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  Mixing Facilities

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Contents


"Perfection of means and confusion of goals seem, in my opinion, to characterise our age." − Albert Einstein.


Introduction

This page is about the facilities that are typically found on a mixer, and what they do.

The primary purpose of mixing facilities is to provide control over the relative level and tonal qualities of each individual sound (i.e. instrument, voice or effect), in order to create one or more sound mixes for supply (after power amplification) to the front-of-house and/or monitor speakers, and/or for recording.

The most basic functions of a mixer are to:

  • Pre-amplify the low-level signals received from microphones and other sources, in order to bring them up to a level suitable for further processing.
  • Modify the signals as required − for example by giving an increased or decreased emphasis to their constituent frequencies (this is called equalisation, or 'EQ' for short).
  • Combine the various different inputs in controlled proportions, in order to achieve the desired sound mix(es).
  • Control the level of the mixed signal(s) that are fed out to the power amplifiers, to provide adjustment of the overall sound level from the speakers.

However, most mixers provide far more facilities than these basic four functions. Read on for more information!

Mixing facilities are generally provided in one of the following three configurations, which differ mostly in the way that power amplification of the mixed signal is achieved:

  • Mixer (or mixing desk) − incorporates no power amplification facilities and so requires the use of separate power amplifier(s). Sometimes referred to as an 'unpowered' mixer.
  • Powered mixer (or powered mixing desk) − a mixer that incorporates one or more power amplifiers.
  • Mixer-amplifier − a power amplifier that incorporates mixing facilities (usually rather limited ones).

For further details on the differences between these three types, see Amplification Facilities.

A further major difference in design is apparent between analogue and digital mixers (see Digital mixer), although the basic functions and facilities remain the same.

The descriptions below assume the most usual case of an unpowered analogue mixer. Much will equally apply to the other types, though on digital mixers many of the physical controls and switches are replaced by digital controls (usually accessed via a computer-style display screen), and the faders are often motorised types.

Mixers are usually made up of:

  • Channel facilities (see the section below)
  • A Master section providing group, matrix and output facilities
  • In the case of powered mixers and mixer-amplifiers, internal amplification facilities

Channel Facilities

A mixer is equipped with a number of channels (usually a multiple of 4), each of which provides control over a single signal source such as a microphone or an instrument. The set of controls (and sometimes the input and output connectors) associated with each channel is often referred to as a 'channel strip'. Usually all the channels are identical, except that some mixers, in addition to the usual mono channels, are equipped with a small number of channels that are able to accept stereo signals (see Input connections below). When no stereo input channels are available, the preferable approach for connection of a stereo source (such as a CD player) is to use two channels − one for the Left signal and one for the Right signal − and set their Pan controls accordingly. Alternatively, many mixers provide one or more '2-Track' inputs, though these usually have very limited equalisation and routing facilities.

The exact facilites provided on each channel vary from mixer to mixer, but the most likely ones to be encountered are:

Input connections

These provide the means to feed the input signal into the channel. There are usually two types of input connections provided on each channel:

  • Microphone input
  • Line input

The microphone input is usually an XLR connector suitable for the connection of a low impedance balanced microphone or other low impedance balanced source that provides a microphone-level signal (such as a DI box). When a multicore cable is being used, its tails are plugged into these inputs.

The line input is usually a "mono" (that is, 2-pole) " jack socket and is intended for the connection of unbalanced line-level signals, such as would be obtained from an effects unit, a CD player or other similar source. The normal arrangement is that when a plug is inserted into this socket, any signal being fed into the channel via the channel's microphone input is ignored − so it can be seen that the channel cannot accept signals from both of its inputs simultaneously. (Note, however, that on some mixers the mic input remains active, and therefore must not be connected to when the line input is in use − check your mixer's manual.) Some mixers provide balanced line inputs via 3-pole jack sockets.

Some models of mixer (especially large ones) do not have line inputs via jack sockets. Line-level sources must be connected to the XLR inputs, and the relevant Pad switches engaged. In such cases, if any sources are either unbalanced or not suitable for the application of phantom power, it is essential that the mixer is not supplying phantom power on the channels that such sources are connected to.

Stereo channels are generally intended for handling signals from stereo line-level sources such as CD players, stereo effects, etc., and therefore are often equipped only with line inputs. Usually the Left and Right input signals are connected via two separate mono jack connectors (rather than via a single stereo connector); alternatively/additionally phono connectors may be provided, or in some cases two XLRs.

