Comments on: What's up with watts: how much power do you need?

The Audiophiliac offers advice about amplifier power

[pgh]

Watt's wrong with this picture

This is a subject near and dear to my heart. In a former life I used sophisticated test equipment to measure amplifier power for a hi-fi company. The thing about wattage is that it's given as a single number when it varies with respect to frequency, distortion level, and load impedance. So, an amp rated at 100 watts at 1% distortion at 1 Khz is probably not as powerful as an amp rated at 100 watts at .05% distortion at any frequency between 20 hz and 20 khz.

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.

[cromeyeller]

Not that bad a method.

I think the method of measuring power output by driving a resistive load, over 20-20khz, gives you a fair, maybe not perfect, evaluation of an amps capacity. Generally, they specify a level of distortion for that output.

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.

[OziIan]

There is only ONE true comparison

I agree with both the others who have replied so far. A watt is a clearly defined term in electrical and electronic fields.

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.

[buzzvader]

what do you recommend?

I'd like to know what amplifiers you do recommend.....whether switched mode power supply and maybe lower in cost than the big bulky amplifiers. I like your reply and believe the same as you do, although how do you get that sweeter sound with solid-state equipment?
Thanks.

[hawaii18]

Hi, like your remarks, funny. I have always wondered about how power amps worked , why and how is the best way to purchase one. In hi school about 20 yrs ago we had a pv amp and it was rated at 300 watts input, and it pumped out the power shaking the floor at quarter power and it was very heavy, like 40 lbs( i know it's not 200lbs heavy). But now i just seen on you tube a new pv amp that weighs like 3 pounds and is rated at 1500 watts.I don't know where all the heat sinks and transformers went, but i was thinking of getting one of the new pv amps for home, even though the quality might not be there ,but who knows better than sound performance than a live band where these amps are going to perform. At home i have a yamaha 360 watt reciever and does nothing compared to the pv and it's heavy.

[defgibbon]

Actually they get away with it

...because of deceitful marketing and lousy retail outlets. I've
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...

[OziIan]

OH? I didn't realise ...

Oh? I didn't realise you could get 5.1 or 7.1 channel audio systems with pairs of 807's glowing next to the 2kg output transformers. :-) Let's not forget that wonderful sun-tan from the mercury vapour rectifyers needed to drive all those valve amps. THEY were the good old days ..... Pfffffssssttttt - another dream from yesterday shattered. Guess I'll have to be careful what I say in future!!! Hehehe.

[mnemonic]

Keep fighting the good fight, I gave up on it! :)

As a former manager of an independant 'mid-fi/hifi' store (we closed in 2003), I used to try to explain this reality to our customers- I even kept a handy reprint of an article from The Absolute Sound demonstrating that with a pair of '8 ohm' 89dB efficiency loudspeakers, in order to accurately render the full dynamic range of a grand piano you'd need 158 watts per-channel (RMS) at minimum; I'd explain what the IHF '78 power spec. was (see OziIan's first post for details) and what it meant, and how no-one in the 'entry-level' price range even used it anymore.

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...

[jstachowski]

And it's not necessarily just watts

And even if an amplifier can deliver 100 W RMS into an 8 ohm load resistor, speakers are actually much harder to drive. Most speakers have lots of weird inductances that cause the impediance to vary. My Thiel's get no where near my home theatre reciever.

[CliffnMichelle]

Let yours ears judge, not the spec sheet.

First off, I always found the Polk Audio education site useful. There is some good writing about power ratings linked here.

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.