Vhf Protection - Lab.gruppen PLM series Operation Manual

5.3.9 DC Protection
DC protection is implemented on each power output to prevent damage to connected loudspeakers or any
PLM components. DC present at the output will cause the PLM's power output module breaker to blow. In
this instance a red LED will illuminate and NEEDS SERVICE will display on the LCD.
The power output channel modules are independent of
the input voltage. Both 115 and 230 V models have amp
channel breakers. This is not a user-servicable fault
condition and the unit should be returned for repair.

5.3.10 VHF Protection

The PLM includes protection circuits that detect Very High Frequency (VHF) content in the input signal. The
detection is frequency-dependent, initiated from 10 kHz upwards. If VHF signals are detected above the
threshold, the output will mute for approximately 6 seconds before a further measurement is taken. When
continuous VHF signal stops, the output unmutes and the amplifier returns to normal operation.
This protection system recognizes that continuous VHF signals at high levels do not appear in speech or
music. Any such content can therefore be considered as a fault condition. VHF protection is essential to
avoid damage to HF drivers.
VHF protection is dependent on a combination of output power level and frequency. Figure 5-2 shows a
decreasing power threshold, from approximately 10 kHz upwards, which illustrates increasing sensitivity of
the protection system with frequency. When continuous output power above the threshold line is detected,
VHF protection becomes active.
The attack time of the VHF protection circuitry also changes with frequency, becoming shorter at higher
frequencies. This is shown in Figure 5-3.
Figure 5-2: VHF Protection Frequency Sensitivity
Operation and Performance
PLM Series Operation Manual rev 1.1.9
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