Subwoofer hum ground loop issue?

[Speedskater]

But if you use the RCA outer conductor as part of differential input of XLR then that conductor needs shielding, no?

The conductor (coax shield) is also the shield (it's connected to the chassis at the RCA send end).

 

[Lambda]

So what are the chances that the sub has a differential input stage?

Every "normal" Balanced input is a differential input stage.
But some do a better job then others in term of: https://en.wikipedia.org/wiki/Common-mode_rejection_ratio

If you do it "properly" by inverting input to drive the cold side you end up with
Hot=singal+noise
Cold=(-signal)+noise
=Hot-Cold=((singal+noise))-((-signal)+noise)=2*Signal
So not only dose the Noise get canceled out but the signal gets amplified by 6dB.

Personally I would

...use the RCA cable as a cabel pulling cord to pull 2 balanced wires and an extension cord trough the wall
If this is not an option...
this is waht Speedskater wants to do?:

 

[pjug]

Every "normal" Balanced input is a differential input stage.
But some do a better job then others in term of: https://en.wikipedia.org/wiki/Common-mode_rejection_ratio

I don't use balanced connections but my understanding is some equipment just have the xlr connectors on but send it to the same circuit as the RCA.

 

[Lambda]

some equipment just have the xlr connectors on but send it to the same circuit as the RCA.

On the sending side yes. with cheap consumer stuff it is common to use just a resistor on the cold side.
https://www.soundonsound.com/sound-advice/q-how-impedance-balancing-audio-different-normal-balancing

Impedance-balanced Send: Increasingly, manufacturers are using impedance-balanced outputs. The signal is still being transmitted differentially, but only the hot wire carries the (full level) signal. The cold wire is arranged to have the same impedance to ground to ensure proper common-mode rejection. This approach has fewer components and so is cheaper to implement, but it also ensures the correct signal levels are maintained regardless of whether the destination is balanced or unbalanced.

 

[Speedskater]

But if you use the RCA outer conductor as part of differential input of XLR then that conductor needs shielding, no?

Take #2
When the signal travels down the coax it's just an ordinary unbalanced signal and it acts like any other RCA interconnect. Only inside the XLR adapter is it treated as a differential signal.

 

[Speedskater]

On the sending side yes. with cheap consumer stuff it is common to use just a resistor on the cold side.

There is nothing wrong with a impedance/passive balanced output stage. Note that the signal level will probably be lower that with active/powered or some transformer balanced output stages.

 

[raindance]

Jensen stuff is decent.

 

[Lambda]

Jensen stuff is decent.

Sure is. but not cheap.

this would probably also work fine.

 

[RoyB]

"The AVR equipment and subwoofer are on a separate circuit "

Are they on "separate" circuits? Or are both on the same circuit?

If separate, Potential for a ground loop right here.......

 

[thepiecesfit]

"The AVR equipment and subwoofer are on a separate circuit "

Are they on "separate" circuits? Or are both on the same circuit?

If separate, Potential for a ground loop right here.......

Yes separate as in the front of the room where the TV, AVR, amplifier, media playback devices, etc. are on one circuit. Then the back of the room where the subwoofer is by the couch is on another circuit. I understand this creates a potential for ground differential. But due to the room layout being open without redoing the electrical kind of is what it is.

 

[RoyB]

Yes separate as in the front of the room where the TV, AVR, amplifier, media playback devices, etc. are on one circuit. Then the back of the room where the subwoofer is by the couch is on another circuit. I understand this creates a potential for ground differential. But due to the room layout being open without redoing the electrical kind of is what it is.

There's your answer right there......Not sure any of the Hum devices will resolve your issue.

 

[pjug]

On the sending side yes. with cheap consumer stuff it is common to use just a resistor on the cold side.
https://www.soundonsound.com/sound-advice/q-how-impedance-balancing-audio-different-normal-balancing

Maybe this is where I got the idea that just + on XLR is used in some amps. I have no idea if this is common at all, but perhaps some subwoofer amps might do this?

https://benchmarkmedia.com/blogs/application_notes/balanced-vs-unbalanced-analog-interfaces
There are some consumer products that have "balanced" inputs with a CMRR of 0 dB. Let's just call these "fake" balanced inputs. The input is equipped with an XLR connector, but the - pin (pin 3) is ignored. These are just unbalanced inputs wired to an XLR connector.

 

[MrPeabody]

I may have missed it, but I didn't see any mention of whether the AV equipment makes use of the safety ground. Since this wasn't mentioned, most likely it does, however there is lots of AV equipment that does not use the safety ground. I don't know how likely it is that it would help, but if any of the AV gear is not using a safety ground, I would try changing that just to see what will happen. It can't hurt anything, and it will make the equipment safer. You have to replace the two-wire power cable with a three-wire power cable that has the safety prong on the plug. Obviously the safety ground wire (green) needs to be connected to the chassis of the device, but make certain that the point where you are thinking of connecting it is a true chassis ground, by using an Ohmmeter to read the resistance between that point and the outer conductor of RCA outputs. Additionally, not knowing how old your house is, if there is any chance that wall outlets aren't correct with respect to the safety ground, this should be checked and corrected if need be. I am not especially knowledgeable of house wiring, but my understanding is that the safety ground for each receptacle should have a good conductive path to the breaker box, using either a dedicated wire or else conduit if there is conduit, and at the breaker box, these should all be joined to the "neutral" conductor and grounded to earth. If this does not correct the problem, it still is worth doing, if any of the AV components is not already using the safety ground, for multiple reasons to include the fact that it will allow the isolation transformer to work better if this doesn't work and you need to add an isolation transformer. The isolation transformer should be the type that uses a conductive (faraday) shield to isolate the primary and secondary windings and it should be placed as close as possible to the subwoofer so that the cable length between it and the subwoofer will be as short as possible.

