|Originally posted by ppl
Wodgy please explain why you think Feedback Kills micro-details. I keep hearing all this fuss about how good no feedback designs are yet every one I have listened to sounded just like they measure Bad. YMMV
I know the math, and I know it's not supposed to make a difference, but it seems to. Check out Erno Borbely's website for some of his published designs; he always strives to minimize feedback as long as it doesn't negatively affect the measurements too much. (i.e. He seems to believe there is a tradeoff between measured performance and the audible effects of excessive feedback.)
You're also undoubtedly right that a big part of the improvement is because the op-amps have dirty power. Keep in mind this is inside a computer, and the existing power filtering on the card is fairly poor; also, no attempt has been made to separate the analog and digitial grounds. With those handicaps, removing the op-amps is clearly the right engineering choice, rather than attempting major redesign of the board in an attempt just to keep the op-amps in there, but make them sound good.
The other thing is, even in the best circuits, with great power supply design and proper implementations, op-amps are just not transparent sounding.
If we went by their datasheets, we'd have to conclude that good op-amps are basically interchangeable, and should make virtually no difference in sound. Your own PPA (which is presumably a proper, careful implementation), demonstrates that this is untrue. Swapping op-amps in the PPA changes the sound. Thus, we must conclude that even when properly implemented, op-amps are not transparent sounding. This is another argument for getting rid of them, unless there is a concrete engineering reason to keep them there.
(I think it's sort of strange that some people argue that capacitors shouldn't be in the signal path, but then have no problems with op-amps in the signal path. Both should be minimized.)
Iron's measurements clearly have not deteriorated (some improved, some were worse) and what's more, the thing actually sounds better. Seems like a good engineering choice to me.