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

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DYohn] wrote:

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https://www.bcae1.com/measpwr.htm

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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.

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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.

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cim2phat4u wrote:

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... since P = I*V = I*I/R,

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

jeffchilcott wrote:

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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.

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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:

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

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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.

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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."

haemphyst wrote:

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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.

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That's why I said you would need to upgrade the amplifier, to allow increased voltage to overcome the increased resistance, to maintain the same power output without increasing current draw. Is this wrong? What I'm saying is that, to produce the same power output, you can either increase the current, as you describe (ie upgrade alternator), or increase voltage (ie upgrade amplifier). I don't see how this is wrong.

You have things mixed up.

You should study up on Ohm's Law, and this site has some help tools in that regard.  www.bcae1.com is a good site that will explain how it all works together.

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cim2phat4u wrote:

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That's why I said you would need to upgrade the amplifier, to allow increased voltage to overcome the increased resistance, to maintain the same power output without increasing current draw. Is this wrong? What I'm saying is that, to produce the same power output, you can either increase the current, as you describe (ie upgrade alternator), or increase voltage (ie upgrade amplifier). I don't see how this is wrong.

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Partly right... To increase the voltage, yes, you would purchase an amplifier rated for higher power for an equivalent load i.e. 100 watts at 4 ohms vs 200 watts at 4 ohms. The 200 watt @ 4 ohm amplifier would provide you with 100 watts at 8 ohms, yes, with all of the good things that go along with running an amplifier at higher impedance - lower heat, better THD figures, better damping, better S/N ratio, improved efficiency.

Here's where you are (and please, this is only for teaching purposes...) wrong. Whether you upgrade the alternator or not, (wow... loss for words...) umm...

OK, (got it...) the input current demand will ONLY increase if the output power increases... (again, by 1: ingreasing the load (lower impedance) or B: increasing output voltage across a given load) What I am saying here is that the input current will not change unless required by the output power increasing... the DEMAND for input current will not go up (increasing output), just because the input power supply CAN provide it, i.e. a High Output alternator. Just because you put a high output alternator in your car, the amplifier won't automatically increase their output power. I think this is what you are saying... Please, if I am misinterpreting your thoughts, let me know. There are two ways to get more power, no more.

1: Increase the load by lowering the impedance.
2: Swap amplifiers for a higher powered amplifier.

Now, if the input power supply is "stiff" enough, there will be no input power drop, and in all of my illustrations above, I am assuming a "perfect" power supply - infinite current at any given voltage - in the car, 14.4 volts DC. If the power supply is not perfect (and we all know it isn't, that's why a HO alternator - closer to perfect) the input voltage will sag, if the available current cannot maintain said voltage.

Please read my sticky at the top of the page - "How To Choose An Amplifier". I cover lots of this in more depth, (especially the part about running higher than rated impedances) in there.

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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."

cim2phat4u wrote:

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[QUOTE=haemphyst] That's why I said you would need to upgrade the amplifier, to allow increased voltage to overcome the increased resistance, to maintain the same power output without increasing current draw. Is this wrong? What I'm saying is that, to produce the same power output, you can either increase the current, as you describe (ie upgrade alternator), or increase voltage (ie upgrade amplifier). I don't see how this is wrong.

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"To produce the same power output" no matter the load impedance requires the same power input.  If the load impedance changes for the same power output the net power does not change, even if the voltage and current levels do: meaning 100 watts delivered from any amplifier into 4-ohms is the same power as 100 watts delivered into 8-ohms.  Anything else would be a magical perpetual motion machine, and even though Tesla claims to have invented one I ain't seen it yet.    The only way to change the input requirements for any fixed output level  is to use an amplifier with higher operating efficiency.

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