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Avoiding Ground Loop Balanced To Unbalanced

excaliburm

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My sub has 60hz hum, which seems to be caused by a ground loop (only manifests when connecting via RCA cable to the receiver). The sub is powered by a different outlet (no way to work around this), so a ground loop would make sense.

There is quite a bit of discussion on the forum to avoid ground loops, as well as balanced cable wiring, but I am in a somewhat unique situation where my source (AVR) has a balanced output for the subwoofer, but the powered subwoofer itself only has an RCA input. Based on my understanding of balanced signals, if we take the difference between the signals carried on XLR, and output unbalanced signal referenced to the subwoofer ground, the hum will be removed. Is this understanding correct, and is there a box/converter cable which essentially does this (and would this approach allow me to get away without a isolation transformer)? Or is there any other way to take advantage of the fact I have a balanced source (but not sink) to avoid the ground loop?
 

Blumlein 88

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You are not supposed to do this. A three pin to two pin electrical plug lets you not connect the 3rd safety ground pin, and often this makes the hum go away. You could certainly connect this way just for a minute to confirm this is the source of the hum.

You aren't supposed to do it because it could allow voltage on some surface of the sub if certain malfunctions occur. You could make such a thing safe by installing a GFIC type plug for the sub. Even with only 2 pins connected those will work if you touched it in a state with stray voltage, and would disconnect power should that happen.
 

solderdude

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You could try a (well shielded) DI box that converts balanced to SE.
When there is a ground loop it will be broken.
 

LTig

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You could try a (well shielded) DI box that converts balanced to SE.
When there is a ground loop it will be broken.
Usually DI boxes convert SE to balanced, but a passive one certainly can be used the other way round as well.
 
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excaliburm

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You are not supposed to do this. A three pin to two pin electrical plug lets you not connect the 3rd safety ground pin, and often this makes the hum go away. You could certainly connect this way just for a minute to confirm this is the source of the hum.

You aren't supposed to do it because it could allow voltage on some surface of the sub if certain malfunctions occur. You could make such a thing safe by installing a GFIC type plug for the sub. Even with only 2 pins connected those will work if you touched it in a state with stray voltage, and would disconnect power should that happen.
What does converting balanced to single-ended have to do with the safety ground? I am not planning on using a cheater plug with the sub, as I am concerned about the safety issues.
You could try a (well shielded) DI box that converts balanced to SE.
When there is a ground loop it will be broken.
Are you familiar with a box which implements this correctly? If I recall correctly some cables/conversion boxes incorrectly short one of the signal pair to ground (which is not what we want in this case). By DI do you mean doubly insulated? Still confused why there would be mains level in such a box.
 

solderdude

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A simple DI box just has a (or 2 for stereo) small line-level transformer(s) in it.
This simply breaks ground loops through audio connection wiring.
usually they are 1 direction only (some of them have a resistor on the load side) but for a sub they will work in both directions.

A ground loop goes from mains source (some leakage to audio ground or a hard connection to safety ground) and travels through the shield (which also conducts the audio in SE) to the sub which also is connected to mains with its own leakage or safety ground connection.

You break the loop by ensuring the leakage/safety ground difference potential can not flow though the audio ground.

As I have never been afflicted (nearly all power outlets are not grounded in my house, and no I never had jolts or dangerous situations) I do not have any practical experience with which DI boxes work well. I own a cheap one (Milenium DATI) which is truly isolated and works fine for test purposes but never needed it.
 
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excaliburm

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So using a line-level transformer is certainly an option ( I don’t need conversion in that case as my AVR also has a single ended output). I was wondering if there is any way to get the ground loop eliminating benefit of balanced with a balanced source (but single ended sink). This would presumably involve a converter where neither pair is shorted to ground (and no transformers are involved) maybe I am misunderstanding the circuitry required, though.
 
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excaliburm

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My understanding is that DI boxes solve the inverse problem (single ended to balanced) and also are intended for impedance changing. I would expect that the conversion is directional, and converting from balanced to single ended is easier.
 

Blumlein 88

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What does converting balanced to single-ended have to do with the safety ground? I am not planning on using a cheater plug with the sub, as I am concerned about the safety issues.

Are you familiar with a box which implements this correctly? If I recall correctly some cables/conversion boxes incorrectly short one of the signal pair to ground (which is not what we want in this case). By DI do you mean doubly insulated? Still confused why there would be mains level in such a box.
I made it very clear that there are safety considerations. Also that replacing your sub outlet on the wall with a GFIC outlet will make it safe again.

The safety ground or 3rd pin ground is usually where you get the ground current causing hum. Which is why using a cheater plug works.

