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Old 9th December 2008, 09:51 PM   #1
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Question No even order THD?

I've read IRF's - Class D Tutorial.

When Half bridge and full bridge are compared, IRF name an advantage gained when using full bridge, which is:

Superior (No even order HD)

Can anyone elaborate on this? As in why even order THD are missing in this setup?
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Old 9th December 2008, 10:03 PM   #2
Pan is offline Pan  Sweden
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Even order distortion products is a result of non-symmetric waveform distortion.

Think of a sine wave that is slightly flattened but more so on one half of the waveform than the other (the positive half for example).

I the waveform would be distorted/flattened symmetrical the result would be only odd order harmonics.

Now, if a circuit with nonsymmetrical distortion (even order) is bridged with a second identical circuit the new complex circuit will balanced so any nonsymmetrical distortion is cancelled out.

A little simplified but more or less so! :-)


/Peter
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Old 10th December 2008, 11:29 PM   #3
ericj is online now ericj  United States
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That all makese sense, but the general, communal wisdom about harmonic distortion is that it's the odd-order harmonics that sound worst.

It's even been postulated that much of what some people like about 'tube sound' is the 2nd order harmonic distortion.

I'm confused by why IRF would want to say "hey! We only have the bad distortion!"

I realize that it's probably more complex than that, but there is certainly an increase in the popularity of integrated amps that include a tube gain stage and a solid-state power output stage. So far i haven't seen one with a class-D power stage, but it wouldn't surprise me to learn that they exist.
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Old 10th December 2008, 11:49 PM   #4
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Full bridge does indeed cancel out even order harmincs, whatever the class of operation.
The point is that class D distortion, which is mostly about dead time and power supply saging phenomena is inheritely odd order, so there is not so much to cancel.
The real case with bridged class D amplifiers is that switching mosfets are really bad when voltages over 150V are concerned.
On the other hand point of building a class D amplifier of below 100W while one can build a great class AB amp of similar wattage is questionable.
IMO class D is calling to be used in full bridge from the more general point if view.

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Adam
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Old 11th December 2008, 06:34 AM   #5
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Pick up a sound program like Audacity. Find a bunch of sound samples of acoustic instruments. And run it through the spectrum analyzer. You'll find some instruments produce primary even harmonics, others primarily odd - yet they all sound musical. In one case I know of a recorder (closed pipe, odd harmonics) was substituted for a flute (open pipe, even harmonics) and many were fooled thinking it was a flute. In some old music scores, flutes and recorders were used interchangeably.

A amps job is to just amplify, unfortunately nothing is perfect and there is some distortion introduced. But with a lot of these amps there is such a small difference you'd have to be super human to really hear a difference. When somebody starts pushing opinions that their amp is designed to produce even harmonics then walk away. It's not doing it's job of amplifying very well.
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Old 11th December 2008, 08:00 AM   #6
Eva is offline Eva  Spain
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Loudspeaker THD figures are in the 0.1% to 1% range, at least at moderate playback levels because they can rise up to 10% at high levels. They produce both odd and even harmonics at all frequencies and particularly in the lower end, whose harmonics are quite audible because ear sensitivity to midrange is much higher than to the bass fundamental, which is also much stronger than the original midrange components.

Solid state amplifier THD figures (including class D) are well below 0.1% during most of the time, at least an order of magnitude lower than loudspeakers. Harmonics are mostly produced in the higher end, where they are harder to perceive because ear sensitivity is much more uniform... So there is not much to worry about.

The problem is that most amplifier freaks think about speakers as ideal black boxes (same for room acoustics, etc...) You can even see people developing sophisticated ultra low distortion amplifiers and using them with boom box speakers...
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Old 11th December 2008, 08:19 AM   #7
Pan is offline Pan  Sweden
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Reducing 2nd ordeer HD also means lower IMD and that is essential in HiFi gear.

Lower sensitivity to higher frequencies does not mean that we can allow high levels of distortion up there since IM products fall down in the midrange where we are most sensitive.

AFAIK class D amps typically have some nasties in the upper range making them non-transparent in blind tests.

As for looking at amplifiers as ideal black boxes, of course, what would we do otherwise? Use them as effect-processors? ;-)


/Peter
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Old 11th December 2008, 08:32 AM   #8
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for class d there are two mechanisms for distortion, the first is swiching related and down to dead time, ringing etc, and the second is down to frequency of the sample/self oscillation to the amplified frequency.

the first distortion is analogous to crossover distortion in class ab amps;
the second is the number of sample steps you have for the feedback to correct the amplifier error; sampling at 400KHz you have 4000 samples for a 100Hz signal, 400 for 1KHz and 40 for 10KHz- however the distortion at higher frequencies appears to the ear lower, as the 2nd harmonic of 10KHz is 20KHz this above most peoples hearing.
this is why class d amplifiers have lower distortion at lower frequencies and it tends to rise with frequency; but perceived distorion is less
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Old 11th December 2008, 08:34 AM   #9
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Quote:
Originally posted by Pan
Reducing 2nd ordeer HD also means lower IMD and that is essential in HiFi gear.

/Peter

How can you justify this statement? mechanisms for THD and IMD are different, especially in class d
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Old 11th December 2008, 11:09 AM   #10
Gyula is offline Gyula  Hungary
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Quote:
the second is the number of sample steps you have for the feedback to correct the amplifier error
The linearizer effect of a feedback depends on the open-loop gain at the desired frequency, the frequencies of the harmonics. The sample rate influences the stability, because of the spectral overlap.
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