Class D amps for SPL measurements - issue in using a feedback probe in 2-channel software

[DavidR]

I had to replace my trusty old Adcom amp for making speaker measurements, SPL and distortion, so I bought a V3 Mono partly for its low inherent distortion and reasonable price. However, I was concerned about using a feedback probe. As it turned out, there seems to be an issue using a probe with this Class D amp. Probes reference ground, but this Class D output doesn't have the negative side grounded, I guess it's BTL after I did some research. I'm using REW for near-field (close mic) tests and plannded to use my old LAUD system for far-field if I can get my old setup working. However, I also have Soundeasy and can use it for far-field, although its not calibrated for absolute SPL as LAUD is. Having had issues with a new Umik-2 below 80Hz (sent in for repair/replacement) I decided to test Soundeasy for near-field as well.

The probe signal was miniscule, but seemed to work. I say seemed, but the signal was there. But as a test I placed the probe on the other speaker terminal. The probe signal was unchanged except for phase, 180 degree change. Not zero voltage/phase as would be the case with a grounded-negative amp. So I ran a test without the probe. The SPL curve remained the same except the absolute value increased about 6dB 15dB Evidently BTL Class D amps with floating output terminals won't work properly with a 2-channel measurement system. I doubt that I can ground one side of the amp output to have a truly grounded reference for the feedback probe. I will use the amp for electronics testing and close-mic speaker distortion, but evidently I can't use it with a 2-channel system for far-field. A bit disappointing. Unless there's some way to modify the setup to do that.