Decoding power ratings

[paulspencer]

Gentlemen, I'm sure we all know not all power ratings are created equal! Or if they are all created equal, some of them much more than others!

I've started a little study to work out some rough ball park numbers on how to decode the inflated numbers into more realistic ones. Obviously the average non-enthusiast is going to be tricked into thinking the Pioneer near entry level receiver claiming 130 watts is more powerful than the NAD or HK with 80 watts, but is it really true?

So I've been going through actual specs of some amps where they publish different ways of rating power. A couple of ways I've noticed they can rate to make a bigger number:

1. specifying into 4 ohms where others specify into 8
2. rating at 1kHz rather than 20-20k
3. specifying a fairly high THD number where amp is at the onset of clipping around 1kHz
4. rating that can't be sustained by all channels in a HT receiver

So I'm coming up with conversion factors to apply as a rough guesstimate so that you can multiply the rating by this factor to get a more true rating. By that I mean 20 - 20k, all channels driven with distortion at 0.1%THD or lower.

Here's my initial results:
(take with a pinch of salt of course)

4 ohm rating - 0.6 (multiply the 4 ohm rating by this number to get a guess at 8)
1k rating - 0.88
4 ohm rating @1k at onset of clipping - 0.55

Haven't yet found anything comparing all channels driven info yet.

I'd be interested to see what anyone else can come up with here.

I'm not suggesting anyone base too much on this, just that when you are looking at receivers and you see a modestly priced one claiming 130w, how much do you believe it? If you took a guess they inflated it as much as possible then you might multiply it by 0.55 and get 70w instead.

[TrueBlue]

paulspencer wrote:

I'd be interested to see what anyone else can come up with here.

I'm not suggesting anyone base too much on this, just that when you are looking at receivers and you see a modestly priced one claiming 130w, how much do you believe it? If you took a guess they inflated it as much as possible then you might multiply it by 0.55 and get 70w instead.

One thing I have to mention here is how likely is it that all 5 (or 7 in my case) channels are going to be driven to full volume at any given time. I think it may be a matter of quality power here. Just my opinion here.

TrueBlue

[paulspencer]

Good point, come to think of it, I'm not that concerned about all channels being driven simultaneously either.

[brent_s]

If you look at the manuals' specification section, they'll usually tell you how they arrived at their numbers. To avoid lawsuits, you can rest assured that there's a way for the manufacturer to achieve the power rating in the list of features.

The long answer. The FTC has guidelines for power ratings on stereo amplifiers. However, there are no such rules for multichannel. The good news, so far, is that separates manufacturers, at least the ones I've looked at, continue to build/rate their products at full bandwidth all channels driven simultaneously. As TrueBlue said, that's really a most unlikely scenario, however.

Receiver manufactures, on the other hand, tend to take advantage of the lack of FTC guidance and use some form of 2 channel power measurement as defined in the specifications and then advertise it as X by 5/7 channels. Generally, any two of the channels can meet the rated power/frequency/distortion number, but not 3+ channels. Sound & Vision and Home Theater Mag usually publish 5 channel driven clipping power bench tests.

Probably the simplest way to get a ballpark estimate on all channel power is to look at the receivers max power draw rating, usually on the back panel or in the manual. Unless a receiver has a built-in Mr. Fusion, the output power is going to be less than the input power by roughly 30% efficiency losses in Class AB amps + processor power draw. This ballpark number will work with any type of amp as long as you know the efficiency losses. For example, my Adcom GFA-7000 is rated at max power draw of 1440 watts with 200x5 output...yielding 69% efficiency at max sustained power.

In reality, even the little Pioneer 516 that I picked up for $100 on a Black Friday deal can deliver 100+ dBs at 17' from the L/R mains in my 22'x20'x9' living room that's pretty much acoustically open to the 2nd floor catwalk, dining room, kitchen, and master bedroom. Speakers are little Polk R150s and a pair of DIY NHT1259 subs driven off of an 80wpc Yamaha receiver.

-Brent

[paulspencer]

Brent, interesting answer. I'd like to come back to this one when i have time and try that one out with amps that have specified the necessary details.

[jackfish]

0.88 times the power rating at 1 kHz to estimate the power at 20 Hz to 20 kHz is pretty good. My evaluation came up with .8 to .9 with most closer to the .9, so .88 is likely a good average.

The estimate of power at 8 Ohms from a value at 4 Ohms is not bad as well. My evaluation came up with .5 to .75. However, this seems to have some outliers that probably make it less accurate.

I can't seem to find any numbers on how different THD numbers affect power as most amplifier manufacturers specify a rating at a single THD figure. I think there could be a big difference between an amplifier rated at 300 watts per channel, both channels driven into 8 Ohms at 20 Hz to 20 kHz and 0.1% THD and one rated at 300 watts per channel, both channels driven into 8 Ohms at 20 Hz to 20 kHz and 0.03% THD. I'll keep researching to see if I can find some specifications which permit comparison of power ratings at different THD levels.

[lcaillo]

You are trying to generalize where it probably won't work well. First, THD and power ratings are made into primarily resistive loads that do not behave like loudspeakers. Second, amps behave differently based on their design in terms of the relationship between power and THD. Third, THD may not be the all encompasing measure of performance that most assume that it is. I could go on, but trying to find some relationship that takes numbers that are purposely reported to put an amp in its best light and making that relationship work across products is likely a fools errand. Laudable idea, but not likely to bear much fruit.