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Class D Switching Power Amplifiers and Power D/A conversion

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Old 7th February 2013, 10:04 PM   #21
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Hi Nigel,
I agree that it is possible to solve it reasonably good.
But that's not at all the normal situation in class D these days.

Let's have a look to a real life filter as attached with values that are not exotic.
In case of 8R the signal will be boosted more than 1.6db at 20kHz.
In case of 4R the signal will be defeated more than 1.7db at 20kHz.
Obviously many output filters will affect the frequency response in a non neglectible way.
Furtheron the difference of 3.4db between 4R and 8R tells that the tweeter is driven with a pretty high impedance. Effective Z is in the range of multiple Ohms. Means a damping factor far below 10.
I am not at all a damping factor fetishist, but such low values simply do not ensure voltage drive for the tweeter.
Someone might now say that speakers are not purely resisitive.
True. But better don't hope that correct equivalent circuits of real speakers would cure things.

Of course the behavior can be improved by higher fs and a faster filter and also by post filter feedback, but there are many designs out there which show similar short comings as the attached simulation.
Attached Images
File Type: jpeg FilterSchem.jpeg (89.4 KB, 572 views)
File Type: jpeg FilterResponse8R.jpeg (170.8 KB, 568 views)
File Type: jpeg FilterResponse4R.jpeg (188.2 KB, 559 views)
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Old 7th February 2013, 10:12 PM   #22
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IcePower amps have very high damping factors, so I don't think it's a class-d problem.
The big ones have great bass, too (to my ears). Probably more to do with the power supply than anything else. And yes, I owned a Bel-Canto.
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Old 7th February 2013, 11:45 PM   #23
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So we have something in common, x-class-D ......
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Old 8th February 2013, 06:39 AM   #24
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Quote:
IcePower amps have very high damping factors, so I don't think it's a class-d problem.
At what frequency ? ;-)

But I agree with you that it isn't a general class-d problem but depending on how a class-d amp is designed. Although they can be built chaply in terms of the parts used, designing them is quite difficult.

The damping factor at high frequencies is definitley determined by the NFB loop and the output filter properties and not the PSU.
But there were a lot of class-d amps around with aenemic bass due to the use of weak PSUs.

Regards

Charles
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Old 8th February 2013, 03:01 PM   #25
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Simply boosting the switching frequency higher will give you a nice AM broadcast amplifier and a whole new set of problems, or so I've heard.

I thing Yamaha approached the problem nicely with their EEEngine
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Old 8th February 2013, 05:29 PM   #26
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Quote:
Originally Posted by David_Web View Post
Simply boosting the switching frequency higher will give you a nice AM broadcast amplifier and a whole new set of problems, or so I've heard.
Because of this i've decided to only use class D amps where circumstances allow for ultra short speaker wires, such as active applications. Using a class D amp with normal, long speaker cables changes the sound of other equipment in my room and not for the better.
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Old 8th February 2013, 05:39 PM   #27
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Hi all,

There are certainly some good and bad class D amps out there. But there are topologies / PCB designs that adress the problems with the output filter, EMC, load dependant behavier, ...

I think Bruno Putzeys pretty much nailed it: http://www.hypex.nl/docs/papers/ncore%20wp.pdf
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Old 8th February 2013, 07:27 PM   #28
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The fact that there are multiple ways to milden class D specific issues does not mean that the issues are gone.
All error mechanisms which need special care in classD to avoid substantial inferior performance than classAB are class D related issues.
- Modulator distortion
- Dead time distortion (some analogies to cross over distortion, but not the same. Not at all!)
- Slowish output filter
- Naturally low PSRR of the switching stage
- Parametrization of feedback loop is another animal
- Clipping recovery
- Carrier aliasing or frequency drop at high modulation levels
- .. ..
Each of the topics above can be a fun killer if not solved with special care.
And only few designs these days take care of all that in a reasonably balanced way. And even in designs of higher quality often there is a massive optimization of just very few points, while accepting unpleasant impact on others. That's the best fundament for a successful marketing story, which highlights a few exceptional good properties and doesn't tell the short comings.

To me the key for a good amp design is an overall well balanced performance.
It is up to everybody to judge on his own, whether a shown design is promoted with a very narrow set of properties or with a set properties that covers very various situations.
Finally the link from all to the numbers to our ears is hard to predict and listening has to be the final criteria.
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Old 8th February 2013, 09:13 PM   #29
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Quote:
Originally Posted by phase_accurate View Post
At what frequency ? ;-)
That's a very good question and I do not have a good answer for it! It may be in the spec sheets, I think it is. How much does damping factor matter in the midrange and highs?
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Old 9th February 2013, 03:23 AM   #30
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Quote:
Originally Posted by ChocoHolic View Post
The fact that there are multiple ways to milden class D specific issues does not mean that the issues are gone.
All error mechanisms which need special care in classD to avoid substantial inferior performance than classAB are class D related issues.
- Modulator distortion
- Dead time distortion (some analogies to cross over distortion, but not the same. Not at all!)
- Slowish output filter
- Naturally low PSRR of the switching stage
- Parametrization of feedback loop is another animal
- Clipping recovery
- Carrier aliasing or frequency drop at high modulation levels
- .. ..
Each of the topics above can be a fun killer if not solved with special care.
And only few designs these days take care of all that in a reasonably balanced way. And even in designs of higher quality often there is a massive optimization of just very few points, while accepting unpleasant impact on others. That's the best fundament for a successful marketing story, which highlights a few exceptional good properties and doesn't tell the short comings.

To me the key for a good amp design is an overall well balanced performance.
It is up to everybody to judge on his own, whether a shown design is promoted with a very narrow set of properties or with a set properties that covers very various situations.
Finally the link from all to the numbers to our ears is hard to predict and listening has to be the final criteria.

Very technical and beyond me for this but interesting reading..

I completely decide on the equipment I want to own by listening to it..
It's hard to understand that theres other ways to decide?
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