| Understanding
and eliminating ground loop problems.
What does it look like?
From light to dark horizontal lines or bars, slowly traveling through the picture,
from the bottom to the top of your TV screen or monitor. This is sometimes called
video hum. Severe cases can cause tearing of your picture.
This image illustrates a moderate video ground loop problem. Note the
three gray horizontal bands. The bands move up and down through the picture. |
| 
| This
shows a moderate
ground loop problem on an
oscilloscope. Video & 60
Hz. | | This
shows a restored video
signal after the humbucker is
installed. |
| Ground
loop interference looks like a wavy line superimposed on the normal video signal.
| |
What does it sound like?
Audio hum, 60-cycle noise, 60 Hz hum, similar
to the lowest note on a bass guitar or bass drum.
What causes the
problem?
Relatively long audio or video transmission cables (usually from
room to room or longer), with 120 VAC powered equipment at both ends of the cable
that is connected to different circuit breakers. This is the typical scenario
but any length cable hooked to any equipment can have this problem. The name given
to this problem is a ground loop.
What is a ground loop?
A difference in the ground voltage between two electronic devices powered by 120
VAC and connected together with your audio or video transmission cable. This difference
(common mode) voltage will drive a current through your cable and superimpose
itself onto your audio or video signal showing up as hum or lines. This is an
addition of voltages. A similar effect can occur when an AC magnetic field comes
close to your cable. The AC field can induce a current directly into your cable
and superimpose itself onto your audio or video signal showing up as hum or lines.
This is related to the workings of a transformer.
How are ground loops
corrected?
Simply disconnect one side of the coax cable and insert a
Hum Bucker between the coax cable and the equipment. The Hum bucker will block
the current flow and restore the audio and/or video to normal.
The
fix for standard equipment is a video Isolation transformer.
The standard
Hum Bucker blocks the current's path with a Video Isolation transformer.
A
standard quality Hum Bucker is used for removing hum from most sources of video.
It passes about 20 Hz to 4 MHz, and its hum isolation is greater than a choke.
The transformer is primarily used in CCTV: security, avionics, display, etc.
Both transformer and choke can pass camera control signals.
The
fix for premium equipment is a video isolation choke.
The Premium Hum
Bucker blocks the current's path with a video isolation choke.
A broadcast-quality
Premium Hum Bucker is used for removing hum from the finest sources of video,
it passes DC to hundreds of MHz. The choke provides protection against moderate
levels of 60 Hz hum, and protection against very high levels of RF noise. The
choke is primarily used in high quality applications like Broadcast TV & ENG
(electronic news gathering) etc. because it passes the DC component of the signal.
Do not remove the middle prong of your 3-prong electrical plugs!
This is very hazardous and a violation of UL code.
This increases the shock
hazard and will not fix the problem. It is difficult to predict where a ground
loop will occur.
Ground loops are usually an "after the fact" problem where
the end user blames the installer, the installer blames the manufacturer, and
the manufacture blames the electricians. Often nobody knows how to fix it. A ground
loop problem may occur at multiple points in your system. Each problem must be
corrected individually. This type of interference can affect cameras, video recorders,
monitors; video effects generators, switchers, computers and much more.
Details about the conditions causing ground loops.
The 120 VAC power
wiring in all buildings has losses in the copper wiring. You can see this in the
fast speed of an electrical saw at the outlet and a slower speed at the end of
a long extension cord. The losses show up as voltage differences. You can actually
take a voltmeter with 30-foot leads and measure a voltage between the ground on
the 120 VAC outlet in one room and the ground on the 120 VAC outlet in another
room. The ground (of course) is the screw that you can see and touch, that holds
the plastic plate onto the electrical conduit box. The voltage gets higher with
multiple power panels and high amperage loads. The equipment that is plugged into
these outlets carries the same ground voltage on their own chassis. When a video
transmission cable is connected between the two chassis it now becomes a current
carrying conductor. The voltage difference causes current to flow through the
coax inducing a (common mode) voltage into the video. This is seen and heard as
Hum and usually displays itself in the form of horizontal lines or a disturbance
floating through the picture.
| 
| Another way
to look at Ground Loop problems.
