5. Channel Strip Details
The gain control or pot (short for potentiometer) can normally be found at the top of the channel strip near the input connector. Gain is also sometimes referred to as 'trim', as it is the control used to adjust the input signal to the correct level for the rest of the mixers circuitry. Careful adjustment needs to be made to ensure that the signal is not too hot which can result in distortion from clipping or overloading the channel, and also that it is not so low as to give a poor signal to noise ratio. Signals which are weak at this point will need boosting further down the audio path, which will then increase noise levels. Correct adjustment of the gain control is normally assisted by some form of visual metering, from a single clip LED on budget models, to full LED bargraph displays or VU meters at the top of the range. Even budget models will usually have reasonable metering on the master section, and this can be pressed into use for individual channels by using PFL (see below).
EQ is used to boost or cut specific frequencies of a sound. Although the term was originally intended to describe the correction or equalisation of a sound, it is now just as often used as a way of enhancing a signal. There are a number of different EQ types, but mixers invariably offer some kind of parametric EQ on the channel strips, and sometimes a graphic EQ on the master section. Parametric EQs offer up to three control knobs per band gain (which is known as cut in its negative stages), centre frequency, and bandwidth or Q which determines how far around the centre frequency the gain control is effective. Note than any gain or cut applied will have a bell shape around the centre frequency. EQs with all three of these controls per band are known as fully parametric; those with only gain and frequency as semi-parametric; and those that operate at fixed frequencies are not really parametric at all, more glorified tone controls, although these can still be very useful. Graphic EQs operate at a number of fixed frequencies, each controlled by its own slider.
High Pass Filter (HPF)
Also usually found near the top of the channel, a High Pass Filter is an invaluable tool, allowing the attenuation of low frequencies which can otherwise clutter up your mix. They are particularly useful for removing rumbles picked up by microphones, either from low frequency on-stage sources such as bass cabinets and kick drums, or footsteps transmitted through mic stands. Most high pass filters are fixed at a specific frequency, normally around 100HZ, although some offer separate control over this.
Most condenser microphones and DI boxes require electrical power in order to operate, which can sometimes be supplied via a battery, but most often by 'phantom power'. Depending on the mixer, phantom power is switched on and off either independently per channel, or globally across a number of channels, and is distributed to the source device by the same cable that is carrying the audio signal. Although it is generally not a problem to send phantom power to microphones that dont require it, some ribbon mics can be damaged by it, so make sure you choose a model with individual switches if you intend using these. Phantom power is often labelled as +48v, which is an abbreviation for the standard required voltage of 48 volts do not be alarmed by this as the currents involved are very small!
Direct outputs are typically configured pre-EQ (although this is sometimes switchable) and are normally found on every channel of mixers that have them. By connecting the signal directly to a multi-track recorder or DAW, they enable each channels basic signal to be recorded without processing - these can then be returned to the mixer later on for mixdown. Direct outs can also be put to good use in live environments for making recordings to be mixed at a later date.
Insert connections provide both an input and output, typically on a single TRS (tip-ring-sleeve) jack connector. They are usually located next to the input connections and are specific to the associated channel. As they send the whole signal, they are normally used for the connection of dynamics processors such as compressors and noise gates.
Auxiliaries or auxes are the normal point at which to connect outboard effects processors, and are used to send a copy of a signal from one or more channels to an external device such as a reverb unit, the level of the copy from each channel being controlled by its own knob the channels aux send. A desk will usually have a master control over the mix of aux sends from all the channels. An auxiliary return is a stereo input designed for the return signal from an effects unit, and is normally fed directly to the mix bus. Larger desks have multiple auxiliary busses for use with multiple effects units. Another common use for aux sends is to provide separate headphone mixes in the studio, or on-stage foldback mixes in a live environment. For this kind of use, it is important to use pre-fade auxes see the Important Terms section for an explanation.
Pan knobs (short for panorama) control the balance of each channels signal that is sent to the left and right outputs a central position sends equal amounts to both, resulting in the sound appearing to come from the middle of the stereo field.
PFL (Pre-Fade Listen) and AFL (After-Fade-Listen) are both means of overriding the normal signal routing, or soloing one or more channels for various purposes. PFL sends a signal to the monitor outputs independent of the position of the channel's fader, and simultaneously mutes the other channels. In other words, PFL allows you to solo a channel even if the fader is pulled all the way down. AFL performs the same function, but maintains the effect of the channel fader. Note that on most consoles, these processes affect the monitor outputs only, and do not interfere with the main, tape, or auxiliary outs. Both PFL and AFL (but more commonly AFL) are also known as solo.
This is a button which mutes the selected channel(s) irrespective of the fader position. After muting the user can quickly return the channel to its previous state without having to reset the level again, as would be the case if the fader was used to silence it.
At the bottom of each channel strip will be either a fader or rotary pot controlling the level of the signal sent to the main output - higher-end products tend to have longer throw faders with higher resolution.