Mackie 406M, Owner'S Manual

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26
GENERAL PRECAUTIONS AND CONSIDERATIONS
NEVER bypass the AC
plug’s ground pin. This is
dangerous!
It is recommended that a stiff (robust)
supply of AC power be used because the amp-
lifiers place high momentary current demands
on the AC line. The more power that is available
on the line, the louder the amplifiers will play
and the more peak output power will be
available for cleaner, punchier bass. A suspected
problem of “poor bass performance” is often
caused by a weak AC supply to the amplifiers.
AC Power Distribution
The majority of AC outlets encountered in
homes and clubs (in the U.S.) are served by a
240VAC center-tapped service entrance trans-
former. This provides two phases of AC power
on either side of the center tap, at 120V each.
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, or light-dimmer switches, are used).
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.
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
disconnected.
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
must be grounded. You can
check it by connecting the adapter to the outlet
and then plugging in your handy-dandy AC
outlet tester.
Thermal Considerations
The PPM Series Powered Mixers contain
two powerful built-in amplifiers. All amplifiers
produce heat, and the more power it produces,
the more heat is produced. It is important to
dissipate the heat away from an amplifier as
quickly as possible. This results in increased
reliability and longevity for the amplifier. That’s
what the massive die-cast aluminum heatsink
on the rear panel is for.
Heat from the output transistors is coupled
to the heatsink, which draws the heat away
from the transistors and out to the heatsink
fins. Cool air is drawn through the fins due to a
process called convection, warming the air and
taking the heat away.
In order for this convection cooling to work
efficiently, it is important to provide adequate
airspace behind the mixer. We recommend
leaving at least six inches of air space behind
the mixer when you set it up.
In the unlikely event of the amplifier overheat-
ing, a thermal switch activates and shuts down
the mixer until it cools off. Double-check the
speaker load on each amplifier and make sure
that the total impedance of each load is 2 ohms or
greater. If the load impedance drops below 2 ohms,
it can cause the amplifier to overheat. If the load
impedance is okay, overheating may be caused by
excessive ambient temperature (the room’s too
hot!). Try using a fan aimed at the heatsink to
move air through the fins faster.
AC Power Considerations
Be sure the PPM Series Powered Mixer is
plugged into an outlet that is able to supply the
correct voltage specified for your model. If the
voltage should drop below 97% of the specified
line voltage, the built-in amplifiers will no
longer be able to supply rated power. (They will
continue to operate down to 75% of the rated
voltage, but won’t reach full power, resulting in
lower headroom.)
Under typical conditions, reproducing music
where musical peaks are just below clipping,
the powered mixers draw the following average
currents:
Average Current
Amplifier Loading 406M, 408M, 408S 808M, 808S
8
Ω
per side 2.0 A 2.2 A
4
Ω
per side 3.1 A 3.3 A
2
Ω
per side 4.8 A 5.0 A
(Above current based on using 120V AC line)