The mixer is the central device in any sound studio. Although you can do a lot without it, sooner or later you are going to want to bring all of your materials together to make a piece of music, and for that the mixer is essential.
The mixer is also the most formidable looking device in the studio. Pictures of recording studios always show the mixer because there is nothing more impressive than a couple of acres of knobs. The functions of a mixer are actually quite simple; all you are doing is combining a few channels of signal into one or more outputs. Along the way, the signals are amplified and equalized if necessary. The mixer looks complicated only because these functions are repeated many times.
The functions of a mixer are simple: 1) Process input signals with amplification and EQ, and 2) Combine those signals in a variety of ways.
ORGANIZATION
Although most mixers have similar features and organization, there is some variety to meet special purposes. This is generally a matter of which features are duplicated. Thus a "stage mixer" has several auxilary outputs, a "studio mixer" has very low noise, a "broadcast mixer" has cue functions and stereo faders, etc. The basic layout of the mixer is described in terms of the number of main inputs and outputs, such as 8X4.
A very popular configuration is 8 or 12 inputs with four subgroups and a stereo master (8X4X2).
A mixer is really a traffic manager. A signal is connected to an input, and you steer it to one of several possible outputs. Some mixers have several stages of mixing, where inputs are mixed to submixes, or GROUPS, and then the groups are further mixed to a stereo output. For instance, you might have five mics on a drum set, and group these so one fader controls all drums. Trombone, tuba and trumpet might go to a group for horns, etc. Groups usually have the same assignment flexibility as individual inputs.
There are several parallel mixers, (called busses), to allow different combinations of inputs at once. You might send piccolo, clarinet and bass to the right channel, clarinet and xylophone to the left channel, and the clarinet and tuba to a reverb unit. The output of the reverb may then be brought back and sent to the left and right channels in equal amounts.
When you look closely at a mixer you will usually see three kinds of module: INPUT MODULES, GROUP MODULES, and MASTER MODULES.
NPUT MODULES
INPUT MODULES contain circuits that modify the signals before they are mixed.
MICROPHONE PREAMPS: boost the signal from microphones to levels compatable with the rest of the studio. The quality of the preamps separates the tools from the toys in the mixer world. (So-called keyboard mixers do not have mic preamps.)
PADS or ATTENUATORS: A pad is a simple push button that cuts the microphone signal by a fixed amount, usually 20 db. An attenuator is another name for the same thing, and sometimes offers a choice of attenuation. This is necessary because when you put mics close to an instrument you often get an output strong enough to overload the mic preamp.
MIC/LINE SWITCHES: allow you to choose either a microphone or signal from the patch bay as the input. Sometimes a third position selects the outputs of a multitrack tape deck.
TRIM adjusts the signal coming from the mic preamp and/or the line input to be compatable with the rest of the mixer. This should be set so that there is no distortion when the fader is all the way up.
FILTERS and EQ adjust the frequency response of the input modules. These may range from a simple high cut to several parametric sections. EQ adds a lot to the cost of the mixer. There is often an EQ BYPASS, because even the best eqs always change the signal slightly.
PAN KNOBS and ASSIGN BUTTONS make the actual connections between the input modules and the group or master modules. In most modern mixers the PAN function gives a continous sweep from one output to another, with various combinations possible depending on how the ASSIGN switches are set. Some mixers allow an input to be assigned directly to the stereo master , others only allow assignment to the groups.
FADERS conveniently control the relative levels of the various input signals. This is where you control the balance of the mix.
EFFECTS, FOLDBACK, ECHO or AUX are all examples of auxiliary SENDS, which mix signals from the input modules to special outputs independently of what is happening on the main busses. There is a trend in modern mixers to add a switch which connects a tape output to the cue mix, to reduce patching with multitrack tape decks.
Auxilary sends may be PREFADER, meaning the signal will get through even if the fader for that input is down, or POSTFADER, where the fader controls the amount of signal sent.
SOLO is a button that connects the input module output to the monitor output (see below) to the exclusion of all else. This is very useful for checking on the operation of a single microphone. PFL or prefade listen, is similar to solo, except that the signal is monitored at full volume. On some broadcast consoles, this feature is called CUE, and may be triggered by pulling back on the fader.