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Class D Switching Power Amplifiers and Power D/A conversion

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Old 28th April 2009, 09:28 AM   #21
Eva is offline Eva  Spain
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Now replying in a funnier way to the first post of this thread:

- The efficiency of class D is something easily measurable (even comparing temperatures with a finger one heatsink burns, the other doesn't).
- The purity of class A is a purely subjective quality. Most exotic designs tend to be strong distortion generators actually.
- Class D roughness is again a subjective quality, I was stuck on that way of thinking until I tried it a few years ago. There are many bad designs out there, too. It's easy to think that way when you don't understand how it works.

Probably half of the people that I could indirectly blame for indirectly teaching me to think in this and many other ways, that later proved to be wrong, write regularly on this forum. Fortunately some are evolving towards a more open mind too.
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Old 28th April 2009, 09:46 AM   #22
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Evan, Eva,

Thanks for your insights, which, as I understand, come from experience.

As I mentioned before, I just started to read about Class D and possible combinations and of course have a lot to learn. Having heard only class A, AB amplifiers, I might be biased in taking a plunch into fully class D designs. I have played with the idea of trying a couple of Hypex modules, but never actually did.

As technology progresses very fast, the Audio Physic AD approach may be outdated. As I understand, it was a way of combining the best of both worlds into a single design, which was quite ok and maybe succesful for a short while.

As you mention Eva, there are a lot of crappy class D designs available and for me it makes it even more difficult to distinguish good or best from mediocre. In that regard some advice as what to read or look for is appreciated. I have already done some reading in the class D thread, but may not have hit truly valuable information yet.

Thanks so far.
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Old 28th April 2009, 12:02 PM   #23
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Quote:
Originally posted by courage


As technology progresses very fast, the Audio Physic AD approach may be outdated. As I understand, it was a way of combining the best of both worlds into a single design, which was quite ok and maybe successful for a short while.

I donīt believe that technically achievements and improvements in sound quality happen in an strict "evolutionary" way. Marketing, fashion and production cost all have an important in the process of manufacturers decisions and consumerīs behavior. With time we may have a general improvement for those who cares as well
as a negative scenario for the wast majority....
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Old 28th April 2009, 12:15 PM   #24
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Quote:
Originally posted by The golden mean


I donīt believe that technically achievements and improvements in sound quality happen in an strict "evolutionary" way. Marketing, fashion and production cost all have an important in the process of manufacturers decisions and consumerīs behavior. With time we may have a general improvement for those who cares as well
as a negative scenario for the wast majority....
I totally agree. Technological improvements do not neccesarily mean improvements in sound reproduction. I think that this is also the main reason why there are, among the many, only a few "good sounding" class D designs.
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Old 28th April 2009, 12:38 PM   #25
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BTW. Evolution takes time and contrary to many peoples belief Charles Darwin didnīt get the idea on evolution while visiting the Galápagos Islands but much later after returning to England and under the influence of currents of the minds of others e.g. Thomas Malthus and Adam Smith.
An while the evolution in nature goes on and is by no means finished so does the evolution in audio... But my remark as a believer in evolution is that evolution dosenīt necessarily means that something better manifests itself....better or worse; these concepts belong only to human evaluation and are hard to decide upon.
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Old 28th April 2009, 02:30 PM   #26
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Quote:
Originally posted by Evan Shultz
Sure, it was a cool idea many years ago but class D has advanced since then and this idea is just plain outdated.

IMHO it was never (more so in the past) a particularly good idea as a class D + class A concoction will still lag behind the overall technical performance and efficiency that can be achieved with well established linear class B design techniques.

There are ways of significantly reducing the dissipation in class A designs, for the goal in itself of chasing ever better measured performance specs at ever increasing power levels, but they do not include class D.
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Old 28th April 2009, 02:53 PM   #27
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Class B and AB with their glitches and abrupt discontinuities in open loop gain near zero current crossing (and not zero voltage) are the worst of all Try to match and flatten low-level open-loop-gain vs. current and voltage plots for class B and AB and you will get plenty of headaches, while class D inherently results in a straight line. To make it worse, the shape of these discontinuities and glitches is signal frequency dependent.

In class D, amplification is done on the "time domain" rather than on "voltage/current domains", and it's time linearity what matters, which is usually excellent. In the end, MOSFET switching times are a linear function of drain current, particularly when body diodes are not allowed to store substantial charge during on-time.

Time discontinuities may arise due to dead time (or body diode conduction) but they don't happen at or near zero crossing, and zero (or almost zero) dead time operation is possible (some of us are doing it that way).

In my biggest amplifier (170Vpk, 70Apk), discontinuities (actually narrow glitches on a straight line) due to slight dead time don't happen until +8.5A and -8.5A, and open loop linearity in this range is already excellent. Distortion products due to these discontinuities lay typically outside the audio band. Then add some dozen dB of negative feedback too. Then add output filter "glitch" attenuation above 20Khz, which you don't have on class B or AB.

Controlling switching timing rather than linear voltage or current has unique advantages.

It seems that you have a long way to go until you can really understand these facts.

BTW: Class B and AB efficiency with typical full-range music signals at low to moderate output levels is not much better than class A efficiency. It only improves substantially (up to 65%) when output level approaches clipping.
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Old 28th April 2009, 03:16 PM   #28
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Quote;"In my biggest amplifier, discontinuities (actually glitches) due to slight dead time don't happen until +8.5A and -8.5A, and open loop linearity in this range is already excellent. Distortion products due to these discontinuities are typically outside the audio band. Then add some dozen dB of negative feedback too. Then add output filter "glitch" attenuation above 20Khz, which you don't have on class B or AB."

Outside the audio band, but maybe it can influence performance inside the audio band? Just a hypothesis. Negative feedback is a servo system that is intended to correct what has already happened in an unwanted way so this is partly a time domain problem. A more or less sharp filter at 20 Hz may be the course
of less than perfect treble reproduction, again I say maybe.
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Old 28th April 2009, 04:17 PM   #29
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ClassAD, IR make one and said they like the sound result

http://www.google.com/patents?id=tUu...s+2007/0063764
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Old 28th April 2009, 07:46 PM   #30
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Quote:
Originally posted by G.Kleinschmidt


There are ways of significantly reducing the dissipation in class A designs, for the goal in itself of chasing ever better measured performance specs at ever increasing power levels, but they do not include class D.
This would mean that if this is possible, we can reduce the heatsink space to half or even lower and the huge krell-like amps would make room for smaller, but still good sounding, power amps.

Do you have examples of such amps?
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