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Mixers in PA Systems

The Mixer is the heart of the system. It is where the most important processing decisions - gain structure, channel EQ and mix - take place.

What it is

A mixer is a device that combines two or more separate signals. Mixers range from a couple of variable resistors with knobs to the big and complicated-looking consoles used in the largest multi-performance events.

What it does

Although - at its most basic level - a mixer simply combines a set of individual signals, most mixers do quite a lot of other things too. In most applications, signals are generated from a range of sources (instrument pickups & instrument amplifiers, electronic instruments, and microphones covering everything from soft voices to loud snare drums). It is the mixer's job to combine these so that they work together effectively. The channel controls of a typical mixer may include any or all of the following (not necessarily in the same order):

1.  Mic/Line. Most mixers have a switch to select between the mic (XLR) and line (1/4" Jack) inputs. Inserting a jack plug disconnects the XLR socket on some mixers. On others (e.g. most Allen & Heath mixers), the Mic/Line switch doubles as a Pad switch (see below).

2.  Gain. The signals arriving at the mixer vary substantially in level. The gain control adjusts the level of the signal so that it is appropriate for the mixer's circuitry. Some mixers also have a Pad switch, to reduce the level of signals that are too big when they arrive. If the mixer has a Phase Reverse (Ø) switch it will usually be near this, at the top of the channel strip.

3.  EQ. Any signal arriving at the mixer may need some tonal adjustment. Even the most basic mixers normally have some kind of channel EQ facility. The following varieties are common:

a.  High and Low (fixed frequency)

b.  High, Mid and Low (fixed frequency)

c.  High and Low fixed frequency, variable frequency ("sweepable" or "semi-parametric") Mid

d.  High and Low fixed frequency, two variable frequency ("sweepable" or "semi-parametric") Mids

e.  High and Low fixed frequency, two variable frequency and variable bandwidth ("fully parametric") Mids

f.   Four-section fully parametric EQ.

Generally (but not universally), the highest and lowest frequency bands are shelving EQs, while the middle bands are peaking EQs.

There may also be a High Pass (or Low Cut) switch - typically at the top of the channel strip, but sometimes in the EQ section - to remove the lowest frequencies from signals that don't need low-frequency amplification. In most live applications, only kick drum (and occasionally low toms), bass guitar and keyboard will need any amplification below 100Hz or so. The High Pass switch also helps to reduce any stage rumble a mic might pick up.

Many mixers also have a switch on each channel - useful for comparison - to include or bypass the channel EQ. Before using the EQ on any channel, check that it is switched in! If you are moving the EQ level control beyond the 9 o'clock or 3 o'clock positions and cannot hear any effect, check this first. If the EQ is switched in & you still can't hear the effect, check you have the right channel!

4.  Monitor Send. This enables a separate and different mix to be sent to on-stage monitors. Budget mixers (including those with built-in effects) often have a dedicated Monitor control on each channel. Most other mixers have an Auxiliary Send (Aux) section on each channel, with Prefade and Postfade sends (often this is switchable, either for each send individually, or for one or more groups of sends). The number of Auxiliary sends typically varies from two (often one Pre, one Post) to eight or more (often individually switchable between Pre and Post). Monitor sends are usually Prefade so that movement of the channel fader does not affect the monitor levels. Each prefade send allows a further independent monitor mix. Sometimes there is a choice of whether the Prefade send is Pre- or Post-EQ. If it is Prefade and Pre-EQ, changes to the channel EQ will not affect the monitor sound.

5.  Effects Send. Some channels may need an effect (e.g. reverb) where others do not, or some may need more and others less. Again, budget mixers may have a dedicated Effects control, while others use sends in the Auxiliary section. Where this is the case, effects sends are normally Postfade: you want the amount of reverb to rise and fall with the channel output level. Postfade means the signal is taken after the channel fader, and so is subject to the fader position.

6.  Pan. Most modern mixers (even the little ones) are stereo, and the Pan control positions the signal in the stereo soundstage. On desks which have groups (see below), this will also apportion the signal between pairs (1-2, 3-4, etc.) of groups.

