11
Protection Features
Any time a channel is driven into hard, continuous clipping, the GCL circuit will automatically reduce the channel
gain t
o a level just slightly into clipping, guarding the speakers against the damaging high power continuous
square waves that may be produced. Situations that may activate the GCL circuit include uncontrolled feedback,
oscillation, an improper equipment setting or malfunction upstream from the amplifier. Normal program
transients will not trigger the GCL, only steady, excessive clipping will. The GCL LED will flash on peaks when GCL
is active, and will flash brightly with clipping when GCL is disabled.
Gain Comparator Limiting (GCL)
Internal fans keep the amplifier operating well within its intended temperature range under all normal conditions.
I
f either channel's heat sink temperature reaches 90°C, both channels of the amplifier will shut down until it has
cooled. During this time, the THERMAL PROTECT LED will light up and the fans will continue to run at high speed.
Thermal Protection
The CPX 4 amplifier incorporates several circuits to protect both themselves and loudspeakers under virtually any
situa
tion. Crest has attempted to make the amplifiers as foolproof as possible by making them immune to short
and open circuits, mismatched loads, DC voltage, and overheating. If a channel goes into the Gain Comparator
Limiting or GCL gain reduction mode, the speaker load remains connected, but clipping percentage is instantly
reduced. DC voltage on the output, excessive subsonic frequencies or thermal overload will cause the channel’s
output to disconnect from the speaker load until the problem is corrected or the amplifier cools down.
Short Circuit
If an output is shorted, the overcurrent protection circuit will engage and reduce the output of the amplifier to
pr
otect the output devices. Solid red GCL LEDs and dim signal LEDS is an indication of a short circuit and should
be investigated immediately. To find the cause of the overload, start by disconnecting one output at a time, and
then one speaker at a time until the bad cable or damaged speaker is isolated.
DC Voltage Protection
If a DC or subsonic voltage is present at the outputs, a crowbar circuit engages to prevent loudspeaker damage.
At power-up, the amplifier stays in mute mode with outputs disconnected for several seconds while the power
supplies char
ge and stabilize. When power is removed, the mute mode engages so that no thumps or pops are
heard.
Turn-On/Turn-Off Protection
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