So far, we have considered the basic functionality that typical small mixers share. This time, we will look at the features you can expect from an intermediate or large mixing desk.
A quick note about something that’s important to understand when using a mixing desk:
Decibels
On a guitar amp or hi-fi system, the level controls are usually labelled so that the number ‘1’ represents the lowest level of volume and higher numbers represent higher levels. Bizarrely as it may seem, then, when the level controls on a mixer are centred, the label they will point to will be ‘0’. Anything above this is prefixed with a plus sign and anything below with a minus sign. This is because professional audio systems work using Decibels.
At ‘0’, the signal is not affected; at minus values it is augmented and at positive values it is amplified. So, as long as a signal is going into a channel, you will still hear it when the fader is set at ‘0’. The lowest possible position of a level control or fader, the position at which the signal is completely turned off, will usually be labelled as – infinity Decibels.
Mixers often have a couple of useful features for connecting microphones: ‘phantom power’ and ‘low cut’.
Phantom Power
Condenser microphones require a small voltage, known as phantom power, which is sent to it from the mixer via the microphone lead. Without this, a condenser microphone will not produce any signal at all. Phantom power is turned on either with a button at the top of each channel or by a single button or switch which applies phantom power to all channels simultaneously. Phantom power is usually harmless to devices that don’t require it, but it is still better left off unless using a condenser microphone.
Low Cut
Low cut, also labelled as ‘High Pass Frequency’ or ‘HPF’, applies a frequency filter to the input of a channel. A microphone can pick up very low frequency breath sounds or vibrations from the stand they are on being knocked. By turning on the low cut, all frequencies below around 100 Hz are filtered out, removing these unwanted sounds. Like phantom power, it is turned on with a button at the top of each channel.
Pre and Post Sends
In the previous article, we discussed auxiliary sends. Larger mixers will have more than one aux send but their functionality remains the same. There is, however, one concept which is important to understand: the primary difference between ‘Pre’ and ‘Post’ aux sends, also known as ‘Pre-fader’ and ‘Post-fader’ sends.
The pre and post here refers to whether the signal is split off to the aux output before or after that channel’s level control or fader. This is important because a pre aux send will continue to send its signal to the aux output even when the level control is turned all the way down. Conversely, the signal from a post aux send will be reduced when the level control is turned down. Pre aux sends are suitable for providing monitor mixers to performers since the level at which they hear themselves is independent of the main mix which the audience hears. Post aux sends, on the other hand, are for using with effects units; when the level of that channel is turned down the aux signal is no longer sent out of the aux output.
Aux sends may be hardwired as pre or post or switchable by the user. Pre sends may be labelled as ‘Monitor’ and post sends as ‘Effects’ or ‘FX’ for convenience.
Parametric EQ
In the last article, we discussed mixers which had EQs with a simple control that boosted or cut a signal at a set frequency. More expensive models will instead have what are known as ‘parametric EQs’ i.e. EQs that have more than one control per frequency. With intermediate models, the high and low EQ controls are usually at a set frequency whilst the mid frequency EQ has two knobs allowing the user to set both the frequency at which the EQ is applied and the amount by which the frequency is amplified or augmented. This is great for pinning down and reducing the level of harsh frequencies.
The low and high frequency EQs can, and often do, have this feature and some mixers may have four or more parametric EQs per channel, often splitting off the mid frequency band into ‘High Mid’ and ‘Low Mid’.
Mute and Solo
On intermediate mixing desks, next to the fader, you will the ‘Mute’ and ‘Solo’ buttons. Mute turns off that channel completely and no signal is sent to the outputs. Solo, on the other hand, turns off every channel other than the channel where the solo is turned on, making it useful for fine tuning the mix of a particular instrument or microphone. It can be useful for diagnosing where a hum or background noise is coming from.
Graphic EQ
So far, we have discussed single knob EQs and parametric EQs. Larger mixers will have a ‘graphic EQ’ on the ‘master section’, i.e. the section to the right of the channel section which is applied to the entire main mix. Unlike the EQs mentioned up to this point, graphic EQs have a row of several small faders - at least five but usually eight or more - all at a set frequency allowing the user to fine tune the mix by altering the level of very specific frequency bands.
Returns
As well as aux sends, mixers will often also have one or more aux ‘returns’. The idea of this is that the signal that is sent out of an aux output to an effects unit is returned to the mixer via its aux return inputs. This feature is simply a matter of convenience; Aux returns function the same way as stereo channels in that they combine the signal with the main mix, however, they will usually have just a level control rather than the full series of controls found on each of the normal channels.
USB
Traditionally, the tape in and tape out on a mixer were used for recording and playback from a cassette desk. In the present day, however, tape has been replaced by the use of a PC or laptop which communicates with the mixer digitally over a USB lead. Mixers with USB connectivity work the same way that older mixers worked with tape machines: a digital copy of the main out is sent to a computer and the computer sends back a stereo signal for playback. Mixers allow the user to select if the ‘USB in’, i.e. the signal the computer sends back, is sent to the main outs, headphones, or both.