[polar316]

I know that standard (non-audiophile?) surround sound systems and car systems are measured on a different basis than what most people on this forum consider their power output. could anyone explain this to me?

[JimJ]

You'll see "peak" power ratings that really have no bearing in reality. It's an advertising game.

Car audio manufacturers can now elect to follow the CEA-2006 standard, but even that can be played with.

[markC]

Continuous power is the true measure. That means that if an amplifier is rated @ 100 Watts true  RMS, it can do that all day. Not just briefly. (Very broad explaination).

[JLM]

Most use a non-reactive, steady 8 ohm load at 1,000 Hz, which is by far the easiest possible way to test.  This is almost completely opposite of what a speaker load playing music really is.  Musical peaks (transients) are the most severe load for an amp and why you should either oversize the amp or use tubes (that overload is a more "speaker friendly" manner).  Even a tiniest flea amp can power nearly any speaker to "comfortably loud" levels at steady levels, its the transient loadings that show the need for more power to provide a commanding grip on the speaker.

BTW, wattage itself is very misrepresentative.  A better measure of power is decibels of gain, or dBW as it correlates to what we hear.  The relationship between watts and dBW is logarithmic, so ten times the wattage only sounds twice as loud (+10 dB) and twice the wattage only sounds half again as loud (+3 dB).  Unfortunately this is a seldom used specification, but it can easily be calculated.

1 watt = 1 dBW
5 watts = 7 dBW
10 watts = 10 dBW
20 watts = 13 dBW
50 watts = 17 dBW
100 watts = 20 dBW
1,000 watts = 30 dBW

Adding the rated speaker efficiency with a room factor (from -2 dB for tiny/highly reflective rooms to +6 dB for residentially huge rooms) and the amp's dBW should add to 105 - 110 dB to cover the peaks found in real music.

[Duke]

My understanding is that a stereo amplifier must be rated with both channels driver to the same power level, while the rules for rating a home theater amplifier only require the other channels to be on, and then the power delivered by that one channel is multipled by the number of channels.

So suppose I have a power supply that can feed 200 watts of amplifier.  I can build a 100 watt per channel stereo amplifier, or I can build a seven-channel home theater amp and drive one channel to 200 watts while the other six channels are just barely on and delivering less than a milliwatt each.  The rules would allow me to claim 200 watts per channel - 1400 watts total - for that seven channel amp, but its real-world ability to deliver power to all those channels simultaneously actually comes out to about 29 watts per channel.

Now not every manufacturer of multichannel amps takes advantage of this loophole.  The two multichannel amp manufacturers I represent clearly state that theirs are rated with all channels driven.  But the $149 wonders at Best Buy are likely playing the numbers game.

[Wayner]

Congress actually pass a law that home electronics manufacturers are supposed to publish power specifications like this example: 250 watts RMS per channel 8 ohms, 20 to 20,000 cycles @ .02% harmonic distortion with both channels driven.

Of course, many are skirting the rule by publishing 4 ohm specs or come up with some goofy "peak" power rating. Do you really think a 7.1 HT receiver can put out 120 watts for all the channels? I don't and not for one country second. Car audio is a complete joke with total reckless abandon. 500 watts per channel, yeah right. With 50% harmonic distortion, maybe.

Wayner 

[opnly bafld]

Car audio is a complete joke with total reckless abandon. 500 watts per channel, yeah right. With 50% harmonic distortion, maybe.

While I agree many companies vastly overstate the power output of their amps  (a simple check of the external fuse rating is usually quite telling), you might want to check this link and rethink your statement.

http://www.caraudiomag.com/specialfeatures/0202cae_high_power_amp_test/index.html

Lin

[Wayner]

At a quick glance, I never saw one specification quoted that was to the "standard".

Wayner

[opnly bafld]

At a quick glance, I never saw one specification quoted that was to the "standard".

Wayner

Whether you want to believe it or not there are car amps capable of 500w/ch with less than 50% distortion.

Lin

[JimJ]

At a quick glance, I never saw one specification quoted that was to the "standard".

Wayner

There are kW+ amplifiers out there that are under 1% distortion...

Best Buy isn't the whole of the marketplace

[Niteshade]

I absolutely love lower end surround receivers spec's: They proclaim something like 700 watts out, have a skinny cord and only take something like 275 watts to run! AKA: Perpetual motion machine!

Congress actually pass a law that home electronics manufacturers are supposed to publish power specifications like this example: 250 watts RMS per channel 8 ohms, 20 to 20,000 cycles @ .02% harmonic distortion with both channels driven.

Of course, many are skirting the rule by publishing 4 ohm specs or come up with some goofy "peak" power rating. Do you really think a 7.1 HT receiver can put out 120 watts for all the channels? I don't and not for one country second. Car audio is a complete joke with total reckless abandon. 500 watts per channel, yeah right. With 50% harmonic distortion, maybe.

Wayner