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I've built up a little stereo speaker system with USB-powered speakers(just powerd by USB, sound still comes from the 3.5mm jack) and the RPi 3. When I use the speakers on a seperate outlet everything works just fine, but when I connect them to the RPi3's USB, I get a constant low frequency hum from the speakers. I've seen the same problem when using the same external battery pack for the RPi3 and the loud speakers.

What is this noise? From other sound equipment I'm used to the mains hum when something is not built properly, but I'm not sure how the RPi could create this noise when the connected parts are both DC powered..!? Is this an electrical/engineering problem of the RPi? And more importantly, is there a way to get around this?

  • Are your speakers connected to the 3.5mm jack output? – goobering Nov 6 '16 at 21:06
  • Sorry, totally forgot about that. Yes, they are. – Izzy Nov 6 '16 at 21:29
  • Hmmm -- "I use the speakers on a seperate outlet everything works just fine". I think maybe this is a ground loop, which is the same thing that causes "mains hum", but that does not mean it requires AC to happen. Although there would be AC involved somewhere in that loop anyway... – goldilocks Nov 6 '16 at 21:36
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It most likely is a ground loop which is caused by the Pi's PWM line oscillating about a different audio ground to the USB speaker's audio ground. This problem can be caused by switch mode supplies (such as the Pi's 5V to 3.3V step down switching regulator).

There are many solutions which would work in your situation, however you may only find one of them is suitable. Here are some of the solutions :

  • Use a USB isolator. This will restrict the amount of power your speakers can draw and depending on their requirements may not be suitable.
  • Use an audio isolating transformer. This solution will introduce some nonlinearity into the audio signal, but may be acceptable in your use case. This solution is nice and simple and doesn't chew lots of power.
  • Try to connect the two PWM audio grounds together possibly with a simple buffer circuit. This may require you to open the USB speakers and find their audio ground. You can then use a high order low pass filter with a very low cut off frequency to connect the two audio grounds together. This approach forces both audio grounds to match. It has advantages over both other approaches however has a higher degree of difficulty in implementation.

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