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Old 21st January 2015, 09:58 PM   #1
tigox is offline tigox  Portugal
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Default Problem with filtering in speaker amplifier

Hello, i own the "supertooth Disco" portable speakers and lately am hearing a lot of nasty noise on the right channel, so after opening the speakers, i found a TPA3004d2, and started probing with my DS1054z oscilloscope; at first i didn' now how in the world i was going to see the waves because of how class d amplifiers work, but after AC coupling the inputs and enlarging the horizontal scale i found that there really is something up with the right channel:

Click the image to open in full size.
Here is shown the left chanel in yellow and the right channel in blue, and as you can see there is major interference in the right(blue) channel.

After attempting to take a closer look at actual peak i zoomed in and got this:

Click the image to open in full size.
So, as i am new to class d, my one and only guess is a bad filtering stage somewhere, maybe the LC filter at the output? or maybe a cap somewhere?

Opinions are appreciated
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Old 21st January 2015, 11:02 PM   #2
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Try replacing the capacitor in the filter first.
That would be a cheap easy fix.
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Old 21st January 2015, 11:10 PM   #3
tigox is offline tigox  Portugal
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which capacitor, input network, output?
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Old 21st January 2015, 11:11 PM   #4
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Output filter.
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Old 21st January 2015, 11:16 PM   #5
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Is your amplifier well decoupled ?
If it wasn't that would be a source of noise too.

I found class d to be fussy about decoupling and pcb layout.
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Old 21st January 2015, 11:39 PM   #6
tigox is offline tigox  Portugal
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the amplifier is from a speaker system (supertooth disco) i am trying to repair the noise, so i would assume it is well decoupled, but yes i will check the output capacitor, and also, i am sorry to ask but are you referring to the decoupling cap or the LC filter?
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Old 21st January 2015, 11:40 PM   #7
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The LC output filter.
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Old 21st January 2015, 11:46 PM   #8
tigox is offline tigox  Portugal
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thank you so much, tommorow i will try to replace it and will report back if any good result
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Old 22nd January 2015, 11:23 AM   #9
tigox is offline tigox  Portugal
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So... I replaced the capacitors and the problem remains.. Hum, but also I got a better sound at what the noise sounds like, it sounds like the highs are there but the mids and low ends like bass distort, any ideas? And also what is the ideal way to measure the output of a class d on an oscilloscope?
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Old 22nd January 2015, 03:09 PM   #10
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There might be a problem at your input. Guessing, that INRN is coupled to GND via a capacitor, there might be a problem. The scope pics look like that. You may check that. (broken cap)
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