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Old 10th July 2011, 03:11 PM   #1
epilot is offline epilot  United States
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Default Power measurment of a Power amplifier

Hello guys,

Just 2 questions for experts in the field.

How to measure the output power of an amplifier? I want to measure the output power of LM3886T and LM3875T,Please can you tell me how to do so?
I have a 2 channeled 20MHz scope, a 3MHz function generator and 2 multimeter.

The other question by an example:
the power of LM3886 is 68W or for LM3875T is 56W, How they measure it and for which parameters it is? I mean how many voltage and so on..

Thanks for any help
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Old 10th July 2011, 03:12 PM   #2
epilot is offline epilot  United States
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By the way I am not sure if these said powers are RMS or P-P values?
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Old 10th July 2011, 03:56 PM   #3
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
the maximum output voltage is limited by the clipping of the output signal. This is directly related to the Power Supply voltage and the voltage loss through the chipamp.

As the voltage from the mains varies, the apparent maximum voltage at the output will appear to vary.
If want to be able to make comparable measurements, then you must find a way to eliminate the mains voltage variation or at least minimise it's effect on your measurements.
Doing all your measurements at a similar time of the evening may remove much of that error.

Now to measuring the chipamp.
You measure voltage while checking to ensure that the signal is not entering clipping.
A properly designed and assembled chipamp will operate over the full range of output voltage without any load on the speaker terminals.
This is the easiest maximum voltage you can measure.
Set up your input so that it is finely adjustable.
Monitor both the input signal and the output signal on a dual beam or dual channel scope. Check that both waveshapes are substantially similar.
Increase the input until the output just clips. Reduce the input until the output is just below clipping. Measure the Vac across the output terminals.

For all other voltage measurements, you need dummy loads over the range you are interested in.
I suggest 2r0, 4r0, 8r0 and 16r0 for testing a chipamp capable of driving an 8ohm speaker.

You will need to develop a strategy/method that does not overheat the chipamp nor overheat the dummy load, otherwise your measurements will be worthless.

You only measure voltage, not power. Measure the rms voltage at the load. Assume that you can hold your load at substantially the same resistance as it is at cold. Calculate the power from the equation: average power = rms Voltage squared / load resistance, P=Vrms^2/R
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regards Andrew T.

Last edited by AndrewT; 10th July 2011 at 04:03 PM.
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