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Grounding chassis through RCA

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Beranga

Beranga

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Joined 2008
#1
Thanks you for bearing with me and my newbee question:

I have a DAC-Pre-Amp setup with ground loop issues that go away when using cheater plugs on the DAC and Pre, and leaving only the Amp safety grounded. I understand this setup is dangerous, my question is:

As I understand it, the path to ground of the cheater plugged equipments is though the shield of the RCA cables connecting them to the grounded Amp, and that shield will probably not hold in case one of the ungrounded chassis goes live, but what if I make an RCA interconnect with no signal wire, only the shield consisting of a 10AWG wire? would it make my setup safe?

I have checked conductivity with a DVM and it seems that the RCA grounds, the chassis and the safety grounds, are connected inside on all three equipments with a thick bare copper wire.

I guess connecting all chassis directly by screwing the 10AWG wire is another alternative, but using the RCA plugs would be more practical. Would this be a much better way to do it?
 
#2
Instead of doing the above, why don't you lift the grounds from the chassis? Add a 33-50 ohm high wattage (20W-100W) resistor from the power earth/ground wire to the chassis for all three of your equipment. This will still maintain safety and prevent/reduce ground loops.
 
#8
tomchr said:
A ground separator would do the job better than the resistor. See D1, D2, R1, C1 of the first figure on this page.

~Tom
Click to expand...

Why would this work better than a simple resistor and capacitor? I understand the role of the capacitor, which I would also include.
But the diodes? What does he mean by a "major problem"?

I think the resistor value shown is actually a little too low. I would still stick to something in the region of 30-100 ohms.
 
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#9
quikie22 said:
...why don't you lift the grounds from the chassis? Add a 33-50 ohm high wattage (20W-100W) resistor from the power earth/ground wire to the chassis for all three of your equipment.
Click to expand...

tomchr said:
A ground separator would do the job better than the resistor. See D1, D2, R1, C1 of the first figure on this page
Click to expand...

Thanks for the quick responses, I've another question:

Could the resistor/network mentioned be connected between the earth wire and the chassis? or would the chassis need to be directly connected to earth and the resistor/network before the rest of the circuit?

I´m confused because in the ground separator schematic, it looks like there is a connection to the chassis before the R-C-Diode network and I don´t understand why.
 
#12
M Gregg said:
Just for interest,

You must never put anything in a protective Earth. The chassis should always be at Gnd potential.


Regards
M. Gregg
Click to expand...

Gotta agree there. I don't think any electrical code allows for a resistor between the chassis and ground. Now I've heard of putting a small resistor of maybe 10 ohms between the RCA signal grounds and the chassis.

Are both your pieces of equipment plugged into the same receptacle?
 
#14
Thank you

I have the chassis grounded (as well as the r-core tx screen) and experimented with some resistors between the circuit 0V and chassis GND.

The best solution (lower noise) is using a cheater plug in the mains socket... But that is not practical due to safety reasons.

Following this idea : http://sound.westhost.com/project04.htm

Now I am trying to find the best resistor value to put between the circuit -V and chassis GND.

10r is not very effective... 100r is better but not perfect.... can I try 220r without expecting any issues ?

Some suggest that we could only use the two diodes (or a rectifier bridge) and avoid the resistor....

I am a bit confused and it seems there is no consensus.... Is this a try and error thing ?
 
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#15
quikie22 said:
if you study the ground separator circuit, you will notice that it is grounded twice.. one is to the chassis, the other one after the ground separator is to the circuit ground.
Click to expand...
I agree with what you are saying that the separator is after chassis Gnd.

Just in answer to Earthing. I agree some countries my vary.

The protective earth must go to the chassis first. Then it is possible to "lift" circuit ground after this.

It is not allowed to connect a chassis requiring an earth via a signal cable to a Grounded chassis, the supply must disconnect first when the earth is disconnected via a plug and socket. That is why the earth pin is longer.

Each Chassis should have a dedicated earth.
The circuit would be protected by the fault clearance of the circuit fuse back to the Tx. Even if it is "Lifted" from Gnd.

There must never be a potential difference between each chassis under fault conditions.

Don't want to get boring here. The reason for this is that the clearance times of fusing is effected by any resistance in the earth cable and anything in circuit with it. Also semi conductors should not be used as a path for fault current.

This is not about applying engineering ideas! It is normally a legal requirement in most countries. You should not fit fuses, circuit breakers, semi conductors etc in a protective earth.

The engineering side is making the circuit function with the correct earthing. Not altering the earthing to make the circuit work.

Regards
M. Gregg

 
#16
RCruz said:
Some suggest that we could only use the two diodes (or a rectifier bridge) and avoid the resistor....
Click to expand...


You can omit the resistor. Keep the cap. Use diodes rated so that your component mains fuse will go first in the event of a circuit fault. You don't want your RCA common to carry high voltage.

That said, this is DIY. The best thing is to rework your internal grounding schemes to minimize loops.

Sheldon
 
#17
Sheldon said:
You can omit the resistor. Keep the cap. Use diodes rated so that your component mains fuse will go first in the event of a circuit fault. You don't want your RCA common to carry high voltage.

That said, this is DIY. The best thing is to rework your internal grounding schemes to minimize loops.

Sheldon
Click to expand...

Much better than disconnecting chassis Earth.

Or you could just connect Gnd at one end of the RCA.


Regards
M. Gregg
 
#18
Sheldon said:
You can omit the resistor. Keep the cap. Use diodes rated so that your component mains fuse will go first in the event of a circuit fault. You don't want your RCA common to carry high voltage.

That said, this is DIY. The best thing is to rework your internal grounding schemes to minimize loops.
Click to expand...

Hi Sheldon

I worked a lot in the internal GND layout and it works very well... The issue comes from the loop created because all system components are grounded to the same mains earth point and also connected by the rca cables.

As I am working in a double mono preamp, I can not pass without all RCA returns connected otherwise I can increase cross talk.

I will try the diodes and the cap alone 🙂
 
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