The Trupower™ Limiting System; Low- And Mid-Frequency Limiters; High-Frequency Limiter; Very-High Frequency Limiters - Meyer Sound MILO Operating Instructions Manual

CAUTION: The strain relief stud fitting must
!
be used only to secure system cabling. This
fitting is not intended to be used with system rigging
or a pull-back motor (pulling the bottom of the array
backward to increase downward tilt). The point
is mounted to the side of the amplifier area so as
not to interfere with the rain hood (if fitted) and the
amplifier fan exhaust area.
THE TRUPOWER™ LIMITING SYSTEM
Conventional limiters assume a constant loudspeaker
impedance and therefore set the limiting threshold by
measuring voltage only. However, this method is inaccurate
because the loudspeaker's impedance varies throughout
its frequency range, changing in response to the frequency
content of the audio source. In addition, the impedence also
changes due to temperature variations in the voice coil and
magnet. Consequently, conventional limiters begin limiting
prematurely, which under-utilizes system headroom and
lessens the loudspeaker's dynamic range.
In contrast, TruPower Limiting (TPL) accounts for varying
loudspeaker impedance by measuring current as well as
voltage to compute the actual power dissipation in the
voice coil. TPL improves performance before and during
limiting by allowing each driver to produce maximum SPL
across its entire frequency range.
NOTE:
TPL only reduces the signal level
to keep the voice coil below 180 degrees
Celsius, hence the peaks are unaffected.
In addition, TPL eliminates power compression when the
system is operated at high levels for extended periods, and
also extends the driver life cycle by controlling voice coil
temperatures.
The actual power is monitored for three of MILO's four
amplifier channels. When the safe continuous power level is
exceeded, the TPL limiter controlling that amplifier channel
engages. TPL activity is indicated by the LEDs on the
user panel (Figure 2.4). The very-high frequency channel
is controlled by a sophisticated average and peak voltage
limiter.
Figure 2.4: MILO Limit LEDs
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Low- and Mid-Frequency Limiters

MILO's left and right 12-inch cone drivers are powered by
separate amplifier channels, each with a power detector
but routed to one limiter; the limiter tracks both channels
and uses the higher of the two values to engage. By limiting
both amplifier channels equally, any anomalies in the
frequency range shared by the drivers is eliminated during
limiting. The LO TPL and MID TPL LEDs on the user panel
indicate TPL activity for these two drivers.

High-Frequency Limiter

The single 4-inch diaphragm high-frequency compression
driver is powered by one amplifier channel; this channel has
both TPL and peak limiters. When engaged, the peak limiter
prevents signal peaks from causing excessive excursion
in the driver as well as distortion in the amplifier channel,
preserving headroom and maintaining smooth frequency
response at high levels.
The High TPL LED is used to indicate any limiting activity
for this driver. When the LED turns on and off in rapid
succession, it indicates peak limiting; when it turns on and
off slowly, it indicates TPL activity.

Very-High Frequency Limiters

The three 2-inch diaphragm very-high frequency
compression drivers are powered by the fourth amplifier
channel. The VHF limiter prevents excessive continuous
voltages and signal peaks from causing excessive heat
and excursion in the drivers as well as distortion in the
amplifier channel. The limiter also helps preserve headroom
and maintain smooth frequency response at high levels.
The VHF LED indicates average and peak voltage limiting
activity for these drivers.
All limiters cease operation when the power level and
voltage for the channel returns to normal – below the
limiter's threshold. The limiting circuitry utilizes optical
limiters that add no noise and have no effect on the signal
when the limiter is not engaged and the LED is inactive.
MILO is performing within its acoustical specifications and
operating at a normal temperature if the limit LEDs are lit for
no longer than two seconds, and then go off for at least one
second. If an LED remains on for longer than three seconds,
that channel enters hard limiting, with the following negative
consequences:
 Increasing input level will not increase volume.
 Distortion increases due to clipping and nonlinear driver
operation.
 The lifespan of the driver is reduced because it is
subjected to excessive heat and/or excursion.
CHAPTER 2
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