Bombaywalla:
I was trying to keep it simple to make a point. Most amps, powerful or not, have anywhere from three to five gain stages (very few have two), and we all know that using global feedback with such designs makes the amp builder's job a lot easier. I do not agree that feedback ultimately lowers distortion - it changes the types of distortions to those that are different from the harmonics of live instruments (which is why it screws things up so much). Nelson Pass wrote the following:
"We have seen that nonlinear distortion becomes larger and more complex depending on the nonlinear characteristic of the stages, the number of cascaded stages, and the number of spectral elements in the music.
Negative feedback can reduce the total quantity of distortion, but it adds new components on its own, and tempts the designer to use more cascaded gain stages in search of better numbers, accompanied by greater feedback frequency stability issues. The resulting complexity creates distortion which is unlike the simple harmonics associated with musical instruments, and we see that these complex waves can gather to create the occasional tsunami of distortion, peaking at values far above those imagined by the distortion specifications."
Source: www.passdiy.com/pdf/distortion_feedback.pdf
PS - Pass makes a no-global-feedback amp that is even more powerful than the X600 monos: the X-1000, which also has only two gain stages.
Kijanski:
I really respect Ralph Karsten's (Atma-Sphere's) designs, and I have no doubt that his amp with 40 tubes is really good for what it is, but this is precisely the type of amp that will be noisy ("mucky"!) compared to an amp having, say, 4 tubes. 40 tubes?! Or 40 transistors? Have you ever heard an amp with that many tubes or transistors in a really resolving system? More importantly, have you heard an amp with only a small handful of tubes or transistors in a really resolving system? Come on.
I was trying to keep it simple to make a point. Most amps, powerful or not, have anywhere from three to five gain stages (very few have two), and we all know that using global feedback with such designs makes the amp builder's job a lot easier. I do not agree that feedback ultimately lowers distortion - it changes the types of distortions to those that are different from the harmonics of live instruments (which is why it screws things up so much). Nelson Pass wrote the following:
"We have seen that nonlinear distortion becomes larger and more complex depending on the nonlinear characteristic of the stages, the number of cascaded stages, and the number of spectral elements in the music.
Negative feedback can reduce the total quantity of distortion, but it adds new components on its own, and tempts the designer to use more cascaded gain stages in search of better numbers, accompanied by greater feedback frequency stability issues. The resulting complexity creates distortion which is unlike the simple harmonics associated with musical instruments, and we see that these complex waves can gather to create the occasional tsunami of distortion, peaking at values far above those imagined by the distortion specifications."
Source: www.passdiy.com/pdf/distortion_feedback.pdf
PS - Pass makes a no-global-feedback amp that is even more powerful than the X600 monos: the X-1000, which also has only two gain stages.
Kijanski:
I really respect Ralph Karsten's (Atma-Sphere's) designs, and I have no doubt that his amp with 40 tubes is really good for what it is, but this is precisely the type of amp that will be noisy ("mucky"!) compared to an amp having, say, 4 tubes. 40 tubes?! Or 40 transistors? Have you ever heard an amp with that many tubes or transistors in a really resolving system? More importantly, have you heard an amp with only a small handful of tubes or transistors in a really resolving system? Come on.