Fixing ground loops

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We've been discussing how to identify a ground loop. Now it's time to fix what we've identified as a problem. But let me warn you of a couple of things: reread what I have written about identifying them (make sure it actually IS a ground loop), secondly, the cure can be dangerous if not done properly. To review. There cannot be a ground loop with only one piece of equipment. It takes two. The difference in ground potential between the two (ground being at a different level for each) is what causes the hum. Further, you can have one piece of equipment in a many-piece set that is at a different level, while all the others are at a proper level. You will still get hum. To remedy the ground loop we must do one of two things: find the source of the ground difference and correct it (that's the best way), or float the offending piece of gear from the others. Here's a good example. The most common ground loop problem comes from a cable TV. Their grounds are rarely at the same level as our home's grounds (although they are supposed to be). Connect the little CATV connector to, say, a cable TV box that is also connected to your stereo or home theater system and bingo! You'd got a ground loop. Remove the CATV connector and the hum goes away (so too does the picture). So, how do we fix this? One quick example is to use a TOSLINK optical cable between the cable TV box and the stereo system, if your system has a DAC. This works because the optical cable cannot have a ground problem since it is optically isolated from the system. Here's another, less desirable way: use a cable TV isolation transformer between the input CATV connector and the cable box. This works for isolation and hum elimination, but with modern high speed internet and cable streams you might lose some quality. These are just a few examples. But now let's say we have only a pure audio system, yet we still have hum. We suspect a ground loop because of two facts we've discovered: the hum is a higher pitched buzz relative to a low smooth hum (I have placed audio examples here) and the two suspect pieces are plugged into different AC outlets. Our first task is to narrow down the offending piece. This is where we want to use our method of elimination, starting at the loudspeaker again. Remember? Connect the power amp to the loudspeaker making certain you haven't any inputs on the power amp - here I am being very literal. I don't mean no signal present. I mean, NO INTERCONNECTS attached. Even without a signal our grounds would still be attached. Is there hum? If there is, you have a problem specific to your power amp, one requiring you to return it to the manufacturer for repair. No hum? Okay, now connect the preamplifier with the same restrictions. NO INPUTS. Hum? No and you start adding inputs in one at a time till you find the culprit. Hum, yes? Then you must find out if that hum goes up and down with level of the preamp. If it does, it's not a ground loop. If it doesn't, it may likely be. Let's leave of till tomorrow the next steps (I don't like the posts getting too long).
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Paul McGowan

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