Honestly I do not have a lot of knowledge in this area, and one thing that I don't really understand in your description of the problem is why disconnecting the safety ground at the subwoofer eliminated the problem.

 

[raindance]

I may have missed it, but I didn't see any mention of whether the AV equipment makes use of the safety ground. Since this wasn't mentioned, most likely it does, however there is lots of AV equipment that does not use the safety ground. I don't know how likely it is that it would help, but if any of the AV gear is not using a safety ground, I would try changing that just to see what will happen. It can't hurt anything, and it will make the equipment safer. You have to replace the two-wire power cable with a three-wire power cable that has the safety prong on the plug. Obviously the safety ground wire (green) needs to be connected to the chassis of the device, but make certain that the point where you are thinking of connecting it is a true chassis ground, by using an Ohmmeter to read the resistance between that point and the outer conductor of RCA outputs. Additionally, not knowing how old your house is, if there is any chance that wall outlets aren't correct with respect to the safety ground, this should be checked and corrected if need be. I am not especially knowledgeable of house wiring, but my understanding is that the safety ground for each receptacle should have a good conductive path to the breaker box, using either a dedicated wire or else conduit if there is conduit, and at the breaker box, these should all be joined to the "neutral" conductor and grounded to earth. If this does not correct the problem, it still is worth doing, if any of the AV components is not already using the safety ground, for multiple reasons to include the fact that it will allow the isolation transformer to work better if this doesn't work and you need to add an isolation transformer. The isolation transformer should be the type that uses a conductive (faraday) shield to isolate the primary and secondary windings and it should be placed as close as possible to the subwoofer so that the cable length between it and the subwoofer will be as short as possible.

Honestly I do not have a lot of knowledge in this area, and one thing that I don't really understand in your description of the problem is why disconnecting the safety ground at the subwoofer eliminated the problem.

Disconnecting the ground on one item to remove hum is the classic definition of a ground loop. You can visualize the shield on the cable as being an extension of the chassis of the equipment. Because it is also tha signal return in an unbalanced connection, you are adding the voltage between the safety grounds to the signal, which in an unbalanced situation, has no ability to reject this voltage, and amplifies it instead. It's a hum because it's either 50 or 60Hz, depending where you live.

Disconnection of a safety ground is a safety issue, so most people choose not to leave it that way.

In many cases, too numerous to count, in fact, a ground loop is caused by cable TV, because the coax, also single ended or "unbalanced", brings another ground voltage into the mix.

Helpful?

A balanced signal does not use the shield for signal at all, it splits it across two conductors that carry mirror image signals and the input side reconstructs the signal by effectively inverting one of the signals and adding them. The cool side effect being that any noise picked up by both conductors sums to zero effectively. In balanced, the shield is optional at one end.

 

[MrPeabody]

Disconnecting the ground on one item to remove hum is the classic definition of a ground loop. You can visualize the shield on the cable as being an extension of the chassis of the equipment. Because it is also tha signal return in an unbalanced connection, you are adding the voltage between the safety grounds to the signal, which in an unbalanced situation, has no ability to reject this voltage, and amplifies it instead. It's a hum because it's either 50 or 60Hz, depending where you live.

Disconnection of a safety ground is a safety issue, so most people choose not to leave it that way.

In many cases, too numerous to count, in fact, a ground loop is caused by cable TV, because the coax, also single ended or "unbalanced", brings another ground voltage into the mix.

Helpful?

A balanced signal does not use the shield for signal at all, it splits it across two conductors that carry mirror image signals and the input side reconstructs the signal by effectively inverting one of the signals and adding them. The cool side effect being that any noise picked up by both conductors sums to zero effectively. In balanced, the shield is optional at one end.

I thought I was being helpful, and was trying to be anyway, but apparently not. Sorry.

 

[raindance]

I thought I was being helpful, and was trying to be anyway, but apparently not. Sorry.

Huh? I'm trying to be helpful and help with your understanding of the issue at hand. Not sure what I said wrong.

 

[Speedskater]

Sure is. but not cheap.

this would probably also work fine.
View attachment 117482

Dangerous! Not permitted by NEC/UL

 

[Lambda]

Dangerous

why?

Not permitted by NEC/UL

there is a commercial overpriced snake oil product doing this.

 

[thepiecesfit]

Anyone know what the Emotiva CMX-2 product accomplishes based on the design? Does it lift the ground?

 

[Speedskater]

why?.

Because it defeats the basic purpose of the Safety Ground system! In a Ground Fault (short circuit) failure it may be required to carry be required to carry 1000 Amps for a fraction of a second unit the circuit breaker trips. The Safety Ground must be all copper and as large as the Hot & Neutral.

there is a commercial overpriced snake oil product doing this.

and does it have the required 'UL' or other safety agency listing?