Allen&Heath MixWizard3 14-4-2 - Manual (Page 11)

WZ 3 14:4:2 User Guide 11
Audio Connections
The MixWizard uses professional grade 3 pin XLR, 1/4" TRS (3 pole) jack and RCA phono
sockets. To ensure best performance, we recommend that you use high quality audio
cables and connectors, and take time to check for reliable and accurate cable assembly. It
is well known that most audio system problems are due to faulty or sub standard
interconnecting leads. The following plugs may be used to connect audio to the console:
Avoid reversing + and - on balanced connections as this will result in reversed polarity (out
of phase) signals which may cause signal cancellation effects.
Where long cables runs are required, balanced interconnections should be used. However,
line level interconnections between more affordable 2-wire (signal, ground) unbalanced
equipment and the console are unlikely to cause problems if the cables are kept shorter than
10 meters or so. Refer to the wiring diagrams on the opposite page.
Dealing with Ground Loops, Buzz and Interference
For optimum performance all audio signals should be referenced to a solid, noise-free
ground (earth) point, frequently referred to as the ‘star point’ or ‘clean earth’.
A ground loop is created when potential differences exist between grounds at different
points in the system, and the signal has more than one path to ground. In most cases
ground loops do not result in audible problems. Should you experience hum or buzz caused
by a ground loop, check first that each piece of equipment has its own separate path to
ground. If so, operate ground lift switches on connected equipment in accordance with the
instruction manuals. Alternatively disconnect the cable screen at the destination end only.
This breaks the offending loop while keeping the signal shielding down the length of cable.
WARNING For operator safety do not remove the ground (earth) connection
in the power lead of the console or connected equipment.
To avoid interference pickup keep audio cables away from mains power units and cables,
thyristor dimmer units, computer equipment and mobile phones. Where this cannot be
avoided, cross the cables at right angles to minimise interference
A note about balanced connections
A differentially balanced connection has two signal wires, signal + (hot) and signal - (cold)
and a shield. The signal source generates positive going polarity down the + wire and
negative polarity down the – wire. The destination input stage accepts the + signal on its
non-inverting (+) input pin, but it inverts the – signal, adding it to the + signal. The result is
that the wanted signal is boosted. Now examine what happens when unwanted interference
(hum and noise) is induced into the cable. The noise is induced equally and with the same
polarity into both wires. At the destination input the – wire signal gets inverted and added to
the + signal. Because the polarity is the same on both input wires the noise cancels itself
out at this input. For this interference rejection to work it is important that the source, the
cable and the destination input are all balanced. Balancing provides greatest advantage
with low level signals such as those produced by microphones.
An impedance balanced output provides similar interference rejection, but without the signal
drive on the - wire. It does not generate a negative polarity signal at its – output. Instead,
the – wire has no signal but is held at the same impedance as the + wire. This means that
both wires pick up the noise equally resulting in cancellation as described above.