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Old 4th February 2013, 01:31 PM   #8351
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I see where you're going but in the end it's a matter of what the ear-brain system thinks of all that
anyway the interviews (more like short quizzes actually) aren't about class D, that's just one of the questions.
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Old 4th February 2013, 02:29 PM   #8352
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I took this extract below from the Cambridge Audio manual . It says a white paper available for patent application GB050524.0 . I couldn't find it . I suspect the patent didn't go through . Any ideas ? Perhaps it is the old class A op amp trick revisited ( resistor from output to rail ) . Oxford University used that with 741's in the 1970's .

By actively displacing the crossover point this technology creates a
region of pure Class-A operation where the crossover zone would
otherwise be before moving into an enhanced form of Class B at higher
levels. It should not be confused with Class AB, which gives a small area
of Class A, but at the cost of higher distortion as soon as the signal level
moves outside the AB area. Class XD circuitry not only removes
crossover distortion from the zero-crossing point but also reduces
distortion in other parts of the amplifier's output range
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Old 4th February 2013, 02:37 PM   #8353
gootee is offline gootee  United States
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There is more about it, here:

Amplifier Class-XD

And here:

Google
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Old 4th February 2013, 03:03 PM   #8354
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what do you thing about the other responses from the guys in the article?
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Old 4th February 2013, 03:32 PM   #8355
dvv is online now dvv  Serbia
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It seems to be yet another sliding bias scheme.

I have yet to hear anyone else's solution other than Krell's which actually works well. Most I don't even notice in operation either way, just another amp.

But it looks great in ads.
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Old 4th February 2013, 03:35 PM   #8356
dvv is online now dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
...
By actively displacing the crossover point this technology creates a
region of pure Class-A operation where the crossover zone would
otherwise be before moving into an enhanced form of Class B at higher
levels. It should not be confused with Class AB, which gives a small area
of Class A, but at the cost of higher distortion as soon as the signal level
moves outside the AB area. Class XD circuitry not only removes
crossover distortion from the zero-crossing point but also reduces
distortion in other parts of the amplifier's output range
OK, fine, so make it have low distortion when it does go into class B, and you're home and dry.

So, what's new?
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Old 4th February 2013, 03:45 PM   #8357
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Quote:
Originally Posted by dvv View Post
OK, fine, so make it have low distortion when it does go into class B, and you're home and dry.

So, what's new?
exactly. sounds like marketing talk.
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Old 4th February 2013, 03:51 PM   #8358
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Sorry Mr PP , was designing a PCB today so had littler time to read the PDF link .
Thanks Gootee for XD link .

Reading about the Vincent amp in the Tubecad link . I came to the same reasoning that 4 W is fine for class A operation ( my 0.7A standing current then has transition to AB at that point up to +/16 V rails ) . My feeling is that the crossover doubling effect is acceptable and preferable to being at the sub 1 watt area of conventional class AB . I was pleased to note most of us listen to about 5 watts typical . Thus an op amp with feedback pair outputs and sensible feedback with standing current of 0.7A is a great little amplifier . Use in bridge if wanting more power . I found bias was OK using 2 x 1n4007 and a small resistance ( about 4R7 ) . By chance very stable with 1N4007 fixed with epoxy to the outputs . I did build a Vbe bias , was disappointed that it didn't give me better results although easier to adjust . Cfbp outputs need only 2 x 0.65 V to bias if wondering . Please forgive dia ,it's just my notes ( hope everything is in the right place ) . If trying a similar design try taking the feedback from the op amp alone or half and half . You will find at this current the output stage has very little distortion . OPA 604 was the easiest to get stable I found ( TLO71 and LF351N also ) . The cap on the feedback arm is required for most bipolar input op amps . Even with JFET DC offset is slightly better if it is retained ( 1 mV , 12 mV ) .

Click the image to open in full size.

Last edited by nigel pearson; 4th February 2013 at 03:59 PM.
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Old 5th February 2013, 10:21 AM   #8359
dvv is online now dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
Sorry Mr PP , was designing a PCB today so had littler time to read the PDF link .
Thanks Gootee for XD link .

Reading about the Vincent amp in the Tubecad link . I came to the same reasoning that 4 W is fine for class A operation ( my 0.7A standing current then has transition to AB at that point up to +/16 V rails ) . My feeling is that the crossover doubling effect is acceptable and preferable to being at the sub 1 watt area of conventional class AB . I was pleased to note most of us listen to about 5 watts typical . Thus an op amp with feedback pair outputs and sensible feedback with standing current of 0.7A is a great little amplifier . Use in bridge if wanting more power . I found bias was OK using 2 x 1n4007 and a small resistance ( about 4R7 ) . By chance very stable with 1N4007 fixed with epoxy to the outputs . I did build a Vbe bias , was disappointed that it didn't give me better results although easier to adjust . Cfbp outputs need only 2 x 0.65 V to bias if wondering . Please forgive dia ,it's just my notes ( hope everything is in the right place ) . If trying a similar design try taking the feedback from the op amp alone or half and half . You will find at this current the output stage has very little distortion . OPA 604 was the easiest to get stable I found ( TLO71 and LF351N also ) . The cap on the feedback arm is required for most bipolar input op amps . Even with JFET DC offset is slightly better if it is retained ( 1 mV , 12 mV ) .
This is a small power amp, right?

I would suggest strongly you couple it with TI BD 249/250 C transistors. They are rated at 25 A continuous, 40A impulse (50A if by SGS-Thomson), they have unexpectedly short Ton, Tstore and Toff times, in C version do 100V, and a nominal Ft of >3 MHz. This last piece of data might put off some people, but it's at their loss.

These are some very musical devices, trust me on this, and are usually very reasonably priced. Their gain factor isn't very high, so I suggest some powerful drivers, MJE 15030/15031 should do it.

I think you would be surprised at what you would hear in the end.
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Old 5th February 2013, 08:47 PM   #8360
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As you say . D44/45 even . It could be a preamp with attitude . Put a fan in and it could drive speakers when asked from the preamp . That would be useful . Change bias to suit task . Try it with no feedback to outputs , distortion is very good if enough current flowing .
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