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2nd March 2012, 06:13 PM  #1 
diyAudio Member
Join Date: Feb 2012

When companies state watts, is it rms or peak?
Just a general question when trying to compare amps from different vendors. Is it standard convention to state your power in rms or peak?

2nd March 2012, 06:56 PM  #3 
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Join Date: Sep 2008
Location: Near Seattle Wa

Oh my. I expect this to be a long drawn out thread. RMS is the right answer buttttt there is much hype, misdirection, and dowright falshood on the part of advertisers. Even if you read the specs carefully and know what to look for, one can be misled on this subject. Best corse to follow is to buy from reptuable manufactures, with respected names, who will most likely quote RMS, with a specific signal input, at a specific load, for a specific leignth of time. Dont even get me started on car amp specs.
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2nd March 2012, 08:05 PM  #4 
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Join Date: May 2007

Apart from a 1970s(?) misleading standard, there is no such thing as RMS watts. What people often mean is the power you calculate from RMS volts or current: this gives average power. Peak power for a sine wave is twice this.
For amplifiers there may be extra confusion because an unregulated supply may mean thet a short tone burst or transient can have more power than a continuous signal. All this means that 'peak power' could be up to about 4 times the continuous average power. A reputable manufacturer will say exactly what their number means. 
2nd March 2012, 08:46 PM  #5 
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Join Date: Feb 2007
Location: Fl

take the fuse amp rating... multiple by voltage... add in efficiency...
ex. 20 amp fuse * car alternator 14.4 * average class ab amp 60% 20*14.4*.6 = 172.8 watts rms not a very exact measurement but its easy to calculate power watts rms is not everything... its gain and what power is needed to properly push a speaker. 
2nd March 2012, 09:23 PM  #6 
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Join Date: Feb 2012

Sorry I don't mean this to become some long drawn out debate now am I saying watts is everything. I'm just in the market for a pro amp and want to compare power to my receiver. For example, my current receiver, a Denon AVR 890 claims 105W per chanel. I'm looking at a QSC 850 which is 200W per channel (into 8 ohms). Just wondering if they are measuring power the same way and if I can really expect double the power from the QSC. Right now my speakers are rated 110W nominal and despite the AVR890's claim of 105W it seems kind of weak for my speakers.

2nd March 2012, 09:41 PM  #7  
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Join Date: Sep 2006

Quote:
It soooh obvious that the product of two rms quantities must also yield an rms quantity. Good luck anyway
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2nd March 2012, 10:02 PM  #8  
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Join Date: Dec 2004
Location: Next door

Quote:


2nd March 2012, 10:15 PM  #9 
diyAudio Member
Join Date: May 2007

If 100W is not enough, then 200W won't be much better. You will only just hear the difference. You can assume that home entertainment equipment will misleadingly overstate output power, but stay (just) within the bounds of honesty.
My system has about 20 genuine watts per channel and is plenty loud enough. 
2nd March 2012, 10:28 PM  #10 
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Join Date: May 2003
Location: quebec

For a resistive load with a pure AC signal : peak voltage multiplied by peak current will give you peak power.
For a resistive load with a pure AC signal : RMS voltage multiplied by RMS current will give you mean power. For a resistive load with a pure AC signal : peak power is twice mean power. Mean power is the correct term for power in an AC system, RMS power is not exact. If the load is not a resistance you will have some phase shift betwen voltage and current, and mean power will be lower. Then mean power is given by : peak voltage multiplied by peak current multiplied by the cosine of the phase shift angle and divided by 2. Or mean power is given by : RMS voltage multiplied by RMS current multiplied by the cosine of the phase shift angle. 
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