Bedienungsanleitung Allen-Heath GL2800 (Seite 13 von 42) (Englisch)

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GL2800 User Guide 13

Audio Connections

The GL2800 uses professional grade 3 pin XLR and 1/4" TRS (3 pole) jack 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 metres 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 power supply unit or connected equipment.

To avoid interference pickup keep audio cables away from mains power units and cables,

lighting 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 not as much

maximum drive capability as the differentially balanced output, typically +20dBu versus

+26dBu. 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 the advantage of noise cancellation as described

above when connected to a balanced input stage.

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