Solving the humming problems of computer soundcards and home theatre systems

Getting rid of ground loops is not easy. Finding them is even harder. LiveDV magazine has published a good tutorial Soundings: Getting Wired (January 1996) how to find and solve ground loop problems in typical home AV system which consists of computer and other AV equipment (receiver, amplifier, VCR etc.).

The only acceptable method for testing is to remove and disconnect everything. Then reinstall the equipments and connection piece by piece. Stop when the hum becomes audible, and fix the problem in the last piece of equipment that was installed.

If you have equipments connected to many power outlets, then try to connect them all to one grounded extension cord and then plug this cord to one grounded outlet. This solves usually many problems, because this creates star-like grounding scheme for those equipments.

But the following star grounding scheme is effective if the equipments do not have any direct connection to other equipment grounded elsewhere (transformer isolated connections are ok).

Computer soundcard problems

Typical computer soundcard ground loop situation

Typican ground loop humming problem occurs when you connect your computer sound card to a stereo receiver which is connected to wall antenna plug. The computer is connected to grounded power outlet which causes that the sound card audio ground is connected to mains ground. The receivers gets connected to antenna ground because it is wired to the antenna plug in the wall. The mains ground and antenna are tied together (ar at least should have) somewhere else in the house. Unfortunately there is all kinds of electricla interference which can be coupled to those grounding wires and cause small potential differences between the ground on the antenna outlet and your mains outlet. The small potential difference causes small current flowing through the equipments and cables which causes hummign to them.

antenna         +----------+               +----------+
 <--------------| receiver |===============| Computer |
plug            +----------+  audio cable  +----------+        
If you want to get rid of this annoying humming problem then first remove the antenna connection of your receiver. If removing antenna cable helps to get rid of humming then get an antenna isolator to the antenna connection of your receiver or an audio isolator to the audio cable between your computer and the receiver.

Typical computer audio problem case

I have my computer soundcard connectoed to my HiFi system. I connected my TV to my AV receiver and I get very loud buzzing even if the TV is off. Same also happens if I connect my VCR to the AV receiver.

I decided to go through the routine ground-loop check by disconnecting everything and putting everything back together. The check gave following conclusions:

Basically, the hum/buzz goes away when any of the following altered:

How the isolators can be used to solve the problem

The most effective way to stop this 50 Hz or 60 Hz noise is to break the galvanic connection which causes the ground loop. You can either connect an isolator to the antenna wire:

      isolator  +----+               +-----+               +----------+
 <-------**-----| TV |===============| AMP |===============| Computer |
 cable TV       +----+  audio cable  +-----+  audio cable  +----------+
or putting on isolator to the audio line:
                +----+               +-----+   isolator    +----------+
 <--------------| TV |===============| AMP |=====**========| Computer |
 cable TV       +----+  audio cable  +-----+  audio cable  +----------+
This is the only effective and safe solution to problems which deal with grounded outlets and radio/TV cable/antenna connection.

TV and audio system

If you connect your TV or VCR to otherwise humless audio system you will quit epropably hear a humming noise (50 Hz or 60 Hz depending on the mains voltage frequency) caused by current flowing between your equipment and the antenna connection in the TV. The following example is a typical home setup where you have your multimedia computer TV and connected to your stereo system:

                +----+               +-----+               +----------+
 <--------------| TV |===============| AMP |===============| Computer |
    cable TV    +----+  audio cable  +-----+  audio cable  +----------+        
Most people have their surround sound receiver connected to a "home entertainment center" with at least a TV and cable. If this is the case, try disconnecting the cable. If your stereo system has also a receiver which is connected to wall antenna plug then remove it also because the receiver antenna connection can also be a source of humming. If removing antenna cable helps to get rid of humming then get an antenna isolator to all your antenna connections or an audio isolator to the audio cable between your computer and the amplifier.

