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Class D Switching Power Amplifiers and Power D/A conversion

LTspice Ncore simulation
LTspice Ncore simulation
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Old 6th October 2018, 02:18 AM   #21
IVX is offline IVX  Ukraine
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OMG, analogspiceman is here, like in good old time
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Old 7th October 2018, 06:29 AM   #22
boB_K7IQ is offline boB_K7IQ  United States
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Looks like this could have a bit more LP filtering to reduce the HF peak and the extra switching noise although that noise probably doesn't affect the sound.
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Old 7th October 2018, 07:21 AM   #23
analogspiceman is offline analogspiceman  United States
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One of the improvements claimed of the Ncore topology over the original Ucd self-oscillating class-d amplifier is that the modulator transfer function is more linear. I created a couple of simulations to check this out and, indeed, the Ncore transfer function is very linear until just under saturation whereas the original UcD design has a more tube-like, rounded transfer function with a bit of an s-curve. To reproduce these curves, the x-axis must be changed from "time" to "V(a)" by hovering the mouse pointer over it until the ruler icon appears, then click and edit the axis parameter.

The Ncore also has about 20dB more loop-gain across the audio band (to see this, switch to an ac analysis for both simulations and click on the "fb" node).
Attached Images
File Type: gif UcD Xfer.GIF (43.0 KB, 438 views)
Attached Files
File Type: asc UcD Ncore xfer.asc (5.8 KB, 37 views)
File Type: asc UcD xfer.asc (2.8 KB, 29 views)
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Old 7th October 2018, 06:07 PM   #24
analogspiceman is offline analogspiceman  United States
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Some astute readers may have noticed that the self-oscillation frequency predicted by the ac simulation is close to, but higher than the actual operating frequency in the transient simulation.

As Bruno Putzeys pointed out, the UcD is not a sine wave oscillator, so the additional phase shift of harmonics influences phase crossover for the composite waveform. Oscillation occurs at the frequency where if the loop were broken (and driven as before the break), the signal on both sides of the break would be the same shape and phase relationship.

At quiescence, at the input to the power comparator the output square wave has been heavily low passed, so it appears as a slightly distorted sine wave, so the influence of the harmonics is very small and the ac analysis is quite close. However, when the output level is near the rails, the output is a narrow pulse rich in harmonics that must be taken into account.

In the ac simulations (which are open loop), I set the ac source to a value so that when traversing the feedback loop, zero crossing of magnitude would occur at the same point as phase crosses zero. I have done this simply to make the results appear more intuitive by mimicking what the closed loop would do naturally through feedback.
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Old 7th October 2018, 07:34 PM   #25
bwaslo is offline bwaslo  United States
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LTspice Ncore simulation
Quote:
Originally Posted by boB_K7IQ View Post
Looks like this could have a bit more LP filtering to reduce the HF peak and the extra switching noise although that noise probably doesn't affect the sound.
As feedback is taken after the LP filtering, more filtering probably wouldn't do much to the HF peak other than reduce the feedback there (and possibly degrade phase margin)
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Old 8th October 2018, 12:07 AM   #26
boB_K7IQ is offline boB_K7IQ  United States
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Quote:
Originally Posted by bwaslo View Post
As feedback is taken after the LP filtering, more filtering probably wouldn't do much to the HF peak other than reduce the feedback there (and possibly degrade phase margin)
Yes. Of course there could be added filtering after that output feedback point that is not in the loop.

I doubt it would make it sound better. I would welcome HF boost in that high range although I can't hear quite that high now anyway.

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Old 8th October 2018, 04:25 PM   #27
Reactance is offline Reactance  South Africa
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I would like to start building a variation soon, I'm more interested how the fault protection works with the PIC18F26K20, one of the parameters is monitoring the PWM oscillation for drift.
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Old 26th October 2018, 01:49 PM   #28
SSassen is offline SSassen  Netherlands
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OMG, analogspiceman is here, like in good old time
I rejoiced as well, we're in for a treat!
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Old 30th October 2018, 01:26 AM   #29
kartino is offline kartino  Indonesia
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Old 6th November 2018, 07:21 PM   #30
analogspiceman is offline analogspiceman  United States
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Perhaps if I have the time I will attempt to create a simulation with a linear version of the N-core with the same response as the switching version (except for the switching, of course). Why? Then I can run the same audio .wav file input into both simulated amplifiers and difference their outputs, saving the result as a .wav file.

If the responses for moderate signal levels are truly identical the difference will yield no output (at least at low to moderate input levels). However, with the original UcD, for signals with outputs that exceeded about half of full power, the difference was no longer zero. This difference is the coloration added by the UcD switching process. I believe these .wav files should still be available here on the diyAudio website. They are interesting to hear although I don't claim to know what they mean.

Is anyone interested in this exercise for the N-core, assuming it can be done? What other simulations with the N-core might be interesting?

Last edited by analogspiceman; 6th November 2018 at 07:30 PM.
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