Lab.gruppen fP 6400 - 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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The AFS TM circuitry continuously monitors the mains current. If the average mains current is too high for
a 16 ampere slow blow fuse, as it can be during so-called bench tests, the AFS™ limiter will limit the
mains current to a safe level.
If the AFS limiter is activated, this will be indicated by the AFS LED on the front panel. The AFS
limiter will not be activated during normal use, even under sustained heavy clipping.
AFS
TM is an abbreviation for Automatic Fuse Saver. The main reason for its presence is to avoid
blowing the mains breaker if a bench test is made at continuous full power.
The AFS
TM circuitry is a current limiter, limiting the mains power current draw to 20A (230V) or 40A
(115V). The circuitry has the same characteristics as a mains breaker, so it allows for high peak currents
without significantly reducing the long-term average power, but limiting the current if the amplifier
draws a high continuous current. It is the continuous average current that blows the mains breaker.
The AFS LED on the front panel indicates the AFS
TM circuitry limiting the mains power current or the
regulated power supply going out of stabilization. The power supply will go out of stabilization at low
mains voltage in combination with low load impedance and during the start-up sequence. Since 1997,
when this feature was introduced, not a single user has reported that the AFS
TM has been activated
during music performance, even in high power demanding situations.
If the average current draw exceeds what the mains breaker can take (e.g. during bench test), the output
power is getting reduced by 2dB. This is the same as an fP 6400 being reduced to the power of an fP
3400 (but again, not a single user has reported that the AFS has been activated). In reality this is much
better than the mains breaker blowing and the amplifier being turned off. A listener can hardly hear this
reduction, as only the headroom will be reduced. If the peaks are around 120 to 130dB, a 2dB reduction
can hardly be noticed.
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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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