The purpose of any sound reinforcement system is to amplify a source, whether that is a single speaking voice, a group of singers, a collection of musicians, a piece of pre-recorded music, or other sound sources, and to deliver that to the listener.
Each individual piece of equipment in the signal chain between the source and the loudspeaker can affect the level of the signal.
That means that the mixing console, outboard effects processors, crossovers, and amplifiers (obviously) can all add to or subtract from the signal level.
The mixing console can almost be considered to be an audio system in itself, containing microphone preamplifiers, equalizers, and perhaps even dynamics processing.
Each includes a gain stage that can amplify the signal. Then there are the input channel faders, subgroup faders, and the master faders. Again, each can add gain to the signal.
Each component part of the mixing console, and piece of audio equipment in the following signal chain, also adds noise to the system, since all electrical circuits produce some level of noise. The amount of noise depends on the circuit design, and may be negligible in the case of a digital device or may be a great deal, especially where analog gear is involved, and particularly with older or poorly maintained equipment.
If you turn everything up in your system with no signal going through it then what you hear coming out of the loudspeakers—that hiss—is the sum of the noise being introduced by everything in the signal path (the equivalent of the tape hiss we mentioned).
The level of hiss that you hear is known as the noise floor. It may also include buzzes from grounding problems or other interference, but those are issues for a separate article.
At the other end of the scale, too much gain introduced by any device in the signal chain will “clip” the signal, which means that the signal peaks are flattened by the circuit’s inability to handle the level, resulting in a distorted sound.
With analog circuits a little distortion is tolerable. Digital clipping, however, just plain sounds nasty, and is to be avoided at all costs.
There is a window in which the system operates at its optimum, where the noise floor does not mask a signal that has been set too low and high-level signals do not distort.
The fundamental aim when setting the gain through a mixing console is to ensure that the loudest signal passes through at a level just below clipping, while also allowing for signal peaks. That extra leeway is known as headroom.
Let’s take a basic signal path, a microphone connected to the input of the mixing console, the output of which is connected to an amplifier and a loudspeaker. To set up the console, begin by turning down all the input gain (also known as trim or input level or input sensitivity) controls. Disengage the pad switches, if there are any.
Pull all the faders—inputs, subgroups, and masters—all the way down. Set the equalizer level controls to the 12 o’clock position or switch the EQ out of the signal path. Turn all of the auxiliary sends down and set the pan (also known as balance) controls to the 12 o’clock position.
On the relevant input channel, select PFL or Pre Fade Listen. This switch routes the input signal to the meter (and usually the headphone output) from before the fader, so the fader position has no effect on the signal.