Split output connection

This connection is usually only found on the larger-sized desks. It provides a balanced microphone-level output for connection to equipment such as a monitor mixer or recording mixer. This output is an exact copy of the input signal, and is not affected by any of the mixer controls; often it is hard-wired to the input XLR socket. Sometimes an earth lift switch is provided.

Direct output connection

This connection is usually only found on the larger-sized desks. It provides a line-level output for connection to equipment such as in-ear monitoring transmitters, multi-track recorder, or a recording mixer. This output is taken after the pre-amplification provided by the channel. It is typically taken after the EQ, but this may be switch-selectable (or determined by an internal jumper). The level of the output will be affected by any changes to the Gain (or Trim) control. Some mixers provide a 'Direct PRE' switch, that controls whether or not the output is affected by the channel fader; when no switch is provided the output is usually pre-fader. Setting the output to post-fader enables it to be used to feed an outboard parallel effects unit that is dedicated to that particular channel. (Contrast with the use of the Insert connection for connection of effects − see below.)

Insert connection

This connection is usually a 3-pole) " jack socket (sometimes inappropriately referred to as a "stereo" socket). It is provided to allow an outboard serial effects unit to be connected into the channel's signal path, so the effect will be applied to that channel only. Insertion of a TRS plug into this socket breaks the internal link between the output of the channel's pre-amplifier and the rest of the channel's circuitry, and so allows the pre-amplified signal to be diverted out to the effects unit and then returned to the channel via the same plug and socket. The level of the output will be affected by any changes to the Gain (or Trim) control.

Insert points are usually pre-EQ (or this may be switch-selectable), but are sometimes post-High pass (check the block diagram in the handbook!) − this means that effects units connected this way are handling the un-equalised signal. This may be an important issue in some cases, such as with dynamics processors (but using their side chain EQ can help).

When no Direct Output facility is provided, the Insert connection can usually be employed for that purpose, either by partially inserting the jack plug (not good practice), or by using a special cable which has the tip and ring terminals of the jack linked together. However, use of the Insert connection as a Direct output is not suited to feeding a channel-specific parallel effects unit, because Insert outputs are pre-fader.

Pad switch

This switch provides the facility to reduce the level of the input signal by a fixed amount − usually by 20 dB. This is useful when the level of the signal is so large that overload of the channel would occur even with the Gain (or Trim) control at its minimum setting.

Phase switch

A switch that (despite its name) reverses the polarity of the signal. Sometimes two or more channels of a mixer handle different versions of the same sound source. For example, when a guitar amplifier is DI-ed into the PA system as well as being miked-up, or a snare drum is miked from below as well as from above. In such situations, there can be complex differences between the versions of the source signal handled by each channel. Apart from differences in level and tonal content, the signals may have opposite polarity, and such a polarity difference would result in unwanted cancellation of fundamental frequencies if the signals were summed without the polarity of one of them being reversed.

In addition, there may be timing differences between the signals, e.g. caused by differences in the distance between the source and the relevant mic(s). As timing differences produce shifts in phase (different amounts of shift at different frequencies) it can be useful to reverse the polarity of the signal on or more of the relevant channels in order to improve the way in which their signals combine. The phase switch may also be useful to correct an accidental reversal of polarity elsewhere in the system.

Gain (or Trim) control

This control is usually labelled 'Gain', though some some mixers label it 'Trim'. In conjunction with the Pad switch, it sets the amount of initial amplification (or attenuation) that is given to the input signal by the channel's pre-amplifier. To minimise noise and to avoid distortion, it is important that the input signal is amplified (or attenuated) by the amount necessary to bring its level to within the range of levels that the rest of the channel's circuitry is designed to handle. Therefore, to enable the Gain control to be set correctly, some means is usually provided to indicate the level that is being obtained after the initial amplification/attenuation (but before the fader and auxiliary send controls), with the current settings of the Gain control and Pad switch. See the next item in this list for advice on how to use these indications to help you to set the gain.

