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Author Topic: Noisy output from Raspberry Pi  (Read 305 times)

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thomashenry

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Noisy output from Raspberry Pi
« on: November 19, 2020, 05:13:11 am »
Not sure the best place to post this, but I guess it's here. I've got an arcade build in progress, the electronics of which can be seen below. It's a dual switchable system, combining a JROK jamma board and a Raspberry Pi. The problem I've got is that the audio from the Pi via the headphones is extremely noisy, and contains all sort of interference. I'm not expecting high fidelity sound of course, but the noise is way, way in excess of what I'd expect.

myarcadeboard by Tom D, on Flickr

The parts, as numbered are:
1. 6PDT switch with 3 connections: it switches power to the pi/jamma, and audio L R from the pi/jamma to the amp. Green wire goes off to the 5V switching PSU.
2. Amp, taken from some Logitech PC speakers. Powered by its own 9V supply (blue/white wires). Red/Yellow/Black are speaker cables.
3. iPac2
4. Raspberry Pi with VGA hat
5. Input from the control panel, inputs go through diodes to both the jamma board and iPac
6. JROK
7. Scanline generator

As you can see, ground is common and daisy chained around. When I'm using the Jamma board, the sound is perfectly acceptable. when using the RPi, there's way too much interference. Any ideas what I can do to sort this out?  There's no way the headphone out should be this noisy, so I must have done something bad.

Reading other threads elsewhere indicated to me that the problem could be the ground wire to the Rpi is thin and long, and therefore has too much resistance?  Though I don't know why this would impact the audio. Or is the Pi simply too close to the amp?
« Last Edit: November 19, 2020, 05:40:31 am by thomashenry »

lilshawn

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Re: Noisy output from Raspberry Pi
« Reply #1 on: November 19, 2020, 11:18:59 am »
typically noise is either the result of dirty power or bad grounding (or ground loop.)

try tying all the grounds together between the pi and amp and don't rely on just the shield of the headphone for grounding.

and either tie the power supplies together so they share a common ground (might not be possible given the amp you have going on there.) or completely isolate the amp with a "ground loop isolator"

something like so:

https://www.amazon.ca/AUKEY-Ground-Isolator-System-Stereo/dp/B01M1671I0

thomashenry

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Re: Noisy output from Raspberry Pi
« Reply #2 on: November 19, 2020, 12:35:44 pm »
typically noise is either the result of dirty power or bad grounding (or ground loop.)

try tying all the grounds together between the pi and amp and don't rely on just the shield of the headphone for grounding.

and either tie the power supplies together so they share a common ground (might not be possible given the amp you have going on there.) or completely isolate the amp with a "ground loop isolator"

something like so:

https://www.amazon.ca/AUKEY-Ground-Isolator-System-Stereo/dp/B01M1671I0

Thanks for the reply!!

"try tying all the grounds together between the pi and amp and don't rely on just the shield of the headphone for grounding."

What does 'tying all the grounds togther mean'. Sorry, I'm clueless when it comes to this stuff. The headphone sheilding actually goes nowhere, it's not connected to anything.

As it came from PC speakers, which had a stereo jack input, it has 3 inputs, LRG. The G here is coming via the iPac, via the JROK board.

chopperthedog

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Re: Noisy output from Raspberry Pi
« Reply #3 on: November 20, 2020, 06:00:54 am »
Has nothing to do with grounds. Pi  3.5mm audio signal is awful. Get a small USB audio dongle.


good day.

thomashenry

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Re: Noisy output from Raspberry Pi
« Reply #4 on: November 20, 2020, 08:18:05 am »
Has nothing to do with grounds. Pi  3.5mm audio signal is awful. Get a small USB audio dongle.


good day.

No, the sound from the jack is fine when using headphones.

thomashenry

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Re: Noisy output from Raspberry Pi
« Reply #5 on: November 20, 2020, 08:22:28 am »
typically noise is either the result of dirty power or bad grounding (or ground loop.)

try tying all the grounds together between the pi and amp and don't rely on just the shield of the headphone for grounding.

and either tie the power supplies together so they share a common ground (might not be possible given the amp you have going on there.) or completely isolate the amp with a "ground loop isolator"

something like so:

https://www.amazon.ca/AUKEY-Ground-Isolator-System-Stereo/dp/B01M1671I0

Well, with your help I have largely fixed the issue. I had been proceeding with the matra that ground is ground is ground....

I change things so that the ground/shielding on the jack cable from the pi connects to the ground/sheilding on the input to the amp. That random black wire coming from the amp is connected there too. Now the noise is 98% gone. The only thing that remains is interference when the wifi/sd card is being worked. This modem noise type interference is independent of the amp volume, and quite low level.

What I don't get it how the audio from the Jamma board still works,but it does!!! haha