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Owner’s Manual
Owner’s Manual
3. PROTECT Indicator
If the PROTECT LEDs are on, the amplifier’s output
section has shut down. That, of course, means you won’t
hear anything until you rectify the situation. Three
things can cause the PROTECT circuit to engage:
1. Powering up the amplifier. A built-in delay circuit
saves your speakers (and ears) from the thumps or
pops that can sometimes occur when powering up a
system. During this three-second delay, the PRO-
TECT LEDs light up.
2. A short circuit (or near short) in either of the
outputs. Both the PROTECT and the SHORT LEDs
light up.
3. The temperature in the amplifier has risen to an
unsafe level. The PROTECT and the OVER TEMP
LEDs light up.
The M-Series amplifier draws its ventilation air in
from the rear and out through the front. The amp needs
plenty of fresh air to stay cool. DO NOT BLOCK THE
VENTILATION PORTS. See “Thermal Considerations” on
page 16 for more details.
Note: In the unlikely event that the power transformer
overheats (in general, this cannot happen during nor-
mal use — but could occur when bench testing at high
power levels for an extended period of time), the PRO-
TECT LED will light, but not the OVER TEMP LED. It
takes from one to two hours for the transformer to cool
to a safe temperature, when normal operation resumes.
4. SHORT Indicator
If this LED comes on, the amplifier has detected a
short circuit (or near short) in either of the outputs,
meaning that the hot (+) and cold (–) speaker wires
are touching, or a speaker itself is shorted out. Such
a condition causes the amplifer to engage its Protect
mode, muting all signals at the amp’s outputs.
This short-circuit LED is a Mackie exclusive and can
save precious minutes of your troubleshooting time.
Without it, you’d still have speaker and amp protection
(via the PROTECT circuit), but you wouldn’t be able
to determine the source of the problem. But with the
SHORT LED, the M-Series amplifier comes right out and
tells you!
WARNING: The SHORT LEDs
indicate an unsafe condition for the
power amplifier. When the short-
circuit protection is activated, the
SHORT LED lights, then the PRO-
TECT LED lights and the SHORT LED turns off. After
about four seconds, the protection circuit turns off and
the amplifier resumes normal operation. If it senses the
shorted condition again, the cycle repeats until you fix
the problem.
Typical causes for a “short” indication would be either
a shorted speaker cable or too many speaker cabinets
connected to the amplifier (i.e., the load impedance
is too low). If a “short” is indicated, please check your
cables. If the cabling is OK, then reduce the number of
cabinets driven by the amplifier.
Note: When using the amplifier in BRIDGE mode, one
or both SHORT LEDs may light under shorted or low im-
pedance conditions. Regardless of whether one or both
LEDs light, it’s an indication of a problem that requires
further investigation.
5. OVER TEMP Indicator
OVER TEMP (short for over-temperature) is another
feature designed to keep your mind at ease. Under
extreme conditions the amplifier may overheat. You may
ask, “What kind of extreme conditions?”
Overheating problems are usually caused by one of
the following situations: improper ventilation, high
ambient temperatures, overdriving the amplifier into
clipping, driving the amplifier hard into low-impedance
loads, frayed or partially shorted speaker cables, or
defective or internally shorted speakers.
The heaviest load the M-Series amplifier can toler-
ate is 2 ohms per channel (4 ohms in bridged mode). If
you’ve got a set of speakers wired in parallel, be sure the
load isn’t adding up to less than 2 ohms. Anything below
2 ohms can cause the SHORT LED to light and trigger
the PROTECT mode, or cause the amplifier to overheat,
which also triggers the PROTECT mode.
Note: Since each channel has its own
temperature sensor and protection
circuit, it is possible for only one
channel to go into PROTECT mode
while the other channel continues to
operate.
As the internal temperature of the amplifier rises, the
fans turn on at 50°C (122°F), and they kick into high
speed at 65°C (149°F). More air moves through the
constant temperature gradient cooling tunnel to remove
additional heat from the output transistors. However, if
the internal temperature of the amplifier should exceed
85°C (185°F), the OVER TEMP LED turns on, and both
PROTECT LEDs shine. The output of the amplifier is
muted — at this point the amplifier is in Protect mode
and remains there until the internal temperature cools
off to a safe level (60°C or 140°F). When this occurs,
the OVER TEMP and PROTECT LEDs turn off, and nor-
mal operation resumes.