11. SIG/LIMIT LED
This bi-color LED illuminates green whenever there is
a signal present at the main input connector. It senses
the signal just after the level control, so adjustments to
the level control will affect the sig/limit indicator.
The HD1521 has a built-in limiter that helps to pre-
vent the amplifier outputs from clipping or overdriving
the transducers. The sig/limit indicator lights in yellow
when the limiter is activated. It's okay for it to blink
yellow occasionally, but if it blinks frequently or lights
continuously, turn down the level control until it only
blinks occasionally.
Excessive limiting may lead to overheat-
ing, which in turn trips the thermal protect
circuitry and interrupts the performance. See
‘Thermal Considerations’ on page 9 for more informa-
tion.
12. POWER
Use this switch to turn the HD1521 on and off. The
LED next to the switch will illuminate when powered
on. The front panel LED will also turn on, but only if the
power light on switch (9) is engaged.
Press the bottom of this switch to put the speaker into
standby mode. The HD1521 will not function, but the
circuits are still live. To remove AC power, either turn
off the AC mains supply, or unplug the power cord from
the speaker and the AC mains supply.
As a general guide, the powered speakers
should be turned on last, after the mixer and
other sources. They should also be the first
things turned off. This will reduce the possi-
bility of any turn-on, or turn-off thumps in your speak-
ers.
13. IEC AC Receptacle
This jack accepts the supplied 3-prong AC power cord.
Before you plug the AC power cord into the
powered loudspeaker, make sure that the
voltage of your unit (listed above the IEC AC
receptacle) is the same voltage as your local
AC mains supply. Use only the power cord supplied.
Also, disconnecting the plug’s ground pin is dangerous.
Don’t do it. And no running with scissors either. Let's be
safe out there!
7. HIGH
This knob allows up
to 3 dB of boost or cut
at 12 kHz, with no
change in level at the
unity (U) mark. This is
a shelving EQ.
8. LEVEL
This controls the overall signal level at the input to
the built-in power amplifiers. It ranges from –6 dB to
+6 dB of gain. The center detent is 0 dB (unity gain).
9. POWER LIGHT ON
Press this switch in to turn on the front panel power
LED if a visual indicator is preferred. The LED next to
the switch will light as a reminder.
If this switch is out, and the HD1521 is turned on, the
LED on the front of the cabinet will not light, nor will
the LED next to the switch.
10. THERMAL LED
The HD1521 is equipped with a thermal protection
circuit that monitors the internal temperature of the
amplifiers and heatsink. If the temperature exceeds a
safe operating level, this indicator lights and the input
signal is muted to allow the amplifiers to cool. When the
temperature cools to a safe level once again, the thermal
protection circuit deactivates, the thermal LED turns off
and the HD1521 returns to normal operation.
When the HD1521 is in thermal protect mode, the
power LED (12) will remain lit, indicating that the unit
is still powered on despite the lack of output. If power
light on (9) is engaged, the front panel power LED will
go out when in thermal protect mode.
Activation of the thermal protection circuit
is an indication that you should take steps to
avoid continued thermal problems. See “Ther-
mal Considerations” on page 9.
20
Hz
100
Hz
1k
Hz
10k
Hz
20k
Hz
–3
–2
–1
0
+1
+2
+3
HD151
HD15
21