How to prevent audio hum and video bars.
What is a Ground Loop?
When you hear hum in audio or see bars in video, it's often caused by a ground
loop effect between two or more pieces of gear, plugged into two or more 120 VAC
outlets.
One half of a ground loop is formed by connecting two electronic
components together with a cable, such as, a DVD player is connected to a TV,
with audio and video cables.
The second half of this ground loop is formed
by the 120 VAC plugs of the same electronic equipment and the unseen ground wiring
in the house, such as, the TV's power cord is plugged into one outlet and the
DVD player is plugged into a second outlet and the wires in the wall connect the
two together.
This loop is from a DVD player, through audio and video
cables, to the TV, through the TV's power cord to the house wiring, through the
house wiring to the outlet that the DVD is plugged into, through the DVD players
power cord and back to the DVD player where we started.
How to Fix
Ground Loops
For complex systems you may need to repeat these steps starting
with a different piece of equipment in various combinations to locate the problem:
1. Strip the system down to one central piece, such as a mixer, by disconnecting
all interconnects and AC cords except for the mixer.
2. Add one piece
of equipment at a time; hook up AC and interconnections (making sure all grounds
are connected and in good condition) then listen for hum or look for noise.
3. You can turn on and off the power each time you switch equipment to avoid
pops and shorted outputs.
4. Proceed until you find the offending piece(s)
causing the problem.
5. Plug the The Ground Loop Isolation Transformer
in all lines between the offending equipment and the rest of the system.
6. Another common path for ground loops is through a chassis into the rack
and then into another chassis. Test this by removing the chassis from the rack.
7. What about cutting or lifting the AC ground?
Caution: Removing
or disabling the AC ground can cause electrocution and:
· Can
cause distortion due to floating signal references
· Can cause noise
to be dumped into another piece of equipment.
8. What about cutting
or lifting the signal ground?
Cutting the shield at one end of the
interconnecting cable:
· Can distort the signal return.
·
Can alter the cable's frequency response.
· Can defeat the shielding
effect.
Ground Loop Isolation Transformers are the safer and better solution,
they break the ground loop and keep all AC power grounds intact. |
More filter information.
Hum
Eliminators can do more than just prevent AC hum from being picked up by your
sound systems. Special units can be used as one of the most cost-effective ways
to convert unbalanced signals to true balanced signals.
Ground Loops
– Hum Eliminators prevent ground loop antennae from forming and picking up
AC hum. AC hum & noise can be caused by a loop antenna effect across signal
lines between two or more pieces of gear, or by long unbalanced cables picking
up noise. Loop antenna(e) are basically a type of radio antenna, which tend to
pick up the 60 Hz (and harmonics) AC signal being broadcast by a building's electrical
current. These loop antennae are closed circuits usually along the ground wires
and hence are commonly called ground loops.
Balanced and Unbalanced
- Unbalanced signals are more susceptible to picking up electrical noise &
RF interference than balanced signals are. The longer the unbalanced cable, the
greater the chance of a problem. Hum Eliminators are one of the most cost-effective
ways to convert between unbalanced signals and true balanced signals.
MIXERS
and RECORDING
Many mixers have unbalanced or "impedance balanced" monitor
outputs. Use Hum Eliminators to get true balanced outputs for noise-free performance.
When using more than one mixer (or submixing), there’s a good chance of getting
ground loops (and buzz). Use Hum Eliminators to prevent these ground loops. Eliminating
hum is one of the biggest challenges facing studios. Many studios will run all
of their line level gear through Hum Eliminators to help ensure noise-free recording.
KEYBOARDS, SAMPLERS and SYNTHS
Most keyboards, samplers, and synthesizers
have unbalanced -10 dBV outputs. Use a Hum Eliminator to balance the signals if
used with other –10 dBV equipment. To use a –10 dBV unbalanced device
with a +4 dBu balanced device, use a Line Level Shifter.
LIVE SOUND and
DJ MIXERS
A common problem with live performance is long unbalanced cables
picking up hum along the way. Balanced signals are more immune to picking up noise.
