24
The extension cord used to supply power to
the “amp rack” should have a third wire safety
ground to avoid presenting a safety hazard. We
also recommend using a cord that has conduc-
tors large enough to avoid severely limiting the
amplifier’s ability to supply high currents on
transients. With this in mind, we recommend
using the following for a fully loaded 15A service:
Extension Cord Length Wire Gauge
25 feet
14 AWG
50 feet
12 AWG
100 feet
10 AWG
Power amplifiers can have
momentary peak current re-
quirements many times
above the nominal average
current draw. According to
Ohm’s Law, the greater the resistance of the
linecord, the more input power is lost between
the AC outlet and the power amplifier (across
the linecord). And to further aggravate the
matter, this relationship is nonlinear. The
amount of power lost across the linecord in-
creases exponentially as the current demand
increases. So if the current demand doubles
momentarily because of an exceptionally loud
bass note, the amount of AC input power lost
across the linecord increases four times. You
can see that it is very important to keep the
resistance of the linecord to a minimum. That’s
why we recommend using a heavy gauge exten-
sion cord (like 14 gauge or bigger). Remember,
wire gets thicker as the gauge number gets
smaller (10 gauge is thicker than 14 gauge).
AC Power Distribution
The majority of AC outlets encountered in
homes and clubs are served by a 240VAC cen-
ter-tapped service entrance transformer. This
provides two phases of AC power on either side
of the center tap at 120V each.
In order to minimize ground loops, the safety
grounds for all the outlets should be connected
to a common (“star”) grounding point, and the
distance between the outlets and the common
grounding point should be as short as possible.
If lighting is used in a show, it is preferable
to power the lights from one leg of the service,
and power the audio equipment from the other
leg. This will help minimize noise from the
lights coupling into the audio (particularly if
SCRs are used).
When setting up for a show, oftentimes you
are plugging into an AC power distribution sys-
tem you know nothing about. You may even be
faced with 2-wire outlets that are missing the
third safety ground pin. It’s a good idea to have
a three-wire AC outlet tester in your toolbox so
you can check the outlets yourself to make
sure they are wired correctly. These testers will
tell you if the polarity of the hot and neutral
wires is reversed and if the safety ground is dis-
connected.
Don’t use an outlet if it is wired
improperly!
This is to protect yourself as well
as your equipment.
If you find that you must plug into a two-
wire outlet, you will need to use a two-wire to
three-wire adapter (cheater plug). These come
with a metal tab that you put underneath the
center screw that holds the AC outlet faceplate
in place. This center screw should be grounded.
You can check it by connecting the adapter to
the outlet and then plugging in your AC outlet
tester.
INPUT WIRING
Use a high-quality 3-conductor shielded
cable to connect the signal between the signal
source (mixing console, equalizer, etc.) and
the balanced inputs to the amplifier. If you’re
using the unbalanced inputs, use a high-quality
2-conductor shielded cable. Your Mackie
Dealer can recommend a suitable cable for
your application.
If you want to build your own cables, refer to
the “Connectors” section in
Appendix C
.
OUTPUT WIRING
Use heavy gauge, stranded wire for connect-
ing speakers to the M•1200/M•1400 amplifier’s
SPEAKER OUTPUT
terminals. As the dis-
tance between the amplifier and the speakers
increases, the thickness of the wire should also
increase. Speaker wire has resistance, and
when electricity passes through a resistor,
power is lost. The thicker the wire, the less re-
sistance it offers, and the more power actually
gets to the speakers.
TRANSFORMER
120V
120V
240V
HIGH VOLTAGE POWER LINE
EARTH
GROUND
(NEUTRAL)
PRIMARY
WINDING
SECONDARY
WINDING
240V Center-Tapped Secondary
