Jcarr, I think you may find this article by Nelson Pass to be interesting:
https://passlabs.com/articles/audio-distortion-and-feedback
It seems to re-enforce some of your comments above. Norman Crowhurst wrote about some of this as well, although I'm don't have a link handy, I suspect I could find the article on Pete Millet's website given enough time :)
Essentially though, the issue as I see it is that as you add feedback, the problem/solution is in the way the 5th 7th and 9th harmonics are handled, but you also have concerns with intermodulation at the feedback node. This can result in a harmonic and *inharmonic* noise floor, with harmonics up to the 81st (although the circuit may lack the bandwidth for that).
The concern I have has to do with human hearing rules. The 5th, 7th and 9th are measured by the ear brain system to determine how loud the sound actually is; so if they are altered even in very tiny amounts the sound will be artificially louder and brighter than it really is.
There is also the issue of detail- due to the ear's masking rule, louder sounds can block the presence of quieter sounds. If the louder sounds are distortion then you will have less detail. It turns out that the one exception to the masking rule is the ability to hear into a noise floor composed of hiss; the ear can hear about 20 db into such a noise floor (something usually ignored by digital advocates). I suspect that this may be because analog hiss has a lot in common with wind noise, something with which our ear/brain system is very familiar. But if the noise floor is composed of harmonic and inharmonic noise, the detail below it will be lost as the ear cannot hear into that kind of noise floor.
If you want a link to Crowhurst's comments regarding this feedback phenomena, I can find it but may not be able to take the time until after CES.
Have a good show!
https://passlabs.com/articles/audio-distortion-and-feedback
It seems to re-enforce some of your comments above. Norman Crowhurst wrote about some of this as well, although I'm don't have a link handy, I suspect I could find the article on Pete Millet's website given enough time :)
Essentially though, the issue as I see it is that as you add feedback, the problem/solution is in the way the 5th 7th and 9th harmonics are handled, but you also have concerns with intermodulation at the feedback node. This can result in a harmonic and *inharmonic* noise floor, with harmonics up to the 81st (although the circuit may lack the bandwidth for that).
The concern I have has to do with human hearing rules. The 5th, 7th and 9th are measured by the ear brain system to determine how loud the sound actually is; so if they are altered even in very tiny amounts the sound will be artificially louder and brighter than it really is.
There is also the issue of detail- due to the ear's masking rule, louder sounds can block the presence of quieter sounds. If the louder sounds are distortion then you will have less detail. It turns out that the one exception to the masking rule is the ability to hear into a noise floor composed of hiss; the ear can hear about 20 db into such a noise floor (something usually ignored by digital advocates). I suspect that this may be because analog hiss has a lot in common with wind noise, something with which our ear/brain system is very familiar. But if the noise floor is composed of harmonic and inharmonic noise, the detail below it will be lost as the ear cannot hear into that kind of noise floor.
If you want a link to Crowhurst's comments regarding this feedback phenomena, I can find it but may not be able to take the time until after CES.
Have a good show!