Alesis Car Amplifier MULTIMIX User Manual

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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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