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Old 6th January 2006, 11:01 PM
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Default How do I measure the power output of my amplifier?

How do I measure the power output of my amplifier?

The simplest set up needs, preferably a dual trace oscilloscope, signal generator and 8ohm load resistor. You can check at all audio frequencies or just a few to give you an idea of whether the amp is likely to be working to spec.

Connect the load resistor to the amplifier across the + and negative speaker terminals, connect one probe of the oscilloscope to the o/p of the signal generator and the other to one of the positive and negative loudspeaker terminals. Feed a sine wave signal into the amp increasing the input level until the first sign of clipping or other distortion can be seen on the scope trace at the loudspeaker terminal. Measure the voltage on the scope trace as a peak to peak measurement. Divide this measurement by 2.82 to convert to RMS volts and use Ohms Law to give watts, EG:

peak to peak volts = 44v, divided by 2.82 gives 15.6v RMS into the 8ohm load


Using I = V/R
I = 15.6v divided by 8ohms = 1.95Amps

then using W = Amps x Volts
15.6V x 1.95A = 30.42Watts


Alternatively, since power = voltage squared divided by R, therefore:

Power = 15.6 x 15.6 / 8 = 30.42 Watts

Last edited by John Caswell; 16th July 2011 at 04:48 PM. Reason: Tidied up text