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Missed a variable or two....

Not only does the power amp have to contend with the back EMF from the loudspeaker, it has to contend with yet another signal that gets generated independantly from the direct input signal to that amp channel: sound waves from other speakers in the room, which would include the OTHER speaker of a stereo pair, or a subwoofer, or any other active speakers in the room.

In order to create these conditions in a test of a power amp, I do not believe it is sufficient to use another power amp as the generator source into the DUT power amp channel. I believe that it would be much more realistic to use an actual loudspeaker system, and have it be exposed to the output from yet another speaker, in order to fully expose the DUT power amp to what it experiences in the real world.

Thus, the DUT power amp channel would be exposed to:
the original input signal,
the back EMF from it,
the acoustic output of the other speaker generating a completely different signal feeding into the output of the DUT power amp channel,
AND
the sound wave vibrations from both speakers!

A test bench and a load resistor, or even just another power amp, is NOT going to duplicate this more complete scenario.

Of course, the icing on the cake would be to use some Phi Spectral Multitone test signals for both of the stimuli, using different frequency bands on each of the independant stimuli so as to more clearly see where the distortion is originating from.

Phi Spectral Multitone info can be found in:
AES preprint #4803
"A NEW CLASS OF IN-BAND MULTITONE TEST SIGNALS"

with some info at:
http://www.geocities.com/jonrisch/PhiSpectral1.htm

Jon Risch

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