Ground Loop Hum from HDMI devices!?

[ss_blake]

I need help fixing a stubborn ground loop hum issue affecting my 5.1 A/V system. The 60hz hum comes from a Sunfire HRS12 subwoofer that is at the back of the theater room, plugged into a different AC outlet than the rest of my gear (but shares the same circuit on the breaker panel). The sub uses a 3-prong AC plug. The ground loop forms when grounded (3 pronged) HDMI devices are connected to receiver via HDMI cable, continuing on via the coaxial RCA cable to the subwoofer.

After a lot of online reading and trial-and-error, I have made the following observations:

1) If I disconnect the RCA coaxial cable (a high quality, shielded, 50' cable) from the subwoofer, hum goes away. So it's not a sub issue.

2) The hum is present if ANY one HDMI device that uses a 3-pronged AC plug is connected to my A/V receiver via its HDMI cable (ex. TV, Oppo Blu Ray, satellite box). If I unplug that device's HDMI cable (or its power cable), the hum goes away.

3) All devices (other than the sub) plug into a Panamax 5400 power conditioner. The AV receiver (Marantz SR7008) uses a 2 pronged AC plug. I tried two other power bars with no difference.

4) The hum goes away if I plug the sub into the same outlet as my other gear OR to another outlet on a different breaker circuit (impractical to run long extension cord).

5) If I lift the ground pin on the subwoofer (using a cheater plug adapter): no hum. I like this solution but I gather it's not safe, as a short within the sub could energize the back metal panel which someone could touch and get electrocuted.

Is there any way to fix the undergoing problem here?

I can't understand why a subwoofer, plugged into an outlet just 30 feet away on the SAME panel circuit, would causes a ground loop issue.
I wish the Marantz receiver used a 3 pronged AC plug: I suspect this would fix the hum as the receiver must be at a different ground potential than my HDMI devices?

P.S. I tried running a wire from the receiver's metal case (also tried the "signal ground" screw) to the ground lug on the Panamax power center, but no difference.

Would appreciate ANY suggestions!

 

[MidnightSensi]

My head is too fried to visualize your setup really..but.. along those lines:

Do you connect anything on analog inputs to the AVR? Maybe a device doesn't get a good ground near an analog signal cable?

I used to have a Sunfire sub a long way back, was so sensitive when it came to noise and grounding issues.

 

[selden]

Since the hum goes away if you plug the sub into the same outlet as the other audio equipment, this indicates an electrical system problem. Some possibilities are
1) there is very high resistance between the ground of the sub's power outlet and the ground of the power outlet used by the audio equipment
2) there is grounding problem in the circuit breaker box
3) they are not in fact connected to the same phase of input power.

In any of these cases, you need to have a certified electrician inspect and fix it.

 

[MidnightSensi]

Since the hum goes away if you plug the sub into the same outlet as the other audio equipment, this indicates an electrical system problem. Some possibilities are
1) there is very high resistance between the ground of the sub's power outlet and the ground of the power outlet used by the audio equipment
2) there is grounding problem in the circuit breaker box
3) they are not in fact connected to the same phase of input power.

In any of these cases, you need to have a certified electrician inspect and fix it.

What about a change in potential from the 30ft distance between the outlets?

 

[TLS Guy]

I need help fixing a stubborn ground loop hum issue affecting my 5.1 A/V system. The 60hz hum comes from a Sunfire HRS12 subwoofer that is at the back of the theater room, plugged into a different AC outlet than the rest of my gear (but shares the same circuit on the breaker panel). The sub uses a 3-prong AC plug. The ground loop forms when grounded (3 pronged) HDMI devices are connected to receiver via HDMI cable, continuing on via the coaxial RCA cable to the subwoofer.

After a lot of online reading and trial-and-error, I have made the following observations:

1) If I disconnect the RCA coaxial cable (a high quality, shielded, 50' cable) from the subwoofer, hum goes away. So it's not a sub issue.

2) The hum is present if ANY one HDMI device that uses a 3-pronged AC plug is connected to my A/V receiver via its HDMI cable (ex. TV, Oppo Blu Ray, satellite box). If I unplug that device's HDMI cable (or its power cable), the hum goes away.