Overload or level indication

To enable the Gain (or Trim) control and the Pad switch (where provided) to be set correctly, a means is usually provided to indicate the level of signal being obtained on the channel after the initial amplification given by the channel's pre-amplifier (and, usually, after the equalisation), but before the fader control (i.e. 'pre-fader'). The nature of this pre-fade indication varies widely from mixer to mixer, so you should consult your mixer's handbook to determine the correct way to interpret the indicator(s) provided. However, as a rough guide, the most common kinds of per-channel level indicator are:

  • An LED which lights when the pre-fade level is too high (usually labelled PEAK or CLIP). Set the Gain control and Pad switch to just avoid this LED lighting when the highest input levels to be expected are being input to the channel. (Usually, the occasional slight flicker is not a problem.)
  • An LED which lights when the pre-fade level is just adequate (usually labelled 0 dB or ZERO LEVEL). Set the Gain control and Pad switch so that this LED only goes out when levels lower than those normally expected are being input to the channel.
  • An LED bargraph indicator which gives a more accurate indication of the pre-fade level (usually provided only on the larger and more expensive mixers). Set the Gain control and Pad switch so that normal peaks light the 0 dB LED.

When there is no per-channel indication of level provided, it is usually possible to monitor the pre-fade level of the channel on the 'main' meters by operating the channel's PFL switch; in this case set the Gain control and Pad switch as per the handbook (or otherwise so that normal peaks register 0 dB). Failing the provision of any facility to indicate or monitor the channel's pre-fade level, then as a last resort adjust the gain control to obtain a roughly correct level of the channel in the main mix when the fader is set at its 0 dB setting (usually about two-thirds the way up), taking into account the position of the Main (or Master) fader. Also see the Setting gain controls item on the FAQ page.

Equalisation

Commonly referred to as "EQ", this facility consists of controls which allow adjustment of the relative level of selected frequency ranges of the signal being handled by the channel, whilst leaving other frequencies essentially unaffected. Most EQ controls allow the relevant frequencies to be either cut or boosted.

When used with discretion, these controls are able to compensate to some degree for inadequacies in the signal source or to usefully modify its tonal qualities for creative effect. However, when misadjusted they can make a potentially good signal source sound awful. To use these controls really effectively requires considerable skill, which can only be learned with practice. Some combination of the following facilities will usually be provided:

  • High-pass (or Low-cut) switch and/or control(s). Provides the facility to reduce the level of frequencies that are lower than a particular frequency value. Very useful for mic channels (especially vocals), to reduce unwanted or excessive bass content, handling noise, etc. Various forms are found, depending on the mixer's level of sophistication:
    • A simple switch; engaging this cuts frequencies below a fixed value, usually around 100 Hz. This is the form usually found on fairly basic mixers.
    • A single control that provides adjustment of the amount of cut that occurs below a particular frequency. With the control at '0' (usually fully clockwise) the amount of cut is zero, so there is no effect on the signal.
    • A single control that provides adjustment of the frequency below which reduction occurs; frequencies above the set value are essentially unaffected. If no low-cut is required, set the control to the lowest possible frequency (usually fully anticlockwise) or, if provided, set the associated switch to bypass the low-cut facility.
    • Both of the above controls − i.e. variable amount of cut and variable cut-frequency. If no low-cut is required, set the 'amount' control to '0' or, if provided, set the associated switch to the 'bypass' position.
  • High frequency cut or boost control, often labelled 'HF', 'Hi' or 'High'. Only affects frequencies above about 8 kHz. Essentially a "treble control".
  • Low frequency cut or boost control, often labelled 'LF', 'Lo' or 'Low'. Only affects frequencies below about 100 Hz. Essentially a "bass control".
  • One or more controls that enable a cut or boost of mid-range frequencies (the frequencies that lie between bass and treble). These controls come in one of two different arrangements:
    • Fixed frequency. In this arrangement there are one or more controls, each of which provides a cut or boost of the frequencies centered around a particular frequency. A common arrangement is a "low mid" control, which affects frequencies in the region of, say, 350 to 450 Hz, and a "high mid" control, affecting frequencies in the region of, say, 2.5 to 3.5 kHz.
    • Swept frequency, often called "sweep EQ". In this arrangement there are one or more pairs of controls, each pair consisting of a "cut or boost" control and a control which determines the centre-frequency of the band of frequencies that is affected by the "cut or boost" control. Usually, when there is more than one pair of controls, each pair is arranged to provide control over a different part of the mid-range spectrum. For example, one pair may have a frequency control capable of adjustment between 250 Hz and 2 kHz ("low-mid"), while another pair may have a frequency control capable of adjustment between 1 kHz and 8 kHz ("hi-mid"). More advanced mixers may additionally have a Q control associated with one or more of the pairs of other controls − this determines the width of the band of frequencies that is affected by the "cut or boost" control. Another name for sweep EQ without Q controls is 'semi-parametric EQ'.
  • 'EQ Bypass' switch. A switch that allows the whole EQ facility of the channel to be switched on or off. Useful (during sound-checks) for establishing exactly what effect the current EQ settings of a particular channel are having on the sound.

