What type of shield works best against 60-cycle hum from power transformers and AC cables?
The sad truth is that the most offensive “hum-producing” frequencies (60 and 120 Hz) generally emitted by transformers and heavy power cables are too low in frequency to be stopped by anything but a solid tube of ferrous (magnetic) metal—iron, steel, nickel, etc.—none of which contribute to the flexibility of a cable! For magnetically-coupled interference, the only solution is to present as small a loop area as possible. This is one of the reasons that the twisted-pair configuration generally used in balanced-line applications became popular. Fortunately the high input impedances generally found in unbalanced circuits minimize the effects of such interference. Don’t run instrument cables parallel to extension cords. Don’t coil up the excess length of a “too-long” cable and stuff it through the carrying handle of a amp—this makes a great inductive pickup loop for 60Hz hum!
