Derating of Amplifiers - Reduction of maximum output level

All amplifiers are rated with a maximum output level in relation to the number of allowable channels the amplifier was tested with. This is referred to as the amplifiers’ channel loading ability. An earlier specification for amplifiers was known as DIN45004B.

The specification will show the maximum output level with a loading of two channels only. However, many commercial grade amplifiers will show the output level specification using the Cenelec 50083 standard with a 42 channel loading capacity in line with European and new Australian standards. If an amplifier specification only shows a 2-channel loading in its load chart then we can use a de-rating graph to work out the calculation for derating an amplifier for 42 channels.

Figure 5 allows you to determine the maximum output level the amplifier can be set to. Most well designed systems can have more channels added without any adjustments to the amplifiers. This is because the system was designed with amplifiers already de-rated to more than the current number of channels being distributed in the network. Most amplifier output levels are now rated at the "42 Channel" operation. As we increase the number of distributed channels we must decrease the output in dB according to the figure below to maintain distortion products at minus 60dB.

Figure 5: Derating of Amplifiers

Example:
The Polytron amplifier HA40125N has a rated output of 125BuV @ 2 channels
Therefore the new output level when loaded to 32 channels will be 113BuV. De-rating factor is 12dB.

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