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If a signal above 12kHz is detected at the amplifier outputs for more than five seconds at full output
power, the VHF protection mutes the input signal. This is indicated by the LED’s on the front panel
labeled Protect. After five seconds the outputs will un-mute and return to normal operation, unless the
output signal has remained unchanged, in which case the VHF protection will re-initiate.
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All Lab.gruppen amplifiers are completely short circuit protected. The protection circuitry permits very
high peak currents, but still holds the output devices within the safe operation levels. If a short circuit
remains to be present, the channel effected will eventually go into thermal protect cycle until the short
circuit has been removed.
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If the AC mains voltage is lower or higher than the allowed operational voltage (over- or under-
voltage), the power supply will automatically shut down. Once the mains voltage is above the minimum
start voltage and below its maximum operating voltage the amplifier will restart.
It takes a couple of seconds to self-check (this is known as the "soft start" or "slow start" sequence), the
gain will slowly and gradually be increased back to the previously set level, and the fans will first blow at
high speed before going into "idle". The four green ”ON” LED’s indicate that the power supply is
operating properly.
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There are two types of DC protection:
•
Fuses on the supply branches of each channel.
•
A DC crowbar protection that shorts the output if more than 10 volts DC are being detected on the
outputs.
Either of these protections comes into effect once DC voltage is being detected, independently for both
channels.
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