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Old 3rd December 2010, 02:07 PM   #1
nar is offline nar
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Default XA.5 evolution anyone ?

Hello all,

Extracted from the X.5/XA.5 litteraturefrom PassLabs, I wondered if anyone tried to guess the new circuitry schematic ???

The improvements are:

Cascode JFET Symmetric front end input differential MOSFETs have been replaced by the same JFETs we use for the input devices on our MM and MC phono stages. They have very much less noise, higher input impedance, and greater linearity. They operate self-biased in a symmetric quad topology, each device cascoded and level shifted to form the full voltage gain stage of the amplifier.

More power output devices Greater quantities of matched complementary power Mosfets operated as followers, giving more current and power handling, and also improved linearity.

Single-ended Class A bias The output stage not only uses a heavy bias current for push-pull Class A operation to a large portion of the rated power, but also has a single-ended bias source for single-ended Class A at low wattage.

New bias circuit A newly developed bias generator has eliminated three sources of variation in bias current in the output stage. It also allows approximately 20% greater power output for a given supply voltage.
Improved power supplies The power supplies have been given greater storage capacitance, larger / quieter power transformers, fast recovery rectifiers (twice as many), and improved harmonic filtering in both the primary and secondary supply circuits.

Better layout All portions of the amplifier circuitry have been refined with respect to wiring and circuit board layout so as to achieve lower noise and distortion and greater reliability and serviceability.


From what I understand, the X/XA distinction is not so clear - for example, the article states that all voltage gain comes from the self biased diff pair, which is inaccurate regarding old XA base schematic, where the output power mosfets do have positive voltage gain. However, it is possible that Nelson and his stuff developed a new output stage technology for XA.5 amps, that could be described as "push pull circuitry with additional single ended Class A bias CCS", maybe I'm wrong, but I have in mind the article "Leaving Class A" where the following can be read:

Subsequently in the X amplifier series we retained a small amount of single-ended Class A bias on our output stage as a means of controlling the amount and character of the distortion at the lowest power levels that all-important first watt.

By setting the value and direction of a single-ended bias current in a push pull power stage you can reduce the distortion by canceling the second harmonic nature of imperfectly matched Q1 and Q2 halves, or you can arbitrarily select a desired ratio of second to third harmonic, depending on subjective evaluation of the resulting sound.


Any hints ?



Best regards,

nAr
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Old 3rd December 2010, 03:23 PM   #2
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I`m almost sure there was a thread about the X.5 type already a few years ago (2007?) but I just can`t find it anymore. As far as I can remember even a preliminary schematic has been proposed.
It may also be that my brain consists of already more holes than solid stuff and that I just dreamed this, I don`t know.
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Old 3rd December 2010, 04:58 PM   #3
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Maybe you're right But definitely, I wouldn't mind understanding of the new current capabilities (output stage) or even the jfet cascoded CCS input gainstage
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Old 3rd December 2010, 05:31 PM   #4
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Quote:
Originally Posted by nar View Post
The improvements are:

Cascode JFET Symmetric front end input differential MOSFETs have been replaced by the same JFETs we use for the input devices on our MM and MC phono stages. They have very much less noise, higher input impedance, and greater linearity. They operate self-biased in a symmetric quad topology, each device cascoded and level shifted to form the full voltage gain stage of the amplifier.

Sounds relatively direct. So, they are 389/109s - 170/74s. "Symmetric front end" sounds like differential? Or, complimentary Push Pull? "Quad topology", Ahhh, sounds like the dual differential, as in John Curl's favorite?
Quote:
Originally Posted by nar View Post
More power output devices Greater quantities of matched complementary power Mosfets operated as followers, giving more current and power handling, and also improved linearity.
How much output current do you need? I need not say more...
Quote:
Originally Posted by nar View Post
Single-ended Class A bias The output stage not only uses a heavy bias current for push-pull Class A operation to a large portion of the rated power, but also has a single-ended bias source for single-ended Class A at low wattage.
Now this point has me thinking? Just what's going on there? How could you do that? Does N.P. have a patent with something like this???
Quote:
Originally Posted by nar View Post
New bias circuit A newly developed bias generator has eliminated three sources of variation in bias current in the output stage. It also allows approximately 20% greater power output for a given supply voltage.
Improved power supplies The power supplies have been given greater storage capacitance, larger / quieter power transformers, fast recovery rectifiers (twice as many), and improved harmonic filtering in both the primary and secondary supply circuits.
More thinking. Hmmm...
Quote:
Originally Posted by nar View Post
Better layout All portions of the amplifier circuitry have been refined with respect to wiring and circuit board layout so as to achieve lower noise and distortion and greater reliability and serviceability.
Us DIYer's do that all the time...

Any hints ? Not yet but I will be thinking about it

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Last edited by flg; 3rd December 2010 at 05:33 PM.
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Old 3rd December 2010, 06:43 PM   #5
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The only thing I don't know is the new bias circuit, but I definitely won't be sharing the rest of the circuit with anyone.
I think it goes against Nelson's philosophy of sharing the circuit details when the product is no longer for sale. SInce this product is still for sale I don't think it would be right of anyone to detail the circuit details on a public forum.
When you work it out keep it to yourself.

Last edited by Melon Head; 3rd December 2010 at 06:47 PM.
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Old 3rd December 2010, 07:03 PM   #6
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Quote:
Originally Posted by nar View Post
for example, the article states that all voltage gain comes from the self biased diff pair, which is inaccurate regarding old XA base schematic, where the output power mosfets do have positive voltage gain.
That is not accurate. Probably inadequate proof-reading.

As to the bias circuit, I can only think of two types of
variation it addresses.

If you haven't read "Leaving Class A" I suggest that with regard to the
application of single-ended bias.

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Old 3rd December 2010, 08:10 PM   #7
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as usual - sky is a limit

seems that Papa unchain enough pigeons from cage , to help us

UGS 1 -2-3-4 , TL431 , optobias , cascodes , BA fronts , BA ends


etc

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Old 3rd December 2010, 08:21 PM   #8
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That is symetric BA fronts or dual differential.
You might also get some clues looking at A75

Last edited by Melon Head; 3rd December 2010 at 08:26 PM.
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Old 4th December 2010, 05:12 AM   #9
nar is offline nar
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Quote:
Originally Posted by Nelson Pass View Post
That is not accurate. Probably inadequate proof-reading.

As to the bias circuit, I can only think of two types of
variation it addresses.

If you haven't read "Leaving Class A" I suggest that with regard to the
application of single-ended bias.

Thanks for answers Mr Pass.
I think the literature paper is only a "literature" and can't be taken so seriously, e.g. the paper mostly focused on the X.5 amps front end (and not necessarly the XA.5 front ends)

And yes, I'm reading "leaving Class A" in detail in the next days, for single ended bias source

Regards,


nAr
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Old 4th December 2010, 06:03 AM   #10
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Quote:
Originally Posted by nar View Post
Thanks for answers Mr Pass.
I'm reading "leaving Class A" in detail in the next days, for single ended bias source
nAr
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