is THD+N in class D amps HD or noise limited? - diyAudio
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Old 21st September 2010, 09:17 AM   #1
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Default is THD+N in class D amps HD or noise limited?

I haven't followed D-amps for a couple of years, but quick browsing of manufacturer datasheets and this forum leads me to believe that even with feedback, D-amps are still much worse than average class AB amps, i.e. a good D-amp will have roughly 0.02 % or - 74 dB THD+N, no matter whether analog input or digital. If I have overlooked any designs that perform significantly better, I'd be glad for a pointer.


Datasheets will ususally give THD+D vs. power and sometimes vs. frequency, but I am not aware of any FFT data with a sine input. Again, I'd be glad for any pointers.

The best thing I found was the TAS5630 data sheet that shows about 0.02% THD+N over much of the operating range. It gives an FFT of the noise floor, which seems to sit at about - 135 dB. This would imply that THD+N is dominated by HD, contrary to what I expected. Comments?


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Old 21st September 2010, 12:03 PM   #2
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I am not an engineer, but I want to point out that 41Hz's implementations of Tripath TK2050 and TA3020 amps have been measured with 0.007-0.008% THD+N:

41Hz Amp4
41Hz Amp15
Amp15-PS measurements

When the Amp15-PS BBM timing was lowered from 80nS to 40nS, "the THD+N at 1KHz went down from around 0.008% to around 0.005%." (source). This heats up the output FETs a great deal, to the point of blowing them up in some cases, so that the 0.005% THD+N distortion figure may not be realistically attainable in all configurations.
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Old 21st September 2010, 01:04 PM   #3
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Thanks. Especially the last link with the measurements was very helpful. While it shows that the noise floor is excellent, it seems to indicate that THD+N is mostly defined by a -80 dB second harmonic (ok, maybe -82 dB if you take into account signal generator noise).

This is a number you would expect from a poorly designed AB amp with high feedback, or a well-designed AB amp with little or no global feedback.

Are the distortion mechanisms understood in class D?
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Old 21st September 2010, 02:27 PM   #4
AP2 is offline AP2  Italy
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Hi all,
This FFT is also an excellent outcome reported to an amplifier
in class D.
Attached Images
File Type: png FFT-44K-8R-20w.PNG (18.6 KB, 599 views)
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Old 21st September 2010, 07:50 PM   #5
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Quote:
Originally Posted by AP2 View Post
Hi all,
This FFT is also an excellent outcome reported to an amplifier
in class D.
Thanks for the pic - what amp is it from though? (I'm pretty interested in the IMD figures, do you have those?)
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Old 22nd September 2010, 07:36 PM   #6
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Quote:
Originally Posted by AP2 View Post
Hi all,
This FFT is also an excellent outcome reported to an amplifier
in class D.
Yes, it might be good for class D, but -86 dB 2nd harmonic at 1 kHz is at best low end for class AB.
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Old 22nd September 2010, 08:06 PM   #7
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Quote:
Originally Posted by capslock View Post
Yes, it might be good for class D, but -86 dB 2nd harmonic at 1 kHz is at best low end for class AB.
Hmmm .. Can you give a reference to a "low-end" class AB amp with significantly better THD specs?
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Old 23rd September 2010, 09:49 AM   #8
AP2 is offline AP2  Italy
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Quote:
Originally Posted by capslock View Post
Yes, it might be good for class D, but -86 dB 2nd harmonic at 1 kHz is at best low end for class AB.
Hi,
yes, this is not very good for audiophile performances but... see full spectrum please. this is a good commercial audio amplifier class D,300-400w(4R) with PSU on board. you can see FFT at 1KHz from annother amplifier, spectrum is full of harmonics. very clean output has to work very hard.
many harmonics affecting the upper audio frequencies to listen, the voice is as shrill.
I not see clean FFT of others Amp, as the A-B class.
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Old 23rd September 2010, 10:42 AM   #9
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does the LM3886 count as low end?

I am also LM3886!
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Old 26th September 2010, 08:42 AM   #10
Pabo is offline Pabo  Sweden
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capslock

There are two basic families of class d. Clocked and self oscillating.

Clocked versions have a theoretical loop gain that is constant vs output power (given that the power supply is firm) which makes THD fall all the way to clipping similarly to a class B amp. The big problem is that their loop gain is low giving a high THD to start with. Adding integrating loops around a clocked amp gives a big increase in high order harmonics unless the fsw and harmonics are filtered out of the fed back signal.

Self oscillating amps have a much higher loop gain at low output powers but the loop gain falls as output signal is increased (due to dropping fsw) giving a rising THD at higher powers. It is somewhat easier to add integrating loops to a self oscillating amp but one has to make sure that the amp does not start switching at the resonance frequency of the output filter.

There are of course many more factors arising from the practical implementations.

One can achieve good audio performance with both clocked and self oscillating amps but it is much more complicated and expensive to do it with a clocked amp.
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