Lab.gruppen fP 3400 - Manual (Page 17)

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If the amplifier is driven very hard into a low impedance load, the cooling fans will run at high speed. If
the causing conditions continue, the “Temp” temperature indicator(s) will illuminate indicating that the
amplifier is about to go into thermal shutdown.
After approximately five seconds the amplifier will go into thermal protection by muting the input signal.
After another 15-20 seconds the amplifier should have cooled down enough for the amplifier to come out
of shutdown and operate as normal. If the load conditions remain unchanged, the thermal protection will
be reinitiated accordingly. Thermal protection starts, if the Intercooler
® heatsink reaches a temperature
above 90 ° C.
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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 VHF. 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 two 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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