

By establishing a good gain structure from the start, you you'll get a strong , clean signal flow with a nice range and great control of the mix or recording. If your gain is too low, you will over-compensate with the channel and master fader bringing noise in the signal. If your gain is too high , you will have distortion and channel clipping.

Meet the auxiliary knobs

Most mixers are equipped with one or more auxiliary circuits. This is a convenient way to redirect part of the input signal to one or more independent outputs. Auxiliary outputs are controlled with corresponding auxiliary knobs on each channel of the mixer. Here the main purposes for the auxiliary outputs:

1) Monitoring
Because auxiliary outputs are independent from the main outputs of the mixer, you can actually create an alternative mix with the AUX knobs. For example, you can have a separate sound system for stage monitoring controlled by the same mixer.

2) Outboard FX
With AUX outputs you can also send part of the input signal to an external FX processor like a reverb. You can return the processed signal to the mixer into the AUX return input or another available channel input to mix the reverb.

3) Direct recording output
Because of the independence of the AUX circuit, you can use AUX outs to divert your inputs to a recording device such as a computer without affecting your main mix.

Written by:
Eric Warlaumont