Buying Advice - PA Audio Mixers
(Please note: the following information is for
guidance purposes, and you should always verify that a piece of gear
is suited to your specific needs.)
"It s all in the mix" as they say, so getting the right
mixer for the job makes will help you make the best impression
PA audio mixers come in a huge range of sizes and for differing audio applications.
As with the PA speaker article, we are looking at PA mixers for live bands, duos
and solo artists performing in pubs, clubs, smaller music venues
and function type events that include hotel function rooms,
community halls, marquee tents etc. As with all these things,
purpose is the key dictating factor in what feature set is most suitable.
Here, we go through the basic features as you would find them set
out on a typical mixer input channel and output section so that you
can get an idea of what features you need for your situation.
At the end of this article you will find
a checklist you can go through to ascertain what type and size of
mixer would meet your requirements.
Mixer input channels
For each microphone you use you need an XLR type
balanced input. Some keyboards, sound modules and computer audio interface units
have XLR balanced outputs too, as do line-outs from some
guitar & bass amps, DI boxes, amp simulation units etc., so take all of this
into account when calculating the number of XLR inputs you need.
This sets the input level of the signal into the channel.
When set too low you will add noise to the mix, when set too high
the channel will overload and distort the signal. On all but the
most basic mixers there will be an LED commonly labelled "Clip",
"Level" or "Signal" that will flash (usually red)
when the gain level is set too high.
Line level inputs
Line level inputs are used mostly for
keyboards & sound modules with jack outputs and acoustic
guitars with built-in pickups. Line level inputs are quite often
stereo channels with left and right inputs for stereo
signals such as a keyboard etc. into one channel allowing easier control
of stereo signal sources. These channels feature a balance control rather than
a pan control for left/right positioning in the mix.
Although most mixers also have an additional dedicated stereo input for a CD or MP3 player
which can be useful for backing tracks etc., these don't usually
offer any control over the sound, so if you need to have backing
tracks that you have some control over, you'll need a mixer that has either
two extra line level channels for the left & right signals
or at least one stereo input channel.
EQ - Equalization
EQ is mixer terminology for tone controls. The most
basic mixer will have "lo" and "hi" EQ controls,
the equivalent of bass and treble controls on a hi-fi, which is
just enough to give some basic adjustment to your sound.
However, you don't have to go far up the price ladder to find mixers
with a "mid" EQ control as well, which could also be "sweepable"
or "semi-parametric" (see below).
Sweepable or semi-parametric EQ
Some smaller mixers feature a sweepable EQ section,
sometimes called "semi-parametric EQ".
This lets you select the frequency at which the cut or boost
is applied. On budget mixers this is usually just on the "mid"
frequency control, more featured mixers may have a hi-mid and
a lo-mid with sweep, high end mixers may have several
sweepable EQ controls that also offer bandwidth control for each frequency
(this is full parametric EQ). Sweepable EQ is particularly
useful for eliminating feedback from a specific source and for tuning
the EQ for different voices.
AUX sends allow you to send controlled amounts of
signal from each channel for a specific purpose, for example
creating monitor mixes, adding effects or for recording a different
mix to the main mix. More featured mixers allow you to select whether
the signal being sent to the AUX bus is "pre" or
"post" fader. The "pre" setting
is useful for creating monitor mixes as the monitor volume isn't changed
when the main mix fader is adjusted. The "post" setting is useful
for adding effects as when the main fader is adjusted, it also controls the
effect volume simultaneously, so for example, you won'tt end up with a
vocal swimming in reverb as you reduce its volume.
If you are using monitors as well as out-front speakers,
you will want one "AUX send" bus for each different monitor mix you require.
AUX busses give you control over the level of each channel going
to each monitor sepeartely, allowing a different mix in each monitor.
(i.e. if you are a duo and need two different monitor mixes you will need
two "AUX sends" for this purpose).
Using effects processors
If you want to add signal processing to multiple
channels to improve your sound you will need an additional AUX send
for each external effects processssor in your set-up.
(See "Insert points" below also.)
Built-in effects processors - FX
Many smaller mixers also have a basic on-board
digital effects processor which usually has a dedicated "FX send"
knob on each channel to control the amount of effect applied to that
channel's signal (this is basically a dedicated
AUX send specifically for this purpose). These effects processors typically
have a range of different effect types but with the limitation
that you can only use one at a time in the mix.
As mentioned above, to use external effects processors you will
need AUX busses to control their levels. In some situations however,
you only want to apply processing to one signal source, i.e.
compression and gating on a vocal mic. In this instance you will
want a mixer that features "insert" points on at least
some of the channels. These usually take the form of stereo jack
sockets that allow you to insert your processor into the signal
path at the optimum point to maintain signal quality, using a special
lead that splits the stereo cable into two mono cables, one to, and
one from the processor.
For a smaller set-up a powered mixer might be the ideal solution.
These mixers have an integrated power-amp making set up very simple,
just plug your speakers in and away you go.
But there are some disadvantages with powered mixers too. Firstly, weight;
they can be quite heavy, especially when in a decent flight case. Secondly,
if either the mixer or the power amp fails, then you have lost the use of both in one go.
Keeping your mixer and power-amps separate limits the impact
of component failure and allows more flexibility in upgrading or
adapting your set-up to different situations.
Desk or box form?
Powered mixers come in two main types, desk form
or in a box similar to an amp head. Generally, the box type are
simpler mixers more suited to smaller situations. The main advantage
thay have is that they are far more robust when being moved around
without a case, they will fare much better in the back of a van or car with
loads of other gear. The desk style mixers can have many more
channels and other features that the smaller front panel of box
style mixers can't accommodate, the most obvious being volume
faders rather than knobs which apart from greater audio quality make it
much easier to control and see where the channel volumes are set.
How many XLR type inputs do you need?
How many line type inputs do you need & should some of these be stereo channels?
How many different monitor mixes do you want?
How many different types of signal processing do you want to use simultaneously?
Do you need insert points for compressors and/or noise gates?
Do you want on-board effects?
Still to come:
Output section info
On-board graphic equalizers