Hum, Ground Loops and Safety

Electricity is a little bit like water, enough so that we can use water as an analogy. If you have two reservoirs of water, one higher than the other, and the physical block between them is removed, then the water in the higher reservoir will flow to the lower reservoir, until they are at equilibrium. If you have two electrical conductors that are charged with electricity, and one has more charge than the other, then if you remove the blockage between those two conductors, the electricity will flow from the conductor with the higher charge into the one with the lower charge, until they are at equilibrium. The difference between the charges in the conductors is what we call "voltage." It's also called "potential," because there's potentially a current flow there, it's just waiting for a conductor path to a place where there's a lower charge.

Imagine that you have a very, very sharp butcher's cleaver, and several shelves that are uneven, slanted, and covered with slippery grease. If you set the cleaver on one of the shelves, it has the potential of sliding off, and if it hits someone on the way to the next shelf down, it could hurt them very badly. The highest shelf has the greatest potential to hurt someone, because if the cleaver slides off that shelf, it could fall all the way to the floor, and by the time it hits the floor, it's going to be moving very fast -- it could chop through a lot of things, including a finger. You don't want that. Assuming we have to put the cleaver on one of the shelves, what's the best way to ensure that the cleaver will not fall and hurt someone? Put all the shelves on the floor. In that way, there's absolutely no possibility that the cleaver will fall if it does slide off a shelf, it just slides harmlessly right onto the floor. There's no potential energy in the cleaver if the shelf it's on is already down on the lowest level in the room.

Follow the analogy, now: With electricity, the lowest level in the room -- the floor below which nothing can fall any farther -- is "ground." If everything is at ground level, there's no potential, there's no voltage, and no current will flow. No cleaver will fall. If any conductor is "raised off the floor" at all, then current will flow to the floor -- to ground -- if it's given a path to do so.

With electrical gear, the way to put all the shelves on the floor is to connect them all with a conductor, so all the barriers to current flow are removed, and the electricity seeks its own level. There will be no voltage among the pieces of gear. Then, to make sure that the shelves really are "on the floor," and there's no lower level to which the current could go any farther, we connect the conductor that connects all the gear to the earth. The earth is absolutely the lowest level, the basement floor, when it comes to electrical potential. No electrical cleaver can fall any farther than that.

A bass amp, like a lot of music equipment, has a three-wire power cord: One conductor is "hot," meaning it is charged with electricity which is looking for a path to a place with lower charge. It's black. Another conductor is grounded, which is commonly referred to as the "neutral" -- it isn't really neutral, it's grounded. It's white. What it does is give a place for the electrical charge in the "hot" wire to go, and in between the hot and neutral, we put the electrical circuits that do work for us -- amplify our bass signal, in this case. The path between hot and neutral is intentional, it's where the work gets done. The last conductor is the grounding wire. It's green. This wire connects to the chassis and cover of the amp. It's connected to the same place that the neutral wire is, back at the building's electrical service panel, but the difference between it and the neutral is that there's no intentional connection between it and the hot wire, there's no work done between it and the hot wire, it is purely a safety conductor to take current to ground in case there's an accident of some kind -- for instance, if there's an unintentional connection between the hot wire and the chassis of the amp. If that were to happen, then the chassis of the amp is hot, and the electrical charge in it is looking for a path to a place with a lower charge. If, for instance, you became that path to ground, then the current would flow from the hot chassis through you to ground, and if that current flowed though your heart or brain, you'd be killed. But, since the grounding wire is there, connected to the chassis, the current flows directly from the hot chassis to the ground, and you don't become part of that circuit -- or at least, you're not the path of least resistance, and the bulk of the current will go elsewhere. Now, if the building is properly wired, there will be a fuse or circuit breaker wired into the hot wire, and it will blow when there's a dead short between hot and ground, and the current will stop flowing. If the building's not properly wired, and say for instance the grounding wire is not connected to actual earth, then everything stays hot and the circuit breaker won't pop. That's why you should always take a circuit tester with you and check your AC supply.

Now, going back to the shelf-on-the-floor analogy, the way to get all the electrical shelves harmlessly on the floor is to connect them with a conductor that eliminates the chance for voltage potential to develop. That conductor is the grounding wire in the power cords of your various pieces of gear. Assuming all your gear is plugged into the same AC circuit, and the AC circuit is itself properly grounded, then all the gear's grounds are connected with one another through the AC receptacles. Theoretically, this puts them all "on the floor," there's no potential among them, and no current will flow among them. They're all "bonded."

But the theory doesn't translate perfectly to reality. Conductors are not perfect, they're not at zero Ohms. Connectors, like the three-prong plug and the AC receptacle, can have some resistance. Therefore, there can be some blockage in the flow from higher-charge areas to lower-charge areas. Some potential can exist. The shelves are not all on the floor, some may be raised a little.

