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safety
6
TourClass
®
protection
Just like Crest Audio’s highly acclaimed Pro series amps, LT and ST amps incorporate
TourClass protection features. Derived from Crest Audio’s extensive experience with the
world’s largest sound rental companies, the TourClass group of circuits sets the industry
standard for assured protection of internal amplifier circuits and all connected loads.
ACL ACTIVE CLIP LIMITING
At the amplifier’s full power limit, or clipping point, ACL will be activated.This is indicated
by illumination of the
ACL / IGM LED.The channel gain is automatically reduced, protecting
the loudspeakers from potential damage from the high power, continuous square waves that
would otherwise be produced.
ACL may be activated by uncontrolled feedback,oscillations,
improper equipment gain settings, or an equipment malfunction upstream from the amplifi-
er. Only steady or excessive clipping
(not normal program transients) will trigger ACL. The
circuit is virtually transparent in operation and full signal bandwidth is maintained.
IGM INSTANTANEOUS GAIN MODULATION
IGM is an innovative circuit that allows the amplifier to operate safely into loads as low as 2
ohms.When the amplifier sees a load that overstresses the output stage,the
IGM circuit adjusts
the channel gain to a safe level. This gain control circuit is inaudible in normal use. In addition,
if extreme and sustained low impedance is encountered,the amplifier’s output relay will open.
AutoRamp
Auto Ramp operates every time the amplifier is turned on or is reactivated after a protect
condition is corrected.This exclusive Crest Audio feature gradually increases gain to the atten-
uator setting avoiding unnecessary stress on the loudspeakers.
Thermal Protect
Abnormally high heat sink temperatures will engage the Protect circuit and disconnects the
output relays. During this time the Prot LEDs will light red.The cooling fan will continue run-
ning at high speed. Normal operation resumes once the amplifier cools to a safe level.
Short Circuit
If an output is shorted (i.e. defective speakers or crossed speaker wires) the channel will shut-
down and can be re-set by turning the amplifier off, then on.
DC Voltage
If an amplifier channel detects DC voltage at its output terminals,the output relay will immediately
open to prevent loudspeaker damage. The protect LEDs illuminate.
Subsonic Frequencies
Built-in high pass filtering provides subsonic frequency protection for each channel.In addition,
a relay will open if excessive subsonic energy appears
at the output.