For a list of mixing desks see the Mixing Desks category.
The heart of any sound system is the Mixing Desk, or mixer, which takes all the ‘sources’ (e.g. Microphones, CD player, etc) and allows us to control each one and literally ‘mix’ the individual sounds together. Mixing Desks initially look very complicated, but can easily be divided into manageable sections, as follows:-
The first section is the Input, which is where all the different sources come into the desk and are manipulated individually. On the desk, there is a channel for each input, usually running in a strip down the desk. This Channel Strip is repeated for each input.
It is important to note that the layout of every desk is unique. Some larger consoles may have pan controls below the gain controls, they may also have double controls for eq/aux, where each control has 2 functions.
At the top of the channel strip is the Gain knob, which basically allows each individual source to be matched during the set-up.
Below this lies a selection of control switches. These switches can alter the polarity of the input. Apply attenuation to the input. Switch Direct outs on and off or select pre/post fade, eq, insert etc.
On most desks you will then find a HPF (High Pass Filter) control. For most desks this is a switch which places a filter on the input, the frequency varies from desk to desk but is usually between 75Hz and 125Hz. Larger desks tend to have a variable HPF control to adjust the filter frequency. This will often be combined with an in/out switch.
Next is the Equaliser or ‘EQ’ section, which works in a similar manner to tone controls on a hi-fi, allowing different frequency bands to be boosted or cut. Most mixers have preset frequency bands such as high (treble), middle and low (bass), sometimes with one or more extra control in the mid band(s) to adjust the frequency at which the boost/cut occurs. This is referred to as a Sweepable Mid or Semi-parametric Mid. As mixing consoles get larger in size and function the eq section can develop even further. All bands of eq may become semi-parametric. In addition to this some or all bands may have Q adjustment. This can be a basic switch to switch between a narrow and wide Q or a seperate fully adjustable control. this is known as Fully Parametric eq. This gives the most control but often comes at a high price. Some consoles will have an in/out switch to switch the eq circuit on and off. if you are not using an eq then it may be wise to switch this off as some circuits (even expensive ones) can introduce unwanted noise.
Next is the Auxiliary or ‘Aux’ section, which allows the source signal to be sent to any of the auxiliary outputs of the desk. For example, this would allow for a feed to a stage monitor, effects unit, backstage loudspeaker or to a recording device. Some people will run delay speakers, in/outfills, or even subs off of an aux send. With larger consoles auxiliary controls can be switched between pre fade, where the movement of the channel fader does not affect the aux send, and post fade, where the aux send relies on the channel fader. Other types of switching can put aux sends pre/post EQ, pre/post Hi Pass Filter, etc. Sometimes each aux can be switched in this way individually although quite often this is switched in pairs or even larger groups.
Next, there is usually a Mute or ‘on’ switch, which does exactly what it says. This is followed by buttons for Routing, allowing the signal to be sent to any of the mix outputs. An important note for mute/on switches, be sure to understand which the desk has before you start to use it. An ON switch will light up when the channel is activated, a MUTE switch will light up when the channel is MUTED. if you forget this in the dark things can go horribly wrong. Sometimes a mute/on switch will mute the channel to the VCA or subgroup/LCR only, sometimes to the aux sends aswell. It may also mute the Direct out sockets. All this may be switchable.
At the bottom of the channel strip is the channel Fader, which allows the level to be controlled. Alongside this is usually a switch marked PFL (Pre Fade Listen), which allows the operator to preview the individual channel when required, without it being heard at the output.
All of the individual input channels come into the Output or Master section.
On the most basic desks, everything is mixed to the LEFT and RIGHT or Master stereo fader, allowing the overall level to be adjusted. The signal then leaves the desk and goes on to the next stage of the chain.
Sometimes, channels can be first mixed to a Subgroup or Group, each with an overall fader to control the combined level. These groups can also be used as additional outputs. Larger desks make use of VCA's which are explained in the relevant section. At first sight a VCA seems to operate like a normal subgroup but on closer inspection there are some very important differences.
Other typical facilities in the master section include the master aux controls, which usually have a switched marked AFL (After Fade Listen), which works in a similar way to PFL but instead of previewing the signal, it only allows listening to the level at the output.
At the top of the master section is the VU Meter, which gives a visual indication of the output level or, when PFL or AFL are pressed, the level of that individual signal. Sometimes in the master section there can also be additional inputs, designed to be used as Returns from effects or playback equipment for example.
At the back of the desk (or sometimes on the top), is the connection section, which is where all the inputs and outputs are connected.
More advanced features include:-
- The mic pre-amp is a low noise amplifier on each microphone input to the mixer. The level coming from a microphone is very low in level and therefore prone to interference. The mic pre-amp raises the microphone signal to the standard working level for the mixing desk. Typically, the mic pre-amp feature a control that allows the operator to adjust the sensitivity of the input, allowing microphones with different sensitivities and output levels to be matched at the desk.
- The hi-pass filter is a feature on some desks allowing unwanted very-low frequency sounds to be removed, this may be on outputs and/or inputs.
- Insert Points are provided on most desks, allowing additional outboard equipment to be connected into the signal path.
- Subgroups can commonly be mixed back into the master output as well, allowing several similar sources to be sub-mixed first before going to the main mix.