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Follow up on the need for short balanced cables

For today’s Tech Tip we’re following up on a Tech Tip and related question that was posed back on 4/3/03. The Tech Tip related to the benefits and needs for balanced cables. The question posed at the end was:

“Is there any benefit to using balanced wiring for very short audio cable runs? In other words, is it really true that balanced cables are ONLY needed with long cable runs? The answer will be covered in a future Tech Tip. If anyone would care to venture a guess now feel free to e-mail us. A yet-to-be-determined prize will be awarded for the first exceptionally good answer to this question.”

No one came up with exactly the answer I was looking for. However, Dan Simpson and David Patterson were both awarded a couple of different prizes to choose from for their submissions.

The gist of their responses are posted here in edited form, followed by the not-so-obvious answer we were looking for.

Response 1

“There is an advantage to using balanced cables on short runs – IF both the source unit and the receiving unit have balanced connectors. The advantage is that the dual signal (one reversed) nets to 6 dB of extra gain, so you get a better signal to noise ratio.”

A valid point, but potentially misleading because it isn’t really considered gain – though that distinction is somewhat of a technicality. The balanced input expects the levels it receives. It’s only when you drop one of the conductors do you lose 6 dB of level. Further, this doesn’t necessarily relate to a better S/N ratio unless you are talking about inside the wire itself, where the difference would be negligible and hard to quantify, because….practically speaking the Common Mode Rejection provided by being balanced is where the improved noise performance comes from.

Response 2

“It depends on your environment and your equipment. Are you operating under a radio tower or near a military communications center? Then balance everything. Trust me, I’ve done shows in Red Bank, NJ near the army’s east-coast comm center. The gear involved is another deciding factor. Transformer-balanced equipment must be connected with balanced lines. Other electronic balancing schemes also work better if kept balanced: the CMR can worsen with one signal lead grounded (my Ashly boxes can get noisy unless all leads are properly balanced.) Of course, certain “balancing” schemes can behave just the opposite, being susceptible to ground loops unless connected unbalanced or with telescoped balanced leads. It all depends upon the internal grounding. I carry all manner of different cabling when using an unknown house system.”

All valid points, some of which begin to touch on what I was thinking.

Our Answer

One major difference between balanced and unbalanced cables is how the shield is used. In unbalanced cables the shield is a conductor of the signal itself, and is very important for continuity. Cut the shield and you will (usually) lose so much of your signal the cable will be useless. In a balanced cable, however, there are two conductors for the signal. The shield is not needed for signal continuity. This means that the shield can be cut at one end without disrupting the signal, which provides a potentially good way to get rid of ground loops. The shield can still be pretty effective at doing its job (it is still connected to ground at one end of the cable). This is where the concept of the telescoping shield comes from. Many installations take advantage of this in their wiring schemes. And that would be a really great benefit of having balanced cable runs in circumstances where they might not apparently be needed. Ta-da!

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