HELP: Have amps on another circuit plugged into my Anthem AVM70 via XLR and causes hum on my subs

[xaxxon]

So.. if I get a ground lifter do I lose the noise cancellation properties of the XLR cables? What is the downside to a ground lifter - and if there is none why wouldn't they just all be designed that way?

 

[atmasphere]

I have a 5.1 system and my surrounds/rears are far in the back so I run 50' (canare cable ) XLR to them and then have an AHB2 amp plugged in to the wall in a different circuit back there. However, when the XLR cables from/to my rear amp is plugged in to my AVM70 (all others are fine) I get the 60hz hum from my subs (connected via RCA - no XLR option on the subs). Unplug the two XLR cables for the surrounds, hum is gone.

I thought XLR made hum not be a thing for stuff like this. Is something defective or do I just have to run a really long extension cord, too? (or wire an outlet in the back on the same circuit)

Thanks.

The XLR cables I'm using are https://www.amazon.com/gp/product/B08YR64JHQ I'm happy to open them up and look at them to see if they're built wrong or something but I'm not sure what I'd be looking for.

What is happening here is you've exceeded the Common Mode Rejection Ratio (CMRR) range of your equipment, by plugging in the amps on the opposite side of the AC line, even though everything might be balanced.

XLR cables are supposed to prevent ground loops only because the equipment itself is supposed to ignore ground. With many high end audio and home theatre products, this simply does not happen because ground is referenced by the signals in the cable (IOW the manufacturer may have ignored the balanced line standard; I've seen this a lot in high end audio). Put another way when you want to get the most out of balanced operation, the balanced standard known as AES48 should be supported by the equipment involved.

However! If the CMRR range is exceeded you'll get a buzz anyway. At this point you have 2 options. The first is the best- get an electrician to move the AC power feeding the amps over to the same side of the breaker box as the rest of the system. The second is to start lifting grounds, either the AC ground on the power cords of the amps (which isn't the best idea to say the least since it introduces a shock hazard) or lift the ground of the interconnect cable between the equipment (and leave it grounded only on one end so it still provides shielding). Since in a balanced system ground is supposed to be ignored this should work just fine.

 

[xaxxon]

What is happening here is you've exceeded the Common Mode Rejection Ratio (CMRR) range of your equipment, by plugging in the amps on the opposite side of the AC line, even though everything might be balanced.

XLR cables are supposed to prevent ground loops only because the equipment itself is supposed to ignore ground. With many high end audio and home theatre products, this simply does not happen because ground is referenced by the signals in the cable (IOW the manufacturer may have ignored the balanced line standard; I've seen this a lot in high end audio). Put another way when you want to get the most out of balanced operation, the balanced standard known as AES48 should be supported by the equipment involved.

However! If the CMRR range is exceeded you'll get a buzz anyway. At this point you have 2 options. The first is the best- get an electrician to move the AC power feeding the amps over to the same side of the breaker box as the rest of the system. The second is to start lifting grounds, either the AC ground on the power cords of the amps (which isn't the best idea to say the least since it introduces a shock hazard) or lift the ground of the interconnect cable between the equipment (and leave it grounded only on one end so it still provides shielding). Since in a balanced system ground is supposed to be ignored this should work just fine.

My problem isn't with my rear speakers, though. It's with the subs that are already on the same circuit as everything except the rear amp.

So not sure how common mode rejection is in play.

And yes, I could just run the circuit to the back... but that's not the solution I'm really hoping for. The ground lifting XLR plugs sound good, I just am confused by them.

 

[atmasphere]

My problem isn't with my rear speakers, though. It's with the subs that are already on the same circuit as everything except the rear amp.

So not sure how common mode rejection is in play.

And yes, I could just run the circuit to the back... but that's not the solution I'm really hoping for. The ground lifting XLR plugs sound good, I just am confused by them.

It sounds like I misunderstood.
What you need to sort this problem out is this device, probably a pair of them if you are running a pair of subs; this is what I use in my system which is all balanced except for the subs:
https://www.jensen-transformers.com/product/sub-1rr/
This is a transformer that converts from balanced to single-ended and is optimized for subwoofer operation so it will not rob the bass of any extension or impact.

 

[xaxxon]

It sounds like I misunderstood.
What you need to sort this problem out is this device, probably a pair of them if you are running a pair of subs; this is what I use in my system which is all balanced except for the subs:
https://www.jensen-transformers.com/product/sub-1rr/
This is a transformer that converts from balanced to single-ended and is optimized for subwoofer operation so it will not rob the bass of any extension or impact.

Yeah, that sounds closer to what I was thinking.

 

[antcollinet]

My problem isn't with my rear speakers, though. It's with the subs that are already on the same circuit as everything except the rear amp.

So not sure how common mode rejection is in play.

And yes, I could just run the circuit to the back... but that's not the solution I'm really hoping for. The ground lifting XLR plugs sound good, I just am confused by them.

Here is what is happening:

You have an AVR or AVP connected to mains (presumably at front of room). This is connected to your subs via unbalanced RCA. Your subs are also connected to mains, and grounded there.

You have (long) xlr across the room (through roof space??) to the rear speaker amp at the back of the room. This is also connected to mains at that end of the room.

There is an earth connection between the two mains connections - either around the room (if on the same circuit) or meeting back at the distribution board if on separate circuits.

So you have a ground loop:

Mains to AVR and/Or Sub - Through RCA shield to Sub - to mains (sub mains connection) - through mains earth connection in wall/back to distribution board - back to mains at back of room - to rear speaker amp - through XLR shield and finally back to the AVR.

This is a huge loop. Any stray magnetic field (eg from mains currents) passing through the loop gets picked up (Think of the earth loop as a single turn secondary of a transformer) and generates AC currents around the loop. This AC current passes through the shield of the RCA connection and causes an AC voltage difference across the impdeance of that shield. Since the connection is unbalanced that appears as an AC noise voltage at the input to the sub (added to the signal from the AVR).

Same is also happening on your XLR cable shield - but since that is a balanced connection, the common mode voltage is added to both the hot and cold signals - so it gets cancelled when cold is subtracted from hot at the input to the amp. So the XLR connections are doing what they are supposed to do else you would get noise on the rear speakers as well as the subs. This is the common mode rejection.

 

[xaxxon]

ended up going with ebtech hum-x (now called morley hum exterminator)

https://www.amazon.com/Morley-Hum-Exterminator-Ground-Loop/dp/B08NPY6CBN

Definitely solved the hum. Not sure if I'm over pulling from them (rated for 6A 120v) but I'm guessing I'm not pulling that much from each of my subs. Also not sure what happens if you pull too much through them.