In reference to our recent tech tip and WFTD on ground loops and ground lifts (inSync 3/4/99) a reader responds with the following question:
“Regarding your comments about using a ground lift plug to lift the ground, my control room and my studio have different AC circuits (the studio was built long before the control room). As a result, when I use my keyboard in the studio, running the line outs directly to my mixer in the control room, I end up with hum. How else can I get rid of the hum other than using the adapter plug to defeat the ground or running a 50′ AC through sound proof doors?”
That the hum goes away when you lift the AC ground proves it is a ground loop and not some other problem (not all hums are ground loops). To some extent the answer to your question depends upon whether you want to die. I hate to be so melodramatic about it, but if you believed that using your equipment without being properly grounded could kill you (it can, by the way) then you wouldn’t think twice about running a 50 ft. extension cord. Remember, this type of hum results from your equipment having two different paths, and path lengths to ground. The way to solve it is then to provide only one path to ground. Removing the electrical ground connection from one of the items is one way to do it, albeit an unsafe one. Another would be to make sure everything is plugged into one electrical circuit (which should have only one ground). This may require running extension cords or having your electrical service rewired to accommodate your studio needs. Another way is to interrupt the “other” path signals are taking to ground, which is through the audio cables. This basically means breaking the ground, or shield, connection in the audio cables between your keyboard and the equipment in the control room. If you are running balanced cables this can be as simple as cutting the wire connected to pin 1 (shield) on XLR cables, or the sleeve connection on TRS cables. If you are using a direct box, they sometimes have a ground lift switch on them for this purpose. If you are running unbalanced cables, or if for some reason these other solutions aren’t workable for you, you can buy isolation transformers to completely isolate the studio’s audio from the control room. In fact, there are some pretty cost-effective products that do this (for more details contact your Sweetwater Sales Engineer). There are other solutions to these kinds of problems. Electrical studio wiring has been a topic of interest and debate for years. Consequently all manner of wiring and grounding schemes have been deployed over the years to try to reduce the likelihood and severity of ground loops and other problems.