Perhaps we should change the name from Technical Tip of the Day to Hum of the Day. Here’s the latest:
“Is there an economical way to isolate or shield patch cords in the rear of a rack from a transformer on a processor? This is a four space rack that I use on the road and I use a rack mounted electrical distribution device but if a patch cord comes even close to that transformer it picks up a bad hum from it. I’ve tried grounding different pieces and lifting different pieces in different combinations and switched out numerous cables. It seams to affect one side of the amp channels worse than the other. Could it be a bad transformer in some way?”
Those darned transformers, wall-wart or otherwise, are known for reeking havoc on audio systems. Believe it or not this is actually another function of a ground loop (see yesterday’s TTOTD). The EMI problem in this case comes from the magnetic field being produced by the offending transformer. It is possible that the transformer in the device could be shielded better than it is, and that would help. You can even try to add shielding of your own with grounded aluminum around cables and the cases of the equipment. Sometimes a sheet of aluminum between rack units can help. Also different types of cables can be more or less susceptible to magnetic field induction. In yesterday’s inSync we reported on a way unbalanced cables can be wired that can help. Certainly the way cables are routed in the rack is of paramount importance. Your problem is that you are dealing with a very intense field. One that is way worse than what a typical ground loop picks up. Magnetic fields obey the inverse-square law; as you double the distance from the transformer the field drops to 1/4 strength. In other words, nothing works better than distance. I suggest making patch cords that are just long enough to allow routing them to the side of the rack, away from the offending transformer. You can attach the cables the to the rack with cable clamps (Radio Shack # 64-3028) or screw mount, tie-wrap anchors. Some of these other tricks may help a little, but in this case distance is your best ally.