skipskip, I am with almarg on this one, I don't see evidence the OP is defeating the safety AC cord ground.
In the US/Canada, unless the latest NEC/CEC has changed, it is acceptable to connect an isolated DC ground to chassis/safety ground, and technically if the voltages are >50V, it is a requirement you do, though I can't comment on the subtleties unless I delve back into that. In Europe, if my memory serves, on the DC isolated ground of an AC connected unit, you cannot connect the DC ground to the Earth/Safety ground for most applications (though for battery systems you do). Everything I do is isolated so I have not needed to delve deeply into this in a while, so if someone is more up on current regulatory requirements, please pipe up :-)
quanghuy147, as some have stated, maybe your solution is better, maybe not, but you believe it is, so lets make that assumption. Within any given piece of equipment, there are parasitic capacitors, potentially many of them, between PCBs, wires, and the chassis. The impact of those capacitors is how close the PCB/wires are to the chassis, and the frequency of both the signal (and noise sources). If you have some equipment that is poorly designed, then a connection from the DC ground to the chassis ground can eliminate some of those capacitors and make your system better. This could be the case with sensitive analog circuits, but also systems with digital noise, switching power supplies, etc.
Shorting the two grounds can also shunt common mode noise into the safety ground, instead of sending it out over your RCA cables.
If you are concerned with safety, but want to accomplish much of the same thing, instead of wiring directly from the one RCA to the AC safety ground, connect them with a capacitor. Ideally that would be what is called a Y-Rated safety capacitor. You can get them up to 1uF. Typically these capacitors are small to prevent leakage current at 50/60Hz AC frequencies. At 120V, 1uF in series can pass 45mA, more than enough to kill. However, we are assuming you are keeping your AC cord safety grounds intact.
In the US/Canada, unless the latest NEC/CEC has changed, it is acceptable to connect an isolated DC ground to chassis/safety ground, and technically if the voltages are >50V, it is a requirement you do, though I can't comment on the subtleties unless I delve back into that. In Europe, if my memory serves, on the DC isolated ground of an AC connected unit, you cannot connect the DC ground to the Earth/Safety ground for most applications (though for battery systems you do). Everything I do is isolated so I have not needed to delve deeply into this in a while, so if someone is more up on current regulatory requirements, please pipe up :-)
quanghuy147, as some have stated, maybe your solution is better, maybe not, but you believe it is, so lets make that assumption. Within any given piece of equipment, there are parasitic capacitors, potentially many of them, between PCBs, wires, and the chassis. The impact of those capacitors is how close the PCB/wires are to the chassis, and the frequency of both the signal (and noise sources). If you have some equipment that is poorly designed, then a connection from the DC ground to the chassis ground can eliminate some of those capacitors and make your system better. This could be the case with sensitive analog circuits, but also systems with digital noise, switching power supplies, etc.
Shorting the two grounds can also shunt common mode noise into the safety ground, instead of sending it out over your RCA cables.
If you are concerned with safety, but want to accomplish much of the same thing, instead of wiring directly from the one RCA to the AC safety ground, connect them with a capacitor. Ideally that would be what is called a Y-Rated safety capacitor. You can get them up to 1uF. Typically these capacitors are small to prevent leakage current at 50/60Hz AC frequencies. At 120V, 1uF in series can pass 45mA, more than enough to kill. However, we are assuming you are keeping your AC cord safety grounds intact.