An Amplifier without feedback called Pax, What about Vitus SS-101? - diyAudio
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Old 28th August 2008, 04:14 PM   #1
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Default An Amplifier without feedback called Pax, What about Vitus SS-101?

Hi

I was reading Vitus Amplifier SS-101 specifications. It was written "Zero feedback". I look up this forum and found this thread
Zero feedback amp

There, everybody had claimed the feedbacks are Local.

But in Elektor Magazine Apr-2008 an amplifier has been described which does not uses feedback, instead using an Error correction method. The designer is a very nice guy that you can ask your questions from him in Elektor forum. (here: http://www.elektor.com/forum/elektor...n.432667.lynkx)

1- I do Not know, How the quality will be. Maybe U experts can foresee!
I have extracted the pages from the magazine here for you.

http://www.4shared.com/file/60807816/187af4ea/PaX.html

2- What do you think about the voltage amplifier Stage. It uses The Supply current to Amplify the Input voltage;

3- Here Some Info about Vitus Amp SS-101. They claim:

"We do not employ any form of negative feedback. This is one of the really hot topics amongst different manufacturers, and most seem to be in favor of using feedback.

http://www.ultraaudio.com/features/2006_10_01.htm

Do you think Vitus is using the same Topology as Pax? If Not, What do U think?

ThanX
Shahriar
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Old 28th August 2008, 04:31 PM   #2
lineup is offline lineup  Sweden
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hello man.

good questions and interesting.

anything dealing with error correction
in my Lineup Book
deals with output using feedback, of one or another kind

simply because without some form of feedback,
there can be no knowing what the signal at output looks like

I post the simplified diagram of the PAX error correction (feedback) diagram

Regards Lineup from Sweden - a very clever man, if you ask him
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Old 28th August 2008, 07:12 PM   #3
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Quote:
The designer is a very nice guy that you can ask your questions from him in Elektor forum.
The designer is also a member here, Janneman
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Old 28th August 2008, 08:00 PM   #4
darian is offline darian  France
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Default This is sort of feedback

Hello,

I have been reading about all this Hawkford style error correction a lot because I was interested in that Pax amplifier. My conclusion is that it is a feedback loop. In classical feedback you have gain in the forward path and no gain in the feedback loop. In this error correction type you have no gain in the forward path but gain in the feedback loop. It's not so different, except that the feedback in the Pax can be negative or positive. And in the Pax the feedback is also local and not global. There is still no way of total distortion cancellation (contrary to feedforward technology) and the problem of high frequency instability. In fact, just this instability problem is the testimony of the feedback principle of this design.
Anyway, it's a very seductive design and I would like to hear about some listening impressions. The Halcro amps use the same principle and overall they have raving reviews, but some say they can sound too cold and clinical (even if they uses Mosfets).
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Old 28th August 2008, 09:26 PM   #5
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Default I have readed almost all the informs written by Jan Didden


And i think he is the designer.

Very interesting... a pitty that it is so complicated to my knowledge and material to assemble... i would like to listen to see how big is the sonic advantage.

I am very curious about.... i will receive some operational amplifiers and maybe i will try...but not sure...i really feel it too much complicated to me.

But curious.... very curious..... how much better can be?

All my life i could perceive small differences from standard amplifiers compared to high end units... i hope this one can offer us something easy to perceive.

Nice... interesting.... very nice.

regards,

Carlos
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Old 28th August 2008, 09:40 PM   #6
dmills is offline dmills  United Kingdom
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Am I missing something?
I count at least three or four feedback paths in that schematic:

1: The overall feedback path back to the input of U4, polarity is uncertain and it appears to possibly be current mode.

2: The emitter resistors R64 and R66 provide local feedback around the power transistors by effectively adding IR drop to Vbe (in addition to that caused by the inherent Re value of those parts). This is effectively current mode feedback.

4: I assume C7 and C8 are pole splitting caps (which implies negative feedback at high frequency).

3: The bootstrap caps C11 and C12 provide audio frequency voltage feedback (positive) but the gain should be very low.

I am concerned by the lack of output filtering which potentially allows RF picked up on the speaker leads back into the input of the amp via the overall feedback path, and the lack of short circuit protection could also be a problem, albeit one easily rectified.

I must be missing something if this is a design without feedback, because it seems to have loads of it to me.

An explanation would be appreciated.


Regards, Dan.
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Old 28th August 2008, 11:43 PM   #7
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Default Re: This is sort of feedback

Quote:
Originally posted by darian
Hello,

I have been reading about all this Hawkford style error correction a lot because I was interested in that Pax amplifier. My conclusion is that it is a feedback loop. In classical feedback you have gain in the forward path and no gain in the feedback loop. In this error correction type you have no gain in the forward path but gain in the feedback loop. It's not so different, except that the feedback in the Pax can be negative or positive. And in the Pax the feedback is also local and not global. There is still no way of total distortion cancellation (contrary to feedforward technology) and the problem of high frequency instability. In fact, just this instability problem is the testimony of the feedback principle of this design.
ABSOLUTELY CORRECT

Thank goodness there are people in this forum who understand control systems.
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Old 29th August 2008, 12:09 AM   #8
darian is offline darian  France
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Thanks a lot!!!
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Old 29th August 2008, 12:11 AM   #9
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Quote:
Originally posted by dmills
Am I missing something?
I count at least three or four feedback paths in that schematic:
No your are not missing anything.
But the editors of Elektor certainly seem to have!
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Old 29th August 2008, 12:27 AM   #10
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Quote:
Originally posted by lineup
anything dealing with error correction
in my Lineup Book
deals with output using feedback, of one or another kind

simply because without some form of feedback,
there can be no knowing what the signal at output looks like
Also absolutely correct.
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