Vhf Protection - Lab.gruppen PLM series Operation Manual

7. Operation and Performance
The power output channel modules are independent
of the input voltage. Both 115 and 230 V models
have amp channel fuses. This is not a user-
servicable fault condition and the unit should be
returned for repair.

7.3.8 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
allowed threshold, the output will mute for approximately 6 seconds before a further measurement is taken.
Once no further continuous VHF signal is detected, 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 7.3.8a below
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.
Figure 7.3.8a: VHF protection frequency sensitivity
The attack time of the VHF protection circuitry also changes with frequency, becoming shorter at higher
frequencies. This is shown in figure 7.3.8b.
Figure 7.3.8b: VHF protection attack time variations
The VHF protection circuit is NOT a limiter and does not alter the PLM's frequency response. It is imple-
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PLM Series Operation Manual rev 1.1.3