Watt You Must Remember

Sydney Morning Herald

Monday December 21, 1998

GREG BORROWMAN

THE power output of hi-fi amplifiers and receivers has traditionally been an area where manufacturers have played fast and furious with the truth. Nowhere was this more evident than in the US during the '70s, where claims escalated from the merely improbable to the downright impossible.

The US Federal Trade Commission eventually decreed that enough was enough. It stepped in and developed its own standard for measuring amplifier power. Any manufacturer wishing to sell a hi-fi amplifier or receiver in the US had to measure its power output by following a strict procedure, and quote that figure in specifications.

Here in Australia, 30 years later, despite there being an Australian Standard for power output, it is common to see improbably high-power output figures in advertisements, particularly with regard to mini-systems and portable equipment.

About the only thing all these claims will have in common is the word "watt". The watt is the unit of electrical power. However, different companies have different methods of obtaining watt ratings, most of which revolve around manipulating the test conditions. For example, if you see that the power output of an amplifier has been measured under "dynamic", "peak" or "transient" test conditions, you can be assured that amplifier will not be able to maintain the claimed output power for more than a few seconds.

If you see that the power has been measured at only one frequency, usually at 1-kHz, the amplifier will not be able to produce the same amount of power at lower or higher frequencies.

Another popular power output qualifier is a claim for "music power", which is meaningless. Some manufacturers continue to state power outputs according to a peak music power output (PMPO) "standard". Regrettably, no such standard exists.

Many consumers do not realise that the results of measurements of an amplifier's power are directly related to the load resistance placed across the amplifier's speaker terminals when making the measurement. Because of this, it is possible to make an amplifier's power output seem higher than it would be in practice by connecting a low-value resistor in place of the standard 8-ohm value. An amplifier that can produce 50 watts into a 4-ohm resistor will generally produce only 25 watts when connected to an 8-ohm resistor.

All claims for power output are supposed to give the result for just a single channel, but now that multi-channel systems are common, it's becoming equally common for manufacturers to add the outputs of all the channels together to arrive at a highly inflated figure. How is it that a modestly rated 20-watt five-channel home theatre system could sound as loud as a fire-breathing 100-watt five-channel system? It could if the manufacturer multiplies a 20-watts per channel figure by the five available channels, to arrive at a 100-watt "total".

You will sometimes see the letters RMS after a power output claim. RMS stands for root mean square, and is an accurate method of measuring an alternating waveform; one that delivers an effective mean power level. However, because even this measurement can be made over an unrealistically short time (one full cycle), the RMS value should be averaged over several minutes, to return a continuous RMS power rating.

Generally, if you look carefully at an instruction manual, you will find, hidden among all the conflicting claims, a figure that is measured in watts RMS continuous per channel, while two or more of the channels are driven into 8-ohm loads. This is the real power output figure, the one you can trust.

All of which begs the question of how much power is required in a hi-fi system. There is no definitive answer. If you live in a flat and listen at background music levels, 25 watts per channel should be more than adequate. To listen at high sound pressure levels in a large room, 100 watts or more may be required.

Remember that the sensitivity rating of your loudspeakers will also dictate the amount of power you require. A speaker with an 84dBSPL rating requires a 60-watt amplifier to achieve a sound pressure level of 102dB, whereas a speaker with a 90dBSPL rating could achieve the same sound pressure level with only a 25-watt-per-channel amplifier.

Although amplifiers that can produce 1,000 watts and more are available, commonsense dictates an upper limit for domestic applications. Few living rooms would require an amplifier capable of producing more than 120 watts RMS per channel. Irrespective of power output, a good-quality amplifier should return a trouble-free life of 20 years or more, so it's worth investing in the best amp you can afford.

Greg Borrowman is the editor of Australian HI-FI Magazine.