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To accurately measure output power, you will need:
1.A regulated D.C. power supply capable of powering any amplifier that you need to test
2.A set of high power resistors. They must be rated to handle the power output of any amplifier which you need to test.
3.An oscilloscope to view the waveform.
4.Some sort of tone generator. A sine wave generator is a good choice but you may be able to get by with a CD player and a test tone disc.
5.A digital voltmeter capable of measuring A.C. voltage. A true RMS meter would be nice.

NOTE:This information is only to let you know how the test is done. If you have all of this equipment, you will probably already know how to make this test.


Connect the 12v power supply to the power wires on the amplifier.

Connect the power resistors (dummy loads) to all of the channels of the amplifier.

Set the gain controls for all channels of the amplifier to the same level. Either all of the way up or all of the way down will probably be the easiest. You can make fine gain adjustments at a later time.

Connect the tone (sine wave) generator to all of the amplifier channels. Turn the output level of the tone generator all of the way down.

Turn the power supply on. If it is adjustable, set it where you want it (12 volts, 13.8, 14.4 your choice).

Power the remote terminal of the amplifier.

Measure the output voltage of the D.C. power supply (it should be the same as before the amplifier was turned on).

Set all equalization to the off position and set all crossovers to full range.

Set the output frequency of the generator to any frequency that you want. I generally use 100hz. You should always use the same frequency or at least make note of the frequency used during the test.

Slowly increase the output of the tone generator until the output of the amplifier is approximately 2 volts A.C. Now set all of the gains to match the output of all of the channels.

Connect the oscilloscope to any channel (they should all be the same since you tweaked them).

Monitor the output voltage of the power supply, either with your volt meter or by the digital meters on the power supply. If the power supply doesn't have digital meters, use a digital multimeter.

Slowly increase the output of the tone generator while watching the oscilloscope. Increase the level until the top and/or bottom of the sine wave flattens out. Reduce the level until the wave is 'clean' again.

Now see if the power supply voltage has held to the preset value. Reduce the output level of the generator. If the D.C. voltage changed during the test, you must take this into account. A well regulated supply will have held the preset voltage.

Now connect the A.C. voltmeter to the output terminals of one channel. The channel connected to the scope would be a good choice.

Again Increase the level of the generator until just before the amplifier starts to clip. Make note of the voltage reading.

Turn the power supply off.

Quickly disconnect the dummy load of one channel and measure its exact resistance while it is still hot. The value of the resistor will change slightly as it heats up.

Now use the Ohm's law formula, P=E^2/R. If we found that the amplifier drove 35 volts A.C. across the resistor and the resistor had a D.C. resistance of 4.08 ohms, the amplifier produced 300.25 watts RMS.

P=300.25 watts RMS

1.If the D.C. power supply does not hold its preset voltage, the power output will be lower than the amplifier is capable.
2.This test cannot be done using speakers in place of the dummy loads. The speakers will give you a false high reading at most frequencies.

Test Data:
The following data was taken in a test to show how calculating power when using the voltage across a speaker's terminals instead of the voltage across a non-reactive dummy load will result in false output data. The frequency is the test frequency. The voltage is the voltage that was measured across the terminals of the load (either speaker or dummy load). The power is the calculated power using the speaker's nominal impedance (4 ohms) and the voltage across the speaker's terminals at the various frequencies. The voltage across the dummy load will be virtually the same across the range of frequencies so only one voltage reading was necessary. The amplifier is capable of producing only 205 true watts (RMS power). When using a speaker, it appears to be able to produce as much as 376 watts (which it clearly is not capable of doing).

Frequency Voltage Power
35 33.7 284
40 35.86 321
45 35.53 315
50 37.19 346
55 38.0 361
60 38.8 376
65 38.5 371
70 35.88 322
75 33.2 276
Dummy Load 28.6 205

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