3) All devices (other than the sub) plug into a Panamax 5400 power conditioner. The AV receiver (Marantz SR7008) uses a 2 pronged AC plug. I tried two other power bars with no difference.

4) The hum goes away if I plug the sub into the same outlet as my other gear OR to another outlet on a different breaker circuit (impractical to run long extension cord).

5) If I lift the ground pin on the subwoofer (using a cheater plug adapter): no hum. I like this solution but I gather it's not safe, as a short within the sub could energize the back metal panel which someone could touch and get electrocuted.

Is there any way to fix the undergoing problem here?

I can't understand why a subwoofer, plugged into an outlet just 30 feet away on the SAME panel circuit, would causes a ground loop issue.
I wish the Marantz receiver used a 3 pronged AC plug: I suspect this would fix the hum as the receiver must be at a different ground potential than my HDMI devices?

P.S. I tried running a wire from the receiver's metal case (also tried the "signal ground" screw) to the ground lug on the Panamax power center, but no difference.

Would appreciate ANY suggestions!

Ground loops are due to a potential between grounds. You get rid of ground loops by getting rid of grounds not adding them.

If the Marantz had a ground it would almost certainly make it worse.

There must be increased resistance at the outlet your sub is plugged into or at your Panamax. There is nothing wrong with lifting the sub ground, as long as the sub remains connected to the AVR. It will ground fine through the shield and through the other grounded devices.

 

[selden]

What about a change in potential from the 30ft distance between the outlets?

That's essentially the same as my item #1.

 

[selden]

Ground loops are due to a potential between grounds. You get rid of ground loops by getting rid of grounds not adding them.

If the Marantz had a ground it would almost certainly make it worse.

There must be increased resistance at the outlet your sub is plugged into or at your Panamax. There is nothing wrong with lifting the sub ground, as long as the sub remains connected to the AVR. It will ground fine through the shield and through the other grounded devices.

Until you unplug it. Then it grounds through you. If there's a fault in the device that you just disconnected, that could be fatal.

 

[ss_blake]

Since the hum goes away if you plug the sub into the same outlet as the other audio equipment, this indicates an electrical system problem. Some possibilities are
1) there is very high resistance between the ground of the sub's power outlet and the ground of the power outlet used by the audio equipment
2) there is grounding problem in the circuit breaker box
3) they are not in fact connected to the same phase of input power.

In any of these cases, you need to have a certified electrician inspect and fix it.

Seldin - Is 30 feet of space between (properly wired) outlets enough resistance difference to introduce ground loop hum ? Now I am worried there could be an electrical issue in my house. I purchased an outlet tester for $10 from Home Depot, and plugged it into every receptacle on this circuit. It lit up OK for all of them (no polarity, ground issue faults). I do see a green wire going into my breaker box.. perhaps there is an issue with earth grounding outside? Another thing I noticed - if my central vaccuum motor is running (its on its own dedicated circuit) there is a loud hum on the subwoofer as well! Is this to be expected?

MightnightSensi- I can reproduce the ground loop hum when the ONLY analog connection to the receiver is the subwoofer RCA cable. Plugging in any HDMI device (that uses a 3-prong AC) to the receiver introduces hum. Even if I touch the outside of the HDMI cable to the receiver!

 

[TLS Guy]

30 ft is quite enough distance to increase the resistance to cause hum. House grounds use thin copper and are daisy chained all over the place.

To prevent ground loops you need copper wire like this.

Even then I use a star cluster ground arrangement.

I find in my other systems I have to lift all grounds except one to keep the system quiet. More than one ground in a system frequently causes hum.

My guess is that an electrician will find nothing wrong with your outlets. House wiring is not, does not have or need to be of a standard to prevent this sort of thing.

If you are really worried, the you can install a GIF outlet like you would have in your kitchen or bathroom for your sub and lift the ground. However I have lifted grounds for years and I have never had a shock from it.

The other issue is that your neutrals are bonded to ground at the panel. So if your sub did short to the chassis, it would still blow the line fuse and or the panel breaker.

Ideally there should be just ONE ground in the system.