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AUDIO INPUT
The MTS-4A presents a 10 kΩ balanced input impedance to a
three-pin XLR connector wired with the following convention:
Pin 1
220 kΩ to chassis and earth ground (ESD clamped)
Pin 2
Signal
Pin 3
Signal
Case
Earth (AC) ground and chassis
Shorting an input connector pin to the case can
form a ground loop and cause hum.
Pins 2 and 3 carry the input as a differential signal; their
polarity can be reversed with the input polarity switch on the
user panel. If the switch is in the up position, pin 2 is hot
relative to pin 3, resulting in a positive pressure wave when a
positive signal is applied to pin 2. Use standard audio cables
AMPLIFICATION AND PROTECTION CIRCUITRY
Each driver in the MTS-4A is powered by one channel of the
Meyer MP-4, a four-channel 2480 W amplifier (620 W/ch)
utilizing complementary power MOSFET output stages (class
AB/H). The following sections discuss the MP-4's limiting
circuitry and the four-fan cooling system.
T
P
™ L
S
RU
OWER
IMITING
YSTEM
Conventional limiters assume that the resistance of a speaker
remains constant and set the limiting threshold by measuring
voltage only. This method is inaccurate because the speaker's
resistance changes in response to the frequency content
of the source material and thermal variations in the
speaker's voice coil and magnet. Conventional limiters begin
limiting prematurely, which under-utilizes system headroom
and deprives the speaker of its full dynamic range.
The TruPower limiting (TPL) system accounts for varying
speaker impedance by measuring current, in addition to
voltage, to compute the power dissipation and voice coil
temperature. TPL improves performance before and during
limiting by allowing the speaker to produce its maximum SPL
across its entire frequency range. TPL also extends the lifetime
of the drivers by controlling the temperature of the voice coil.
Sub Limit, Lo Limit, Mid Limit, and Hi Limit LEDs on the
user panel indicate TPL activity for each of the four amplifier
channels. When any channel exceeds the safe continuous
power level, that channel's limiter engages, ceasing operation
when the power level returns to normal. The limiters for each
channel function independently and do not affect the signal
when the LEDs are inactive.
The MTS-4A performs within its acoustical specifications and
operates at a normal temperature if a given TPL LED is on for
no longer than two seconds, and off for at least one second. If
an LED remains on for longer than three seconds, that channel
with XLR connectors for balanced signal sources.
NOTE: If abnormal noise (hum, hiss, popping) is produced from
the loudspeaker, disconnect the audio source from the
speaker. If the noise stops, then the problem is not
within the loudspeaker; check the audio input and AC
power.
A single source can drive multiple MTS-4As with a paralleled
input loop, creating an unbuffered hardwired loop connection.
Make certain that the source equipment can drive the total
load impedance presented by the paralleled input circuit. For
example, since the input impedance of a single MTS-4A is
10 kΩ, cascading 20 MTS-4As produces a balanced input
impedance of 500Ω. If a 150Ω source is used, the 500Ω load
results in a 2.28 dB loss.
is hard limiting with these negative consequences:
• Increasing the input level will not increase the volume.
• The system distorts due to clipping and nonlinear driver
operation.
• Unequal limiting between the drivers alters the frequency
response.
• Driver and amplifier components are subjected to maximum
heat, which shortens their life-span.
The TPL LEDs can indicate an imbalance in a system of
speakers by functioning like a spectrum analyzer. If speakers
in a subwoofer, mid-bass, or mid-hi subsystem begin to limit
before reaching the required operating level for the entire
system, then that subsystem needs to be supplemented with
additional speakers.
NOTE: Although the TPL limiters exhibit smooth sonic
characteristics, we do not recommend using them
for intentional compression effects. Use an outboard
compressor/limiter to compress a mixed signal.
F
C
S
ANS AND
OOLING
YSTEM
The MTS-4A uses a forced-air cooling system with four fans
to prevent the amplifiers from overheating. The fans draw
air in through ducts on the front of the cabinet, over the
heatsinks, and out the rear of the cabinet. Since dust does not
accumulate in the amplifier circuitry, its life-span is increased
significantly.
A foam insert filter, in combination with the entire front grill
surface, acts as an air filter for the cooling system. Despite
the filtering, extensive use or a dusty operating environment
can allow dust to accumulate along the path of the
airflow, preventing normal cooling. We recommend periodically
removing the grill, filter, and amplifier module and using
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