# Testing wattage?

Discussion in 'AV Receivers' started by Jonathan Larson, Mar 28, 2004.

1. ### Jonathan Larson Extra

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Hello,

When a manufacturer claims 100 watts x 2 how do you really know that your channels are 100 watts? Is there some type of device for measuring the wattage?

Thanks

2. ### Nick.S Stunt Coordinator

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I to would like a way to test this. Would really like to test on my car audio amps.

Nick.S

3. ### Craig_Kg Supporting Actor

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They stick resistors across the output (usually 8 ohm, sometimes quoted for other resisitance) and then measure the RMS voltage across them at the THD they want to quote (using sine waves). Then P = V * V / R.

4. ### Robert Cowan Supporting Actor

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you can get some big power resistors from partsexpress.com. they are pretty cheap and will do 100W or 200W i think...(two differnt models).

basically, you put a sine wave into the receiver, put it on max (or before it clips), measure the voltage. the wattage will be the voltage squared, divided by the load (8 ohms). so, if you get 10 volts, thats 100/8 = 12.5 watts.

by this rule, you can tell that the wattage is very dynamic at times... for insance, 10 volts is only 12.5 watts, but 20 volts is 50 watts. it goes up quickly. with this resistor you can amaze yourself by how much (little) power you actually use. try playing a cd at normal volume into a resistor. you may be surprised to find you are only using 5 watts. on normal cone speakers i have only ever seen 15 watts used. planars are different, i have measured them at peaks of 150-250 watts.

back to your question though... doing this on a receiver could prove difficult. you need a few things first... 1.) to know when the amp is actually clipping, 2.) getting a full bandwidth line level signal into the amp (if you send a higher than line level signal, your measurement wont be as accurate), and 3.) testing all channels driven. the resistors are only \$20 each, but if you have 7 channels, that gets costly just to test how much power your receiver is over-rated by

if it sounds good, rest easy. if it doesnt sound good, upgrade.