By Ray Rayburn
Ray Rayburn answers this questions. “Why would we ever lift one end of a cable shield?”
Someone recently posted to the SAC list that even on sound systems that were quiet with both ends of the cable shields connected they would go in and lift one end of the shields. That comment prompted this post.
Why would we ever lift one end of a cable shield? As I see things, these are the reasons:
- The equipment at one or both ends of the interconnection has a pin 1 problem. As manufacturers catch up with current knowledge on this issue such as the June 1995 AES Journal or some of the AES Standards that touch on this subject, there is less equipment on the market with this problem. None the less equipment with pin 1 problems continues to be made and sold, and we sometimes find it in our systems.
- The power systems we are forced to use have significant ground potential differences between different portions of our system, and the funds, and/or the willingness to address the power problems does not exist.
- The cabling we are using has both high Shield Current Induced Noise (SCIN), and has significant current flowing in the shield. Most cables with a big SCIN problem are foil shielded with a drain wire. By avoiding the use of such cable where possible we can greatly reduce the chances of facing a SCIN problem.
Since it sometimes helps reduce certain hum and buzz issues, why not always lift one end of every cable shield? Here are the reasons why not from my perspective:
- Lifting one end of a cable shield makes that shield much less effective at RF. This can be mitigated by terminating the lifted cable shield end in a capacitive or resistive/capacitive network, but adding such a network adds considerable cost, and there may be no good place to put the network.
- Lifting one end of the cable shield raises the question of “which end” should be lifted. I believe it has been conclusively shown that if one end must be lifted, it should be the signal destination end of the cable, not the signal source. However, not everyone does it that way, and it is easy to wind up with a cable that has the shield lifted at both ends.
- One end of a cable shield should only be lifted if the equipment at both ends already has a common ground such as via a power system safety ground. Cable shields where at least one end is floating such as a microphone, must never have one end lifted. As a result it is almost always a bad idea to lift the cable shield at one end of portable interconnect cables, since they are likely to get used in places where the lift will cause problems. If a portable cable must have the shield at one end lifted, the cable must be clearly so marked.
Often those who believe in always lifting one end of cable shields will raise the issue of “ground loops”. Many times when people talk about “ground loops” they really mean “something (anything) that causes my system to hum” no matter what the actual mechanism is. Most of the problems blamed on “ground loops” are totally unrelated to actual loops in the ground system. That said, almost all systems contain hundreds if not thousands of real ground loops. Unless the systems also have pin 1 problems, and/or ground potential differences, and/or bad SCIN, those loops can exist without causing any problems.
I used to believe that lifting one end of cable shields was a good practice, but have come to believe it is only a band-aid that tries to mask the actual problems. Sometimes a band-aid is needed, but it should never be used unless actually needed.