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Clearing Up The Mystery & Confusion With Amplifier Power Ratings

The question "how many watts?” can have many answers...

Power Ratings

There are many examples of power ratings outside of audio.

Some examples include AC generators, motors, light bulbs, and horses – and many, many others. In every case, the rating given is assumed to be continuous, meaning a sustainable, guaranteed value.

When you buy a 5,000-watt generator, you expect to get 5 kW continuous, not short-term peaks of 5 kW.

A good horse can generate one horsepower continuous (746 watts) for a period of time long enough to complete a task. This provided a guideline for replacing it with a steam engine.

Audio amplifier power ratings got crazy in the 1970s when some manufacturers began basing their ratings on “peak” or “instantaneous” values rather than “continuous” output. This gave them an advantage in the marketplace, because “more is better.” Right?

The Federal Trade Commission stepped in and established a standard to level the playing field. It placed requirements on the bandwidth and distortion level of the audio signal from the amplifier. So did the (now defunct) Electronics Industries Alliance or EIA rating method (now CEA-426-B R-2005).

In short, there are many ways to test both amplifiers and loudspeakers to get power ratings. The oldest and most conservative method for amplifier testing is the use of a continuous sine wave, with the amplifier connected to a resistive load and operated for a specified period of time (”continuous” implies “indefinitely”).

I am a big fan of this method, for several reasons.
1) It’s a simple test. There’s no way to fudge it, fool it, fake it or misrepresent it.
2) The rating can be easily verified in the field, with simple instrumentation.
3) I can easily determine the output power with other waveforms (music or speech) by substituting the crest factor of the program material for that of the sine wave. This always results in less power flow.
4) The distortion of a sine wave is much easier to measure than the distortion of more complicated waveforms, so it is easy to determine when the amplifier is at maximum level.
5) Large power amplifiers can drive distributed loudspeakers systems without the use of a step-up transformer.

These systems base their ratings on the sine wave. If I need “70 volts” or higher to drive such a line, then I need a sine wave rating for the amplifier to know whether it can get there.

As a system designer, I want a simple, no nonsense conservative rating of what the amplifier can do. I can increase the crest factor (de-rate the amplifier) to know the level produced by music or speech.

Now, before the letters to editor start pouring in, I’m smart enough to know that it is unlikely that the amplifier will ever have to pass a full-scale sine wave into a load for an extended period of time unless I intend to use it to drive a shaker table or as a regulated electrical outlet on the test bench (seriously – got a 60 Hz oscillator?).

I’m also smart enough to know that most “embellishments” to this simple rating method are to achieve higher power ratings and an advantage in the marketplace. (The 1970s revisited. Do we really have to go through that again?)

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I also understand that some amplifiers can produce more than their continuous sine wave rating if I pulse the test signal. Fine. I’ll take it. It’s only a few dB. In fact, the decibel is the key to understanding and comparing power ratings.

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