Bridge Mode - Crest Audio CPX1500 Owner's Manual

Industrial & commercial installations
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 adequate
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 ampli-
fier 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, however, the
amplifier thermal-sensing system may cause temporary shut down of the unit, indi-
cated by the PWR LEDs on both channels going dark. Inadequate cooling 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 cor-
rect 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 amplifier, then steps should be
taken to provide a cooler environment for all the amplifiers. 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.

Bridge mode

The Bridge mode on stereo amplifiers is often misunderstood relating to actual
operation and usage. In basic terms, when a 2-channel amplifier is operated in the
Bridge mode, it is converted into a single-channel unit with a power rating equal to
the sum of the power rating for each channel, at a load of twice that of the single-
channel rating. For example, the CPX 1500 is rated at 750 watts RMS per channel
into 2 ohms. The Bridge rating is 1500 watts RMS into 4 ohms (minimum load).
Bridge mode operation is accomplished by placing the MODE switch in the
BRIDGE position, using only the BRIDGE Speakon connector or the red binding
posts for the output, and using the CHANNEL A input. All CHANNEL B input func-
tions are defeated and serve no purpose now. Bridge mode operation can be used
to drive sound distribution systems in very large public address applications.
Another common use for the Bridge mode is in subwoofer applications where very
high power levels are required to reproduce extremely low frequencies with ade-
quate headroom. Such enclosures usually contain 2 or 4 loudspeakers to handle
the power levels involved. For Bridge mode usage, the enclosure impedance must
be 4 or 8 ohms?never below 4 ohms.
Application details
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