I seem to remember Modejeski discussing that Futterman 60dB feedback figure in one of the threads on the Music Reference AudioCircle Forum (though dormant, all posts are still viewable), and maybe in one of his three seminars at The Burning Amp Festival, which I encourage everyone to watch on YouTube. All contain a wealth of free information and wisdom, as does the MR AC Forum.
Are all amps being built wrong?
The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?
Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)
So ... why are amplifiers not built as a high impedance current source?
This is an interesting read: https://www.current-drive.info/
Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)
So ... why are amplifiers not built as a high impedance current source?
This is an interesting read: https://www.current-drive.info/