Ground-Loop Hum Troubleshooting

One of the most annoying and difficult problems to solve with audio systems is a persistent low-level humming sound coming from the speakers.  This is often not a fault of the equipment itself, but of the AC power connections to the the equipment, as well as the audio cables that connect the different components together.   Sometimes, slight differences in the "ground" voltages of the various audio components will cause current to flow in the ground conductors of the cables that are supposed to carry only the sound.  Such noise is technically called "common-mode noise," but it is also known as a "ground loop." 

This is most often a problem with the common RCA connectors, which have a center pin surrounded by a circle of four or more metal tabs.  Ground-loop hum does not occur with "balanced" connectors, which have two signal wires plus a ground conductor.

Here are some ways to troubleshoot and correct such problems.

  1. The first step is to isolate the source of the trouble.  Start by turning on your system and turning the volume up to the point where you can hear the hum.  Then, stop the music on whatever source component you are listening to.
  1. Turn off your amplifier or receiver (the component where the speaker wires connect) and disconnect the cables coming into it from whatever source component you are listening to.  Turn the amplifier or receiver back on and see if the hum is gone.  If so, that component is the one at fault.  Skip down to Step 1 in the next section.

  2. If not, turn the amplifier or receiver back off and disconnect the cables from another component to see if it's the problem.  Keep in mind that the problem can be caused by cables from components other than the one you are listening to.  I know it seems counterintuitive, but the ground currents from some other component can add hum to your system, even if it's turned off!

  3. Continue this procedure until you have found the offending component.  Alternatively, you can start with only your speakers connected to the amplifier or receiver and add cables one at a time until one of them brings the problem back.

Now that you've found the source of the problem, it's time to try some things to make it go away.

  1. Reconnect the problem component and confirm that the hum is still there.

  2. If possible, use a digital connection to the receiver or amplifier.  Digital optical, digital coax, HDMI, and DVI cables all carry signals in a digital form, which is immune to common-mode noise:  They either work perfectly or not at all.

  3. Make sure that the problem component is plugged into the same wall outlet as the amplifier or receiver.  If you don't have enough wall outlets for all your components, plug them into a single outlet strip and plug the strip into a wall outlet.

  4. Check the outlet wiring.  AC receptacle testers are available for $5.00 and up from Radio Shack, hardware stores, and so on, and so on, and so on.  All you need is a cheapy with 3 lights on it to indicate good connections of the hot, neutral, and ground wires.  Reversed hot and neutral wires can allow all your equipment to function but still cause hum and safety problems.

  5. Try different outlets.  Paradoxically, ground loops can sometimes be improved by plugging individual components into different outlets in different parts of the house.  I once used this method to get rid of a nasty hum from a giant Marantz receiver used with a Soundcraftsmen amplifier.  I guess the orange extension cord connecting the amplifier to a kitchen outlet looked tacky, but it seemed important to have all 1160 Watts available in a one-bedroom apartment...

  6. Try shorter audio cables or cables with heavier shield (ground) wiring.  This doesn't mean you have to go out and buy the $200 platinum-plated litz-wired mono-directional "Godzilla" cables that your big-box store recommends.  These have much more to do with marketing and sales commissions than they do with quality.  Just try to find some cables with rugged connectors and heavy outer shield wiring and only buy one set.  Remember, you already eliminated all your other cables as possible sources of this problem.

  7. Add a separate ground wire between the offending component and the amplifier or receiver.  This can be done by getting some garden variety 12 ga. TW wire from your local Home Despot store.  Wrap one end under a screw on the back cabinet of the source component and the other under a screw on the back of your amplifier or receiver.  This has the effect of decreasing any ground voltage differences between the two units; it may reduce or wholly eliminate the hum.

