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14
CHAPTER 3
Very-High Frequency Limiters
The two 2-inch diaphragm very-high frequency compres-
sion 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.
NOTE: 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 not lit.
MILO 120 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 follow-
ing 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 sub-
jected to excessive heat and/or excursion.
NOTE: The limit LEDs indicate when the safe
power level is exceeded. If any channel on
an entire system of MILO/MILO 120 loudspeakers
begins to limit before reaching the required sound
pressure level (SPL), consider adding more loud-
speakers to satisfy the SPL requirements without
exposing the drivers on that channel to excessive
heat and/or excursion.
FANS AND COOLING SYSTEM
MILO 120 uses a forced-air cooling system with four fans
to prevent the amplifier modules 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. Because dust
does not accumulate in the amplifier circuitry, its lifespan is
increased significantly. The front grille surface acts as an air
filter for the cooling system and should always be in place
during operation (Figure 3.5).
Figure 3.5. Airflow through MILO 120
Because the grille acts as a filter, it should be inspected and
cleaned routinely – once every six months, or once every
three months in a dusty environment – to assure proper
airflow and cooling. The grille can be vacuumed to remove
dust.
TIP: If your MILO 120 loudspeaker is weath-
er-protected, the grille must be removed in
order to the clean the additional foam underneath
the grille, which covers the air inlets.
CAUTION: When operating a weather-pro-
tected MILO 120 loudspeaker be sure the
rain hood is fully open. Leaving the hood closed
or partially open will limit the airflow through the
amplifier, which could cause it to overheat and shut
down.