Dc Protection; Hi Impedance Warning (Open Load); Lo Impedance Protection (Short Circuit) - Lab.gruppen C 68:4 Operation Manual

signals do not appear in "natural" sources such as
music. Any such signals can therefore be considered
as a fault when present. VHF protection is essential
in avoiding damage to high frequency drivers.
The VHF protection operational area is dependent
on output power level and frequency. The illustration
below shows a decreasing threshold on the output
power level, starting at approximately 10 kHz and
rising with a -6 dB slope. This defines the VHF protec-
tion area. When continuous output power above
the threshold line is detected the VHF protection
becomes active.
Output
power
20 Hz 10 kHz
Slow
Fast
20 Hz 10 kHz
The Attack time for the VHF protection is increa-
singly 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 imple-
mented 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
6 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
Max. Peak
output power
VHF protection
operation area
- 6 dB slope
Max. Continous
output power
20 kHz 30 kHz
Attack time
VHF protection
20 kHz 30 kHz
OPERATION AND PERFORMANCE 7
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.

7.5.4 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.

7.5.5 hi impedance warning (open load)

A high impedance (open load) condition is indicated
when an input signal above approximately -29dB
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 Hi-impedance detection
initially triggers only when the input signal
rises above -29dB, it might cause the
indicator to first turn green, and then red,
even in situations where no speaker is connected.
7.5.6 Lo-impedance protection (short circuit)
A lo-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 protection
will sequence at 6 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 -4dB 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.
C Series Operation Manual 19