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I've read some questions and answers here about making 2 separate grounds - one for signal, second one for power. Every question was about the case where the grounds meet each other in one point.

I came across mixer schematic and there are 2 grounds: channel ground (signal ground) and power ground. They aren't joined together, the thing that joins them is a capacitor. First question is: can it work like that? The answer is probably yes, because it's a schematic of a working device that is on the market. Second question is - would it be better to join the grounds by resistor not by a capacitor? I've read and article about HBR (hum breaking resistors) and maybe this would be a proper application of such element?

And final question is - how to run 2 grounds on the PCB? Should power ground or signal ground be distributed as a plane? Or no ground planes?

Extra question is - to which ground should i connect the shield of the shielded cable?

schematic

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    \$\begingroup\$ I bet that page does not show the whole story. Please link to full schematics for us to see. \$\endgroup\$ – Justme May 24 at 18:24
  • \$\begingroup\$ it tells the whole story about how those grounds are linked, signal ground and power ground are joined by a capacitor \$\endgroup\$ – cubix May 24 at 18:29
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    \$\begingroup\$ I'd like to confirm that myself by looking at the whole schematics, thank you. I mean, where would the phantom power supply ground would come from, it certainly would not work if there is only the capacitor connecting the grounds. There must be something else, somewhere. \$\endgroup\$ – Justme May 24 at 18:41
  • \$\begingroup\$ we.tl/t-cBioH1poHT - here it is \$\endgroup\$ – cubix May 25 at 12:13
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    \$\begingroup\$ As I suspected, all the grounds are connected together at the power supply board, so the whole system and all boards have single point star ground. For example, the CHAN-GND only connects to J44-1. The other signal grounds are also at other pins of J44. POWER-GND only connects to J43-6. At the power supply PCB, the ground connector with all the grounds are J1, so the CHAN-GND is J1-1. At the power supply, the POWER-GND is the J107-6. \$\endgroup\$ – Justme May 25 at 18:59
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The schematics you show tells how the grounds are connected at this board only, not how they are connected in the whole system.

So answer to your first question is, no, it can't work like that by itself. In this mixer, the system designer has chosen to use star grounding, where all the different grounds are only connected at one point only, on the power supply board, to avoid ground loops.

The capacitor is there to allow for AC signals (the audio) to have a short return path, so that they are not affected by wire inductance.

So the answer to your second question is, no, it is not a good idea to use a resistor, because current through resistor creates a voltage difference. In this mixer, the system design has chosen a 0 ohm resistor basically, as the wires have extremely small resistance so that small currents do not cause much of a voltage difference between different grounds.

The star grounding is the key to make a quiet system here, so for example the headphones output ground return wire is a separate wire to power supply board, so that for example, sensitive mic input ground wire does not carry the headphone ground return currents.

As to your final question, just run signal ground where you need it, and power ground where you need it. Audio signals are not that prone to couple between tracks, but you can use ground planes if you like. Some systems can work with single ground plane for both signal and power, but it still requires some careful thought how to route them, so that signal grounds do not carry power grounds and cause coupling that way. Basically, that answer would require a book about the subject, and there are books about grounding and audio devices.

Shield of a shielded cable such as RCA input is return path for the signal, so it belongs to the signal ground. The mixer uses signal ground for the XLR input as well.

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The question of separating or joining grounds is not easy to answer, because both methods have important pros and cons.

Usually you want to seperate grounds for reducing interference (=galvanic isolation). But this comes with the cost of building loops which induce currents (=inductive coupling).

So it depends on the circuit and function you need to achieve.

Again: There’s no general answer.

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  • \$\begingroup\$ well, i'm considering making separate grounds to reduce crosstalk and noise pickup \$\endgroup\$ – cubix May 24 at 17:48

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