The main uses of the EQ facility are:

  1. To modify the way in which the signal level changes, as its fundamental frequency (the basic "pitch" of the signal) changes. For example, to compensate for an unevenness in the pick-up of a piano microphone, across the whole range of the keyboard.
  2. To modify the emphasis given to the various harmonics of the signal, that is, to modify the "tonal quality" of the signal.
  3. To assist in the reduction of unwanted components of the signal, such as hum, hiss, and sounds from sources which are appearing on the channel unintentionally (e.g. an electric bass being picked-up by a vocals mic).

The last of these three is fairly self-explanatory, but the first two are more complex and somewhat interrelated, and further explanation is given in the following paragraphs. The explanation is a little theoretical, and at the end of the day the important thing is just that your EQ settings produce the sound that you want. Nevertheless, you are unlikely to hit on the perfect settings by accident, as there are endless combinations of the various controls, so some understanding of the theory can help in knowing where to start. If you don't want to bother though, just understand that often a change to the EQ will unavoidably have an effect in both of the first two respects listed above. Each of these two effects will be significant to a greater or lesser degree depending upon the nature of the signal and upon the particular ranges of frequency that are being cut or boosted.

Let's consider firstly the changes in level of the signal, as its fundamental frequency changes. For example, the fundamental of a male vocals signal may vary, during a set of songs, between 100 and 350 Hz. It is quite normal and acceptable for the signal level to vary as its frequency varies across this range, but there may be particular ranges of fundamental frequency, within the overall range, where the signal tends to be over-emphasised or under-emphasised, and the EQ can be used to apply some degree of correction to this. In the case of our male vocals example, if there tends to be an over-emphasis of the signal when its fundamental frequency is in the region of say, 150 Hz, then this can be corrected by applying some cut around that frequency. (It should be pointed out that this is a rather simple example, because the human voice is monophonic − it can only have a single fundamental at any point in time. In the case of a polyphonic instrument such as keyboard, there will generally be several fundamentals present simultaneously, but this does not affect the EQ principle just described. The EQ may still be used to modify the balance between the levels of fundamentals of different frequency, because the EQ acts properly on each frequency that is present, regardless of the presence of other frequencies at the same time.)

Now let's consider the harmonics of the signal. At any point in time, a monophonic signal such as our vocals example will generally consist of a single fundamental and a large number of harmonics, each harmonic being an exact multiple of the fundamental frequency. The level of each of the harmonics, relative to the level of the fundamental, determines the tonal quality or "colouration" of the signal at that point in time − whether warm, nasal, hollow, harsh, etc. By applying a cut or boost to ranges of frequency within the frequency range occupied by the harmonics, we can modify their relative level and so affect the tonal quality of the signal. For example, a boost at around 4 kHz will tend to improve the clarity of a vocals signal.

The greater the degree to which the range of frequencies being cut or boosted lies within the fundamental frequency range of the signal, the more significant the effect on the balance between fundamentals of differing frequency will be. Our example cut at around 150 Hz will have no effect on the harmonics in our example signal − it will affect only fundamental frequencies, because 150 Hz is well below the frequency range into which the harmonics of this signal fall. But, this cut will also affect the tonal quality of the signal! The reason is that, for signals around 150 Hz, the level of the fundamental has been reduced without a corresponding change in the level of the harmonics of those signals, so making these harmonics more significant. In contrast, a boost at around 4 kHz will have no effect on the balance of the fundamental frequencies in our example signal − it will affect only the tonal quality, because 4 kHz is well outside the frequency range occupied by the fundamentals of this signal.

