Phase of Pass Labs amps!

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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.
 
phase shift

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

:nod: :nod: :nod:
 
here we are

Nelson Pass said:
On the top end, we tend to roll off our amps - 3 dB at 100 KHz,
single pole. Someone else can calculate the phase that results.

-> ~10° at 20kHz -> 3/16 inches ~ 5mm offset

Nelson Pass said:
On the low end, we roll the Aleph 5 off at -3 dB @ 0.15 Hz, single
pole, with unity gain at DC.

-> ~0,6° at 20Hz -> 1 1/16 inches offset ~ 28mm offset


Uli

:nod: :nod: :nod:
 
yepp

Hi mrothacher,

I calculated in the following way:

(maybe not quite correct, but the region is enough)

an rc - combination has one pole at 1/(2pi*r*c)

ok this means a 45° phase shift as the capacitive

impedance part is equal to the resistive one (= r) but 90°

phase shifted.

You now calculate the capacitive impedance x for the

desired frequency (x = 1/(2pi*f*c) and now:

phi = inv tan(r/x) here we are!

delay: (wavelength/360)*phi

wavelength is 340/frequency in meters.

Uli

:nod: :nod: :nod:


PS: dont use this calculation for filters with more than one pole!!!

as their phase depends heavily on the Q!
 
Incidentally, I know I make some folks 'round these parts nervous when I build circuits with DC to light bandwidth. This is a good example as to why I do so. Phase shift is cumulative and by the time you string a phono stage, a line stage, an active crossover, and a power amplifier in series, you've got a bit of a problem.
The wider the bandwidth the less the phase shift, in the manner indicated above. Granted, people who live in more RF prone areas may have problems, but those of us who live in parts of the world that are barely out of the stone age have at least that one aspect going for us.

Grey
 
Yeah, I know. The real world is a bummer sometimes. But that's one of the glories of the DIY mindset--you can tailor the circuit to local needs and conditions. Not to mention oddball prejudices such as my DC to light conviction. It's not that I ask anyone to come along; it's just something I like to do for my own purposes. I figure you can always cap the thing off at any arbitrary bandwidth that you might want, but it's not so easy to stretch the bandwidth out after the fact. At least, not so easy for those who don't want to cheat by using NFB to do the job.

Grey
 
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