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I am somewhat new to the world of electronics, but am now trying to design my first more complex PCB. I am trying to make a set of battery powered earphones that take an input from a 3.5mm audio jack and mix that with a signal generated by a microcontroller. Luckily the DAC that I use can take a mono input and do the mixing for me.

However, I am currently trying to figure out how to properly input the signal from the audio jack. For my circuit I use 3 AAA batteries in series and use their negative terminal as ground and use an LDO to get the main 3.3V. So I am thinking, that this ground would be different than that of the device that's plugged into the audio jack (phone or laptop). In my mind this would create a ground loop and introduce noise or even damage my devices. Also, since the audio signal from the audio jack would move around ground of the audio jack, if I just used the signal with my ground reference, wouldn't the signal be shifted by the differences of the grounds?

I now read up about differential amplifiers and using them as "line drivers". But online there is more talk about using them to convert single ended signals to balanced signals. I researched that and it does not seem to be what I want it to do. So I am somewhat lost now and can't find any more info on the internet or in this forum. Surely, I can't be the only person encountering this problem - so am I overthinking it? Maybe you can point me in the right direction here.

Schematic of DAC, audio jack headers and battery input: schematic of dac, audio jack and battery input

AUXS is where the (in my mind) corrected audio signal would enter the the DAC. Disregard the Power Button in the lower left.

Schematic of power supply: schematic of power supply VBUS is an alternative power source, when the PCB is connected to USB. That is just used for programming and debugging so also disregard this for the moment.

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    \$\begingroup\$ Battery powered devices have no external connections unless you make them, so they can't have a ground loop unless you have another ground connection between devices. And how would be the signals offset then? Draw a schematic because you might not even have a problem or there might be some misunderstanding. \$\endgroup\$
    – Justme
    Sep 11, 2022 at 16:07
  • \$\begingroup\$ @Justme I edited my original question to clarify my point. \$\endgroup\$ Sep 11, 2022 at 17:01
  • \$\begingroup\$ A ground loop can be closed on three nodes, not two. Think about it. \$\endgroup\$ Sep 11, 2022 at 23:11
  • \$\begingroup\$ The audio input plug seems to be connected nowhere. \$\endgroup\$
    – Justme
    Sep 12, 2022 at 1:04
  • \$\begingroup\$ The audio input (J1) is just a header that is going to be directly connected to a 3.5mm TRRS port. \$\endgroup\$ Sep 12, 2022 at 9:09

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There is no reason you cannot connect the grounds of two independently powered systems to each other. They are independent, and isolated because they are powered by different batteries and are thus floating relative to one another. When you connect their grounds together they then float together.

Your bigger problem is that your DAC is probably unipolar, not bipolar. This means that when you connect the DAC's ground to your earphone ground, it only produces voltage signals above ground, not above and below ground your ear phones may be expecting.

However, your earphones may include AC-pass/DC-blocking capacitors to remove the DC-offset. If they don't you can include them yourself.

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  • \$\begingroup\$ Okay, I am using the negative terminal of three triple A batteries in series as my "ground". If I understand correctly that would be a negative voltage relative to earth ground and probably a negative voltage to a lipo powered device such as my phone right? That's why I assumed that there would be a harmful discharge if I connect the two together. \$\endgroup\$ Sep 11, 2022 at 16:16
  • \$\begingroup\$ The DAC is made for earphones. It is a PCM1770PW from TI. In the common application schematic there are two caps on either output, so I included them without much knowledge of what they did. But that makes sense - thanks! \$\endgroup\$ Sep 11, 2022 at 16:18
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    \$\begingroup\$ @meisterlampe - Hi, It's not clear why you believe earth ground has anything to do with your system. Please follow the request in the comment on your question and edit your question to add as much as possible of a schematic (at least a block diagram) of your system, showing at least all the interconnections and power sources in detail. (Use Ctrl+G (if you are on a PC) in the edit box, to start the uploading of an image - must be <2MB so not a full res photo.) Otherwise we are having to guess about important details. Thanks. \$\endgroup\$
    – SamGibson
    Sep 11, 2022 at 16:20
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    \$\begingroup\$ @meisterlampe Please edit what your first comment is describing into your question post. It is unclear. A battery has one end labelled + and another end labelled -, but this is relative to each other, not to anything else. It does not become relative to anything else until you connect one of those ends and fix it to the same potential as something else. \$\endgroup\$
    – DKNguyen
    Sep 11, 2022 at 16:20
  • \$\begingroup\$ Okay, I see that that was probably not correct. Of course earth ground has nothing to do with it, I just wanted to make clear why I believe that there would be a difference in grounds. As I said I am somewhat new to that stuff. I added some clarification and schematics. \$\endgroup\$ Sep 11, 2022 at 16:47

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