how do you test amplifier output wattage

Hey, I have a voltmeter and I know that P=IV. I want to find out roughly what my amplifier's output wattage looks like when music and test tones are playing, and compare it to the rated power handling of my speakers. Basically I want to know how much wattage is used to produce a certain level of performance/loudness in my system, and how much headroom my speakers have as I change the gains. What's the best way to do this? I figure I could just disconnect the speaker from the amp and put the voltmeter across the +/- speaker terminals on the amp and play some tones. This voltage should get bigger with louder tones or sounds right? But having a rough idea of the voltage, what current do I use to calculate the actual wattage that would normally be going into the speakers? Thanks in advance

you will start by using AC current

DYohn wrote: http://www.bcae1.com/measpwr.htm

Dave, Dave, DAVE... you would send business to a competitor? I prefer this calculator. You can find the voltage, you KNOW the impedance, so it'll be easy. In your case, you'd use voltage squared, divided by resistance, to arrive at power.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Hmm I was just thinking... since P = I*V = I*I/R, that means that at the same power, increasing the resistance means decreasing the current. Therefore, suppose I want to drive a subwoofer with 1000 Watts. Then using a 4 ohm sub would demand less current than a 2 ohm sub, though both are putting out 1000 Watts worth of sound power. Does this mean you shouldn't need as powerful an alternator if you were running a 4 ohm sub? And on top of that, running it at 4 ohm is supposed have better sound quality? I think I must be missing something, or else why wouldn't everybody use high resistance speakers which would minimize current draw and improve sound quality? I know that you would need to upgrade the amplifiers to achieve the same power output at higher resistance, but on the other hand you decrease the current draw which means you don't need to upgrade the alternator as much. So if you wanted to run a sub at a particular high wattage (suppose 1000 watts rms), either way you have to choose whether to buy a new amplifier or new alternator. Since a sub at 4 ohm is supposed to produce better sound quality than at 2 ohm, and also since a new amplifier can easily be reused in a new car whereas a new alternator cannot, clearly it seems better to buy a new amplifier instead of a new alternator and use a 4 ohm sub, right? I am assuming that you cannot buy both a new amplifier and new alternator, and that they cost about the same. Is my logic correct? If it is, why do people upgrade to expensive alternators when they could spend the money on an amplifier instead, and run higher resistance speakers, all on the stock alternator, with all the advantages I mentioned, and the same power output either way? Did I miss something?

No, in the long run it will always take power to make power, no matter what the resistance of the woofer is. To make a good UNCLIPPED 1000 watts of power you will generally need 75-100 amps of current.2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

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cim2phat4u wrote: ... since P = I*V = I*I/R,

Sorry I meant to say P = I*V = I*I*R

jeffchilcott wrote: No, in the long run it will always take power to make power, no matter what the resistance of the woofer is. To make a good UNCLIPPED 1000 watts of power you will generally need 75-100 amps of current.

From the way I see it, according to the formula P = I*I*R, you can increase the power two ways. Either increase the current, or increase resistance. If you increase current, you will need to upgrade the alternator. If you increase current, you will need to upgrade the amplifier. Either way, this gives you the same power, and I don't see how using one method would produce clipping compared to the other. Am I wrong? Most people go to upgrade their alternator so they can run their subs at 2 or even 1 ohm, thinking it is increased performance. However, my argument is that it is better to upgrade the amplifier, run the subs at high resistance like 8 ohm, and you can get the same power from your stock alternator due to decreased current, plus you have the benefits of better sound quality and a reusable amplifier. Somehow, my argument seems strange, but I can't figure out how it's wrong.

cim2phat4u wrote: If you increase current, you will need to upgrade the amplifier.

Should read "if you increase resistance, you will need to upgrade the amplifier." God I keep making typoes today SORRY!

OK... You are very confused, it seems... If you increase the resistance say, from 2 ohms to 4 ohms, you have to increase the VOLTAGE to overcome the additional resistance to drive the current THROUGH the resistance to make power. If all you do is increase the resistance, you cannot get the same power through it, as the current will drop, because the voltage was not increased to overcome the increase in resistance. It all goes hand in hand. Plug the numbers into the formulae, and you'll see. Amplifiers are current devices, they regulate the current output to try to maintain a given VOLTAGE on their output rails. This is why power ratings increase when the load increases (this means lowering the impedance), the voltage remains the same, but when you halve the impedance, the amp will try to double the CURRENT to maintain the same voltage across the terminals. Twice the current with the same voltage, means twice the power. It all plugs in together.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."