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Class D Switching Power Amplifiers and Power D/A conversion

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Old 16th December 2003, 06:38 AM   #41
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Quote:
And yet you can measure harmonic distortion at their output.
Agree on that !

Furthermore, Lars is still owing us the answer how he comes to his conclusion.

Regards

Charles
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Old 16th December 2003, 09:14 AM   #42
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Nelson: Maybe from the sine wave generator?

Charles: can you be a little more specific in your question, so i can give you a precise explanation... thanks!
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Old 16th December 2003, 10:12 AM   #43
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Hi Lars

You always mention that some switching amplifiers wouldn't generate harmonics of the input signal (i.e. harmonic distortion) but rather noise only.
My imagination tells me that such an amp doesn't exist. While an ideal switching amplifier wouldn't introduce any error in the band of interest, a real-world one definitely will - due to timing errors, overshoot, PSU sag, nonlinearities in the "nonswitching part" of the amp etc.

The PWM amp I once developed did indeed have some HF noise that was more noticeable than THD (which was mainly 2nd order) when it was used to listen to music (which it wasn't intended for BTW). Which in turn doesn't mean that it wouldn't be cool to reduce BOTH.
The sound was nevertheless pleasing (OK, OK, OK - no parent has an ugly child....), though we could only test it in mono since there was only one amp. Stereo is usually more revealing for evaluating sound quality.

I did some simulation work on delta sigma amplifiers using a fixed sampling rate (which is the most usual implementation of a sigma-delta modualtor). The "noise" generated by the noise-shaping was mainly consisting of odd order harmonics which correlates well with a statement in a famous book:

Delta-Sigma Data Converters: by Steven R. Norsworthy, AT&T Bell Labs; Richard Schreier, Oregon State University; and Gabor C. Temes, Oregon State University

I assume your amp is working the following way: you have a 2nd order feedback-loop like the ones used for 2nd order noise shapers. Instead of a fixed sampling rate (by the use of a comparator and a subsequent clocked D-flip-flop) you just use a comparator, and the gains and time-constants of the integrators (and therefore loop-gain and phase-marging) are set in such a way that the thing oscillates by itself (feedback is taken from the output of the switching stage).
Even though I don't like self-oscillating things, I have to admit that such a topology could make a very fine amplifier.

But saying that such an amp would just give noise instead of IMD and THD, without mathematical or measurement proof (or at least some hint where the "gut feeling" comes from), is a very BOLD statement !!

Regards

Charles
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Old 16th December 2003, 12:36 PM   #44
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Hi Charles

Your description of my amplifier is mostly accurate.

I don't use a D-flip flop to control timing, since it would add rounding errors to my timing. (Quantization noise).

The topology that sigma-1 (one for you Guido T) people refer to as Class D, uses a triangular wave for input to the comparator. This allows for the comparator's CMRR (in dB) to be translated into harmonic distortion.

Assuming the comparator, operates at 0 mV on both input terminals, the CMRR can not translate into distortions, but only the noise will modulate with the amplified signal.

This is what i mean, in real life - depending on the type of comparator and integrator used - you may of course get HD anyway. That is why my claim is that

'SOME types of PWM amplifiers don't produce HD, but instead random noise'.
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Old 16th December 2003, 01:42 PM   #45
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O.K.
I can agree that you have one source of distortion less, since you are not relying on the linearity of a triangle-generator/comparator topology.


Regards

Charles

P.S.: Ever tried to take feedback from the output filter ? I know that you use a core with an air-gap (I used the same principle with my amp) to reduce nonlinearity but it might still be advantageous to include the filter into the feedback loop.
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Old 16th December 2003, 01:45 PM   #46
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Hi again Charles

I have made a THD measurement on an amplifier of the type we are discussing. Below shown is the THD with an RC output filter (to measure THD of the modulator and amplier alone). The amplitude is equivalent to 10W RMS.
The THD is close to the limit of my THD analyzer, around 0.001%, as can be seen mainly 2nd order.
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File Type: jpg zpmodthd.jpg (56.4 KB, 791 views)
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Old 16th December 2003, 02:00 PM   #47
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And here is the system alone (without PWM amplifier).

The reading still shows 0.001% THD. It is a HP339A.

With a LC filter in the output, the THD is exceptionally higher, about 20 times around 0.02%
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File Type: jpg thdsys.jpg (57.1 KB, 761 views)
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Old 16th December 2003, 02:17 PM   #48
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Charles: My above measurements would surely support your suggestion to take the feedback signal AFTER the output coil. However i think it would be sacrificing some of the inherent stability towards capacitive and inductive loads.

I know of one company doing the feedback-after-the-choke thing, the result of that is also pretty good.

When it comes to feedback loops, though i have an idea of keeping them as short as possible, and with as low delay as possible, in order to keep IMD down.
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Old 16th December 2003, 02:25 PM   #49
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Hi Lars

If your first integrator is of the inverting and summing type then I have a simple suggestion for a mixed feedback. I.e. feedback partially from before AND after the filter which is less effiecient regarding THD reduction but less detrimental to stability.

Regards

Charles
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Old 17th December 2003, 12:31 AM   #50
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Seems the best specs, and best listener feedback, has been with post output filter designs: Spectron, Icepower, Philips SODA and UcD. Perhaps Mueta, once they release evaluation samples, will also fall within this group.
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