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Old 8th October 2006, 04:54 PM   #1
ash_dac is offline ash_dac  United Kingdom
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Question Class XD....

Hi,

http://www.cambridgeaudio.com/assets...r8-2-06web.pdf

Not sure I understand it. Link may have been posted elsewhere on forum but I couldn't find it.
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Old 8th October 2006, 06:18 PM   #2
mikeks is offline mikeks  United Kingdom
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Messrs Bramble and Self state the following:

Quote:
We can see that in Class B it would be better if the crossover region were anywhere else rather than where it is.

If the crossover were displaced away from its zero-crossing position, then the amplifier output would not traverse it until the output reaches a certain voltage level.

Also below this transition point the performance could be pure Class A; above it the performance normal Class B.

This is the basic Class XD principle, and itís a very simple one: develop a topology that displaces the crossover point to one side of zero crossing.
This is approach inherently flawed because the crossover glitch occurs at zero crossing only if the load is a pure resistor.

Otherwise, with an invariably reactive 'speaker load, the location of crossover will be displaced by the same angle as the phase difference between load current and voltage.
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Old 8th October 2006, 06:26 PM   #3
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No matter if the crossover point is displaced in time or not, you STILL get crossover distortion. Pure resistive load or not, it matters not.

Having the crossover point outside the 'normal' listening zone will decrease distortion.

Just upping the bias gives you two distortion points, something Doug Self doesn't like.

Class XD is an interesting idea, as to if its actually any good.....
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Old 8th October 2006, 06:38 PM   #4
mikeks is offline mikeks  United Kingdom
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Ultimately, with real 'speakers, this displacement will occur anyway, and as such demonstrating that it can be obtained with resistive loads, while fascinating, is of dubious utility in practice.
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Old 8th October 2006, 10:41 PM   #5
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remember Graham Maynard's amplifier?

This idea basically changes THD vs. power characteristic (on any load), which might be good indeed, because low level signal will be less distorted, while at high level distortion goes up.
This might give impression of "good" dynamics, 'silence' more silent and loudness more loud, old class A trick.
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Old 9th October 2006, 10:14 AM   #6
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How can Douglas Self patent an idea he has already disclosed!?

The picture is from his homepage.
Click the image to open in full size.

What I can see is a modulating CCS in the lower end reminding of Class XD, though I can't be really sure as I have not seen his schematics yet.

Cheers Michael

PS. EDIT: Funny the picture gets 3 times bigger when inserted here while it's only 300x400 pixels at his homepage???
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Old 9th October 2006, 10:24 AM   #7
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I once modelled an amp with a modulated current source and a source follower in the output stage (i.e. both MOSFET). The results were quite promising (O.K it was just a simulation ....).
But is was DC coupled unlike Self's example.

Regards

Charles
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Old 9th October 2006, 07:15 PM   #8
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Quote:
Originally posted by mikeks
Ultimately, with real 'speakers, this displacement will occur anyway, and as such demonstrating that it can be obtained with resistive loads, while fascinating, is of dubious utility in practice.
I looked out my window and a pig flew by, so I guess it's safe
to agree. If Doug Self weren't so self-referencing, he could
list numerous examples of prior art, some of them mine.

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Old 9th October 2006, 07:40 PM   #9
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pig or pigeon
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Old 10th October 2006, 12:59 AM   #10
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Nelson Pass
If Doug Self weren't so self-referencing..

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