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Application details
4
Industrial & commercial installations
GCL
Crest's patented GCL (Gain Comparator Limiting) circuit enables the sound tech-
nician to maximize the performance of the amplifier/speaker combination by pre-
venting the power amplifier from running out of headroom (clipping). This limiting
system is activated by a unique circuit that senses signal conditions that might
overload the amplifier and activates compression (reduces the channel gain)
when clipping is imminent. The threshold of limiter is clipping itself, and no specific
threshold control is used. This technique effectively utilizes every precious watt
available for the power amplifier to reproduce the signal, while at the same time
minimizing clipping and distortion. GCL significantly reduces the potential of loud-
speaker degradation and damage, and is the most effective, automatic, hands-off
approach to the problem of power amplifier clipping.
Since CPX series power amplifiers use a circuit breaker for overcurrent protection,
the GCL limiter system plays an even more important role in continuous perfor-
mance by preventing each channel from clipping and overload. Continuous opera-
tion at clipping can cause the circuit breaker to trip, but with the GCL activated,
this problem is minimized. For this reason, the GCL compression system should
always be enabled.
For commercial and other installations where sustained high power operation is
required, the amplifiers should be mounted in a standard 19" rack. It is not neces-
sary to leave a rack space between each amplifier in the stack since each fan
pulls air in from the rear and exhausts the hot air out the front. However, an ade-
quate cool air supply must be provided for the amplifier when rack mounted. The
internal fan must have a source of air that is not preheated by other equipment.
The amplifier will start up in low speed fan operation and will normally stay at low
speed unless sustained high-power operating levels occur. Then, as temperatures
in the amplifier heat sinks increase, the automatic thermal-sensing circuitry will
cause high-speed operation to occur. Depending upon signal conditions and amp
loading, high-speed fan operation may continue or the fan may cycle continuously
between high and low. This situation is quite normal. If cooling is inadequate, how-
ever, the amplifier thermal-sensing system may cause temporary shut down of the
unit, indicated by the PWR LEDs on both channels going dark. Inadequate cool-
ing may be due to preheated air, reduced air flow resulting from blockage of
inlet/outlet ports, severe amplifier overload, or short circuit conditions. Depending
upon the available cooling air, operation should be restored relatively quickly, and
the power LEDs on both channels will again be illuminated. In any event, action
should be taken to correct the cause of the thermal shutdown. If the amplifier is
not severely overloaded or shorted and air flow is normal in and out of the amplifi-
er, then steps should be taken to provide a cooler environment for all the ampli-
fiers. As a general rule, the cooler electronic equipment is operated, the longer its
useful service life.
In most low to medium-power applications, the amplifier can be mounted in any
configuration. It is desirable that, if at all possible, the power amplifier be located at
the top of an equipment stack. This will prevent possible overheating of sensitive
equipment by the hot air rising from the power amplifier. As a general rule, most
home and studio requirements will never cause high-speed fan operation. High-
speed operation may indicate that you have not taken the necessary steps to pro-
vide adequate cooling. Fully closed up in a cabinet, a CPX Series power amplifier
will have severe cooling problems, even at low power levels.
