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Class D Switching Power Amplifiers and Power D/A conversion

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Old 24th November 2005, 01:20 PM   #1
gpapag is offline gpapag  Greece
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Default A fish named Wanda

Hi all
Am I allowed to throw a speculative question here (like talking to myself)?
I wonder (Wanda) if the perceived superiority in resolution, detail, transparency of the the class D amplifiers over class A, AB amps is really attributed to the different way these amps process the electrical signal (hardware level).
What Iím about to say? I will explain myself . But first I have to explain how did I start to question the technical nature of the problem.
They were two cases (one pre and one power amplifier) that were actually modulating the music signal with high frequency signal, in other words, they were oscillating. Nature of oscillation was such that it was sucking power and the music signal was starving low in amplitude. That was the reason they were brought to the bench. Acoustically, they were anaemic, thin, but oddly enough, artificially transparent. After oscillation problem was rectified, signal balance returned to normal, but this transparency went away.
This happened more than 5 years ago.
A few months ago, I had the chance to listen to a class D amp for the first time. The sound was very detailed and transparent. Next, I could not resist to measure it with the scope. To my ignorance and to my surprise, the output signal was modulated by a high freq. signal. It was the participants of this forum that convinced me that this was normal with such amps.
These qualities of detailed and transparent sound I experienced (to a higher degree) again with a TA2020 chip amp.
Now, thinking over why and how is it possible for these amps to produce these results, the case with the oscillating analogue amplifiers came back to my mind.
Is the correlation between the (accidental for the analogue, intentional for the class D) high frequency modulation of the output signal and the perceived detailed-transparent sound a mere coincidence, or is it something more systematic behind it?

What do I imply? Might be the case that it is the nature of the modulated acoustic waves that excites the auditory system in a more efficient way than the non modulated ones? (in technical jargon: does this modulation works to the auditory system like the HF bias works to the magnetic recording media)?
Absurd? Maybe.
Please note the following:
In the field of visual perception the following is true:
We are visually inspecting a certain surface with defined subjective conditions (i.e. lighting, angle of view, distance ect.) trying to locate any small flaw. We can increase our ability to locate such a flaw by introducing a relative motion between our eyes and the inspection surface. This relative motion has to be in a plane normal to the axis of our sight, to be small in amplitude and to be repetitive. The thus increased
discrimination capability of the visual system is said to be attributed to two factors:
The first factor is the electrochemical way that the eye cones and rodes function. They do saturate when the stimulus ( image projected over them) remains unchanged. Literally speaking, this happens only when motional nerves are neutralized by anaesthesia. Otherwise, the eye is constantly under unconscious, unintended micro-motion just for to prevent reaching saturation level. So one can say that this unconscious micro-motion is the way to keep cones and rodes in a condition just over the saturation-no impulse output-level. The intentionally applied relative motion during visual inspection adds to this level, thus increasing the resolution-discrimination- capabilities of the eye.

The second factor has to do with the access priority of the peripheral vision (most sensitive in detecting motion) over the central vision (more specialized in details and colour perception) in reaching and stimulating the alert command section of mamal's brain (this is the way the evolution process has set weighing factors and priorities into the structure of the visual perception system).

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George
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Old 24th November 2005, 01:21 PM   #2
SSassen is offline SSassen  Netherlands
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Paragraphs buddy, learn to use the [enter] key!

Best regards,

Sander Sassen
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Old 24th November 2005, 01:32 PM   #3
gpapag is offline gpapag  Greece
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Right Sasen.
I fixed it. Copy and paste doesn't work always correct.


George
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Old 24th November 2005, 02:48 PM   #4
nitrate is offline nitrate  United Kingdom
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Hi,

I think you need to learn a little more about classD. You are right, the sound does achive a very high purety and sound extremly detailed but it is nothing what so ever to do with the HF bleeding through the output. The HF bleed is mearly a by-product of the amplifiying methods employed in this class of amp. The reason it sounds so detailed and clear is simply because the audio really IS that good. I can't understand why everyone has this perception that classD sounds crumby


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Mad.P
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Old 24th November 2005, 03:35 PM   #5
gpapag is offline gpapag  Greece
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Quote:
The HF bleed is mearly a by-product of the amplifiying methods employed in this class of amp.
Mad P.
Yes. I do have a limited knowledge on anything in electronics, let alone class d, but this i have already understood. What i was really asking in thread #1 is if this mere by-product has by chance the ability to bias the auditory system in a beneficial way. A way for answering this question is having a very effective and ideal PL output filter that zeroes out any residues of HF. If the sound is still transparent and detailed after this filtering, then what i suggest as a possibility is plain wrong.