Use the Hum Eliminator to balance unbalanced signals. Mobile DJs or bands never
know when setting up their system if they’re going to get a hum or not. The
electrical wiring changes from one building to another. What works fine at one
gig might not work at the next. It’s worth the low cost to keep a Hum Eliminator
available as nobody can afford to lose a gig because of hum problems. Some DJs
run everything in their rig through Hum Eliminators to play it safe. The main
outs on most DJ mixers are -10 dBV unbalanced RCA. When using with a +4 dBu power
amp, over 2/3 thirds of the amp's volume can be lost. Use a Line Level Shifter
to fix it.
POWER AMPLIFIERS
Running multiple amplifiers can create
ground loops between the amplifiers, which will pick up the dreaded hum and buzz.
Run each amplifier's input line through a Hum Eliminator to prevent this problem.
GUITARS
Many guitar players use rack and pedal effects. They can get
buzz from ground loops that these devices create. Use a Hum Eliminator to stop
that buzz. When using two or more guitar amplifiers, ground loops can form between
the amplifiers. Because of the tremendous amounts of gain in guitar amps, the
buzz can get so bad that the amplifiers will actually squeal (especially with
tube amplifiers). Simply run the feed to each amp through one channel of a Hum
Eliminator.
COMPUTER SOUND CARDS
Most sound cards have unbalanced
inputs and outputs. Balance these with the Hum Eliminator. Even balanced sound
cards have tremendous susceptibility to ground loops because of the computer's
power supply. If you hear a buzz, try a Hum Eliminator.
BROADCASTING VIDEO
Press Boxes have many feeds all going to different gear, which can create lots
of ground loops and hum. Use a Hum Eliminator on these feeds to eliminate the
problem. Using a Hum Eliminator on the audio feed of a video camera can help prevent
video hum (rolling bars).
Note:
Never use Hum Eliminators between
amplifiers and speakers, equipment may be damaged. Only use the Hum Eliminator
on line level signals.
Another common path for ground loops is through a
chassis into the rack and then into another chassis. Test this by removing the
chassis from the rack. The Hum Eliminator will help but you should also try isolating
the chassis from the rack with electrical tape and insulating the rack screws
with nylon washers.
Most ground loop problems can be solved using the Hum
Eliminator. Patience is a necessity when attempting to solve ground loops! Many
popular mixers with TRS balanced line inputs do not have common mode rejection.
They drop the inverted signal, creating unbalanced inputs! To get the benefits
of balanced inputs, run your balanced line into the Hum Eliminator and run a mono
1/4" line from the Hum Eliminator outputs to the mixer's TRS inputs. This way
you are running a balanced line all the way up to the Hum Eliminator and you will
not lose common mode rejection.
Note
Some audio power amplifiers
have a small amount of hum inherent to the internal electronics. You can sometimes
test for this by turning off the receiver and leaving on the power amplifier (be
sure not to do damage to your system).
A question to ask yourself, have
there been any changes to your video system or your power system just before your
picture or sound got worse?
A ground loop filter will not work if you
have any other wires that start from the same source and connect to the same monitor,
this would 'short out' the filter. Such as audio cables, computer cables, steel
2 x 4s, power extension cables etc.
A good test is to disconnect all cables
from the TV except the power cord and the cable from your ground loop corrector.
The picture may change confirming your system may have a second ground loop, or
a broken filter.
Sometimes a Cable box to your plasma TV looks like a ground
loop problem but it is not.
The problem can be the refresh rate of the plasma
TV that is set wrong.
How to test your CATV system
to determine if a CATV filter will help you.
Some times a bad CATV ground
loop problem will insert hum into your system even when your TV is turned off,
and you are only listening to a CD etc.
A good test is to disconnect your CATV
cable from your TV system.
If this stops your hum it is a good bet the CATV
filter will work for you.
If the hum persists then you could disconnect more
cables one at a time, starting with the longest cable runs. The cables that are
most likely to cause your problem are relatively long audio or video transmission
cables (usually from room to room), with 120 VAC powered equipment at both ends
of the cable.
If your problem clears up after unplug the powered equipment
at the far end of the cable then the HUMplug-e may be a good choice.
If you
find a problem with one or more cables you will find many choices of filters to
use on this page. | |
