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I've looked at several questions, such as:

Isolating audio ground between devices

I want to connect an input and output audio line signal of two different devices. The earth potential of their audio connectors is might be floating, connected to mains earth, or offset by some DC or AC amount, with some resistance and other complications - the usual real-life audio stuff.

Is a simple solution to connect them with two capacitors? The function generator is the line out, and speaker the line in:

schematic

simulate this circuit – Schematic created using CircuitLab

I'd rather not use audio transformers if this will do.

Edit - Additional details

  • The above values are a bit of a hack. The circuit too, so I'd expect problems like pops on power up at least
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  • \$\begingroup\$ Are these balanced or unbalanced inputs and outputs, and will 48V phantom power ever poke its head in? \$\endgroup\$ – ThreePhaseEel Mar 21 at 3:04
  • \$\begingroup\$ It's just home audio - unbalanced, and no phantom \$\endgroup\$ – CL22 Mar 21 at 3:12
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AC mains is a audio frequency so capacitors large enought to pass audio will not effectively block AC hum. use an audio isolating transformer instead.

Small transformers are available for line level signals, and larger ones for loudspeaker signals.

isolation is no needed between a floating and a fixed device unless the floating device must stay floating.

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  • \$\begingroup\$ How do hi-fi separates ensure compatibility? \$\endgroup\$ – CL22 Mar 21 at 3:14
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    \$\begingroup\$ @CL22 -- the thing with "hifi" gear is that it's designed so that everything "floats together" if you will, with ungrounded (Class II, in modern gear) power supplies throughout \$\endgroup\$ – ThreePhaseEel Mar 21 at 3:17
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No, your circuit won't work as expected. The capacitor in the audio ground is connected to the two grounds so if they are at different DC potentials it will simply charge up and stay there at that potential difference, with no current flowing.

Which tells us the real answer; if both audio circuits are grounded and the grounds may be offset, just disconnect the audio return wire with a "ground lift" switch. Note, it is not safe to disconnect an actual circuit/safety ground, just the audio ground connection between the two. So then you just have one wire carrying the signal, with a capacitor in it.

In other words, just avoid a ground loop by not connecting the two grounds together.

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  • \$\begingroup\$ Joining the two grounds does charge the capacitor to keep it at that potential, leaving no current flowing for DC offsets. I'm sure that that is not an issue. It means both are effectively ground for DC which is what the circuit neeeds \$\endgroup\$ – CL22 Mar 21 at 18:36

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