The circuitry inside our gear relies on the flow of electricity from the hot wire to the neutral to do the work we want our gear to do. That means we want a controlled amount of electricity to flow from hot to ground, without that, the gear wouldn't do anything. So, anything that does electrical work has to have some flow from hot to ground, and that means that every working circuit is going to have a path from hot to ground in it. That path should be from hot to neutral, and neutral's connection to ground is back at the building's service panel. So, take your bass for example, the cable from your amp to your bass creates a link to that circuit from hot to neutral, running through your pickups. The hot wire is the center conductor, the neutral is the shielding in the cable. There's a lot of wire and circuitry components between the hot and neutral, and in turn between the neutral and the grounding wire. So, even though there may be a direct connection between the grounded chassis and the neutral back at the service panel, there is some slight blockage in the flow, and therefore, there is a potential there for current to flow. If that potential exists between the grounding wires and the neutral wires in our various pieces of gear and we put a conductor across it, then we've created a "ground loop." If the conductor that creates the ground loop is in a cable that's carrying our desired musical signal, then it can cause noise, usually in the form of 60-cycle hum.

The part of the circuit in which the work gets done, where the desired flow from hot to neutral takes place, we call the "line-level" signal. That's where the music is, it's separate from the power supply. The ground in the line-level signal is separate from the grounding wire that's attached to the third prong in the power cord, it's not a life-saving safety feature in case of accident, it's an actual part of the music-carrying circuit in unbalanced lines (such as a bass cord) or it's there for interference shielding in balanced lines (such as an XLR cable).

If you have a ground loop and are getting hum, then you need to eliminate the differences in potential that cause that current to flow, and/or you need to break the circuit that makes up the loop. To eliminate potential, you need as-close-to-perfect, zero-Ohm connection between the various equipment grounds and the neutral at the service panel. First, obviously, there must be actual connections. There are a lot of circuits that are not grounded, and there are a lot of building AC service panels that are not grounded. Lots of buildings are grounded through their water-supply plumbing, and these days, a lot of PVC pipe gets inserted into those supply lines, breaking the ground connection. A lot of outdoor grounding rods are corroded or are not in conductive earth. So, the first thing to check is that a ground is actually there.

Second, the grounding wires in the AC supply and power cords should be as short as possible, be of a heavy gauge, and be connected without corrosion. Third, all equipment in the signal chain should be plugged in to the same circuit. Power cords should be in good condition, with all prongs in place and connected. There should be no three-to-two power cord adapters, and no "ground-lift" connectors. The power cord grounds should all be "bonded," and go with as little resistance as possible to actual earth ground. This is why ultra-expensive, custom-made studio gear will have a big, thick copper bus bar in it, to which all internal grounds are connected. It's an effort to create zero-Ohm ground paths.

Do not try to cure a ground loop by breaking the ground connections anywhere in the AC power supply. The only ground connections you can break to cure a ground loop are in the line-level grounds. Ignoring this rule can be fatal.

Also, do not rely on line-level patch cables to be the safety ground -- for what should be obvious reasons, by now. The safety ground should not be an intermittent connection -- i.e., through an XLR or 1/4" plug. It should not be through thin wire braid or foil shielding. It should be direct to ground, it shouldn't be through multiple pieces of gear or instrument cables, and it definitely should not have a human being touching it.

To break the loop, you can break the grounds in the line-level connections. These are the cables that carry line-level signal between pieces of gear. You do this by disconnecting the ground at one end of the cable -- only one end, so that the shielding remains connected to ground and can perform its shielding function, but does not create a circuit for the ground loop.

The problem is that you cannot break the ground loop if it's being caused by unbalanced cables, because if you disconnect the ground, you've also broken the circuit that's carrying the music signal. So, the only thing you can do in that case is to try to ensure that all grounds are bonded and at the same potential level. Again: Do not try to break a ground loop by breaking the ground connection in the AC supply wiring.

The guy who posted the question about hum in his bi-amp set-up has a real problem, because the ground loop is caused by unbalanced cables, so he can't break the loop. All he can do is try to get the grounding conductors bonded as good as possible. If he could use a DI that converts to a balanced signal, he could then break the loop by lifting the ground in the shielding, because the shielding is not part of the signal chain. But his bass amps probably do not have balanced inputs, he is limited to using the front-panel, 1/4", unbalanced connectors.

I hope this makes sense. I know that there are more-qualified folks here that can nitpick this on technical grounds, but I've tried to use analogies and simplified concepts so everyone can understand.

Be safe!
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