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I'm a total electronics noob- I've done a project where I put a Raspberry Pi into an old 80's boombox to offer streaming content. I used a cheap LM2596 buck step down converter to deliver the 5v needed for the Pi. I used this model because I plan to run the unit off batteries part of the time and don't want the inefficiency of a linear converter like a LM7805.

I have my wiring set up just like this (borrowed from another thread)

I'm having a big problem with noise in this system. There is a loud constant buzz (measured 240hz) and I'm also hearing a lot of noise from the computer activity (screechings, etc that time with wi-fi & disk read activity). This all goes away as soon as I remove the ground connection from the LM2596. I tested by using separate ac-dc power supplies for the boom box and the computer and all problems are eliminated. Another point of interest is that bypassing the built in linear PS, and using a 12v wall-wart, greatly reduces the volume of the loud buzz, however the computer noise is still present.

I've come across a couple of suggested remedies-

1) In the thread linked to above it is suggested to add a differential amplifier between the unbalanced audio connection from the Pi to the power amp. This seems to have worked for the OP. With my lack of knowledge in EE, I don't really understand what is going on here or what parts I'll need to build this.

2) Someone recommended I adjust my LM2596 to put out 7.5v and then run that to a LM7805 to minimize efficiency loses while reducing switching noise. I'm just not sure I'm experiencing switching noise so I don't know that this will remedy my problem

3) Use separate power supplies for the Pi and the amp. This is easy enough when connected to AC but much more complicated when running from batteries...I'm open to suggestions.

4) Use audio transformers between Pi and amp. Apparently I will be sacrificing a bit of fidelity and it's rather an expensive option so it would be a last resort.

Any and all ideas welcomed. I'd like to get this device up and running! Thanks

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  • \$\begingroup\$ I'm fairly confident that the noise is entering via input wiring to the amplifier and that it's likely power supply currents to the Pi are somehow sharing the input wiring ground feeds. You have to try and isolate these currents from any input signal wiring to the amplifier. \$\endgroup\$ – Andy aka Aug 6 '13 at 7:11
  • \$\begingroup\$ When you say "remove the ground connection", do you mean the ground side of the audio link or the ground to the power supply? \$\endgroup\$ – pjc50 Aug 6 '13 at 10:02
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This is a common problem in audio amplifier connections especially where there is digital noise and switching regulators. There is no ideal way around this problem without using differential inputs.

Why do differential inputs work - the signal return back to the box on the left won't carry any power supply currents because it feeds a high input impedance differential amplifier (or a transformer). Obviously you have to wire it correctly.

Without a differential input (silicon or transformer) it's difficult to find a signal return path that won't pass digital currents.

This problem has nothing to do with dodgy earths in your home or needing to put a decoupling capacitor somewhere.

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A transformer between Pi and your amp will not fix things. The above solution seems interesting but I'm still not sold it will fix all your problems. I have seen issues like this in low speed communication systems between robotics components, causing terrible unpredictable malfunctions. I would suggest a fully isolated DC to DC converter you can build one but its fairly involved. When it comes time to connect the grounds for the audio signal do so with a ferrite bead to eliminate high frequency noise and use ample capacitors. Also you may consider a strategically placed diode to eliminate reverse currents from scattering around on your ground, strange potential differences can build up between connected isolated systems. The best solution would be to isolate and never reconnect the grounds at all and use an audio isolation amplifier IC with high fidelity. Like this http://www.digikey.com/us/en/ph/ROHM/Groundisolationamp.html

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  • \$\begingroup\$ You can readily buy fully isolated DC-DC converters. \$\endgroup\$ – nekomatic Oct 3 '14 at 12:49
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Guy I bet you have problem with your Ground.

Are you sure your home is well grounded ? (some times in the cable are there in the house but not connected to the ground) Are your computer on batteries or plug into the sector ? Is the power source you'r using well connected to the ground?

Any way to know if it is the problem : You can try to connect the ground of your computer with the ground of your boombox with the ground of your electronic. To do that you just need a metallic part of the body of your computer and your boombox to touch (for the safety the ground is connected to the body of stuff) and to make a wire touch your computer and be connected to the ground of your Pi.

If you still have issue you can use a decoupling capacitor ("capacité de découplage" in frensh) but then you will have to show how a schematic if you want help.

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    \$\begingroup\$ These are mainly questions and guesses. Answers are reserved for answers. Put your questions in the comments area under the original question. Also, having a "problem with your ground" is usually solved with re-routing ground and not using a decoupling capacitor in an unspecified place. \$\endgroup\$ – Andy aka Aug 6 '13 at 11:38
  • \$\begingroup\$ I propose a solution to test if there is a ground problem and if it is he will get how to re-routing his ground. And yes a decoupling capacitor can be quite effective if he doesn't find the origin of his problem. But feel free to remove edit or delete the post to make it look like as much as possible as it should be. I see I've made a lot of miss spelling you can check on it too if you want. \$\endgroup\$ – adrienF Aug 7 '13 at 1:40

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