Audio decoupling: Decouple the GND also?

[PeteAU]

Hi All,
Decoupling audio with a cap is pretty obvious, you want to block DC. But on some schematics and designs I've seen, people put a cap also on the AGND (the audio reference). What could this be good for?

[rs20]

I've never seen this before. But, to be honest, if I did see non-negligible DC current flowing on an audio ground lead, that'd be a cause for concern, definitely not desirable, and blocking it with a cap would not be an unreasonable solution.

It'd also be mandatory for any circuit where the local ground is expected to be tied to a non mains-earth potential.

[macboy]

Those are coupling capacitors, not decoupling capacitors. Different name, different function.

But to address the actual question, that is generally not a good idea. You need a solid, common ground reference. I'm guessing these are designs by amateurs found on the internet, not professionally designed products.

[linux-works]

can't ever think of a valid reason to ADD caps to a ground return path. LOL

now, a ground BREAKER, maybe.  example is when you want to 'link' earth ground (green wire on ac 3 wire IEC, for example) and electrical ground.  a 10ohm and .01uF cap (or maybe .1, I would have to check) is often used as a ground breaker; so that at dc its not zero, but as you get up in freq, it shunts ac 'junk' to ground via the cap.  there's also variations using back to back diodes so that if the voltage gets high enough, it 'switches' the current to gnd.

none of that applies to this example you listed.

someone must have just copied/pasted that.  it makes no sense to me as-is.

and if you need true ac coupling, you'd use a trafo so that you don't have a dc gnd path.