What's up with watts: how much power do you need?
Amplifier power is measured in watts, as in "100 watts per channel," but what does that really mean? Do all 100 watt per channel receivers deliver 100 watts? And what about those "1000 watt" home theater in a box systems? Are they more powerful than 2,000 A/V receivers? And what about high-end 100 watt per channel high-end power amps? Are all watts created equal? I don't think so!
Pass Labs 200 watt XA200 amplifier weighs 175 pounds
(Credit: Pass Labs)Unfortunately power ratings are a near meaningless way to compare the loudness potential of one receiver, amplifier, or HTIB against another. That's what power gets you--the more power you put into a given speaker--the louder it will play. More watts aren't necessarily better sounding watts; some hard-core audiophiles get off listening to ultra low power, 7 watt per channel, hand-built, vacuum tube amplifiers. These guys think the purity of the watts is where it's at.
Sure, most of us want more, not less power, but the catch is there's no reliable way for a consumer to learn about a given amp, receiver, or HTIB's wattage. Oh, there's one specification that has served me well over the years: weight. Power is heavy, or to be more precise, the stuff that makes power is heavy. I'm sorry, but a 9 pound HTIB amplifier has about as much chance of putting out 1000 watts as I do. I pretty much guarantee that a 44 pound receiver will clobber a 22 pounder when it comes to dishing out King Kong's room shaking antics. Look under the hood of a high-end amp or pricey A/V receiver and you see a humongous power transformer. Powerful amps need big transformers to pull a lot of power from the wall outlet, and high current transformers are always heavy things. The other power supply related parts are also big, heavy, and, well, not cheap. Fact is, most $500 A/V receivers never come close to delivering their spec-ed power into their seven channels. Some can barely manage a third of their claimed wattage.
The reasons the manufacturers "get away" with this outright fraud are numerous, but the reality is most buyers don't listen all that loud. If you do and/or live a big house or apartment, or have speakers that demand oodles of juice, pony up and buy a serious receiver or better yet, a separates based system with a surround processor and heavyweight power amp. One hundred watts per channel might not be enough, or 200 might be better, but an honest-to-God 300 watts might be what it takes to rock your world. Even if it's just the occasional party where you really want to blast the system, you need to buy enough power. When in doubt, go for more, and don't worry about damaging your speakers with too much power. Fact is, more speakers get fried by underpowered amps than by uber power amps. In other words, you're more likely to "blow up" a speaker with a 25 watt amp than a 250 watter.
Steve Guttenberg is a frequent contributor to magazines and Web sites including Home Entertainment, Playback, and Ultimate AV. He is a member of the CNET Blog Network, and is not an employee of CNET. Disclosure. 
For conventional amplifier designs (class AB the most common design), your relation of mass to power is a good rule of thumb. However there are low-mass amplifiers that can produce huge amounts of power such as class D switching amplifiers, so make sure you are comparing apples to apples.
Weighing the unit is clearly not accurate, since it doesn't account for the use of toroidally-wound transformers vs. heavier laminated core transformers. I'm not sure that heat sink efficiency is that closely tied to weight, either.
I've listened to a couple of low-powered tube amps, and they will not please most people. They will break up severely when driven with a loud bass note or tympani whack.
The ONLY way to compare amplifier to amplifier is to use the same test procedure.
Way back when amplifier watts were meaningful, it was true RMS watts into a stated resistive load; a stated frequency range (20Hz-20kHz); a stated power level deviation over that range (usually -3dB); a stated distortion figure (often 0.1%); a stated signal-to-noise figure (often 90dB or better); and, MOST IMPORTANTLY, both channels driven simultaneously, and from a stated mains input voltage or range. Top quality amps were always rated this way. I usually refer to these watts as "English" watts. Yeah, yeah ... I come from the stereo generation.
"Japanese" watts, or PMPO, are totally meaningless, no matter how much one tries to spin-doctor the reasons for presenting amplifier performance this way.
Anyone who needs more than 50 TRUE "English" watts per channel (as defined above) is either:
a) deaf
b) a poser who wants bragging rights
c) using extremely inefficient speakers
d) trying to re-create the Woodstock experience, (and in suburbia that will have police knocking on your door in 5 minutes flat).
As to weighing amps ... hmmmmmm ... don't think so. That's as illogical as PMPO watts - totally meaningless.
Maybe you haven't heard of SMPS (or swithced mode power supply) where there are NO bulky iron-cored mains power transformers, torroidal or otherwise.
No-one uses output speaker transformers these days, so that is a non-event.
Maybe you haven't heard of fan assisted cooling where there are minimal heatsinks for quite large output powers.
I'll not even comment on valve amplifiers other than to say they make great room heaters or anchors. :-)
As an article for the technically challenged at best it is a very rough guideline. As an article for the technically informed, there are a lot of holes in it.
Thanks.
heard plenty of 1000 watt home theater in a box systems at owners homes that
routinely clip and distort horribly. Customers never question the rated power
(brand x and retailer y wouldn't lie to me!) they live with it, then when it's time
to upgrade they figure they need at least 2000 watts.
BTW Steve, I'd say your audience is clearly on the silicon side of the amp divide...
I'd go on to explain why good-quality power amps were heavy- big power transformers and filter capacitors have some serious mass, and heat-sinking all those transistors meant lots of extruded aluminum or 'pot' metal.
I do take issue with the posts here about 'not needing' those watts- you're forgetting that more watts also = higher dynamic range capability and more transient headroom, as well as keeping the amp circuit within its safe operating range.
As a personal example, my HT system includes a Yamaha reciever running in 'pre-amp' mode, driving Rotel amps rated at 250wpc (8 ohms/20-20K), and even at "reasonable" volume, its dynamic capability routinely outclasses the reciever by itself- and it's something that's easily apparent to listeners, particularly on dynamic source material.
Even an dynamic acoustic guitar recording, like Michael Hedges' Taproot for example, sound much more lively- his percussive sound-board taps, etc. seem to 'jump' out of the loudspeakers. Does everybody *need* this kind of power? Of course not, but then again, most people are also happy with 96Kbit MP3's...
- Let yours ears judge, not the spec sheet.
- by CliffnMichelle August 29, 2007 8:11 AM PDT
- First off, I always found the Polk Audio education site useful. There is some good writing about power ratings linked here.
- Like this Reply to this comment
-
(10 Comments)http://www.polkaudio.com/education/article.php?id=4
At the end of the day, the best method for the consumer is pretty consistent. Always audition your gear, and let your ears be the judge, not the numbers on the spec sheet.