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I'm designing (probably over-designing, but I want the experience) a four-input, four-output, stereo mixing setup for my PC setup, but I'm concerned that just tying all the ground lines from that many inputs is going to cause ground loops and similar issues.

The inputs are going to come from my PC, an FM radio board, and other headphone-jack style input sources, while the outputs are going to go to my PC speakers, a pair of headphones, the line-in jack on the PC, and a spare auxiliary output.

Would capacitors between the inputs and the amplifiers be enough to decouple things, should I use op-amps in differential mode (like this example shows, though adapted for stereo), or is there another option?

Granted, in a couple of cases, like the PC's input, I'll have to tie the ground to actual ground, as I'm also passing the microphone signal through from the headphones, so I can't cause similar problems the other way around. (I should also be able to get away with using the same ground as the radio module, given that I'm actually providing it with a ground reference, and the fact that it's using the same ground for the headphone jack as the one it's given)

I should note that I'm still in the process of designing the whole project, which means that I don't have a power supply chosen, haven't selected volume controls, and don't know what values to use for any of the resistors yet. (yes, I'm asking this question very early in the design process)

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I should note that I'm still in the process of designing the whole project, which means that I don't have a power supply chosen, haven't selected volume controls, and don't know what values to use for any of the resistors yet. (yes, I'm asking this question very early in the design process)

This doesn't sound too early at all!

If you have a single-ended input, a differential amplifier won't help anything; AC-coupling (i.e. with a sufficiently large capacitor) to a single-ended amplifier is what you want to do. Hint: through AC coupling, you can bias the audio signal to a half-supply-voltage potential, and that eliminates the need for bipolar power supply for the opamp.

All the sources you describe are single-ended.

Notice that even if you have any differential amplifiers in your system, a single opamp's output is usually single-ended. (There's opamps with differential outputs, but let's call them special-purpose.)

outputs are going to go to my PC speakers, a pair of headphones, the line-in jack on the PC, and a spare auxiliary output.

Remember that the headphones jack will probably benefit from getting more power than an opamp output can deliver – luckily there's a lot of headphone amplifiers you can choose from. Just stay away from the "every website has a bad example circuit with it" LM386 (it's a thing from the past, and modern amplifier ICs do way, way better).

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  • \$\begingroup\$ Most of the time, I don't need much power for my preferred volume levels in headphones, so how important is an amplifier of this type in the first place? Is it important enough to have TWO auxiliary outputs, one for other devices and the other for another pair of headphones? \$\endgroup\$ Apr 5, 2020 at 5:05
  • \$\begingroup\$ Re headphones: well depending on your opamp, it will not be driving much current. I presume you'd want to optimize your opamps for noise and distortion rather than for drive; a 32Ω load is too strong for most opamps to drive properly. I mean, if you really don't want to invest into a dedicated headphone driver IC, use your opamp, but buffer its output with a single transistor. Honestly, there's opamps that are strong enough, see for example analog devices' app note, but \$\endgroup\$ Apr 5, 2020 at 11:01
  • \$\begingroup\$ is that really worth the additional effort of having expensive dedicated opamps just for a single output, instead of a dedicated headphone driver IC? \$\endgroup\$ Apr 5, 2020 at 11:02
  • \$\begingroup\$ I probably should have also mentioned that this is a personal project, not a device I'm planning to sell, so price is less a concern at the moment. \$\endgroup\$ Apr 5, 2020 at 21:04
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    \$\begingroup\$ then add that headphone driver! \$\endgroup\$ Apr 5, 2020 at 22:11

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