However, these are fairly extreme examples. Often a range of frequencies being cut or boosted will potentially contain both some fundamentals and some harmonics of lower fundamentals, and the EQ is able to make no distinction between them − it simply cuts or boosts the frequency ranges that it is set to cut or boost, and this cut or boost will apply to whatever fundamentals and harmonics occur within those ranges at any given point in time. For example, a cut of frequencies around 300 Hz will reduce the level of the second harmonic of fundamentals in the region of 150 Hz and the third harmonic of fundamentals in the region of 100 Hz, as well as reducing the level of the fundamentals that occur around 300 Hz.

For further advice on setting channel EQ controls, see the Mixing section on the PA Proverbs page.

Auxiliary sends and pre/post fade switches

Auxiliary send controls are provided to allow a controlled amount of the channel's signal to be fed into an auxiliary mix − usually several channels provide some signal into this mix. The setting of the control determines the relative level of the channel's signal in the auxiliary mix.

Because auxiliary mixes are usually mono mixes, there is no need for a pan control associated with an auxiliary send control. On a stereo channel, an equal proportion of the Left and Right sides of the channel's input signal is fed into the auxiliary mix, so forming a mono version of the channel's signal.

Auxiliary mixes may be needed for all sorts of purposes, but the most common one is to provide a mix for stage monitors that is different to the mix being supplied to the front-of-house speakers. In large PA systems several different stage monitor mixes are often needed, so in this case a mixer is required which can produce several auxiliary mixes. So, there will be several auxiliary send controls on each channel, one for each auxiliary mix.

When many different monitor mixes, or different mixes for various locations within a venue, are required in a system having a large number of input channels, it would not be cost-effective to provide the required number of auxiliary send controls on each channel, and such an arrangement would take a very long time to set up. In any case, it is not usually the case that the level of each individual channel must be controllable in each mix. Generally, what is needed is to be able to set the overall level of, say, lead vocals, backing vocals, drums, guitars, keyboards, etc. in each mix. Therefore, the solution usually adopted in such cases is a "matrix" arrangement: The limited number of auxiliary send controls available on each channel is used to create an "intermediate" mix from each of the required subsets of channels, and these intermediate mixes are then further mixed, using the matrix section of the mixer (or using another mixer entirely), to create the actual mixes that are required for each purpose.

When creating an auxiliary mix for stage monitors, you need to be able to retain the set level of each channel's signal in the monitor mix regardless of any adjustments subsequently made to the level of the channel's signal in the main (front-of-house) mix. This means that the auxiliary send control must take the signal before it has been affected by the channel's fader, so such an auxiliary send is called "pre-fade". Sometimes, however, it is required to create an auxiliary mix in which the level of each channel's signal is affected by the channel fader (as well as by the setting of the auxiliary send control). This kind of auxiliary send is called "post-fade". To provide flexibility at minimum cost, some mixers provide a switch which allows you to select whether each auxiliary send (or small set of auxiliary sends) is pre-fade or post-fade.

Effect sends

Effect send controls are simply post-fade auxiliary send controls that are intended specifically to route a controlled amount of the channel's signal to an effects unit. These controls will typically be found on smaller mixers that incorporate one or more onboard effects processors (see Master Section). In such cases there will usually be one send control for each processor. Some mixers that do not have onboard effects have their post-fade auxiliary send controls labelled 'Effect send'; in this case the corresponding Aux Out connector(s) will be labelled 'Effect Send'. If required, several channels may be set up to send the desired amount of their signal to the same processor. The setting of each send control determines the relative level of the channel's signal in the corresponding effect send mix. (Care must be taken not to overload the processor's input.)

PFL switch

The PFL (pre-fade listening) switch allows you to listen on the headphones to the channel's signal, regardless of the position of the channel's fader or Mute switch. This means that you can listen even when the channel isn't mixed into the main mix at all, so you can use it to do things like cueing-up pre-recorded material. Often the PFL switch will route the signal to the main level indicator as well as to the headphones, and this enables you to use these level indicators as an accurate way of setting the channel's gain control − providing that only one channel (that has some signal going into it) has its PFL switch activated.

