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Multimedia Answers: Watts, ohms, kilohertz, distortion? Is the stereo good or not?

Thursday, December 19, 2002

By Don Lindich

Q: Why do I see shelf stereos advertising 500 total watts for $300, and across the store I see 500-watt surround receivers for $1,000? Why so much difference in price if the watts are the same?

A: The power ratings given for receivers and shelf systems are some of the most confusing concepts for people learning about audio reproduction. Anyone would wonder how the complete shelf stereo system for only $200 can boast "300 watts" when a quality stereo receiver from a manufacturer may sell for $400 and may be marketed as having only 30 watts. Is everyone telling the truth? It depends on how you look at it. Some of these examples are definitely stretching it a bit.

The Federal Trade Commission regulates how power ratings are advertised for the marketing of amplifiers and receivers. The amplifier must have a stated power level in watts, a distortion level, a frequency or frequency range where the measurement is taken and an impedance. (The impedance is the electrical resistance of the speaker. A lower-impedance speaker requires a more solidly constructed amplifier, but in return, the amplifier produces more power into the lower resistance.)

The next time you see a shelf system advertising "200 total watts," look at the fine print. It will probably say something like this: 100 watts per channel, 6 ohms, 1 kilohertz, 10 percent distortion.

What is the fine print saying? "This thing doesn't produce anywhere close to 100 usable watts per channel."

Ten percent distortion is an incredibly high amount. Listening to something being reproduced with 10 percent distortion would be very unpleasant.

One kHz is only one part of the audible range. Humans can hear from 20 hertz to 20,000 Hz. If the amplifier specification gives only one frequency, you can bet it produces less over the entire audible range.

Finally, 6 ohms is a low impedance for rating an amplifier. For years, the default standard for amplifier power ratings has been 8 ohms.

What can we conclude from this? This system doesn't produce anything close to 100 real-world watts per channel. Changing the measurement parameters has skewed the power rating to an unrealistically high point. What's the true power rating? There's no real way to find out. When buying a compact system, ignore the power ratings and just buy the one that sounds best to you.

Next, looking at the fine print on the $200 five-channel receiver, we see: 100 watts per channel, one channel driven, 8 ohms, 1 kHz, 1 percent distortion.

This one probably performs much better than the shelf system. The power rating is given into 8 ohms, the industry standard. One percent distortion is a lot less than 10 percent, but is still considered high. It's also measured at the same 1 kHz -- just a small slice of the audible spectrum. Also note that the power is given into one channel.

That isn't going to happen when you are listening to stereo music (two channels) or surround sound (up to five). What this is telling you is that it can't deliver that power all at once to all the speakers.

Finally, let's look at a $1,200 home theater amplifier: 100 watts per channel, all channels driven, 8 ohms, 20 Hz to 20 kHz with less than 0.09 percent distortion.

Here, we have a winner! The power is delivered to all five channels over the entire frequency range, into 8 ohms, at a very low distortion level. Anything under .1 percent is considered undetectable by human ears.

Always read the fine print when comparing amplifiers. There's a reason those watts are so cheap in some of these products. It's because they aren't real watts. They are advertising watts.


Have questions about audio, video, computers or photography, or need some help using and enjoying what you already own? Don Lindich welcomes your questions and will be answering them in this column. E-mail him at don@multimediaanswers.com.

Click here for an archive of previous Interact articles

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