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Avoiding Ground Loop Balanced To Unbalanced

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excaliburm

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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.
Ah, that makes a lot of sense.

So then, what is the purpose of grounding pin 1/shield of the XLR input? Since the op-amp input is differential, the ground reference voltage doesn't matter for our purposes.

Suppose Pin 1/the shield is not grounded to the ground of this circuit (but remains connect to the source ground)? I would expect this solves all the hum problems, but does it induce a safety issue -- we don't care about grounding on the op-amp circuit side since it only deals with 12V? I don't think any RF issues would be induced because we are grounded on the source side.
 

DonH56

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Grounding the shield provides a path for EMI/RFI independent of the signal path. It is to block interference as well as provide a chassis ground reference for all components. It is not uncommon to lift the shield at one end of the cable/connection to break a ground loop while retaining RF shielding.
 

tonycollinet

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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.
Correct - but with the circuit local to the Sub, and with a very short unbalanced connection it is unlikely that the voltage on the earth between the circuit and the sub will be sufficiently large to be audible. It is why we've been able to work for decades with unbalanced connections between stacked electronics. With everything local the problem is significantly reduced.
 
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excaliburm

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Grounding the shield provides a path for EMI/RFI independent of the signal path. It is to block interference as well as provide a chassis ground reference for all components. It is not uncommon to lift the shield at one end of the cable/connection to break a ground loop while retaining RF shielding.
Does the same hold true for the actual ground pin itself (i.e. Pin 1 in XLR)? I.e. both the shield and Pin 1 are only connected on the source end. If this is the case, why do XLR cables ever bother to connect Pin1/the shield on both ends, as it seems only to cause problems.

Correct - but with the circuit local to the Sub, and with a very short unbalanced connection it is unlikely that the voltage on the earth between the circuit and the sub will be sufficiently large to be audible. It is why we've been able to work for decades with unbalanced connections between stacked electronics. With everything local the problem is significantly reduced.
The confusing part (at least to me) is that in the circuit diagram I shared, we are still sharing a ground. I am not sure (looking at the circuit), if the ground loop noise is removed from the RCA side via the common-mode rejection in the op-amp.
 
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DonH56

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Does the same hold true for the actual ground pin itself (i.e. Pin 1 in XLR)? I.e. both the shield and Pin 1 are only connected on the source end. If this is the case, why do XLR cables ever bother to connect Pin1/the shield on both ends, as it seems only to cause problems.
Not usually but it can be, depends upon the system implementation. Since the signal in a completely balanced connection is independent of the ground connecting at both ends can provide a solid safety ground and potentially better shielding for EM fields such as in a channel where signal cables are run with power and speaker cables. Normal XLR cables have everything tied at both ends and that should not cause issues in a well-designed system. It is generally only in a home environment with mixed single-ended/balanced and varied grounding schemes that you have problems. The XLR connection is mostly for pro installations IME. DI boxes handle various single-ended inputs that arise (typically guitars and their effects boxes, some electronic drums and other instruments). However, I recently swapped subs at home to change from RCA to XLR inputs and even with pin 1 and shield connected at both ends the ground loop was broken by the differential signal path. I had plans for the old subs otherwise would have implemented another (cheaper) solution.
 
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excaliburm

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Not usually but it can be, depends upon the system implementation. Since the signal in a completely balanced connection is independent of the ground connecting at both ends can provide a solid safety ground and potentially better shielding for EM fields such as in a channel where signal cables are run with power and speaker cables. Normal XLR cables have everything tied at both ends and that should not cause issues in a well-designed system. It is generally only in a home environment with mixed single-ended/balanced and varied grounding schemes that you have problems. The XLR connection is mostly for pro installations IME. DI boxes handle various single-ended inputs that arise (typically guitars and their effects boxes, some electronic drums and other instruments). However, I recently swapped subs at home to change from RCA to XLR inputs and even with pin 1 and shield connected at both ends the ground loop was broken by the differential signal path. I had plans for the old subs otherwise would have implemented another (cheaper) solution.
Thanks, I'll try the attached circuit with both ground attached and ground floating to see how it works out.
 
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excaliburm

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Update: the balanced to single-ended converter with common-mode rejection fixed my issues (without a ground lift of any sort).
 
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