Lab.gruppen C Series 88:4 - Manual (Page 27)

5. Operation and Performance
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C Series C 88:4, C 68:4, C 48:4, C 28:4 and C 16:4 Operation Manual rev 2.2.3
The Attack time for the VHF protection is increasingly shorter at higher frequencies. For example, an ultrasonic
continuous signal will cause the outputs to mute rapidly, where it will take several milliseconds for a 10 kHz
continuous signal to trigger the output mute. This is shown in the illustration above.
The VHF protection is NOT a limiter and does not alter the amplifier’s frequency response. It is implemented
solely to detect continuous VHF content. The amplifier will always pass VHF peaks at full power, with no effect
on musical “transients”.
The VHF protection is indicated by a yellow LED on the amplifier front-panel, with output muting for six seconds
when in action. It is reported as a fault via the NomadLink network on the DeviceControl GUI.
If you bench test the amplifier using a continuous, full scale
sine-wave input above 10 kHz, the VHF protection will activate
and prevent measurement of full peak output power. (Output
will be muted long before maximum output power is attained.)
To measure the true peak output power, use a burst signal.
5.5.4 DC protection
DC protection is implemented on each output to prevent damage to connected loudspeakers. DC present at
the output will trigger muting and illuminate the fault LED indicator. Any DC present at the output indicates a
hardware malfunction that requires servicing of the amplifier.
5.5.5 High-impedance warning (open load)
A high-impedance (open load) condition is indicated when an input signal above approximately -29 dB is
detected and no functioning loudspeakers are connected to the amplifier. The fault in indicated by a red Sig/
Hi-imp LED. The indicator is green when a valid load is present under the same input signal conditions
Since the high-impedance detection initially triggers only when the
input signal rises above -29 dB, it might cause the indicator to first
turn green, and then red, even in situations where no speaker is
connected.
5.5.6 Low-impedance protection (short circuit)
A low-impedance or short-circuit fault is detected when current draw is high (Current Peak Limiter active) and
when, simultaneously, output signal is low (-4 dB LED does not illuminate). When this occurs, the amplifier
protects the output stage from damage by muting the output signal and bypassing the circuits. Indication of
this fault is a constant orange illumination of the Current Peak Limiter (CPL) LED on the front-panel. The protec-
tion will sequence at six second intervals to re-measure conditions. If the low-impedance fault is no longer
detected, the amplifier will un-mute.
If the CPL turns constant orange, the output is
muted, and the -4 dB signal LED is ON, then the
amplifier has gone into maximum current protection.
This situation is caused by an excessive input signal
and is not due to a short circuit. Turn down the input signal to
avoid or remedy this situation.