Otherwise some of the hum-buster products here should help. To make certain before ordering one, using a cheater plug just to see if the hum is reduced will tell you if these devices will fix your problem.

 

tonycollinet

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I made it very clear that there are safety considerations. Also that replacing your sub outlet on the wall with a GFIC outlet will make it safe again.

The safety ground or 3rd pin ground is usually where you get the ground current causing hum. Which is why using a cheater plug works.

Otherwise some of the hum-buster products here should help. To make certain before ordering one, using a cheater plug just to see if the hum is reduced will tell you if these devices will fix your problem.

I think he was talking about isolation transformers in the signal line.
 

tonycollinet

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Have a look at something like this if you are able to DIY a little. You would need to provide it with a local power supply.

 

Midnight Audiophile

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As I have never been afflicted (nearly all power outlets are not grounded in my house, and no I never had jolts or dangerous situations)
Same here. Good to know there are others living on the edge : )
If I was in the OP's position I'd give this little box a try. I haven't used one myself but I have three other pieces of Rolls gear that perform well.

71ym8EnBvpL._AC_SL1200_.jpg


From the Rolls website:

"The HE18 is a two channel audio hum and buzz remover. Everyone knows how annoying it is to have 60 cycle hum in your sound. Now you can eliminate virtually all ground hum and buzz by simply inserting the HE18 into the audio path. Plus, the unit will convert an unbalanced signal into a balanced signal, and vice-versa. This greatly improves overall noise performance."


For $49 I'd give it a shot. If it doesn't work send it back.
 

Holmz

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What does converting balanced to single-ended have to do with the safety ground? I am not planning on using a cheater plug with the sub, as I am concerned about the safety issues.

@Blumlein 88 post was clear, and he mentioned temporarily to prove the theory.



Are you familiar with a box which implements this correctly? If I recall correctly some cables/conversion boxes incorrectly short one of the signal pair to ground (which is not what we want in this case). By DI do you mean doubly insulated? Still confused why there would be mains level in such a box.

This stuff: https://www.jensen-transformers.com/home-theater/
I would probably do XLR to RCA at the source, and then
 
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excaliburm

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Have a look at something like this if you are able to DIY a little. You would need to provide it with a local power supply.

I think this is exactly what I was looking for. For reference, this is the circuit diagram (from a similar listing)
1661541718608.png

It looks like the differential signal is correctly subtracted. The only thing I am not sure about is that the loop is not actually broken (i.e. the ground is shared on both ends). However, my understanding is that the reason balanced connections defeat ground loops is that the noise is common mode across both lines. Given that this box will be plugged in the same outlet as the Sub, I am assuming that there will be a negligible ground potential difference on the RCA side of this circuit, and the loop should be defeated. Does this seem correct?
 

solderdude

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ground is still fully connected so when there is a ground loop it won't be broken, only the signal will go from balanced to SE in a 'better' way.
 
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excaliburm

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ground is still fully connected so when there is a ground loop it won't be broken, only the signal will go from balanced to SE in a 'better' way.
How do balanced connections avoid ground loops in the general case then (or why are they recommended as a way to avoid a ground loop)?
 

solderdude

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The ground connection, in fully balanced case, is routed via the cabinets so does not enter the audio path.
Only the balanced signal is used for the signal itself and being balanced is immune because hum will be mostly common mode which cancels.
With this circuit the ground circuit is connected to the ground of the RCA so any ground currents go into the audio path.
 

DonH56

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Have not followed this thread, but here is a short look at ground loops: https://www.audiosciencereview.com/forum/index.php?threads/ground-loops-101.7162/

The key as @solderdude and others have said is to isolate the signal ground from the chassis (safety) ground. The usual ways are using a transformer on the signal line (range from ~$20 to ~2k) or a specialized safety ground "lift" such as a Hum-X (~$80) on the power line. Note the Hum-X is only good for lower-power components; I would use it on the preamp or source component and not a power amp or power sub (same thing). You can also use an isolation transformer on the power line but those are expensive and often violate local electrical codes. An inexpensive RCA isolation transformer good to <20 Hz is pretty cheap and provides adequate isolation. DI (Direct Input) boxes will also do the trick but usually cost more than an inexpensive transformer and you have to look for the ones that claim ground isolation (they often have a ground lift switch on them). They come in passive and active versions but passive is more common for this sort of task. As mentioned above, some active boxes only go one way or have separate paths for each direction (cost more), and not all active DI boxes provide ground isolation. They are found mostly on pro audio sites like Sweetwater Sound (in the USA).
 
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