Another way to “cook the books” is in how the load resistance is selected. You can coax a larger power rating from an amplifier by tweaking the load resistor to find a value that yields the highest burst test results.
The problem is that the amplifier will never produce this much power in actual use, unless it is driving a resistive load of exactly this value, which will never happen. “Three kilowatts into 2.65 ohms” is not a useful metric. But, this “watts” rating still goes on the spec sheet and may even be used in the model number.
An additional variable is the amount of distortion tolerated in the measurement of the burst waveform. A 10 percent allowance will yield a much higher power rating than a 1 percent allowance. A device driven into distortion can yield a much higher RMS voltage.
Due to the many variables, meaningful comparisons of amplifiers from burst test ratings is practically impossible. There’s always a way to get a bigger number, and people have taken greater license with the methods than our ‘70s predecessors.
The Physics Of Power
Power is the “rate” of doing work. This means that there is a time element involved. The dimension of power is energy divided by time. While “instantaneous power” has a physical meaning, it’s just a stepping stone on the way to the actual power produced by a source. The integration of instantaneous power over time yields the continuous power. The continuous power rating describes the heat production capabilities of a furnace, and the work capabilities of an amplifier. Since continuous power is based on the RMS voltage, it is often (incorrectly) referred to as RMS power.
Today’s “face value” power ratings have conveniently omitted the time metric, and power ratings, while correctly based on RMS voltage, are only for a blip, not a continuous waveform. The buying public is allowed to assume that the rating is for continuous output. If you leave out the time metric, you only have part of the story. Imagine what would happen if they did that in other fields. Examples could include:
—If the towing power of a truck were only based on what it can pull for a few seconds, there would be a lot of burned-out vehicles on our roadsides.
—If the horsepower rating of a jet engine were only sustainable for short flights, I would be reluctant to board a plane for a long flight.
—If the current output of a welder could only be sustained for a few seconds, you will be making a lot of pauses on a long bead.
—If the audio output power of an amplifier can only be sustained for a few cycles, what happens for continuous signals?
You get the idea. Time matters.
Efficiency
To a physicist, a product draws power from a source and converts it to another type of power. For a vacuum cleaner, utility power becomes mechanical power. For amplifiers, utility power becomes audio power. The conversion efficiency cannot exceed 100 percent, for to do so your component would have to output more power than is input.
The only way to get more power from an amplifier than is drawn from the source is to redefine power, and the only reason for doing that is to increase sales. If the audio power rating is 2-3 times the power available from the electrical outlet, a red flag should raise.
“Watts-Only” Is A Bad Way To Rate
An audio power amplifier is basically a voltage source that will try to deliver current that the load asks for. As such, current is “drawn” by the load, not “injected” by the source. This is why the impedance of the load is critical to amplifier performance, and why a power rating should always include the load impedance.