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Power Supply Noise Problem
Power Supply Noise Problem
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Old 17th August 2019, 09:55 AM   #1
thyristor44 is offline thyristor44
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Default Power Supply Noise Problem

Hello
I have made a streaming music player to equal or better my Panasonic transistor radio, so not A1 quality.
I have a Pi 3 b+ running pCP, feeding a PCM2704 Dac, feeding into a
TPA3116D2 Digital Audio Amplifier Board Module DC 5-28V VBU. Its all in a box with a 7"Hdmi screen. Power supply is 12v from any number of psu's, and 12v is dropped to 5v for the Pi by a 12-5v convertor.
Its noisey. What can I do, any way to decouple anything.
If I run the Pi from a separate stabilised 5v supply, its quiet, almost, or good enough.
Any ideas to keep only 12v input.
Thanks
thyristor44

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Old 17th August 2019, 10:22 AM   #2
avtech23 is offline avtech23  Australia
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Power Supply Noise Problem
Can you elaborate on the 12-5v converter you are using?

Part details & schematic would be useful to get an idea of what the problem might be.
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Old 17th August 2019, 10:48 AM   #3
thyristor44 is offline thyristor44
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I have no details except its from ebay uk.
eBay item number:
142752974294. I think it is a simple buck convertor.
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Old 17th August 2019, 11:48 AM   #4
avtech23 is offline avtech23  Australia
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Power Supply Noise Problem
That module exhibits a voltage drop as it gets loaded up which might cause an issue.

What wattage 12v supply are you using? Is it enough to power the Pi and Dac and Amp?
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Old 17th August 2019, 05:30 PM   #5
thyristor44 is offline thyristor44
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Its 12v 1A, could be a problem so I will try a bigger one.
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Old 17th August 2019, 05:55 PM   #6
thyristor44 is offline thyristor44
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I tried up to 5A at 12v and same noise. As the Pi boots one of its lights flashes and gives a pulse that comes over the amp. There also fizzling sounds.
As the 12v to 5v 3A stepdown module is very cheap I dont expect it has much smoothing.
It runs the Pi at 5v and also the 7" lcd display.
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Old 17th August 2019, 06:55 PM   #7
bobo1on1 is offline bobo1on1  Netherlands
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There is a ground loop in your system, the current draw by both the raspberry pi and the amplifier are being converted to voltages by the power supply wires, and because they share the same power supply, those voltages are being fed into the amplifier's audio input.

You need to break the loop somehow, you can use an isolated DC-DC converter to feed the raspberry pi, you could use a usb isolator, or you could use a line level audio isolator.
When using an isolated DC-DC converter, or a usb isolator, there might be a capacitor connected between the input and the output, which might still cause a ground loop at high frequencies, so that might not fix your problem.
A line level audio isolator does not have such a capacitor, but the transformers in them will add a bit of distortion.

Another option is to use an amplifier that has a differential input, which will filter out the voltages generated by the current draw from the power supply.
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Old 17th August 2019, 09:17 PM   #8
phofman is offline phofman  Czech Republic
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Exactly, the culprit is the ground loop. Fortunately your TPA3116 amp has balanced inputs, on some boards quite easily usable. It worked in my case very well Adding an Amp into Thin-Client PC
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Old 18th August 2019, 11:17 AM   #9
thyristor44 is offline thyristor44
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Thats interesting, the ground loop theory. I have found if I power the player with separate supplies, 5v and 12v there is virtually no noise.
I will check if my amp has balanced inputs but I don't think the PCM2704 dac has balanced outputs.
If I disconnect the input to the amp and connect the ground output from the dac to ground input to the amp there is no noise.
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Old 18th August 2019, 12:06 PM   #10
phofman is offline phofman  Czech Republic
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Look at the link I posted, the soundcard has no balanced output either. Use a standard single-ended to balanced connection. All you need is disconnect the cold-input capacitors from ground and feed them with ground at the DAC output, using proper balanced cable at best (shielded microphone cable, shield connected at the DAC side only). The common-mode ground noise at the DAC output against the PSU ground will be subtracted by the balanced inputs.
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