Phase of Pass Labs amps!

[bbakota2000]

Long time ago I red an Accuphase's article on the net. It said that the phase (or degree of phase inversion) is of most importance for the correct reproduction of instruments timbre. In Nelsons writings I didn't saw anything about phase invertion at the ends of frequency response (high and low frequencies). One University did a research on how do we react on inverted phase below 100Hz, and they said we hear that as unnatural sound, even unrecognizeable. Can someone (maybe Nelson) tell me something more about that in Pass Labs amplifiers. I would also like to hear your opinions on this subject.

[Nelson Pass]

Pass Labs products do not invert phase, although the
original Zen design (not a real product) explicitly does.

[bbakota2000]

How do you achieve that? I mean, is phase invertion something you are trying to avoid in an amplifier with adding something in a circuit, or there just isn't phase invertion because of a good circuit construction?

[bbakota2000]

Is "phase distortion" similar to "phase invertion"? Somewhere on this forum I red that some Pass amps and preamps have phase distortion of 5 degrees. Is that true? Can that be void? Is it important at all for the music reproduction?

[Michael Rothacher]

Are you perhaps asking about phase shift rather than phase inversion?

[bbakota2000]

yes (I guess)! I don't know the proper word. What is the phase shift? Give me just brief explanation!

[uli]

Hi,

an ideal amp with a frequency range of Zero to Infinite

wil have theoretically zero degrees of phase shift.

If you insert poles you get some degree of phase shift:

the input dc-blocking caps add some phase shift to the

lower range of the signal. therefore good designs start

either at pure dc (e.g. X1000) or at the lowest possible

frequency. this cap produces 45° phase shift at its pole

frequency. the same is true for the upper end of the amp´s

range. therefore designers try to make these poles about

the factor 10 away from the audible range thus reaching

phase shift which is very lo, about 5 degrees or so

An amp with ideal square wave reproduction (0 - oo Hz)

does not necessarily sound perfect. this means that in

real life you always have to accept imperfect solutions

the art of design lies in the perfect blend of those imperfections

just as e.g. a SE power amp with one 300B or an Aleph 2!

Uli

[Michael Rothacher]

Very lucid explanation, Uli

[uli]

tx!

Uli

[bbakota2000]

What about Aleph 5? I didn't find any info. Is there any way to minimise phase shift in this circuit without making it sound worse?

PS: Thanks for nice explanation!