How to Test the True Output of Amps

Connect the AC signal input device to the amplifier circuit input interface. For example, if this is an electric guitar amplifier, plug the guitar into the input plug. Ensure that the amplifier is turned off, and that the AC signal input device is either turned off or is not generating an AC signal.

Turn on both digital multimeters. Set the measurement scale on the first multimeter to "Volts AC." Set the measurement scale on the second multimeter to "Amps AC."

Attach the red probe from the first multimeter to one of the output leads on the amplifier circuit. Attach the black probe from the first multimeter to the remaining output lead on the amplifier circuit. Ensure that this multimeter is wired in parallel with any attached electrical load.

Attach the red probe from the second multimeter to one of the output leads on the amplifier circuit. Attach the black probe to one of the terminals on the amplifier electrical load. Ensure that no other wires are connected to this terminal on the electrical load.

Turn on the amplifier circuit. Use the AC signal input device to generate a signal. For example, if you are using an electric guitar as the signal input device, pluck one of the strings to create an AC signal; if you are using an AC signal generator, turn on the signal generator.

Check the measurement on the first multimeter, and write this measurement down as the "Voltage" measurement. Check the measurement on the second multimeter, and write this measurement down as the "Current" measurement. Turn off the amplifier, the multimeters, and if necessary, the AC signal generator.

Multiply the voltage measurement by the current measurement to get the total power (measured in watts) that the amplifier produced.