Trouble Shooting Audio Hum, Ground Loops and Isolation

Patrick Ziegler Wholesale Electronics Inc.

Today's entertainment centers have changed quite a lot from those of several years ago. Today we have MP3's on our computers, phones and Ipods. Satellite and Cable TV can deliver digital music streams as well as theater quality sound to our great rooms and entertainment centers. As well as DVD's, DVR's etc…

The more sources we add to our audio systems the more likely we are to end up with an audible hum. If you have a sub woofer this can become even more annoying.

Electrical hum is generated for the most part by ground loops. For many, ground loops are mysterious and difficult to comprehend and/or fix.

All electrical grounds are not created equal. For instance, from the cable TV pedestal behind your house to the electrical ground at your AC outlet, there is some resistance. The wiring in your house from one outlet to another has some resistance in the wire and connections.

These resistances will create a difference in voltage between the two points, ( Ideally all grounds would be zero volts ) and a small current will flow between the two points.

Typically, these resistances are small and do not create problems. However, it doesn't take much to introduce a hum into your system.

Lets say your computer is in the den and you stereo is in the living room. So, you wire in a patch cord from the computer to your receiver and now you have a hum. There is nothing wrong with the receiver or computer or the patch cord. The trouble is, the ground in the den and in the living room outlets are not electrically the same resulting in a small AC current flowing between your computer's chassis and your stereo receiver's chassis through your audio patch cord's ground conductor.

If you had an AC volt meter with very long test leads you would be able to measure a small AC voltage between the two grounds terminals at the AC outlets. This same voltage difference is now carried to the chassis of your computer and the chassis of you stereo receiver.

This may be as small as 1/10 th of a Volt. However, the line level audio signal you are sending from the den to the living room is only 1 Volt peak to peak. 1/10 th or 10 mV is enough to cause a hum issue.

As of late, the most common cause of ground loop hum in audio systems is cable TV.

It is the same situation. The shield or outer conductor on the cable is not electrically the same as the AC ground that your cable box or TV is grounded to. And you are connecting audio from your TV or Cable box to your stereo receiver and viola, HUM…

The TV or Cable box doesn't even need to be turned on. As a matter of fact you can turn everything off except the receiver and you still have hum.

So, now you have an idea of what causes the problem so, how do we fix it?

What you don't do is start cutting of the third prong on you power cords. You might lose the hum doing this but you have created potentially dangerous situation.

That third prong ties the chassis of most all electronics to earth ground. If an internal component fails and shorts to ground you could now end up with dangerous voltages on the chassis of your electronics. With the third prong intact, this would trip a breaker or blow a fuse.

Also, most surge protection is useless without being tied to earth ground.

Troubleshooting is a process of elimination…

If you have cable or satellite TV connected to your system, disconnect the coax and see if the hum goes away. If it does, check that you have a grounding block where the cable enters the house and that it is in good repair and properly grounded. If this doesn't solve it you can call your cable guy and have him take a look. There are also ways of isolating your cable feed using RF Isolation transformers.

One easy cheap way of making an RF Isolation transformer is to take two TV Matching transformers or baluns and connect them back-to-back. They have to be isolation type baluns. You can check this with an ohmmeter. There should be no continuity from any point on the 300-ohm side to any point on the 75-ohm (coaxial) side. If there is, it is not an isolating transformer.

This cheap and easy fix can cause issues if you have digital cable. You could lose some or all of your channels. It is a cheap an easy fix and worth a try. There are devices specifically designed to isolate digital cable feed. With digital cable and HDTV a broadband isolation transformer is needed.

If you cable TV is not the issue, are there any two prong electrical cords in your system? If yes, unplug it and see if the hum goes away. If it does, try rotating the plug 180 degrees and re plugging it in, if you can't do this because of polarized outlets and plugs. Have an electrician check that the outlet is properly wired.

Do you have any gear plugged in to different electrical outlets or circuits? If yes, unplug them and see if the hum stops. If it does, have an electrician check the wiring for loose or otherwise bad connections from the outlets to the breaker panel on both outlets and /or circuits.

If you cannot fix the ground loop on the electrical side you can isolate your components from one and other on the audio side using one of these type devices. These almost always contain a transformer. Most folks won't care if there is a transformer between your source and your receiver however there is some loss of both level and frequency response with most transformers.

However, hiring an electrician can be expensive and he or she may not be able to find the problem. Remember, it only takes a small resistance to create trouble.

If you find the trouble is generated by your sub woofer here are some possible remedies…

• Try plugging the subwoofer into a different AC outlet in the room, one that isn't supplying power to your components (A/V receiver, TV, cable box, etc.). That might fix it.


• Try reversing the AC plug for your A/V receiver or the powered subwoofer. If it's a 3-wire plug or a polarized plug, which has one prong wider than the other, you won't be able to reverse the plug. For safety, do not use a "cheater plug" to bypass the 3-wire plug.


• With the power OFF, reverse the AC plugs one by one of any other components that have a standard 2-prong AC plug that isn't polarized. Each time you reverse a plug, turn on the system with the attached component and your subwoofer and see if the hum disappears. In some cases, reversing one or more plugs will eliminate the hum.
  

If none of the above steps get rid of the trouble, it is possible that your electronics is at fault. A bad connection or failed component within your receiver or other audio equipment can cause hum in your system.