Handleiding Alesis MultiMix 12R (pagina 43 van 62) (Engels)

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Operating Instructions

Multimix 12R Reference Manual 33

Avoiding noise

If the Multimix 12R is being run as suggested above, the noise of the

mixer itself will not be a significant factor in the noise level of your

system, since the Multimix 12R generates much less noise than the

typical source does. Noise is present in every system, analog or

digital; as operator it's your job to run the system (and particularly the

mixer) so that the noise isn't amplified unnecessarily. But the Multimix

12R cannot remove noise that is already there. If a low-level source

with a poor signal-to-noise ratio is amplified by the TRIM and channel

fader, the noise will be amplified with the desired signal. A system is

no quieter than its noisiest component.

System noise (ground loops, hum, induced

noise)

In today’s studio, with dozens of different pieces of equipment and

computers, there are many opportunities for ground loop problems to

occur. These show up as hums, buzzes or sometimes radio reception

and can occur if a piece of equipment “sees” two or more different

paths to ground, with one of the paths going down a signal line. While

there are methods that can virtually eliminate ground loops and stray

radio frequency interference, such as installing a separate power

source just for the sound system, most problems are easier to solve.

Here are some basic techniques that you should use to keep stray

hums and buzzes to a minimum.

❿ KEEP ALL ELECTRONICS OF THE SOUND SYSTEM ON THE

SAME AC ELECTRICAL CIRCUIT. Most 60-cycle hums happen

because different components of a sound system are plugged into

outlets of different AC circuits. If any noise-generating devices

such as air conditioners, refrigerators, neon lights, etc., are already

plugged into one of these circuits, you then have a perfect

condition for stray buzzes as well. Since most electronic devices of

a sound system don’t require very much power (except for power

amplifiers), it’s usually safe to run a multi-outlet box or two from a

SINGLE wall outlet and plug in all of the components of your

system there.

❡ KEEP AUDIO WIRING AS FAR AWAY FROM AC WIRING AS

POSSIBLE. Many hums come from audio cabling being too near

AC wiring or the power transformers ("wall warts") used by

equipment requiring an external supply. If a hum occurs, try

moving the audio wiring around to see if the hum ceases or

diminishes. If it’s not possible to separate the audio and AC wiring,

make sure that the audio wires don’t run parallel to any AC wire

(they should only cross at right angles, if possible).

Operating Instructions

① TO ELIMINATE HUM IF THE ABOVE HAS FAILED:

A) Disconnect the power from all outboard devices and tape

machines except for the mixer and control room monitor power

amp.

B) Plug in each tape machine and outboard effect device one at a

time. If possible, flip the polarity of the plug of each device

(turn it around in the socket) until the quietest position is found.

C) Make sure that all of the audio cables are in good working

order. Cables with a detached ground wire will cause a very

loud hum!!

D) Keep all cables as short as possible, especially in unbalanced

circuits.

If these basic experiments don’t uncover the source of the problem,

consult your dealer or a technician trained in proper studio grounding

techniques. In some cases, a “star grounding” scheme must be used,

with the mixer at the center of the star providing the shield ground on

telescoping shields, which do NOT connect to the chassis ground of

other equipment in the system.

Note that the Multimix 12R is grounded through its AC cable. Signal

ground is connected to chassis ground, which in turn may be

grounded again by the rack rails. But since almost all of the inputs and

outputs of the Multimix 12R are balanced, the ground does not have to

be made part of the signal path unless you are connecting to

unbalanced equipment. If the Multimix 12R is mounted in a metal

rack, the mixer shares a common ground with the other equipment in

the same rack. In some cases (such as a star ground scheme), you may

need to use nonconductive rack rails or rack isolators to avoid ground

loops.

To avoid the possibility of electric shock, never defeat the safety ground found

on other equipment in the system. When in doubt about proper electrical

grounding schemes or the power to your system, consult a qualified, licensed

electrician.

Telescoping shield: In fixed installations using balanced outputs

where the amplifier is more than 20' from the mixer and on a different

AC power panel, it may be advisable to disconnect (or "float") the

shield at the amplifier end. This keeps the ground potential of the amp

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