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Re: and again

"The proposed test will, after a few seconds, cover all this span of di/dt and dv/dt versus V.
Which is its intended aim."

This would be true ONLY for a split second at a time when the phase relationships of the test signals happened to coincide in a manner similar to a true back-EMF situation.

The condition would come and go SO FAST, that even if you took a snapshot FFT at the exact moment of a particular coincidence, that the actual portion that coincided would be so small that the effect would be buried relative to all of the rest of the signal present.

However, in the real world, the back-EMF current and voltage phase relationships between the amp output and the speaker would be holding steady for any given tone or combination of tones, rather than occuring only for a split moment and then gone again. If this caused heating of the output stage due to those phase relationships, or activation of the VI limiiter circuits, or caused a bias circuit to drift, etc. NONE of that would show up with the injected tones, the fleeting blip of that special relationship that exists with back-EMF would never be there long enough to be able to be measured readily.

"BTW, you could also use a white or pink noise signal to feed the test amp. While it would be good to assess PAs after manufacturing (against some max reference level of the IIM distorsion), it would be less useful for design verification since the casual intermodulation components would just appear like noise."

Of course, with pink or white noise, it would be impossible to separate out the distortion products from the noise floor, while with a test signal like the Phi Spectral, you could get some information as to where the interactions where coming from, and thus have a chance at determining the mechanism for the distortion.


Jon Risch

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