Amplifier power ratings explained

Week of November 9, 2003

Q: The concept of amplifier and receiver power baffles me. How do you decide between 35 and 65 watts per channel?

-Tom Maloney, Minneapolis, MN

A: The amount of power you need is based on a combination of three factors: the sensitivity of your speakers, the size of your room, and how loud you want to play your system.

Taking the latter two factors first, the larger your room, the more power you will need. If you want to play very loudly it requires more power as well.

Speaker sensitivity is a specification provided by all manufacturers of high quality speakers. The sensitivity rating has no relation to sound quality as some of the very best speakers have low ratings. Sensitivity ratings simply tell you how much sound a speaker will produce for a given power input.

Sensitivity ratings are given in decibels per watt at one meter, or db/Wm. So, with an input of one watt, a speaker with a sensitivity of 90 db/Wm will produce 90 decibels of sound at a distance of one meter. A sensitivity of 90 is considered average, with ratings of 87 and below considered low sensitivity and above 93 considered high sensitivity. (Note to editors: in db/Wm W is capitalized, db and m are lower case.)

To increase the volume by 3 db, you must double the power. So, using the example above, to make 93 db you would need two watts, to make 96 decibels, four watts.

Most of the time, your receiver is cruising along producing only a few watts. You need extra power for loud explosions in movies, crescendos in classical music, and other highly dynamic passages. Your speakers may need over ten times the average power to recreate these dynamic passages accurately, and if you are playing loudly to begin with, you may need an awful lot of power if you have speakers with a low sensitivity rating.

So, when you are buying an amplifier or receiver, consider your speakers, your room size, and how loud you want to play. If you have sensitive speakers and a small room, you probably will not need much power at all- even twenty clean watts would probably be enough. If your speakers are only moderately sensitive, your room is large, and you want to play loudly, you will need more power.   It is not a bad idea to buy as much clean power as you can afford.

In closing, please be aware that not all watts are created equal. Some retailers may advertise a $150 receiver with 100 watts per channel, but if you look at the fine print, the 100 watts is at a high distortion level. This means the 100 watt rating is not realistic. Among well-known brands, Onkyo, Denon and Harman/Kardon provide accurate power ratings. That is one reason they cost a bit more than competing models. If you have a budget of $499 or more, consider receivers from NAD, Outlaw Audio, and Rotel. These are audiophile brands with very high quality amplifiers that outperform their specifications.   NAD and Rotel are available in specialty shops. Outlaw Audio is only available direct from the manufacturer, found at www.outlawaudio.com.

For more information about amplifiers, see the December 19, 2002 column archived on my website (see below).

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