Most mixers and consoles include a group of controls labeled “AUX.” What do these controls do, and how do you use them?
AUX (for auxilary) controls have three functions:
1) To add effects (reverb, echo) to an actor’s voice or a musical instrument’s performance.
2) To create a monitor mix – the balance of mic signals heard through a floor monitor loudspeaker.
3) To create auxiliary mixes – say, a balance of microphone signals going to a recorder, to a green-room loudspeaker, or whatever.
In each input channel on the mixer/console, the AUX knob taps off some of that channel’s signal so you can send it to a particular AUX bus (see below). Each AUX bus is a channel containing a mix of several input channel’s Aux signals.
(click to enlarge)
So, if you turn up the AUX2 knobs for the guitar, bass and drums, the AUX2 bus will contain a mix of those three instruments. On the back of the mixer is a connector labeled AUX2 OUT. If you plugged in an amplifier and loudspeaker there, you’d hear the mix of guitar, bass and drums.
Let’s say you want to add reverb to an actor’s voice. Suppose you want AUX1 to be the reverb bus. Connect AUX1 OUT to the reverb unit input. Connect the reverb unit output to the console’s BUS IN, AUX RETURN, or EFFECTS RETURN jacks on the back of the console. Then, to hear reverb, turn up the AUX1 knob in the actor’s mixer channel.
The actor’s AUX1 signal is routed to the reverb unit. The reverberated signal comes back into the console, where it mixes with the direct sound of the actor’s voice. So you hear the voice with reverb added.
Next to each AUX knob, there might be a switch labeled PRE/POST. PRE means “pre-fader”. The aux signal is taken before the fader, so the aux level does not change when you move the fader. POST means “post-fader”” The aux signal is taken after the fader, so the aux level changes with the fader level. The figure above shows AUX1 as post-fader, and AUX2 as pre-fader.
Use the PRE setting for a monitor mix. That way, the monitor mix that the performers hear will not change when you adjust the house mix.
Use the POST setting for effects. When you turn down an instrument’s fader, its reverb will turn down too.
In some consoles, AUX1 is always post, AUX 2 is always pre, or whatever.
Basically, the aux knobs act like a mini-mixer within the larger mixing console. Each input channel has one or more aux controls. They might be labeled AUX1, AUX2, and so on. If you turn up the AUX1 knobs in several input channels, the signals from all those channels combine into an AUX1 submix. A submix is a separate “hidden” mix within a mixing console. It’s not the program mix which the audience hears. The level of each instrument in that submix is controlled by its AUX knob setting.
Your console might let you set up several different AUX mixes. AUX1 can be the signals going to a floor monitor speaker; AUX2 can be be the signals going to a reverb unit, and so on.
You can choose the function of each AUX bus. When you run sound for a concert, you might use the AUX1 knobs for the lead singer’s monitor speaker, use AUX2 for the drummer’s monitor speaker, use AUX 3 for the keyboardist’s monitor, and so on.
Bruce Bartlett is a microphone engineer (www.bartlettmics.com), recording and live sound engineer, and audio journalist. His latest book is Practical Recording Techniques 6th Edition.