# Power measurement of Audio circuit

#### duykwe

Joined Nov 17, 2023
71
Hello everyone,
I am trying to measure the maximum power of the board and the power of the speakers with the oscilloscope. Here is what I mean to do:
First, I get the maximum power of the speakers by using the source of 1000Hz sine wave. Next, I put the 2 probes of the oscilloscope and stuck them to 2 leads of the speaker. Then I use VAC view to see the max voltage. Then I can determine the power by using the formula P =U^2/R.
For the power of the board, I also have a DC power supplier. I still use the input source as same as above. Then, I can get the board power by using the formula P =UI.
Is this right, I hope you guys can help me.
Thank you so much!

#### MisterBill2

Joined Jan 23, 2018
18,584
The voltage measurement connection method is not clear to me. When you say " I put the 2 probes of the oscilloscope and stuck them to 2 leads " is that two channels? With two waveforms displayed?? OR are you using a differential input to the scope??
One other thing is that the speaker impedance is a function of frequency for a standard type of electromagnetic speaker, because it is an inductive load.
Measuring the DC input power at full output and then subtracting the zero-output power will give a reasonable approximation of the power delivered to the load., but it will also include the power converted to heat, which may, or may not, be quite a bit.

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#### duykwe

Joined Nov 17, 2023
71
The voltage measurement connection method is not clear to me. When you say " I put the 2 probes of the oscilloscope and stuck them to 2 leads " is that two channels?
The audio amplifier IC I use has bridged outputs with no ground. So the two probes are put to the two polarity of a speaker.
subtracting the zero-output power
What do you mean here?

#### Ian0

Joined Aug 7, 2020
9,842
I can determine the power by using the formula P =U^2/R.
You could, if the loudspeaker were a resistor, but if it isn't.
If it were a pure resistor, it wouldn't make any noise.
If you need to measure the power, you must measure the voltage and the current, multiply the two together then take the average.

#### MisterBill2

Joined Jan 23, 2018
18,584
You could, if the loudspeaker were a resistor, but if it isn't.
If it were a pure resistor, it wouldn't make any noise.
If you need to measure the power, you must measure the voltage and the current, multiply the two together then take the average.
The power with a reactive device, such as a speaker, would require you to know the impedance of the speaker at the test frequency. So really you need an 8 OHM resistor with an adequate power rating. I have one someplace, it is in an enclosure to keep it from damaging things with the heat..

And, for the TS who asked: "What do you mean here? subtracting the zero-output power" The answer is that the amplifier draws current even when it is not delivering any output. So that power is not related to the output power, meaning that it needs to be subtracted from the power drawn when the amplifier is delivering a signal to the speaker. If the amplifier were 100% efficient this subtraction would not be needed. BUT if the amplifier were 100% efficient it would not sound good. at all.

#### schmitt trigger

Joined Jul 12, 2010
905
Like someone has previously mentioned, a speaker is a complex impedance, which also changes with frequency. The most accurate way is to get a power resistor, of either 4 or 8 ohm, and measure the voltage across it.
If you have a 2 channel scope, use the ChA - ChB function to obtain the total voltage across the load.

#### KeithWalker

Joined Jul 10, 2017
3,097
Hello everyone,
I am trying to measure the maximum power of the board and the power of the speakers with the oscilloscope. Here is what I mean to do:
First, I get the maximum power of the speakers by using the source of 1000Hz sine wave. Next, I put the 2 probes of the oscilloscope and stuck them to 2 leads of the speaker. Then I use VAC view to see the max voltage. Then I can determine the power by using the formula P =U^2/R.
For the power of the board, I also have a DC power supplier. I still use the input source as same as above. Then, I can get the board power by using the formula P =UI.
Is this right, I hope you guys can help me.
Thank you so much!
Does this mean that you get the maximum by increasing the power until the speaker smokes?

#### MisterBill2

Joined Jan 23, 2018
18,584
OK, as the amplifier uses an output not ground referenced, the TS is using the differential mode on the scope. So that part is exactly the right way to do it. And I guess listening and stopping just before distortion is noticable. A reasonable approach.