7.  Channel Level. On mini budget mixers this may be a knob. On most mixers, however, it will be a Fader (sometimes 60mm on smaller budget desks, but usually 100mm). This sets the final level of the channel signal in relation to the level of other channels. On smaller mixers, the signal (as determined by the channel fader) will go straight to the main mix. Bigger mixers allow a subgroup of signals to be controlled by another separate fader. There are two basic ways of achieving this:

a.  Groups. Routing buttons (usually alongside the fader, at the bottom of the channel strip) allow the signal to be sent either to the main mix alone, or to any of a number of subgroups (usually four or eight). The subgroups can be used to feed separate submix amplifiers (or recording equipment), but will usually themselves be routed to the main mix, so that the level of a group of signals - e.g. backing vocals - can be controlled by the group fader. The signal will not be routed directly to the main mix, but to the group (and from there to the main mix). The groups are always configured in pairs, using the channel's Pan control to assign the signal to odd (1, 3, 5, 7) or even (2, 4, 6, 8) groups. For stereo operation this means that a pair of groups is needed for each sub-mix, so that an 8-group desk can only provide four stereo sub-mixes.

b.  VCAs. Voltage Controlled Amplifiers. The overall level of any channel strip can be assigned to an independent fader (again, there are commonly four or eight of these, usually assigned by buttons next to the channel strip). This means that the output level of a group of channels can be controlled by a single fader. However, the signal can now be routed directly to the main mix (with its position in the stereo soundstage maintained). The VCA fader is not part of the signal path (instead, it controls the DC voltage governing the gain of an amplifier stage), and because there are less components in the signal path, VCA desks are inherently less noisy. VCA desks usually have VCAs as well as groups, not instead of them. For a more comprehensive description, see Allen & Heath's (pdf) booklet: Mixing Live With VCAs.

8.  Mute/Channel On. Most mixers have a switch on each channel (sometimes at the top of the channel strip, but more commonly at the bottom) that either mutes (i.e. disables) the channel, or switches it on (enables it). On some desks the channel is on by default and is silenced by the Mute switch. On other desks, the channel is off by default, and needs to be switched On. On some mixers the Mute/Channel On switch has no effect on the channel's auxiliary sends (this arrangement is more common in studio mixers). On mixers with Groups or VCAs, each Group or VCA usually also has its own mute switch, and on many mixers the main outputs are mutable too.

9.  Mute Group. Some mixers allow you to assign any channel to one of a number of Mute Group buttons. This means you can mute (or un-mute) a group of channels with a single button. Usually the selectors for mute group assignment are in the same area as the other routing buttons. However, on desks with a lot of functions, there is generally an Assign button in the master section, allowing a single channel switch to cover a number of separate functions. This helps to reduce the overall number of buttons on each channel strip (which - with 8 or more groups, VCAs and mute groups - would otherwise be excessive).

10.  PFL/AFL. Pre Fade Listen and/or After Fade Listen. This switch routes the signal to the headphones, and usually also to one or more of the meters in the master section. PFL usually monitors the signal after the EQ section but before the channel fader. AFL monitors the signal after the channel fader (to give an idea of the level in the mix). AFL may also be called Solo (or sometimes Solo In Place or SIP), in which case the signal will be monitored after the Pan control.

11.  Channel Meters. Even very basic mixers usually have a single LED to indicate channel overload. Others have more than one LED, ranging from two or three (e.g. one for signal present, one for intermediate levels, one for overload) to full channel metering. Some have multi-segment LED meters alongside the channel fader, while others have a Meter Bridge: comprehensive metering from a separate meter section above the channel strips.

12.  Phantom Power. Most (even the cheapest) desks can supply 48V phantom power, and on larger desks this is independently selectable on each channel. Budget desks commonly have a single switch in the master section (or sometimes on the back panel) to select phantom power globally for all channels. Where global phantom applies, correct wiring of mic leads is essential: 48 volts can make quite a mess of a small coil.

Mixers also have a Master section, from which the main mix and/or various sub-mixes can be controlled. Commonly there are Mute (or On/Off) switches on the groups and main faders. The master section will generally include some or all of the following:

1.  Aux Send Masters. The overall level of each auxiliary send will be determined by a control - usually a knob on smaller desks, a fader on larger ones - in the master section. There may be a PFL/AFL button for each.

