The mixer is the brain of the sound reinforcement system. A variety of sound sources connect to the inputs of a mixer. Vocal and instrument microphones, instruments such as guitars and keyboards, media players, etc. can each connect to the mixer at the same time (depending on the number of available channels). The mixer then processes and balances all of the inputs into one unified mix to send through the outputs to the audience. Although some seem daunting due to the amount of knobs and faders, the mixer is mostly made up of what are known as “channel strips.” Once you understand one channel strip you know them all. They each have identical controls used for each input.
Each input corresponds with a channel on the mixer so individual adjustments can be made to each sound. Most mixers will have XLR, quarter inch, or combo-jacks (both quarter inch and XLR).
Mixers are used to process sound and this includes audio effects. Some mixers come with effects and dynamics processors built in, whereas for others you can use standalone, dedicated effect units.
There are two methods for connecting an external processor to a mixer. The first is into an individual channel via an “insert point” (a two way connection). Insert points are useful for devices designed for use on one channel at a time, like a compressor. The second is into a circuit called an auxiliary bus (a special piece of wiring in a mixer than can be tapped into by other channels or controls), which allows for any channel to make use of the effect. This method is useful for effects such as reverb.
All mixers have at least one set of master outputs that carry the mixed signal to your amplification system. Usually, they’ll also have a second set of outputs called a Monitor Mix which allows you to send a different mix to an amplification system for the musicians or performers on stage so that can hear themselves playing.
The channel strip will tell you a lot about the mixer such as what sort of equalizer it has and the flexibility it has in terms of monitoring and patching effects. At the top is the input section, the point at which the sound source enters the mixer. The strength of the signal is governed by the gain control, sometimes referred as a trim pot (trim, as in adjust the incoming signal; pot, as in potentiometer; a smooth running level control). Some mixers also have a pad switch, an instant attenuation that reduces signal level by a fixed amount.
Learning proper gain structure will ensure the signal entering the mixer is clean and not distorted. It is ideal to end up with all sound sources having equal signal strength. If enough signal doesn’t get into the mixer, the signal won’t be strong enough and increasing the output level of the channel (using the fader) will add noise. Conversely, if too much signal is coming in, the sound will be distorted and no amount of fader attenuation will be able to help it. Use the meters on the channel as a visual guide (some mixers will have a red “clip” light) and of course, your ears as well.
Most mixers feature some sort of tonal adjustment controls. It may be high, mid and low, cut and boost like on a home stereo, or a more sophisticated system where specific frequencies can be isolated and adjusted. Cutting (reducing) the level of certain frequencies can be just as powerful as boosting frequencies when it comes to improving a sound or fixing a problem. If a sound doesn’t need tonal adjustment, don’t make any changes. Boosting frequencies adds noise to the signal chain, and should be avoided whenever possible.
High Pass Filter
Some mixers employ a high-pass filter that will automatically filter out low frequency noise. This switch comes in handy when wanting to reduce wind noise from microphones. It can also protect the speaker from damage if someone accidentally drops the mic.
Many mixers have auxiliary signal buses to add effects to the sound or deliver a mix to on-stage monitors. An aux knob on a mixer is really just another level control. It adjusts the amount of signal from that channel to that particular aux bus.
Aux buses come in two varieties, pre or post-fader. Sometimes they have a switch to determine the operating mode (pre or post). If the aux bus is pre-fader, the sound is sent to the aux bus before it comes under the control of the channel’s fader. This means that adjusting the channel fader will not disturb the amount of signal going to the aux bus. This will prevent a monitor mix from changing just because the engineer is changing the levels in the mix. Post-fader is used when wanting to adjust the level of the channel, while also adjusting the level sent to the effect unit. For example, as the volume is adjusted, the level of delay on a singer’s voice is also adjusted.
Pan controls send the signal to the left, right, dead-center, or somewhere in between the stereo field.
PFL (Pre Fader Listen)
The PFL allows the sound of the channel in headphones to be heard regardless of the channel fader being up or down. PFL can be a lifesaver during a show since the engineer can check an individual channel without disturbing the sound coming out of the main outputs. It’s also helpful when setting initial input levels since it can be done without the sound coming out the main outputs.
The fader is the vertically sliding cap that increases or decreases the channel’s output. Some mixers use knobs for channel level but the result is the same.
Mixers that contain groups will have a button on each individual channel that allows the signal to be sent to that particular group fader. Grouping like-minded sounds together can minimize drastic changes to individual sounds. For example; if the drums are a bit low, moving up the group fader would increase all of the drum sounds (provided they were assigned to the same group).
Master Controls and Monitoring
In the master section of the mixer, the Aux Send can control the overall level of the entire mix being fed to the monitor amplifier and speakers. With more than one aux, the monitor mix can be sent to an effect, and can adjust the amount of effect that is applied by tweaking the Aux Return control. On individual channels, the amount of effect routed to that particular channel can also be controlled. There is also a control that adjusts how much of the auxes are applied to the main mix. Finally, the main fader adjusts the overall level of the main output mix.
Types of Mixers
In addition to the standard analog mixer, there are two other topologies to be aware of. Powered mixers have a built-in amplifier and will typically allow control over how much power is used for the monitor mix vs. the main speakers. If a monitor mix is not being used, all the power can be harnessed for the main speakers. These types of systems are good for small outfits like two or three people with guitars and microphones.
Digital mixers have greater control and flexibility over their analog counterparts. They can be programmed so that settings are stored and recalled later. This is helpful for bands since the engineer doesn’t have to remember the settings for a singer’s mic, or channel fader positions for a certain song. Many have on-screen controls and work with proprietary software. They can also easily be integrated into computer setups. Routing is often flexible which means the performer can plug into channel one and route the signal to any desired channel. Check out the AMS Digital Mixer Buyer’s Guide for more info.