The wattage rating method used for audio products isn't standardized. The only way that wattage can be used in comparing two products is when dealing with models from the same manufacturer. Is a 300 watt Bryston twice as powerful as the 150 watt Bryston? Yes it is. Is a 300 watt amp from "X" twice as powerful as a 150 watt amp from "Y? " It might be, but it's far from certain. This is because of the variables in rating wattage. Below are just a few:
For mass market goods the wattage is largely determined by the marketing department and then the engineering department does the algebra to figure out which variables give them the desired answer. The Federal Trade Commission's major requirement is that the rating method be disclosed to the public.
- The Impedance used to load the amplifier's outputs during testing, rated in Ohms.
- The frequency range being driven. Some manufacturers drive 20Hz to 20,000Hz because that is what many accept to be the human hearing range. Some cheap receivers are driven at only ONE frequency, 1kHz being the norm.
- How many channels are driven AT THE SAME time. Just because a receiver or amp has 5 or7 channels doesn't mean that they were all driven during the test. Nearly all receivers only drive one channel during testing.
- How long was the test. Many times an amplifiers circuit can deliver very high wattage but the power supply and heat sinks won't allow it to continue doing so for long.
- How much "Total Harmonic Distortion" was deemed acceptable during the testing. More distortion allowed means more watts on paper but distortion is also usually the cause of damage to speakers.
For mass market goods the wattage is largely determined by the marketing department and then the engineering department does the algebra to figure out which variables give them the desired answer. The Federal Trade Commission's major requirement is that the rating method be disclosed to the public.
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