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Pro II owner’s manual
p.30
safety
6
TourClass
®
protection
Every model in the Pro II Series incorporates TourClass protection fea-
tures. 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 LED.The channel gain is auto-
matically reduced,protecting the loudspeakers from potential damage from
the high power, continuous square waves that would otherwise be pro-
duced.
ACL may be activated by uncontrolled feedback,oscillations,improp-
er equipment gain settings, or an equipment malfunction upstream from
the amplifier. Only steady or excessive clipping
(not normal program tran-
sients
) 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. Like ACL,
the IGM 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 attenuator setting avoiding unnecessary stress
on the loudspeakers.
Thermal
Abnormally high heat sink temperatures will engage the Protect circuit for the
overheating channel only.
(An output relay disconnects the loudspeakers until
nominal temperature range is restored.
) During this time, the Status LED will
light red.If the power supply gets too hot, its thermal sensing circuit will dis-
connect both channel outputs. During this time, the
Active LED will extin-
guish, the Status
LED will remain red, and the cooling fan will continue run-
ning at high speed. Normal operation resumes once the power supply cools
to a safe level.
Short Circuit
If an output is shorted (i.e., defective speakers or crossed speaker wires) the
IGM and thermal circuits will automatically protect the amplifier.The IGM cir-
cuit 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 load will be disconnected by
the thermal protection circuitry
(the output relay opens).