082 - Outputs: Main L/R Mix, Aux Sends, Matrix
Once we have a good mix we need to route it to the output of the mixer.
Written by Scott Adamson
Let’s break down three important kinds of mixer outputs: main mix, aux sends and the matrix mixer.
MAIN L/R MIX:
The main left/right mix, which, just for the sake of simplicity, I’ll call the main mix, is what the audience hears. It’s a critical filter for the music being played on stage. This is the FOH mixer’s focus during the show.
The main mix is a stereo mix. Left and right are separate channels that have two discrete busses and two separate outputs on the console. The left side goes to the left speakers and the right side goes to the right speakers. Of course, sometimes additional speakers are used to fill other parts of the room.
On very small analog consoles, input channels are often automatically routed to the main mix. On medium-sized or larger format analog consoles, and on pretty much any digital console, each channel must be assigned to the main mix. On analog consoles, this can almost always be found near each fader, probably labeled “L” and “R” for left and right. You’ll likely find additional buttons to assign channels to various subgroups. If the fader isn’t assigned anywhere, it won’t be routed anywhere and won’t be present in the mix.
It’s important to recognize that the main mix is an audio buss. This means we’re able to insert EQs and dynamic processors like compressors, which can have a big impact on the overall sound. Some rooms or PA systems just don’t sound great on their own. Using tools like EQ and compression won’t magically fix a bad system, but can help us achieve a better sounding mix. The 31-band graphic EQ is a great tool for this, but parametric EQ can also be used. And again, because the left and right are discrete busses, they can be processed differently, but usually only when it’s necessary to compensate for problems, so hopefully that isn’t an issue you’ll run into.
AUX SENDS:
Aux sends are another important output. This is how monitors and FX are fed. Aux sends are found on the channel strip, usually as a section of knobs right below the EQ on analog consoles.
As the signal moves through the channel strip, the aux send “picks off” the signal, splitting it off to that aux buss. The signal continues to move through the channel strip and fader, and then on to the left right buss, but by picking off the signal with an aux send, we’re now also building a separate mix. Other channels can be incorporated and we’re able to feed this new mix to monitors or FX processors.
The most important terms we use with auxes are pre and post, short for pre-fader and post-fader. This refers to different points in the signal chain, called pickoff points, where the signal is, well, picked off! The difference in pickoff points makes the aux send beneficial for different applications.
The pickoff point for a pre-fader send comes right after the preamp section. Considering the flow of the channel strip, this means changes in gain, pad, high pass filter, and phase will affect the aux pre level. You can, however, move the faders up and down without changing what’s happening with an aux pre mix. This is a great setup for mixing monitors from FOH, when you don’t want changes intended for the main mix to affect the monitor mixes. (A pre-fader send could theoretically be pre- or post-EQ, but in live sound settings, pre-EQ would make the most sense.)
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Aux pre sends are also great for recording. By setting up a pre-fader recording mix that’s sent directly into a recording device, you can lock it in ahead of time and won’t need to worry about changes to the recording when moving the faders around during the show. If you choose not to include an input in the main mix because of loud stage volume, you can still have it in the mix going to the recorder.
Aside from those situations, you’ll more likely use post-fader sends. This pickoff point comes after the entire channel strip. This means the preamp section, EQ, and fader all affect an aux post send — but this can be helpful for us. For example, when mixing monitors from a dedicated monitor console, you’ll want to be able to use EQ on the channel strip. This can shape the tone of any specific channel, without changing the overall EQ for that mix. Since the auxes are post-fader, you can adjust all the mixes at once.
If an input is too loud for just one song, you could just turn down the channel fader instead of tweaking all the individual aux sends. When the fader goes back to unity, that input will be back to where it was before.
Even when mixing monitors from FOH, it’s best to use aux post for FX sends. When you’ve dialed in a good EQ for any particular instrument, you’ll want that same EQ going to the FX unit. This helps the FX return sound more like the original source, which will help your mix. When sending a channel to a reverb, you don’t want the reverb to stay active as you turn down the channel fader. If using an aux post send, as you turn the fader down on the channel strip, the send to the FX will follow it and will turn the reverb down, too.
Just like the main mix, aux busses have an overall level control. On digital consoles, these are all faders, but on most analog consoles, we usually find knobs. Either way, the aux master controls the overall volume. We can usually just leave it at unity, but if we need to turn the whole mix up or down without changing the individual channels, we can do that at the master level.
MATRIX MIXER:
The matrix is a helpful tool for more complex signal routing. It’s essentially a mixer for the outputs of the console. All of the output busses, like groups, auxes, and even the main mix, are the inputs of the matrix mixer.
Analog consoles don’t really have the space for huge matrix mixers. If you have 4 matrices and each have 16 auxes, 12 subgroups, plus the left right mix, that’s a lot of real estate on a console already full of knobs! Analog console manufacturers don’t usually incorporate a full matrix mixer onto the console, but instead choose which output busses they’ll accommodate. This is usually just the groups and main mix, but could also include a couple auxes.
In professional digital consoles, we have significantly more options. For example, the DiGiCo SD7 has 128 mix busses that you can configure into auxes or subgroups. Then, a 32 by 32 matrix mixer lets you mix tons of those output busses together and even insert processing. (This is way more than we usually need though!)
Many engineers use front fills when mixing shows. A matrix mixer would allow you to have the same mix coming through the fills as is coming through the main speakers, while offering totally separate control. This is done by sending the main mix to the matrix and sending the matrix to the fills. This means you can turn the matrix up or down as much as you want for the fills and you won’t affect the main mix coming out of the main left and right outputs. Again, since a matrix is a buss, you’re able to insert an EQ to control that separately.
Or say you want to add extra vocals to your mix. Since the matrix mixer is for output busses only, you’d need to first assign the vocals to a subgroup. Then, by turning up that subgroup at the matrix, it will boost the vocals.
Matrices are especially helpful for recording. Even if you just want to record a stereo feed of the main mix, this is a great way to send an appropriate level to the recording device. Simply feed the main mix to the matrix and adjust the level going to the recorder as needed from there. If you didn’t have that extra control, a straight left right feed could overpower the recorder. You wouldn’t be able to turn it down during a show because it would also turn down your main speakers!