2.  Aux Returns. Most mixers include extra inputs for effects returns. Often these are stereo, controlled by a single knob or fader. On smaller mixers these may be permanently routed to the main mix. On larger mixers there will usually be buttons to select routing options for each of the returns, and many also include basic EQ facilities - as well as PFL/AFL - on each return. Where extra channels are available, many engineers prefer to use full channels for effects returns.

3.  Matrix. Some desks have a Matrix section, in which different combinations of input can be assigned to different outputs. This allows further submixing options.

4.  VCA Faders. Each fader will control the level of any channel or group assigned to it.

5.  Group Masters. Each group will have its own output from the mixer, but will also have a switch allowing its output to be routed to the main mix. Larger desks may have more comprehensive routing options. Again, PFL/AFL is common.

6.  Main Faders. These control the desk's main mix (Left and Right output). Some desks use a single fader to control the stereo output.

7.  Mono Sum/Sub Out. Some mixers have an extra separate fader providing a summed mono output. This is sometimes used to provide independent control of the sub-bass level.

8.  Talkback. Many desks include an input for a talkback mic. The talkback mic can usually be routed to a choice of outputs, allowing the engineer to talk to individual performers via their monitor.

9.  Headphone/Monitor Section. Most mixers have a headphone socket, enabling the engineer to listen to different parts of the mix. Sometimes the choice of headphone mix is comprehensive, determined by the selection of PFL/AFL buttons on the mixer. On budget mixers, the choice may be more limited, determined by buttons in the master section. Larger mixers usually have a separate output for the engineer's monitor mix, allowing the engineer to hear what is selected through a separate loudspeaker.

How it works

The principle of operation is to combine discrete inputs via buffered outputs (so that changes made to one do not affect any other) to one or more summing amplifiers. As far as the end-user is concerned, however, the principle of operation is less important than the method. Knowing when to put your foot on the brake is what counts, not the fundamentals of hydraulics.

How do you use it?

If all else fails, read the manual!

A full description of how to use a mixer is beyond the scope of this article (there are useful notes on some manufacturers' websites, e.g. the General Guide to Mixing in the Soundcraft Professional Audio Learning Zone). However, there are three main points to observe:

1.  Gain Structure.  Although most mixers have plenty of headroom - often capable of channel levels and summed outputs of +20dBu or more - the capacity of the system as a whole is restricted by whatever point in the signal path has least headroom. Many professional amplifier/loudspeaker systems are designed to produce full output at +4dBu (1.23 volts), which corresponds with 0dB on the main meters of most mixers. Higher levels will simply drive the amplifiers into distortion, risking speaker damage, as well as compromising the sound.

Also, individual channel levels combine to produce a higher overall level. If the peak level in each channel is +20dB, the channel may be (just) below clipping, but the sum of all the channels will be well over +20dB, so the main mix will clip. If you are using 20 channels, using even a modest amount of the headroom in each channel may be enough to overload the mixer's summing amplifiers.

The solution is to set individual channel gains so that the peak channel level is no greater than 0 dB. Where the dynamic range of the signal is too great to make this practical, it is best to use a compressor to compress that signal. If you can't (if you haven't got a spare compressor, for instance), keep any extra gain to a minimum. Aim to set each channel's fader at around the 0dB position (if this isn't indicated, go for somewhere between 2/3 and 3/4 of the way up). Where the mix needs adjustment, reduce the prominent sounds before you resort to boosting the weak ones. However, where the weakest sounds also need to be the most prominent in the mix (this typically applies to vocals), a few extra decibels on the channel fader may be the most practical option once the show is up and running. The most important element of the mix is almost always the vocals. Where backline levels are too high to begin with, try cutting or muting everything except the vocals.

To set the channel gain, select the PFL on that channel (check that no other PFL/AFL buttons have been selected), and adjust the gain until the loudest sounds from that voice or instrument indicate 0dB on the meters. Getting this right will make everything else much easier.

