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Old 16th September 2007, 09:12 PM   #31
Pierre is offline Pierre  France
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The question is... is the sound really better when THD is 0.004% instead of, say, 0.04%? I don't really think so when the THD introduced by most high quality speakers is much, much, much higher than that. (not to talk about the human hear perception of distortion, of course)
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Old 16th September 2007, 09:25 PM   #32
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Pierre: It's a very good question.

When we replace the LM6172 of our input stage to a LM4562, the THD increases from 0.002% to 0.01% (5 times). But the sound gets warmer and more destinct in details.

So i can agree with you that THD is not the only key to good sound.
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Old 16th September 2007, 10:28 PM   #33
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A well done blind test with some "leading" Class D modules built to amps by the manufacturers would be more interesting to me than traditional measurements of distortion with static signals as in THD and IM distortion analysis. Traditional measurements may tell if an amp is sufficient well engineered, but not necessarily if itīs musically satisfying. Of course low distortion figures are a good thing and I donīt believe in euphoric distortion making reproduced music sound more "real".

Designing a blind test can be tricky, but it can be done in a clever way, and maybe a test called F/E here in Sweden (translated; Before/After) can be the tool to get reliable results. In short it is made so that the test signal, usually music, passes or doesnīt pass the object under test. Levels are adjusted so itīs attenuated before passing an amplifying device like a power amp thus no level differences occur that could be detected. The result can be treated with statistical methods to see if the test persons can determine a clear influence of a distorted or "coloured" signal that has passed through an amp.
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Old 17th September 2007, 05:51 AM   #34
Eva is offline Eva  Spain
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Some words on THD:

According to SpectraLAB software, my THD perception threshold is somewhere between 1% and 5% for a 1Khz tone progressively clipped to get odd harmonics (listening at approx 80dB SPL). Adding other frequency components (or music) only makes THD perception harder.

Anybody owning a computer with sound card, a pair of speakers or headphones and some THD analysis software or equipment can do the same test and see the shocking results for themselves.

Arrange the sound card in loopback mode (or monitor the speaker output with a THD analyzer). Close your eyes, then increase the playback level until you feel that you no longer hear a pure tone (clipping), then open your eyes again and check the THD meter

We are really wasting a lot of effort trying to improve amplifier non-linearities that just can't be heard. Tweeters and woofers are the #1 THD generators. Next time get a precision microphone and insert your favourite loudspeaers in the measurement loop. Play loud tones and start crying when 5% appears in the THD meter
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Old 17th September 2007, 06:45 AM   #35
ssanmor is offline ssanmor  Spain
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I totally agree with Pierre and Eva.

Once a reasonable good level of THD is reached (say, below 0.1%), the rest is pure make-up. Not saying that is worthless to try to improve this figure, as a better number means a careful and well adjusted design, but I am only saying that trying to reach ultra-low numbers is a waste of time when other points are much more interesting in Class-D designs that have to be investigated.

To be sincere, I am saying this when we try to make our COLDAMP amplifiers as low THD as possible. By the way, we have also seen higher figures in the ELEKTOR article than what we get at our labs (below 0.02% at 1W/8ohm). They simply didn't have too much time to setup the measurement properly and even to hear the amplifiers. Perhaps it was the same story with NCD?

However, we are not worried about that, as we know that our customers value the sound of our products above all, and some of them clearly prefer them to other modules with lower THD even for very high quality and precision demanding applications such as recording studio monitors, etc.
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Old 17th September 2007, 07:27 AM   #36
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Lars Clausen, what about induktor? Maybe central gap would be better for EMI, than present construction? Did you test your module for compliance to EMI requirements?
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Old 17th September 2007, 07:47 AM   #37
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Quote:
Tweeters and woofers are the #1 THD generators. Next time get a precision microphone and insert your favourite loudspeaers in the measurement loop. Play loud tones and start crying when 5% appears in the THD meter
Keep in mind that the pure percentage figure doesn't say much. The high THD figures of loudspeaker drivers are usually less disturbing than things like crossover distortion of AB amplifiers for instance.

It is therefore not a bad idea to keep the nonlinear distortion of any electronic part of the audio chain as low as reasonably possible.

Rregards

Charles
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Old 17th September 2007, 07:48 AM   #38
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Bender.ru: yes of course.

We made experiments with many types of inductor, even toroid, but so far nothing has rivalled the design we are using. Center gap also didn't give any advantages on performance or emissions.
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Old 17th September 2007, 08:00 AM   #39
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Originally posted by Lars Clausen
Bender.ru:Center gap also didn't give any advantages on performance or emissions.
really?
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Old 17th September 2007, 09:05 AM   #40
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I suspect because the RM core is a fairly open construction, so RF emits more or less equal from the center gap.
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