Meyer Sound MSL-6 - Manual (Page 6)

6
Amplification and Protection
Circuitry
The MSL-6 is powered by the Meyer MP-4, a four-
channel 2480 W amplifier (620 W/ch) utilizing comple-
mentary power MOSFET output stages (class AB/H).
The following sections discuss the MP-4’s limiting circuitry
and four-fan cooling system.
The TruPower™ Limiting System
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. Con-
ventional 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 actual power dissipation in the
voice coil. TPL
• improves performance before and during limiting
by allowing each driver to produce its maximum
SPL across its entire frequency range;
• protects the drivers by controlling the temperature
of the voice coil;
• eliminates long-term power compression when
the system is operated at high levels for extended
periods.
The true power is monitored for each of the four amplifier
channels, which are controlled by two limiters. The
outer two and center high frequency drivers are driven
by separate amplifier channels but are routed to one
limiter: the HI Limit LED on the user panel indicates
TPL activity for the horns. The two low frequency drivers
are driven by separate amplifier channels but are routed
to one limiter: the LO Limit LED indicates TPL activity
for the low drivers.
When any channel exceeds the safe continuous power
level, its limiter engages, affecting both channels equally.
For example, if the channel for the outer two horns
exceeds the safe power level, the high limiter engages,
limiting the center, as well as the outer two horns. The
high and low limiters function independently and do not
affect the signal when the LEDs are inactive.
The MSL-6 performs within its acoustical specifications
and operates at a normal temperature if the limit LEDs
are on for no longer than two seconds, and off for at least
one second. If either LED remains on for longer than
three seconds, that channel is hard limiting with the
following 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 low and high
channels alters the frequency response.
• The life-span of the drivers is reduced because
they are subjected to excessive heat.
The TPL LEDs can indicate an imbalance in a configuration
of speakers by functioning like a spectrum analyzer. If
the speakers in a subwoofer, mid-bass, or mid-hi sub-
system 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.
VHF Limiting
At normal operating levels, the outer two horns receive
a high frequency boost (centered at 11 kHz) to blend with
the center horn to achieve smooth horizontal coverage.
At high operating levels, however, this boost can cause
distortion and premature TPL limiting.
To preserve headroom and maintain smooth high fre-
quency response at high operating levels, the channel
for the outer two horns has a fast-acting VHF (very high
frequency) limiter that reduces the amplitude of fre-
quencies around 11 kHz. Increased acoustical HF
harmonic content generated at high signal levels
compensates for the effects of the VHF limiter, rendering
its operation almost imperceptible. VHF limiting is
indicated by the VHF Limit LED on the user panel; the
same on/off cycle discussed for the TPL limiters applies
to the VHF limiter.