Re: Hum, Ground Loops and Safety
- From: tlbs <tlbs101@xxxxxxxxxx>
- Date: Thu, 3 Jul 2008 08:31:47 -0700 (PDT)
On Jul 2, 10:58 am, Brian Running <brunn...@xxxxxxxxxxxxxxxxx> wrote:
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
...
read more »
Brian is absolutely correct.
I am an electrical engineer by profession, and I play bass in my
church band.
Even though I know better, I still got a nasty shock when I hooked up
a Peavy head with a plug that someone had cut the ground pin off.
While I was playing, as soon as my lips brushed the microphone...
OUCH!
Tom P.
.
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