I am writing for advise about measuring Amplifier Power Output.

I want to find out where I might have gone wrong when I measured the Output Power of an amplifier, and got odd results when compared with the specification.

I will not mention the Model or Make of the amplifier, as I do not think this is relevent and am measuring the output out of my own interest and curiosity for a better understanding.

The Only Output specification (from a service manual) I have available to me is written as follows.

With 8 ohm loads, both channels driven from 20 to 20KHz, rated 120W per channel minimum RMS power, with no more than 0.6% total harmonic Distortion from 250mWs to rated power.

The input sensitivity is not metioned.....only Input S/N= 96dB to CD, Tape, etc...inputs.

Test Equipment In Use

1. Hameg 60MHz HM604 Oscilloscope
2. Leader LAG-120B Audio Generator
3. Leader LMV186A 2 Channel AC Millivoltmeter
4. Audio Test Disc (with many test tones of varying levels)
5. Wire wound 8 ohm resistors X 2.

MY Method of Measurement

1. Input a 1KHz stereo tone at 0dB, from a test CD disc.
2. Turned the volume to minimum.
3. Connected the amplifier outputs to 8 Ohm Dummy resistors.
4. Connected my Oscilloscope across the resistors.
5. Connected an RMS Voltmeter across the resistors....on 100V range.
6. Turned up the volume gradually until the sinusoidal waveform clips and then backed it off to the point just before clipping.
7. Took a note of the volt meter reading.....50V RMS (Scaled in RMS).

I then used the formula for power......P = V squared / 8 ohms.....2500/8 = 312.5 Watts.

This result is well over the 120W RMS quoted per channel in the specification ........
Where do you think I have gone wrong?

The 8 ohm wire wound resistors are very old large High Wattage wire wound type....would this be a problem as it's not a true resistance....maybe the impedance would be a problem?

I also used the Audio Generator with 1 Volt / 2 Volt 1KHz outputs but stll got odd readings .....far off the mark to give a 120Watt mimimun.

I would appreciate any help that could be povided here.

Thanks
Dermot Hayes