PC based DVD systems

PC is becoming a more and more popular signal source for a home AV system. Nowadays when many new PCs are equipped with DVD drives and display cards with TV outputs, so increasing number of people have started to connect their PCs to their home theater systems.

If you are using one ground for your PC system and a different one for your cableTV service then you most likely have Cable TV ground loop. This can be witnessed as an audible hum through your speakers or vertical black bars on a video display.

There are two remedies to correct this problem. The first is to use the same ground for both the Cable TV and A/V system. Unfortunately chanign the grounding of those you well need an electricial and a cable TV installation people. Better grounding practices usually reduce the problem, but quite often does not solve it completely.

The second and usually more viable option is to install high impedance isolation between the the cable TV conductor and the A/V system and the cable TV shield and the A/V system. And the right adapter for this is the same isolator to the antenna wire discussed eariler on this document. This method is usually the most effective and is the most inexpensive solution. The picture below shows one possible interconnection of PC with DVD drive, AV amplifier and a TV:

                +----+               +-----+               +----------+
      isolator  |    |               | AV  |               |          |
 <-------**-----| TV |===============| AMP |===============| Computer |
 cable TV       +----+  video cable  +-----+  audio cables +----------+
                                              video cable

Here is another quite propable configuration you might encounter. This is an example of properly wired system with a TV and an ampldifier with a tuner and both wired to the antenna connection on the wall:

                +----+               +----------+
      isolator  |    |               |          |
 <-------**-----| TV |===============| Computer |
 cable TV       +----+  video cable  +----------+
                                       || audio cables
                                       ||
                    isolator         +-----+
 <---------------------**------------| AMP |                                   
 radio antenna                       +-----+
As you can see, you need two antenna isolators for this case. One for TV input (for solving video humming bars problem) and another for tuner antenna connection (for solving audio humming).

Audio+video and computer systems with many grounded equipments

Even if you have multiple mains sockets in your room, to take the power for everything from a single socket (run it through a mains filter if you want), then connect everything to that as a tree. Sometimes buildings are wired with a lot of electrical distance between the separate sockets in one room, so this trick can avoid many problems. If this doesn't work, call in an electrician to make sure that your buildings earth actually works at all - you could be risking your life otherwise.

Tips for building well working systems

As with any project, planning can have adirect effect on the results. Before you start setting everything up you should decide where your equipment will be placed. Now, determine where to run the cables. For initial testing connect all of your components and run your cables across the floor, out in the open. Once you are satisfied with the way everything is setup, make sure everything is working properly. ou are now ready to permanently install the system and run the cables. Run the cables in the predetermined locations being aware of electrical and coax anteenna wiring. Do not run your audio cables parrallel to these types of cables, because they tend usually be quite strong noise sources for audio cables.

When making connections use the established color coding system for consumer electronics. White is left, Red is Right and yellow is video. When running cables thru walls make sure you use cable types approved for permanent installation.

I have found that it is essential to keep a flashlight handy and work with a second person. You should keep in mind that properly installing and setting up your equipment is just as important as selecting it. Spend a little time planning your system and installation will save time and money.

Multiple room systems

Depending on your local electric wiring regulations, the potential of the ground terminal in the wall outlets will be slightly different in different rooms due to other loads in the building and the potential will also be different than at a grounded antenna or CATV feed. The voltage difference is small, usually about 1 V.

If you have grounded equipment (either through the wall outlet or through the antenna system) in different rooms and you connect an unbalanced cable directly between the equipment, a large current (up to several amperes) will flow in the shield and it still can not completely remove the voltage difference between the rooms and you have hum added to the signal. When this ground loop current flows in the shield, it will also induce some hum current into the signal lead, worsening the situation even further.

So if you are running signals from one room to another it is a very good idea to isolate all the unbalaced connections using suitable audio isolation transformer. Other possibility to get rid of that problem is to use ba÷anced interconnections if your equipments have such (those are uusally only available on professional audio systems equipments).


Tomi Engdahl <Tomi.Engdahl@iki.fi>