Ground loops

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Happy first day of winter. What better subject to warm the heart on a cold winter's day than ground loops. Keeps us grounded and humming along. Ok, I couldn't resist. I mentioned in yesterday's post if we had hum with only a power amp and speaker connected it could not be a ground loop. How do we know that? We know that because a ground loop needs 'two to tango'. In other words, if there's something funny about the ground it won't matter with only a single piece of electronics connected. The loudspeakers are passive and could care less about ground. I think this bit of knowledge may prove useful to you and here's why. Many times I speak with customers who have 'cheated' the third wire ground prong on their power amplifiers to rid the system of hum. And mostly they are unsuccessful (or they wouldn't be calling me). Cheating, or lifting the third wire ground on a power cord is, many times, the answer to breaking a ground loop but it's important to first understand where it comes from. A ground loop can only exist between two or more pieces of equipment plugged into an AC source. One amplifier cannot suffer from a ground loop. An amplifier and preamplifier, tied together through an interconnect cable can hum from a ground loop. A ground loop happens when the ground of one piece in the chain is at a different potential than another. The difference in potential between the two causes a current to flow and this creates noise. It is because there must be a difference between two pieces of equipment that we cannot have a hum problem caused by a ground loop in one piece of equipment. In a perfect world the ground in our home's AC sockets is nothing more than a wire connecting to earth. Earth as in the ground outside in your garden or next to your house. When you're home is built there is a ground rod, or stake, driven into the earth and a wire attached to it. It might look something like this: rod 3 But there are also circumstances where pipes are used as in this picture: BAD-ground-1 This is NOT recommended but who knows how your home is wired? And perhaps you live in a multi-unit dwelling of some kind. Whatever the situation, the idea behind 'ground' is literal. It means attaching your equipment, through a wire into the ground. We do not live in a perfect world. Well, at least most of us don't. And in our imperfect world much can go wrong. Wiring can be old, instead of copper some homes used aluminum, electricians did not actually connect your ground wire to the third prong on the receptacle. I've even seen the neutral tied to the third prong. Home grounding schemes can be a like a hospital. You don't want to look too closely for fear of what you might find. Perhaps the single biggest cause of ground loops is cable TV. Connect a cable TV anywhere in the system and chances are good you'll have hum. Why? Because the cable company uses a ground different than that of your home. But I digress into too much data. The tendency here is to think that whenever two pieces of kit are interconnected together and cause hum it's a ground loop. That's not a great conclusion and tomorrow we'll discuss why.
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Paul McGowan

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