Protection Features - Crest Audio FXA SERIES Manual

FXA
TourClass ® Protection
Every model in the FXA Series incorporates 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 illumina-
tion of the Clip 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 acti-
vated by uncontrolled feedback, oscillations, improper equipment gain settings, or an equipment malfunc-
tion upstream from the amplifier. 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 Impedance Sensing.
IGM (Instantaneous Gain Modulation) 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 cir-
cuit adjusts the channel gain to a safe level. Like ACL, the IGM circuit is inaudible in normal use. In addi-
tion, if extreme and sustained low impedance is encountered, the amplifier's output relay will open.
AutoRamp Protection.
Auto Ramp operates every time the amplifier is turned on or is reactivated after a protect condition is cor-
rected. This exclusive Crest Audio feature gradually increases gain to the attenuator setting avoiding unnec-
essary stress on the loudspeakers.
Thermal Protection.
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 Protect LED will light. If the power transformer gets too hot, its thermal sensing circuit will dis-
connect both channel outputs. During this time, the Active LED will extinguish, the Protect and Clip LEDs
will stay lit, and the cooling fan will continue running at low speed. Normal operation resumes once the
transformer 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 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 load will be disconnected by the thermal protection circuitry (output relay
opens).
DC Voltage Protection.
If an amplifier channel detects DC voltage at its output terminals, the output relay will immediately open to
prevent loudspeaker damage. The Protect LEDs will light.
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.
Page 12 Crest Audio FXA Series Power Amplifiers
Models FXA601, FXA901, FXA1201, FXA2401, FXA3601, FXA4801