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Old 28th August 2015, 11:57 PM   #1
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Default Super High distortion class d amp

Hi everyone,

So...first time here. I´m pretty much a beginner in electronics and specially in audio amps, but I decided to design a class D amp, just to learn something and the results are not what I expected.

(EDIT)Schematic and photos -

There is really high distortion in the output and I struggling to find the problem.

~=20kHz -
Click the image to open in full size.

Click the image to open in full size.

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~= 3Khz
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Click the image to open in full size.

The above signals were measured with ground as reference, it´s not the diferential voltage at the load resistor.

First I thought it was the filter, but I used diferent types of inductors, some get a little less distortion than others, but still huge amounts of distortion.

Then I though it was the feedback, so I changed a few things, no difference, used a second order integrator and not much improvent.
Still, I´m not shure about the feedback ... I calculated the values, used a simulator and it was fine.

The triangular wave seems ok (500kHz).
Click the image to open in full size.

The square wave, with no imput signal, measured at the source of the top side mosfet´s to ground, looks good, but the lower gates signal... looks weird.
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lower gate
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I dont see significant noise in the supply input.
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I wasn´t expecting to make an awesome amp, but this one sounds awfull.
I hope someone can give me a clue about what I'm doing wrong.

Thank you.

Last edited by shinobot2; 29th August 2015 at 09:28 PM.
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Old 29th August 2015, 01:28 AM   #2
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Your schematic is missing several net joints, so it might be inconsistent. Is it just a drawing or have layed out a board it?

Beside this, your pictures do not show up here.
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Old 29th August 2015, 02:23 AM   #3
DUG is offline DUG  Canada
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How much of the switching frequency is getting through on the "error" net?
Doug We are all learning...we can all help
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Old 29th August 2015, 10:28 AM   #4
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I rearranged the schematic.
Pictures and shematic are in the link above.

As for the switching frequency, its 500kHz, and in the feedback path I put a filter with cutoff frequency at 480kHz. The opamp as quite high slew rate (22V/us).

But now that you reminded me, I don´t know if filtering like I did is the best way to do it. I will change and see the result.

Thank you.
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Old 29th August 2015, 03:58 PM   #5
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Did my first class D-attempts with HIP4081. Powered by 12V SLA driving 8 Ohm speaker. Occasionally the PowerMOSFETs burned for no obvious reason. And there was some interference noise that I traced back to the internal charge pump - no way to fix that. So I gave up fiddling with these chips.
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Old 29th August 2015, 04:27 PM   #6
kevinkr is offline kevinkr  United States
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Links are all dead, please, please, please post this stuff to the forum directly.. Thanks!
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Old 29th August 2015, 06:23 PM   #7
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The link I posted earlier, seems to be working, but I will upload the pictures later when I get home.

Anyway, today I removed/changed a lot of things, but still a lot of distortion.
Removed feedback, to see if it was the problem, got alot worst.
Changed filter caps and inductors, no improvement.
Installed a new Hip4080 and two new IRFI4212 MOSFETS, same thing.

So I´m starting to think it´s the PCB.
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Old 1st September 2015, 06:46 PM   #8
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Hey again,
I took a look at the HDEL and LDEL voltages and I see something that is most likely causing the distortion.

So, on the HDEL signal, two big spikes (around 300mVp) repeat every 2us(500kHz).


HDEL_spike1.jpg HDEL_spike2.jpg

The width of the first spike is around 150ns and the second 100ns, which is similar to the deadtime I was expecting with a 100k resistor on the pin.

Now the problem is on the LDEL pin.

LDEL.jpg LDEL_spike1.jpg

There´s one really big spike (above 1V) with 470ns width. and a small one that look moreless like the ones on the HDEL pin.

With one of the low side MOSFET being switched with 470ns propagation delay... hmm. It´s the only thing I see that might justify so much distortion on the output.
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