Quote:
The reason it sounds so detailed and clear is simply because the audio really IS that good.
Please, can you elaborate?
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George
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Old 24th November 2005, 04:14 PM   #6
nitrate is offline nitrate  United Kingdom
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Greetings,

When i say the sound is 'That good' ( clear ) i mean just that. The sound is an accurate representation of the input signal. Very little distortions and very low unwanted noise.

I can see what your getting at with the HF makeing sound appear brighter and cleaner than it really is, but even if this phenomenone does indeed exist ( and it might, i like to think it will certaintly alter our perception of what we are hearing ) I don't think it is at play here because the speakers will be unable to convert the high frequencys to sound waves. We can see it on the scope thats for sure but is the high frequency being transmitted into the air as sound waves? I don't think any speaker is capable of that! The only other avenue of thought here is the possibility of the HF altering the way the speakers are converting the electrical waves into sound waves. Maybe the presence of HF interferes at this point. Who knows, i don't think it is but sombody somewere may know different. I personally don't know much about speakers in this respect so sombody a little more qualified may be able to enlighten us on this.

Regards
Mad.P
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Old 24th November 2005, 06:02 PM   #7
BWRX is offline BWRX  United States
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It's been said before, but lack of any sort of crossover distortion may have something to do with it. Class d never tries to amplify a signal in the sense that the output follows the input signal. It modulates the audio signal (only has to switch rapidly between two voltage levels as opposed to "tracing" the input signal at a higher voltage) and allows the filter to recover that audio information.

The principles of class d operation are based on Fourier's theorem which basically says that a square wave is composed of an infinite number of sine waves of varying frequencies (sounds like music, eh?). Interestingly enough our ears can be thought of as Fourier analyzer.

Off topic a bit, the Tripath chips do add dither to the modulation signal prior to comparing it with the audio signal. In a 1-bit system like an A/D converter, signal information below the least significant bit can be coded if dither is added to the signal. This effectively codes the information in a PWM signal. This can be compared to a class d amp which is similar to a 1-bit system in that the output signal has only two discrete levels but is continuous in the time domain.

I don't know if any of that helped but I threw it out there anyway
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Old 24th November 2005, 06:07 PM   #8
TerryG is offline TerryG  United States
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I have thought long and hard into why class D amplifiers sound better than linear amplifers. Part of the answer is that they don't, a really good linear ampilfier will sound as good. The difference is that class D is easier to get to sound like a very good linear amp.

Now for why I think that class D does make things sound better. First class D does not increase in distortion at the same rate as linear amplification in general. All amplifiers will increase in distortion with an increase in frequency response. This is directly related to frequency response extention. A divice increases in distortion toward is limits, in almost every way, now class A/B B amps can have more distortion at lower frequencies because often their crossover distortion is greater at lower powers. This was one reason why people started to listen to music louder, because the amp had less distortion at higher volumes, other than the discovery of how the Fletcher/Munson curve applied.

At any rate class D ampifiers don't increase in distortion like linear amps do, they do but not nearly on the same degree, or in the same way. These are not absolute statements I am making, more generally speaking. Likewise the frequency extention in the upper range of class D amplifiers can go quite a bit higher than most linear ampilfiers. Our ears precieve location because of the timing difference of when the sound hits each ear, along with amplitude, but the later is secondary. So a frequency response higher than 20K is beneficial in that it helps us precieve a "cleaner" sound stage and imaging of the signal. It gives more amient information to the sound and generally helps things appear clearer.

All in all the reason things sound better is directly related to distortion, the less we have in the signal the better it will sound, and class D amplifiers don't have the distortions that linear amplifers have in the same ways. With class D most of what we measure with THD is often noise, not true distortion. And in real world situations other than test bench situations class D performs better. Understanding this will also help designers design in such a way that they limit distortions, or in other words design with these things in mine.


Terry
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Old 25th November 2005, 06:34 AM   #9
Kenshin is offline Kenshin  China
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Class D is once thought to be nasty. What's the major difference between class D nowadays and old ones?
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Old 25th November 2005, 07:38 AM   #10
TerryG is offline TerryG  United States
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From what I can gather, because I wasn't around in the days it was considered "nasty", is today we have higher switching frequencies availible that enable true full range Class D amplifiers. This is made possible by advances in solid stage devices, like MOSFETs that have become available in the last five years. Likewise their have been many circuit advances in the last five to ten years, as a direct result of the aforementioned.

Terry
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