Solo switch

There are several different types of solo switch:

  • Sometimes this is just another name for a PFL switch − see above.
  • Or it may be a SIP (solo-in-place) switch. This is like a PFL switch but does take into account the setting of the channel's fader − and also its pan control and any effects added. This means that you can listen on the headphones to that channel's contribution to the main mix exactly as it sounds in the main mix. By having several channels' SIP switches activated at the same time, you can compare the contribution of those channels without being distracted by the signals on the other channels − effectively listening to just a "part of the mix". Very useful.
  • Or it may be a "destructive solo" switch. These are usually found only on recording mixers, rather than live sound mixers. What it does is to kill the sound from all other channels, in the main mix (not just on the headphones, like a SIP switch does) − useful during sound checks but not something to press by accident during a live performance!

Mute switch

This switch removes the channel's signal from the main mix, without having to disturb the position of the channel's fader. It may additionally silence all the auxiliary sends of that channel, or none of them, or just the post-fade ones. It may or may not disable the PFL facility on that channel (it usually doesn't, which is very handy). Switches may also be provided to assign the channel to one or more mute groups.

VCA assign switches

These switches are found only on mixers that are equipped with VCA groups. They select which VCA group(s) the channel is assigned to. The level of that channel in all post-fade mixes (main and auxiliary) will be affected by the VCA group fader of every VCA group to which the channel is assigned.

Pan control

In a stereo set-up, this control determines the position of the channel's signal in the stereo sound image of the main mix. When in its central position, an equal amount of the signal is sent to the Left and Right sides of the main mix, so the channel's signal appears to be in the centre of the stereo image. As it is rotated further to the left (i.e. anticlockwise) of centre, progressively less of the signal is sent to the Right side of the main mix than to the Left, so the channel's signal appears to be increasingly left-of-centre in the stereo image. Similarly, as it is rotated further to the right (i.e. clockwise) of centre, progressively less of the signal is sent to the Left side of the main mix than to the Right, so the channel's signal appears to be increasingly right-of-centre in the stereo image.

On a stereo channel, there is no distribution of the input signal between the Left and Right sides of the main mix in the way that there is with a normal (i.e. mono) channel, because the channel has two input signals, not just one. Rather, the Left signal of the channel's input is routed only to the Left side of the main mix, and the Right signal only to the Right side. The pan control of a stereo channel functions as a balance control, allowing for compensation to be made for any difference in the level of the Left and Right signals of the stereo source.

Routing switches

These switches select which mix bus, or buses, the channel's signal is to be mixed into. Switches may be provided to route the signal direct to the main mix and/or to the selected audio group bus(es). Separate main mix routing switches may be provided for 'Left-Right' (stereo) and 'Centre' (mono) mixes (see LCR (1)). The audio group routing switches may also be referred to as group assign switches (not to be confused with VCA assign switches). Normally, each audio group routing switch is labelled with two group numbers, e.g. "3-4". This means that by operating this switch the Left output of the channel is assigned to audio group 3 and its Right output is assigned to audio group 4. Note that in the USA 'routing' is pronounced "r-owt-ing", whilst in the UK it is pronounced "r-oot-ing".

Fader

This sets the relative level of the channel's signal in the main mix. It also affects the level of the channel in any post-fade auxiliary mixes.


Master Section
Group, matrix and output facilities

Facilities to be found in this section may include:

  • Audio group faders
  • VCA group faders
  • Master faders
  • Matrix section
  • Level indicators
  • Graphic equaliser
  • Onboard effects

Each audio group fader (or pair of group faders − one for the Left channel and one for the Right) provides control over the overall level of the mix of signals that have been routed to that pair of audio groups by channel routing switches.

Each VCA group fader provides proportional control over the post-fade level of all the channels that have been assigned to that VCA group by channel VCA assign switches.

The matrix section of the mixer is like a mini-mixer in its own right, enabling different mixes to be created for different purposes, such as for monitors or for additional locations within a venue. Rather than taking inputs from each individual channel, it takes them from the mix-buses of the main part of the mixer (generally including the auxiliary mix buses), and allows the creation of several different 'super-mixes' from those sources. For example, in the main section of the mixer an auxiliary mix may be created for lead vocals, another for backing vocals, and further ones for drums, guitars, keyboards, etc.. The matrix section can then be used to set the level of each of those mixes within each of several different matrix mixes. (When a mixer has no matrix section, a separate smaller mixer can be used for this purpose if required, taking its inputs from the Aux outputs of the main mixer. The Aux mixes on that smaller mixer then become the matrix mixes.)

