Mackie SR40.8 Owner's Manual page 77

Do's and Don'ts of Fixed Installations
If you install sound systems into fixed in-
stallations, there are a number of things that
you can do to make your life easier and that
increase the likelihood of the sound system op-
erating in a predictable manner. Even if you
don't do fixed installations, these are good
practices for any sound system, installed.
1. Do use foil-shielded snake cable for long
cable runs. Carefully terminate each end,
minimizing the amount of shielding
removed. Protect the exposed foil shield
with shrink sleeving or PVC sleeving.
Prevent adjacent shields from contacting
each other (electrically). Use insulating
sleeving on the drain wire (the one that
connects to pin 1) to prevent it from
contacting the connector shell.
2. Don't connect the XLR connector shell to
pin 1 of the XLR connector (unless neces-
sary for RFI shielding). Doing so is an
invitation for a ground loop to come visiting.
3. Do ensure that your speaker lines and AC
power lines are physically separated from
your microphone lines.
4. If you use floor pockets, use separate
pockets for inputs and speakers, or put the
connectors on opposite sides of the box so
that they may be shielded separately.
5. If your speaker lines run in the open, they
should be twisted pairs, at least 6 twists per
foot. Otherwise, run the speaker lines in
their own conduit. (Of course, conduit is not
too practical for portable systems, heh-heh.)
6. Minimize the distance between the power
amplifiers and the speakers.
7. Use heavy gauge, stranded wire for speaker
lines. Ideally, the wire resistance should be
less than 6% (0.5dB power loss) of the load
impedance. Remember that the actual run
is twice as long as the physical length of
the run. See below.
Maximum wire run for 0.5dB power loss in feet
wire res. per
gauge 1000 ft.
10 1.00 60
12 1.59 40
14 2.5 24
16 4.02 15
2 4 8
120 240
75 150
48 95
30 60
8. Ensure that the electrician uses the star-
ground system for the safety grounds in
your electrical system. All of the audio
system grounds should terminate at the
same physical point. No other grounds may
come in contact with this ground system.
9. Ensure that the AC power feeds are
connected to the same transformer, and
ideally, the same circuit breaker.
10.Walk outside – look at the horizon, see any
radio towers? Locate potential sources of
RF interference and plan for them before
you begin construction. Know the
frequency, transmitter power, etc. You can
get this information by calling the station.
Remember that many broadcast stations
change antenna coverage pattern and
transmitter power at night.
11.Don't use hardware-store light dimmers.
12.Don't allow for anything other than micro-
phone inputs at stage/altar locations.
Supplying line inputs at these locations is
an invitation for misuse. Make all sources
look like microphones to the console.
13.Balance (or at least impedance balance)
all connections that are remote from the
console's immediate location.
14.If you bridge an amplifier, don't use
phone plugs for speaker connectors.
Grounding
Grounding exists in your audio system for
two reasons: product safety and noise reduc-
tion. The third wire on the power cord exists
for product safety. It provides a low-resistance
path back to the electrical service to protect
the users of the product from electrical shock.
Hopefully, the resistance to ground through
the safety ground (third wire) is lower than
that through the user/operator to ground. If
you remove this connection (by breaking or
cutting the pin off, or by using a 'ground
cheater'), this alternate ground path ceases to
exist, which is a safety hazard.
The metal chassis of the product, the
ground connections provided by the various
connectors, and the shields within your con-
necting cables provide a low potential point for
noise signals. The goal is to provide a lower im-
pedance path to ground for noise signals than
through the signal wiring. Doing so helps mini-
mize hum, buzz, and other extraneous
non-audio signals.
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