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CHAPTER 3
In contrast, TruPower accounts for varying loudspeaker
impedance by measuring current, in addition to voltage,
to compute the actual power dissipation in the voice coil.
TruPower improves performance before and during limiting
by allowing each driver to produce maximum SPL across its
entire frequency range.
NOTE:
TruPower limiting only reduces the
signal level to keep the voice coil below 180°
Celsius, hence peaks are unaffected.
In addition, TruPower limiting 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 700-HP’s left and right 18-inch cone drivers are pow-
ered 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.
When the safe continuous power level is exceeded in any
channel, the TruPower limiter controlling both amplifier
channels engages. TruPower limiting activity is indicated by
the LED on the amplifier’s user panel.
In addition to TPL limiting the 700-HP also includes a peak
limiter. When engaged, the peak limiter prevents signal
peaks from causing excessive distortion in the amplifier
channel, preserving headroom and maintaining smooth
frequency response at high levels.
The TPL LED is used to indicate any limiting activity. 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 that indicates when the safe power level is
exceeded (Figure 3.4).
Limiters cease operation when the power level and volt-
age 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.
Figure 3.4. The 700-HP subwoofer’s limit LED indicators
The 700-HP performs within its acoustical specifications
and operates at a normal temperature if the limit LED is lit
for no longer than two seconds, and then goes off for at
least one second. If the limit LED remains on for longer than
three seconds, the 700-HP enters
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.
■
The lifespan of the drivers is reduced because they are
subjected to excessive heat.
CAUTION:
While the limiters protect the
system under overload conditions and exhibit
smooth sonic characteristics, we recommend that
you do not drive the 700-HP into continuous limit-
ing. If an entire system of 700-HPs begins to limit
before reaching the required sound pressure level
(SPL), you should consider adding more 700-HP
subwoofers to the system.
700-HP AMPLIFIER COOLING SYSTEM
The 700-HP uses a forced-air cooling system with two
fans (one primary and one reserve) to prevent the ampli-
fier module from overheating. The primary fan draws air in
through ducts on the front of the cabinet, over the heatsink,
and out the rear of the cabinet. Because dust does not ac-
cumulate 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 the 700-HP