Master faders are usually provided for each output of the mixer, e.g. Left/Right/Main, Centre/Mono, and each auxiliary mix output. The auxiliary masters are usually rotary types, rather than slider controls. The Main fader provides control over the level of the main mix, including any effects that are provided by, or routed through, the mixer.

Level indicators keep the operator informed how close the signal levels are to overload.

A graphic equaliser may be of assistance in compensating for deficiencies in the acoustics of a room and/or speakers. However, mixers rarely incorporate the full 31-band equalisers that are needed for precise control.

Onboard effects processors are generally incorporated only on the smaller-sized analogue mixers and on digital mixers; the latter also usually include dynamics processing facilities. The most common types of effects are described on the Effects page. Signals are routed to the onboard effects processor(s) through one or more Effect send controls on the channel strips. The effects section of the mixer will usually include a means of indicating the total signal level being sent to it, or at least a means of indicating that this level is approaching overload. Each processor will be provided with a means to select the required effect, and often one or more controls to vary the effect parameters. It may also have routing switches to select the destination(s) of the processed signal, e.g. main mix, auxiliary mixes, etc.


Amplification Facilities

The differences in the power amplification facilities of the three main mixing configurations (mentioned at the start of this page) are described under the following three sub-headings.

Mixer (or mixing desk)

This incorporates no amplification facilities, and therefore requires the use of one or more separate power amplifiers. This however has the advantage that the amplifiers can be located close to the speakers (so avoiding power loss in long runs of speaker cable), while the mixer can be located in the best position for the operator to hear the sound (ideally somewhere near the centre of the audience area, but often at the back of the audience area).

Powered mixer (or powered mixing desk)

This is a mixer which incorporates one or more power amplifiers − it is most useful in smaller systems, where the amplification requirements are modest. It usually has most of the facilities that would be found on an unpowered mixer, and is intended for operation by a sound technician during the performance; it generally has a horizontal (or nearly horizontal) control panel, just like an unpowered mixer. It often incorporates a graphic equaliser (usually with a very limited number or frequency bands), and sometimes includes a more comprehensive range of effects than might be found on an unpowered mixer.

The main advantage over an unpowered mixer is the speed of assembling a system, as the need to connect up separate amplifiers is avoided (or at least reduced). The main disadvantage is that the speaker cables must run the distance from the mixer's location to the speakers, so this arrangement is generally only used when this is a fairly short distance (say, less than 25 metres). Otherwise, the resistance of the speaker cables is likely to be a cause of a considerable loss in output power, and the sound quality may also be affected. Use of a powered multicore with a powered mixer makes for even simpler cabling, as only one cable need be run between the mixer and the stage.

(Note: Since the increased availability of powered speakers, this main advantage of powered mixers is largely demolished. This is because powered speakers also avoid the need to connect up separate amplifiers, but without introducing the need for long speaker cables.)

Mixer-amplifier

This is a power amplifier which incorporates mixing facilities (usually rather limited). A mixer-amplifier is usually used on-stage, and is intended for setting up prior to a performance and then being essentially left alone (see the Introduction for Performers page). It rarely includes the more sophisticated mixing facilities such as Solo, large numbers of channels or comprehensive grouping. It is often box-shaped (rather than the flat "desk" shape usual for mixers), the controls being located on the vertical front panel. It frequently incorporates a graphic equaliser, usually with a very limited number of frequency bands, and sometimes includes basic effects such as reverb and/or echo.


Choosing a Mixer

Selecting the right mixer for your needs can be a challenging task − not least because some descriptions can be misleading. For example, the number of channels quoted often (especially on the smaller mixers) does not necessarily indicate the number of mics that can be connected. The figure can include line-input-only channels (of which the stereo ones are often counted as 2 channels each) and sometimes even includes the effect return inputs.

To help you through the jungle, here are some questions to ask yourself before you start looking at specific models or talking to a sales-person. If you can come to at least some idea of the answers to these, the job will be made a whole lot easier. These are brief items only − for further information refer to the details given earlier on this page.