2.  EQ. Boosting or cutting the channel EQ will affect the channel level, so recheck the gain after any change to the channel EQ. Generally, you will get a better sound by reducing frequencies that are too dominant (rather than by boosting those that are weak or missing). Often, however, you may find it easier to identify the frequencies that need to be reduced by boosting the midrange, then sweeping the mid frequency control through its range. Having identified the offending frequency, reduce the level (a small amount of cut - typically with the control at 10-11 o'clock - will usually be enough). If you need more EQ (beyond 9 o'clock or 3 o'clock), you probably need to concentrate on problems outside the mixer (choice and/or position of microphones, pickups, speakers, or even instruments or lead vocalists).

3.  Muting and Routing. If it is muted, it won't make a noise. If none of the channel's routing options is selected, the signal won't go anywhere. If you can't hear anything, check these two things first.

It is also good practice, before you start, to set all knobs and switches to the following defaults:

•  All channels, groups, and the main mix Muted or Off.

•  All phantom power Off.

•  All gain controls, auxiliary sends, and faders to their Minimum (generally fully counter-clockwise, or at the bottom of their travel).

•  All EQ controls Centred (0dB of cut or boost), and - if switchable - switched Out.

•  All PAN controls Centred.

•  All routing buttons Out (i.e. no routing selected).

This will get you into the habit of actively selecting what you want to use and where you want the signal to go. It also means you will have checked the status of every knob, switch, button or fader before you set out, so you won't be surprised by absent signals or sudden ear-splitting howls from the monitors (or from the musicians on stage).

Do you need one?

Simply, yes. Even when your requirements are basically simple and modest, gain control is important.

What sort do you need?

This will depend very much on what you want to use it for. A small "notepad" mixer is often fine for speech or background music. For live performance, make sure the mixer has:

•  Enough channels. Check the specifications carefully, as a lot of small mixers marketed as "16-channel" have only 8 mono channels. The four stereo channels that make the total up to 16 may only accept line-level inputs (and probably won't have XLR sockets), so you may not be able to use it with 10 microphones.

•  Enough auxiliaries. You need as many prefade sends as you need separate monitor mixes. Often one postfade send is enough (unless you need to use more than one effect at the same time), although two or more can be useful.

•  Adequate EQ. High and low are not generally enough, and a fixed-frequency mid isn't much better. Aim for at least three-band with sweepable mid. Four-band with two sweepable mids is preferable.

•  Suitable operating levels. Most domestic audio has a nominal level of −10dBV, so if you want your mixer for home recording, −10dBV will be OK. Professional equipment has a nominal level of +4dBu, and −10dBV won't be enough to obtain full output from your power amplifiers (although there may be enough headroom in the mixer's output levels to make up the difference).

•  Balanced outputs. Most power amplifiers have balanced inputs. In most cases, a balanced signal will provide 6dB more headroom (i.e. 6dB more undistorted output) than an unbalanced signal. A mixer without balanced outputs will turn your 1,000 watt system into a 250 watt system.

•  Phantom Power. Most D.I. boxes and condenser mics need it. If you need to use either, a desk without phantom power will at best be  inconvenient.

Other useful - in many circumstances vital - facilities are:

•  Channel inserts. These allow external equipment (e.g. compressors, noise gates, enhancers) to be inserted in the signal path after the signal has been raised to line level by the channel preamplifier.

•  Group and mix inserts. These allow external equipment (e.g. graphic or parametric equalisers, compressors and/or limiters) to be inserted in the signal path before the group or mix faders, and before the meters. This means the level shown on the meters is the level arriving at your crossovers/amplifiers. If you are using the mixer for monitors and have a listen wedge, this also means that what you hear in your wedge is what the performer hears in his. Fewer mixers have inserts on auxiliary sends, but these can be extremely useful too.

•  Auxiliary Returns. Without them you will need to use extra channels to add reverb or other effects.

•  Channel direct outs. This enables the signal to be routed elsewhere (e.g. to a multitrack recorder or another desk). If the mixer has inserts, these can be used as direct outs (assuming they are not needed as inserts) by using a specially wired adaptor.

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