Measuring Amplifier Power Output

[SQ ABSOLUTE]

I was "trying" to measure the power output of my JL Audio 1000/1 amplifier, without a dummy load attached to its output terminals, and came up with the following:

Test Meter: Fluke 87-V
Test Voltage: 12.57 Volts (engine off)*
Test Voltage: 13.99 Volts (engine idling)*
Test Signal: 40 Hz, 0 dB
Speaker Load: 4 Ohm
Distortion: <3% as tested with gain overlap

Using P=V^2/R

Power=68.5*68.5/4

=1173 Watts

What is wrong with the above, seeing that I did not use a dummy load (because I do not have one) Is the above accurate, not accurate, etc.

*Voltage across the speaker terminals is independent of battery voltage. This amplifier has very tight power supply regulation, without the dummy load, of course.

Your input would be greatly appreciated.

As usual, thanks.

[wintermute]

Your meter is measuring in V RMS I would assume.

14V drop across 4 ohms is 3.5A V=IR

P = IV so power = ~ 49W

Even this is not accurate though as you would need to know the resistance of the VC at 40Hz as it may be significantly higher than 4Ohms depending on the resonant freq of the speaker.

Tony.

[SQ ABSOLUTE]

Yes, those are TRUE RMS voltage readings.

I am not following the rest of what you mentioned. I was referring to a resistive load of 4 Ohm's. A resistor's value does not change at varying frequencies. It resistance remains constant. However, an inductive load which a speaker is, its resistance (impedance) will change at different frequencies which is what I understand you seem to be indicating.

That's not the issue here.

[ppia600]

If you used the speaker as a load its a fair indication, of course not exact. I've done similar for checking random amps in customers' cars and the formulas always came close to rated output and were usually slightly above for quality amplifiers.

[cocolino]

The 14V (13,99) You stated is the car`s battery DC voltage (= amps supply DC voltage) and the 68,5V are the (max.) AC-Volts (RMS) which You measured at the amps unloaded speaker terminals, right?

If so the result of 1173 Watts is theoretically correct but with a connected 4 Ohms load this is only possible if the amp can provide the corresponding amount of current (Amperes).
In reality the amps supply voltage will also "sag" quite a bit with such a heavy (>1kW) load which means with an attached load You won`t quite reach this 68,5V(RMS) out You measured and therefore You also won`t quite get this 1173W output power figure anymore. How much the voltage will sag depends on the "stiffness" of the amps power supply. When it`s regulated as You say it will (should) sag only little.

This JL Audio amp is specified at 1000W / 1,5 to 4 Ohm.
That`s slighty under 64VRMS at 4 Ohms load.

So there appears nothing wrong with Your measurement and the 1173W calculation without load (except that nobody ever needs such an insane amount of power in a car, at least not for "only" hearing music ) and it`s reasonable close to the specs with load.

[SQ ABSOLUTE]

Originally Posted by cocolino

The 14V (13,99) You stated is the car`s battery DC voltage (= amps supply DC voltage) and the 68,5V are the (max.) AC-Volts (RMS) which You measured at the amps unloaded speaker terminals, right? Correct.

If so the result of 1173 Watts is theoretically correct but with a connected 4 Ohms load this is only possible if the amp can provide the corresponding amount of current (Amperes). I understand now, when you used the word "theoretically". So to see how much deviation, if any, from the above theoretical formula, we use a dummy load attached to the amplifier's output. This makes sense, now. Thanks.

In reality the amps supply voltage will also "sag" quite a bit with such a heavy (>1kW) load which means with an attached load You won`t quite reach this 68,5V(RMS) out You measured and therefore You also won`t quite get this 1173W output power figure anymore. How much the voltage will sag depends on the "stiffness" of the amps power supply. When it`s regulated as You say it will (should) sag only little.

This JL Audio amp is specified at 1000W / 1,5 to 4 Ohm.
That`s slighty under 64VRMS at 4 Ohms load.

So there appears nothing wrong with Your measurement and the 1173W calculation without load (except that nobody ever needs such an insane amount of power in a car, at least not for "only" hearing music ) and it`s reasonable close to the specs with load.Thanks for your very clear explanation. I understand now. I have searched long and hard and never got an explanation quite like that..

[SQ ABSOLUTE]

Originally Posted by ppia600

If you used the speaker as a load its a fair indication, of course not exact. I've done similar for checking random amps in customers' cars and the formulas always came close to rated output and were usually slightly above for quality amplifiers.

There are two (2) problems with using a speaker as a load.

Problem Number 1
You cannot turn the volume full tilt .

Problem Number 2
The speaker's impedance varies with frequency, so you will get different voltage readings and therefore power output readings at varying frequencies. The readings will be all over the place.

With a dummy resistive load connected to the amp's output you will get a stable reading on your meter as resistance does not change with frequency, provided you use a TRUE RMS meter and the test frequency is within the AC bandwith of the meter.

I believe on Perry's website bcae1.com he has a graph of what happens when you try to use an actual speaker as a load and try measuring the amplifier's power output.

[wintermute]

Sorry I must have misunderstood your post. I thought that you were saying that you did not have a dummy load and were using a 4 ohm speaker as the load (hence the comment about not being accurate as a speaker is not a resistive load). The apparent resistance at the resonant freq of the speaker is much higher.

Secondly I thought you were saying that the voltage measured across the speaker was 13.99V which I rounded up to 14V

for 65.8V which looking again I realise is your voltage across the speaker, it is a different story

I did think 14V was rather low

Tony.

[ppia600]

Originally Posted by SQ ABSOLUTE

There are two (2) problems with using a speaker as a load.

Problem Number 1
You cannot turn the volume full tilt .

Problem Number 2
The speaker's impedance varies with frequency, so you will get different voltage readings and therefore power output readings at varying frequencies. The readings will be all over the place.

With a dummy resistive load connected to the amp's output you will get a stable reading on your meter as resistance does not change with frequency, provided you use a TRUE RMS meter and the test frequency is within the AC bandwith of the meter.

I believe on Perry's website bcae1.com he has a graph of what happens when you try to use an actual speaker as a load and try measuring the amplifier's power output.

Your first post was a lttle back and forth and I totally understand how speaker/driver impedance changes depending on amp frequency output and speaker used. You also list a 4 ohm speaker as being used for your measurement which is better than NO load. I was simply stating your measurement was fairly accurate and now you're going to dispute what I said? Wow, nevermind

[SQ ABSOLUTE]

Originally Posted by ppia600

Your first post was a lttle back and forth and I totally understand how speaker/driver impedance changes depending on amp frequency output and speaker used. You also list a 4 ohm speaker as being used for your measurement which is better than NO load. I was simply stating your measurement was fairly accurate and now you're going to dispute what I said? Wow, nevermind

My apologies for my first post being a little back and forth and misunderstanding the point of your post.

There is really nothing wrong with how you go about your testing. People do it all sort of ways and seem quite happy with the results. If you are happy with the results, then I see no reason to change. Sorry, if you took offense to what I said.

I was just trying to find out if there was anything wrong with the way "I" was going about things, and someone explained everything to me in great detail.

Again, sorry. It was not my intention.