  1. Do you need a digital mixer?
  2. Do you need a powered mixer? (See Amplification Facilities for further information.)
  3. How many mic channels do you need? − That is, channels which have a balanced low-impedance input connection (usually via an XLR connector) that is suitable for connection of a microphone or a DI box.
  4. Do you need pad switches that work on the balanced inputs? (See also the next question.)
  5. How many additional channels do you need, for line-input use only? (Remember that, on most smaller mixers, 'mic' channels are also equipped with jack connectors to accept a line input, but 'line-input' channels cannot accept a microphone input. However, some larger mixers do not have line inputs at all: line-level sources are connected to the mic inputs and the relevant pad switches are engaged.)
  6. Of the line-input channels, how many stereo channels do you need (if any)? If you are running a stereo system, these channels are very useful for playback of stereo pre-recorded material (from CD, MD, DAT, tape, DVD, etc.), and for returns from stereo effects (when you need the facilities of a channel, such as full EQ and/or routing, or when there are insufficient dedicated effect returns provided − see a later question).
  7. Do you need channel Split Output connections?
  8. Do you need channel Direct Output connections?
  9. Do you need channel Insert connections?
  10. What channel EQ facilities do you need?
  11. Do you need channel EQ Bypass switches?
  12. Do you need channel low-cut switches?
  13. Do you need channel phase reversal switches?
  14. Do you need per-channel phantom power switches? Or will phantom power switching in groups of channels suffice − or even just a single overall switch?
  15. Do you need individual channel metering? Or will channel 'Peak' LED indicators and/or 'Level set' indicators suffice? (See Overload or level indication.)
  16. How many pre-fade auxiliary sends do you need, and how many post-fade ones? Would switchable ones be the most suitable arrangement? (See also the effects sends/returns question.)
  17. Do you need aux-mix Insert connections?
  18. Do you need balanced aux-mix outputs? (Note: just because these outputs are via XLR connectors doesn't necessarily mean that they are balanced.)
  19. What PFL/AFL monitoring facilities do you need?
  20. What PFL/AFL metering facilities do you need?
  21. What effect send and return facilities do you need?
  22. Do you need Left-Centre-Right main mix channel routing? (See LCR (1).)
  23. Do you need audio grouping facilities? If so, how many audio groups do you need? (Note that audio groups are usually arranged in pairs.)
  24. Do you need group-mix insert connections?
  25. Do you need balanced group-mix outputs? (Note: just because these outputs are via XLR connectors doesn't necessarily mean that they are balanced.)
  26. Do you need VCA grouping facilities? If so, how many VCA groups do you need?
  27. Do you need mute grouping facilities? If so, how many mute groups do you need?
  28. Do you need on-board effects? If so, how many effects processors do you need and what choice of effects and degree of adjustment of them do you need?
  29. Do you need one or more on-board graphic equalisers? If so, how many frequency bands are required?
  30. Do you need main-mix insert connections?
  31. Do you need balanced main-mix outputs and/or balanced AUX outputs? (Note: just because a mixer has XLR connectors for these outputs doesn't necessarily mean that they are balanced.)
  32. Do you need matrix facilities? If so, how many matrix outputs do you need?
  33. Do you need anything extra? (Such as panel illumination facilities, recording output (what kind?), utility output, digital output(s), dual headphone outputs, sub-bass output, long-travel faders, a flight case, etc. etc.)
  34. Think ahead − how will your requirements be changing over the period that you intend the new mixer to be useful? For example, you might only need 8 mic inputs and 2 auxiliary sends today, but what about in 6 month's time? If buying from new, it may be worth selecting a larger model than you need just at the moment, to avoid the initial depreciation costs that would be incurred if you have to sell it after only a short period of use.
  35. What about quality? This divides into two essentially separate aspects:
    • Sound quality. These days, the sound quality of all but the very cheapest mixers (of the professional or semi-professional variety) will be adequate for most general mixing applications. All mixers from the well-known manufacturers such as Mackie, SoundCraft, etc. etc. are able to give very good results (providing, of course, that other parts of the overall system are not letting you down). The 'law of diminishing returns' applies: you will have to pay a very significant extra amount for that last extra little bit of sound quality. Do you really need it?
    • Build quality. This relates to how long the mixer is likely to last before problems start to develop (usually with the controls, switches and connectors), and how well it will stand up to the rigours of use, transport and storage. This is where the better quality units really score, and is well-worth paying for.

A final important word of advice: don't let impressive features, a 'bargain price' or persuasive sales-talk distract you into selecting a mixer that doesn't satisfy your basic requirements.


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This page last updated 20-Mar-2016.