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Class D Switching Power Amplifiers and Power D/A conversion

Class D amp + EMI reduction
Class D amp + EMI reduction
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Old 13th March 2018, 10:36 AM   #1
djpns is offline djpns
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Default Class D amp + EMI reduction

Hi all! I'm a new electrician and I'm making a radio for myself. I have a radio receiver that works on 5 Volts and has 2 outputs of 3 Watts each. I've recently bought a D-Class amplifier that works on 12-24 Volts and has 2 outputs of 80 Watts each. I have a problem with the EMI effect. On "not so good" stations, when I connect the speakers to the amp's output, the radio signal falls. I can see it from the radio board and I can also hear it from the speaker. If I connect the speakers directly to the radio board, the signal is perfect. I've read that this is a common EMI problem and I need an output filter. I've also heard about the ferrite chokes but I don't know how to choose the right ferrite or how to use them. Can you please help me find a solution? It means a lot to me. Thank you very much!!!
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Old 13th March 2018, 11:22 AM   #2
FauxFrench is offline FauxFrench  France
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EMI (ElectroMagnetic Interference) is an important problem for the combination radio -> class D amplifier. Could you tell what class D amplifier you bought?
Your class D amplifier should already have an output filter, in particular if it is 24V supply/80W output. The problem you observe is likely to be the radiated noise (by air) that partly paralyzes your radio receiver. A class D amplifier generates square-wave signals in the range 300KHz-600KHz and, as they are square-wave shaped, they contain a lot of higher frequency components. Perfect to disturb a radio receiver. Radiation shielding may be the solution to your problem.

Try the following:
1) Use the radio with a less strong radio station and the speaker connected to your radio. Connect a second speaker to the class D amplifier (a class D amplifier should only be turned ON with a load connected). Put your class D amplifier some meters away from the radio and turn the class D amplifier ON. It should make your radio reception less good. The closer the class D amplifier is to the radio, the worse the radio reception should be.
2) Use the headphone output on your smart-phone as input to the class D amplifier and make sure that the class D amplifier works well.
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Old 13th March 2018, 04:21 PM   #4
FauxFrench is offline FauxFrench  France
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I agree with you, this amplifier has got no output filter. That is unusual with the TPA3116D2 chip. Then, the speaker cables will work as antenna for the modulation frequency (300KHz-600KHz).

Try to see if the radio reception is disturbed even with the amplifier meters away.
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Old 13th March 2018, 05:02 PM   #5
djpns is offline djpns
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Well, when I put the amp some meters away, the signal of the radio stations is much better! But not the best signal. Do you know what filter do I need to buy or how to make one?
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Old 13th March 2018, 05:29 PM   #6
Khron is offline Khron  Finland
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Old 13th March 2018, 05:39 PM   #7
voltwide is offline voltwide  Ireland
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For that kind of emi problem I suggest ferrite clamps - the likes you find sometimes glued to USB-cable.

Any cable connected to the class-d-amp should pass a ferrite clamp that is located close to the class-d-amp.
The best option is not one ferrite clamp/cable but a single big common clamp for all cables.
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Old 13th March 2018, 06:09 PM   #8
djpns is offline djpns
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I tested one ferrite clamp from an old power supply. I cut the wires from the supply and then used them to connect the amp's output with the speakers but something strange happened: there was no sound from the speaker! Even if I connect the speaker's with the output's screws that tight the cables on the output. It looks like this ferrite cuts the audio or something. Is there different values of ferrite? Cause I don't know about them a lot.
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Old 13th March 2018, 06:12 PM   #9
OffGridKindaGuy is offline OffGridKindaGuy  United States
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Input isolation helps. Just the fact of breaking the D.C. path makes a difference. These are some of the best that I have found.


JaJaBor Female to Male 3.5mm Aux Audio Noise Filter Ground Loop Noise Isolator Eliminate Car Electrical For Car AUX Speaker-in Bluetooth Car Kit from Automobiles & Motorcycles on Aliexpress.com | Alibaba Group
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Old 13th March 2018, 09:07 PM   #10
voltwide is offline voltwide  Ireland
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This may be useful to reduce humming noise due to ground loop. But not in case of radio interference in the 100MHz range.
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