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Measuring amp output for watts RMS

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Old 07-04-2011, 08:20 PM
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Impedance is not going to be 4ohms when you play a tone through the speaker.

Use an oscilloscope for true RMS.
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Old 07-05-2011, 10:01 PM
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Originally Posted by Zoomer
disconnect the speakers? Then you have no load for the amp to power and all you are doing is measuring open circuit voltage.
True, I goofed there and posted without thinking. Measuring open circuit voltage is not so bad, it's the lack of being able to tell when clipping occurs. If the amp has a regulated supply the voltage rails would not sag much (if at all) under load and so the open circuit voltage could be used for calculations. Even an unregulated amp would give measurements that are fine for what the OP wants to know. I doubt the he needs a high degree of accuracy.

Originally Posted by BradSk88 View Post
Impedance is not going to be 4ohms when you play a tone through the speaker.

Use an oscilloscope for true RMS.
The rated load (4 ohms) is fine for calculations. A scope is the ideal but anyone who asks how to determine power output isn't likely to have access to one.
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Old 07-05-2011, 10:56 PM
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Originally Posted by BradSk88 View Post
Impedance is not going to be 4ohms when you play a tone through the speaker.

Use an oscilloscope for true RMS.
This should have been the first comment in this thread.
Unless you can measure current you'll never know what your amp is outputting. At least the scope will tell you if you're clipping.
Speaker impedance varies too much with frequency, which you can ignore if you know the current, and voltage. Without that, you're just wasting your time. You're not going to get any useful numbers, just numbers that you think are useful.
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Old 07-06-2011, 08:11 AM
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Originally Posted by Njord View Post
This should have been the first comment in this thread.
Unless you can measure current you'll never know what your amp is outputting. At least the scope will tell you if you're clipping.
Speaker impedance varies too much with frequency, which you can ignore if you know the current, and voltage. Without that, you're just wasting your time. You're not going to get any useful numbers, just numbers that you think are useful.
Depends on the degree of accuracy required. The generic shape of a (moving coil) speaker impedance curve is well known. If you have the published Rs you can make a reasonable guess at the optimum test tone frequency to avoid impedance issues. There is a region above resonance where the inductance is not yet significant.

If you only want to know if you are getting close to the advertised output of your amp then a scope is not required.
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Old 07-06-2011, 10:37 AM
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bunch o' nerds in this thread
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Old 07-06-2011, 02:25 PM
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Originally Posted by wasted911 View Post
bunch o' nerds in this thread
LOL... guilty
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