Crest Audio CPX 3800 - Manual (Page 25)

p. 25
protection features
5
The CPX Series amplifiers incorporate several circuits to protect both themselves and loud-
speakers under virtually any situation. Crest Performance 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 or output power are
instantly reduced. DC voltage on the output, excessive subsonic frequencies or thermal overload
will cause the channel’s output relay to disconnect the speaker load until the problem is corrected
or the amplifier cools down.
GCL gain comparator limiting
Any time a channel is driven into hard, continuous clipping, the GCL circuit will automatically
reduce the channel gain to 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 glow brightly and continuously
when limiting occurs.
IGM
™
impedance sensing
The CPX Series amplifiers feature innovative circuitry that allows safe operation into any load.
When an amplifier sees a load that overstresses the output stage, the Instantaneous Gain
Modulation (IGM) circuit adjusts the channel gain to a safe level. This method of output-stage
protection is far superior to conventional, brute force-type limiting found on other amplifiers.
The IGM circuit is sonically transparent in normal use and unobtrusive when activated.
thermal protection
Internal fans keep the amplifier operating well within its intended temperature range under all
normal conditions. If a channel’s heat sink temperature reaches 75°C, which may indicate an
obstructed air supply, that channel will independently protect itself by disconnecting its load
and shutting down until it has cooled. During this time, the POWER LED will go out, the GCL
LEDs will remain lit and the cooling fans will continue running at high speed.
short circuit
If an output is shorted, the IGM and thermal circuits will automatically protect the amplifier. The
IGM circuit senses the short circuit as an extremely stressful load condition and attenuates the
signal, protecting the channel’s output transistors from over-current stress. If the short circuit
remains, the channel will eventually thermally protect itself by disconnecting the load.
DC voltage protection
If an amplifier channel detects DC voltage or subsonic frequencies at its output terminals, its
output relay will immediately open to prevent loudspeaker damage.