What is a mixer?
A mixer, in its purest and simplest form, combines or meshes an array of inputs into a few controllable outputs (hence the name, MIXer). It is pretty much universal that mixers will have at least a volume control on the output. The vast majority will have volume or "level" controls on each input, or "channel." A great many still will have a variety of controls on each channel, from gains or trims to EQ and aux'es and buses and PFL's and more; don't worry though - I will go through each of these at least briefly.
When a new sound guy looks at a mixer for a large church, per se, he may feel overwhelmed by the oceans of knobs that may or may not be there. But here,I will explain what these knobs do, and you will actually find them to be overwhelmingly simple.
The first thing you need to know is that I will be dealing with moderate to large mixing desks and sound boards, with at least 10 channels (available inputs) or more. These are what you will see if you want to "run sound" for a church or venue or record music of bands, etc.
Step 1: Channels
Simply look at one of the black circle inputs (XLR's for microphones and snakes), then follow the column of knobs straight down. This is a channel strip.
Step 2: From the top: Gain (Trim, Sens)
Many people make the mistake of mistaking gain for volume. This is wrong. Gain should be used just like any other control knob: to set the kind of sound you want and the quality, not volume. That is what faders are for. Additionally, any gain changes will also affect the sound in the monitors or other auxes, so be careful. If you don't know what those are, don't worry: I'll explain them later.
A common use for gain is to sort of even out or normalize the signal (calibration). This is done if you have a VU meter, which tells you how loud the output sound is. One would set your fader or output volume control to 0dB and alter the gain until the meter says the sound is at 0dB.
Step 3: EQ: Equalization
Tone Knob: One knob that, when turned clockwise boosts high frequencies and lowers low, and vice versa. Sometimes called contour, and usually found on very small mixers.
2-3 Band: Consists of bass and treble knobs or Low - Mid - High knobs. I don't think I need to explain these.
Semi - Parametric: Usually a 3 or 4 band (Low, Low-mid, High-mid, High) containing at least one sweep. A sweepable control is one that has a pair of knobs: one chooses a frequency to boost or cut, and the other sets the boost or cut. The Frequency knob is called a sweep.The mixer below has one sweepable band.
Fully Parametric: Every band has a sweep to go along with it.
Graphic: These contain sliders instead of knobs. Each slider has a set frequency and is moved to change the presence of that frequency. These are not commonly found on channels, but are often found on the total mix (right before the output goes out). I have seen graphic EQs with 3 sliders and some with 31+ sliders.
As you progress in size from small mixers to large, you will find the amount of control over the EQ of each channel increasing. Many of the large boards, like the Soundcraft MH series, have a 3rd control knob on their fully-parametric EQ bands: Q. Just as the sweep sets the "center" frequency to be boosted, the Q sets how wide the boost range is. If you think of an EQ band as a line with a hill or bump in it, the sweep sets where the bump is, the boost sets how high the bump is, and the Q sets how wide the bump is.
Many digital mixers allow you to actually have a graphic EQ on each channel. You would select the channel on the screen, and set its EQ. These can be troublesome, however, if you need to quickly cut a particular frequency in a live show before it causes feedback and time is crucial.
I have told you how to use EQ, but not how to be good at it. Many times it is best for newbies to leave the EQ on flat (no frequencies boosted or cut) if possible. Mastering use of EQ takes experience and research. Remember, however, that it can make an OK sound sound good, and a good sound great, but it cannot replace what should be in a bad sound.
Step 4: Auxiliary outputs (Auxes)
The most common use for auxes is stage monitors. Lets say that we have Bob on stage singing, and Joe playing guitar, and each has his own monitor. Lets also say we have Bob's voice plugged into channel 1, and the guitar in 2, and Bob's monitor in aux 1 and Joe's in aux 2. So, if Bob wanted more guitar in his monitor, we would go to channel 2 (where Joe's guitar is plugged in) and turn up the Aux 1 knob: this tells the board to put more of the signal in channel 2 into the device plugged into aux 1.
