Re: Hum, Ground Loops and Safety

I don't have time to read this, but Safety is nothing to be "ho-hum" about.

--
-rob Bartlett, TN
O>
/(\)
^^
"Brian Running" <brunning@xxxxxxxxxxxxxxxxx> wrote in message
news:OrOak.16051$Ri.5243@xxxxxxxxxxxxxxxxxxxxxxx
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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