  8. Try "lifting" the ground connection at one end or the other of the audio cables .  This works by eliminating the ground path through the audio cables, while retaining the shielding effect by leaving the shield connected at one end.  You may be able to try this by pulling both of the cables out to the point where the inside pin on each plug is still connected, but the four outer ground tabs are not touching the ground ring on the jack.  If not, you can modify some cheap cables by cutting through the plastic insulation near the plugs at one end, and then carefully cutting through the many strands of copper that make up the shield underneath the plastic.  Don't cut into the inner wire or its insulation.  You can also try reversing the cables end-for-end to see if it makes a difference which end gets grounded.  If lifting the ground works, you can replace the cut-up cable with a ground loop isolator, described in step 9, below.

  9. Try using an audio ground loop isolator in the audio cables between the noisy component and the amplifier or receiver.  These devices contain either transformers or differential amplifiers that pass the audio signals, while blocking any audio ground currents.  The transformer variety give excellent isolation, but careful design is needed for them to faithfully carry the full-range audio signal.  The electronic differential amplifier isolators can have more perfect frequency response, but their isolation may not be total.  Either way, isolators are available from $5.00 to several hundred dollars.  A good test would be to see if a cheapy eliminates the hum.  Then, you can feel better about spending more money for better sound quality.  Jensen-Transformers.com has a good selection of quality isolators.

  10. If you have two or more audio components with 3 pin AC power cords, a ground loop can exist between them.  As a troubleshooting technique, ONLY, and NOT AS A PERMANENT FIX, these 3 pin power cords can be plugged, one at a time, into a 3 pin-to-2 pin grounding adapter, available at any hardware store.  If this helps, then you know you have a ground loop problem that needs to be solved by some other method.  DO NOT LEAVE ANY AUDIO COMPONENTS UNGROUNDED.  None of the commonly used devices for safe isolation are likely to fix this, as they all isolate the Hot and Neutral wires, but they still have to keep the 3rd Ground pin connected for safety.  Still, if you own an isolation transformer (used by TV technicians for safety on the workbench) or a UPS (uninterruptible power supply), you can try plugging the noisy source device into it.  Just be careful to plug the source component into the transformer or power supply.  Plugging the amplifier or receiver into the transformer may exceed its maximum current rating.

Special Cases

Cable TV Ground-Loops:  TV sets and set top boxes for cable, satellite, and fiber-optic TV services can also have ground-loop problems.  This happens because the installer is required by law to install a grounding block where the cable enters the house.  Just like a grounded AC power outlet, the ground connector on the cable can introduce a loop.  In addition to audio hum, these devices can also exhibit hum bars:  horizontal bars that rise slowly upward through the picture.  First, try removing the RF cable(s) coming from outside the house to see if the problems go away.  The sound and picture will go away soon after you do this, so you may need to look and listen quickly to observe the effect.  If the set top box has a DVR, you can play back a recording while disconnecting the cable.  If disconnecting the incoming cable cures the trouble, you can be sure cable-borne hum is at fault.  You can either insert a cable TV ground isolator (Radio Shack, Jensen Transformer, MCM Electronics, etc.) or coerce your cable TV installer into moving the grounding block ground connection to the grounding bar in your electrical box.  Good luck explaining this to him/her...

Turntables:  Hum on a turntable, only, is a different animal.  You can check or replace the two audio cables and ensure that the separate ground wire, if any, is connected to the ground terminal on the back of your amplifier or receiver.  You can also connect the turntable ground lead to any screw on the back of the metal chassis.  If none of this helps, you may have to replace the phono cartridge or have the internal wiring of the turntable repaired.

Light Dimmers:  Dimmer controls on your household lights can also either transmit RF noise through the air or send noise into the AC power wiring in your house.  This is why theater stages use huge (and expensive!) variable transformers instead of cheap light dimmers.  Turn off or unplug any dimmers you may have.  If the problem is gone, you have found the problem.  Remember that touch-sensitive dimmers built into fancy floor and table lamps can also cause noise.

Final thoughts:

No one technique is guaranteed to work.  You just have to keep trying them alone and in different combinations until you find what works.  Often, this will be something that should not work according to electronics theory.  Go figure...