Two other common uses for auxes are reverbs (device that simulate reverberation effects) and subwoofers (loudspeakers designed to reproduce lower frequencies than normal speakers can play).
Step 5: Faders, PFL's and AFL's, Pre/Post Fader auxes
Auxes, you should know, are normally pre-fader by default. This means that changing the level of a channel on the fader will not affect the sound of that channel in the auxes. However, many medium-large boards have a button near the aux knobs that allows you to change them from pre-fader to post-fader or vice versa.
Example of pre/post fader auxes:
You are running sound for a rock band playing in a large room. Being the sound perfectionist you are, you have set up some subwoofers to help properly produce the low frequencies of the kick drum, floor tom, and bass guitar. The band also has monitors set up on stage. If you have the option, you will naturally want the monitors to be pre-fader, so that changes on the faders will not mess with the monitors. You will also, however, want the subwoofer auxes to be post-fader, so that the mains-subs balance remains the same after changing the fader settings. When you turn up the bass guitar fader, you want the bass signal in the monitors to stay the same, but the bass in the subwoofers needs to increase along with the bass in the main speakers.
Most medium-large size desks also have a headphones jack for the engineer to use headphones. You will also see a button near the fader on each channel labeled PFL. This stands for Pre-Fader Listen. This allows you to listen directly to the signal in any channel via the headphones, unaffected by the fader setting. You may also see a button near the aux masters (they set the total volume of the auxiliary outputs) labeled AFL. This stands for, you guessed it: After-Fader Listen. This allows you to hear the exact sound and volume coming out of the monitors or whatever else you have plugged into the auxes.
You should also realize that above the fader, just after the auxes, is a pan knob. This is simply used to "pan" the channel's signal to left or right. All the way to the left would put the signal only in the left signal, and vice versa, with the middle sending equal to both sides.
Step 6: Buses (subgroups), Main mix, VCA's
One very common use of a bus is to assign all the drum channels (floor tom, kick, hi-lo tom, hi-hat, snare, etc.) to one bus. This way, the engineer can change the volume of the drumset in the total mix without having to change each fader on each channel. Other uses include groups of singers (girls, boys; leads, harmony), instruments (main, backup; brass, woodwind, percussion), etc.
Just remember that changing the fader on a bus is NOT equivalent to changing the faders on all of its assigned channels. It simply changes the volume of that particular mix of channels relative to the entire mix. Channels can be assigned to multiple buses, and changing the volume of one bus with channel 6 in it will not change the volume of channel 6 in any other subgroups.
Each of the subgroups also has a button allowing assignment to the main mix. In most cases, you will want everything combined into the main mix so that you can control the total volume of everything with one setting. Sometimes, however, you may leave one subgroup by itself; independent of the main mix, for whatever reason may apply. Usually each subgroup will have a left and a right fader, allowing for individual adjustment of the left and right aspects of that mini-mix. The main mix will also probably be stereo. There will be two outputs on the back of the board for left and right mix.
However, some mixers have another button to assign the main mix and any and all subgroups to a "mono" fader. This is another output on the back that is a single bridged out between the right and left of the mix out. This allows you to have a huge amount of possibilities of setups depending on where you have channels and busses assigned and what you have plugged in to the back. The mono will "bridge" the left and right channels, causing all sounds in each to be played in both. It will also, however, leave alone any left/right settings set before it.
VCA's are confusingly similar to buses. However, it helps to remember that buses route, and VCAs are simply a control mechanism. Contrary to subgroups, a change in a VCA will be exactly like moving the faders of all its assigned channels. Thus, any post-fader outs will be affected: post-fader auxes, the level of that channel in any subgroups, etc. If post-fader auxes are used for other main speakers like subwoofers, than a VCA will allow change in those too; where a subgroup would not.
VCA's are found only in larger boards.