If transformer isolation is not an option, special cable assemblies are a last resort. It is true that connecting both ends of the shield is theoretically the best way to interconnect equipment though this assumes the interconnected equipment is internally grounded properly. Since most equipment is not internally grounded properly, connecting both ends of the shield is not often practiced, since doing so usually creates noisy interconnections. If one end of the shield is disconnected, the noisy hum current stops flowing and away goes the hum but only at low frequencies. If you ve truly isolated your hum problem to a specific unit, chances are, even though the documentation indicates proper chassis grounded shields, the suspect unit is not internally grounded properly.
Many units come equipped with ground lift switches.
In only a few cases can it be shown that a ground lift switch improves ground related noise. Ground lifts are simply another Band-Aid to try in case of grounding problems. If after hooking up your system it exhibits excessive hum or buzzing, there is an incompatibility somewhere in the system s grounding configuration. If you have an entirely balanced system, verify all chassis are tied to a good earth ground, for safety s sake and hum protection. If you have a mixed balanced and unbalanced system, do yourself a favor and use isolation transformers or, if you re cheap, try the special cable assemblies described here and expect it to take many hours to get things quiet.