Pass X2 and negative feedback

BRunenr · 8th May 2002, 11:02 AM

Is the first version build entirely without negative feedback as Balanced Line Stage of SOZ, or is small amount of local feedback used, as in Aleph P 1.7?

Sorry for my bad english.

GRollins · 8th May 2002, 02:39 PM

By definition, the X topology is a feedback system. If you remove the feedback, it isn't "X" anymore.

Grey

Petter · 8th May 2002, 02:39 PM

I would not know, but my guess is that is uses the X-topology which relies on feedback.

Usually you can read up on such things in user manuals from www.passlabs.com

Petter

BRunenr · 8th May 2002, 06:38 PM

In user manual mr. Pass writes: "The first version used diferential Mosfet circuits operated without feedback." But I am unsure, if it means entirely without feedback or only without global feedback as in AlephP 1.7.

Nelson Pass · 9th May 2002, 08:35 PM

When I say "without feedback" I generally mean
no feedback loop, just degeneration if that.

When I say "no global feedback" I mean no loop from
input to output, although there might be local loops.

BRunenr · 9th May 2002, 09:18 PM

Thanks for explanation mr. Pass. I was interested in Aleph P circuit evolution.

GRollins · 9th May 2002, 10:10 PM

Apologies...I tend to think of X and "Super Symmetry" as synonyms, and since X is easier to type (I was born lazy, then had a relapse), I tend to use the one letter when possible.
Silly me.

Grey

AudioFreak · 10th May 2002, 04:10 AM

Grey that's why it was shortened to Su-Sy

Su-Sy is not a destructive form of feedback and should not be viewed in the same light as negative feedback.

GRollins · 10th May 2002, 05:13 AM

By my reckoning, the letter X only requires one keystroke, whereas Su-Sy takes five.
Ma pore widdle fingers can't take all tha' extra poundin'...

Grey

(Still doesn't excuse me for being mentally sloppy. Seems I'm missing a lot of details this week. I'll try to get some sleep and be back on track tomorrow...or the next day...or the next...)

Petter · 20th May 2002, 07:32 PM

There is no such thing as good and bad feedback.

The X-circuits in the big power amps (which only licensees of Nelson Pass can use provided patent holds and no workaround can be found) employ local feedback to around a single voltage gain stage. This is very similar to basic op-amp theory (but most op-amps have multiple internal stages, and feedback around these have become universally detested in audio).

Feedback is a method to reduce noise etc. Degeneration is generally considered the "least harmful". The next level, I will call feedback around single loop for lack of a better name, and this is the method used in the X-series power amps.

Feedback free amplifiers are like Yeti's -- many have seen them, but nobody can produce one.

Petter

8th May 2002, 11:02 AM	#1
BRunenr diyAudio Member Join Date: Apr 2002 Location: Czech Republic	Pass X2 and negative feedback Is the first version build entirely without negative feedback as Balanced Line Stage of SOZ, or is small amount of local feedback used, as in Aleph P 1.7? Sorry for my bad english.

20th May 2002, 07:32 PM	#10
Petter diyAudio Member Join Date: Jan 2001 Location: Scandinavia	Good and bad feedback There is no such thing as good and bad feedback. The X-circuits in the big power amps (which only licensees of Nelson Pass can use provided patent holds and no workaround can be found) employ local feedback to around a single voltage gain stage. This is very similar to basic op-amp theory (but most op-amps have multiple internal stages, and feedback around these have become universally detested in audio). Feedback is a method to reduce noise etc. Degeneration is generally considered the "least harmful". The next level, I will call feedback around single loop for lack of a better name, and this is the method used in the X-series power amps. Feedback free amplifiers are like Yeti's -- many have seen them, but nobody can produce one. Petter

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8th May 2002, 02:39 PM	#2
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	By definition, the X topology is a feedback system. If you remove the feedback, it isn't "X" anymore. Grey

8th May 2002, 06:38 PM	#4
BRunenr diyAudio Member Join Date: Apr 2002 Location: Czech Republic	In user manual mr. Pass writes: "The first version used diferential Mosfet circuits operated without feedback." But I am unsure, if it means entirely without feedback or only without global feedback as in AlephP 1.7.

9th May 2002, 08:35 PM	#5
Nelson Pass The one and only Join Date: Mar 2001	When I say "without feedback" I generally mean no feedback loop, just degeneration if that. When I say "no global feedback" I mean no loop from input to output, although there might be local loops.

9th May 2002, 09:18 PM	#6
BRunenr diyAudio Member Join Date: Apr 2002 Location: Czech Republic	Thanks for explanation mr. Pass. I was interested in Aleph P circuit evolution.

9th May 2002, 10:10 PM	#7
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	Apologies...I tend to think of X and "Super Symmetry" as synonyms, and since X is easier to type (I was born lazy, then had a relapse), I tend to use the one letter when possible. Silly me. Grey

10th May 2002, 04:10 AM	#8
AudioFreak Warp Engineer On Holiday Join Date: Oct 2001 Location: Queensland, Australia	Grey that's why it was shortened to Su-Sy Su-Sy is not a destructive form of feedback and should not be viewed in the same light as negative feedback.

10th May 2002, 05:13 AM	#9
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	By my reckoning, the letter X only requires one keystroke, whereas Su-Sy takes five. Ma pore widdle fingers can't take all tha' extra poundin'... Grey (Still doesn't excuse me for being mentally sloppy. Seems I'm missing a lot of details this week. I'll try to get some sleep and be back on track tomorrow...or the next day...or the next...)

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