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av-geek 03-19-2013 06:33 AM

ground loop / phase / audio hum problem
I am dealing with an annoying 60-cycle hum problem with a sound system in my home office room that started when I added a computer.

Here's the scenario: I turned the basement bedroom in my house into a home office for my wife and I. One one side of the room, I built a desk with a countertop and some cabinets. Her computer is located under the countertop in the cabinet. The computer I have been using is a laptop that I plug both audio and VGA into a 26 inch TV that I use for a monitor.

On the other side of the room, I built a small sound system in a rack that includes an old Mackie 1202 mixer, a CD player, and an amplifier that drives a pair of in-wall speakers. I then pulled audio cables from one side of the room to the other through the inside of the walls. One audio cable connects directly to the audio port on my wife's computer, the other audio cable connects to the audio output socket on my monitor/TV. This way, we can both use the speakers in the room (or listen to a music CD) without having to switch the sound system over. The sound from my laptop connects into the TV, and when I have the computer port selected, the sound passes thru the TV and goes to the sound system. If she wants to watch TV while I'm gone, she just grabs the remote and tunes to a channel. The TV audio then mixes with her computer audio.

This has all worked perfect until I added a desktop computer to the rack with the audio system this past weekend. I pulled a VGA cable through the wall using the same conduit as the audio cables to our desk, and connected it to the TV on my side of the desk so I can view the computer. When I did this, I am now getting a low level hum in the speakers on BOTH of the audio inputs of the mixer from the other side of the room IE her computer, and my monitor/TV. There is NO audio connection from my desktop computer to the sound system.

It doesn't matter if my new desktop computer is powered on or not, or if her computer is powered on or not, but if I unplug either of the two computers, the audio system is dead silent again. It also goes away too if I unplug the audio jack out the back of the TV. The TV itself is not grounded to the electrical system, as it does not have an internal power supply. It runs off an external 12 volt transformer. I have verified that the sound system rack with the computer in it is plugged into a different circuit and a different phase than the electrical outlets under the desk, but both of the outlets have a good earth ground, and in fact if I use my multi-meter and check continuity between the third prong of the outlet my wife's computer is plugged into, and the third prong of the outlet the sound system and my computer is plugged into, I get continuity, and less than 1 ohm (with all the electronics disconnected to break up the grounds), so I am not thinking this is a ground issue. I have also noticed however that I have about a 5 volt difference in the voltages between the phases in my house at times. When I was troubleshooting this, I was getting 118 volts on one phase, and 123 on the other. Would this imbalance cause a voltage difference an hum to appear in the audio system despite them being properly grounded together?

electures 03-19-2013 06:54 AM

Try using a ground lift to isolate the computer power supply.

After that try changing the power cord. Also try a different receptacle. Remember not to leave the ground lift in place when you are done.

bernie963 03-19-2013 07:42 AM

You mentioned you pulled the new audio cable through the conduit with the other audio cables. Did you damage any of the cables or stress an end connector on any connectors? If so, you may have a broken shield or ground conductor.

have you tried to connect chassis grounds between equipment separate from power cord grounds?


av-geek 03-19-2013 11:41 AM

I thought I had damaged a ground too. That was one of the first things I checked. I am going to perform some more tests tonight by lifting some grounds and see what happens. I would like to leave the equipment on different outlets so that I can spread the load of the electronics to different circuits and phases if it's possible

stickboy1375 03-19-2013 04:18 PM

12 Attachment(s)
I would make sure your utilities are all bonded together to the same ground rods as well... I had a nasty audio hum when my CATV company drove a separate ground rod on the other side of the house for their demarc location.

DanS26 03-19-2013 09:51 PM


Originally Posted by av-geek
I thought I had damaged a ground too. That was one of the first things I checked. I am going to perform some more tests tonight by lifting some grounds and see what happens. I would like to leave the equipment on different outlets so that I can spread the load of the electronics to different circuits and phases if it's possible

Try this experiment. Put all equipment on one circuit. The hum you are experiencing is probably a ground loop between phase A and phase B. Even separate circuits on the same phase could be a problem.

Use extension cords from only one outlet to power all equipment. If hum disappears problem can be solved by rewiring room.

MartySvatt 03-20-2013 06:00 AM

Mains induce hum
Hi AV-geek - I think DanS26 is on to something here, I had terrible mains hum on my guitars in my home studio which turned out to be the lighting circuit in the room, nothing to do with the power circuit. If you have a live feed to a lighting circuit and the neutral is 'borrowed' from another circuit it sets up an induction loop which can induce hum in audio equipment.

I found this article that explains it a bit better (half way down the page under the heading 'induction loops':

I rewired the light circuit and got a massive improvement :)

av-geek 03-20-2013 08:15 AM

I performed some more tests last night to try and get rid of the ground loop. I did exactly as DanS stated, and ran an extension cord from the AV rack's outlet strip to the same plug my wife's computer was in, and the hum went away. Also, the hum would go away if I unplugged the audio jack from her computer, or unplugged the VGA cord from my computer, OR unplugged the audio cable from my TV monitor.

The ground loop appears to be coming from my computer, going to the TV, from the TV to the mixing board, from the mixing board, to my wife's computer. Any break in this chain seems to break the ground loop.

I confirmed that the outlet that the AV rack is plugged into is on a different phase than the outlets on the opposite side of the office where my wife's computer (and my TV) is powered on. I also confirmed that the outlets are wired properly, and there's no reversed hot / neutrals. Would a reversed neutral / ground cause this though? I'm not sure what's going on in my 60 year old electrical box, which is scheduled to be replaced next month.

I suspect the ground loop problem is somewhere dealing with my my new computer. The problem has not occurred with my wife's computer being plugged into the mixing board with all the other AV gear on the other wiring phase...It has only occurred when I added my new computer to the AV rack. Could the IEC power cord to the computer be bad? (like a hot and neutral reversed or a broken ground inside the cord...what terminals are supposed to be hot an neutral on an IEC socket?)What about the wiring inside the computer power supply being reversed? I am of the thinking that chassis ground should be bonded with the third prong of the power cord. With so much iffy electronics coming out of China lately, I am thinking anything is possible with this computer here, although some of this is sort of getting out of DIY electrical repair and more into internal electronics repair.

wkearney99 03-20-2013 09:04 AM

Likely a bad power supply (or poorly engineered one) in the PC. I have run across mis-wired wall cords for them, so check that first. But I've run into more bad power supplies than bad cords.

ddawg16 03-20-2013 09:33 AM

To confirmed that the audio center power is on one side of your 120/240Vac.....and your laptop is on the other side...and you confirmed that the neutrals are wired correctly.

Have you used a volt meter to measure the two different neutrals on the ckts? I know you said you verified they were wired right...but did you actually measure the voltage difference between them? I bet your going to see a few volts difference....most likely because you have a good distance between the outlets and your load center.

Assuming you are not pulling a big load...I would go into the load center and swap the laptop phase with another ckt so that it's on the same phase as your audio....

But I think you will find that it does not solve your problem...which means that you have enough load on the two ckts that you get a voltage drop across your neutral and when you connect up the lap top, some of the current is shunting through it...hence the hum.

Measure that voltage difference and report back.

DanS26 03-20-2013 03:15 PM

Your problem is probably not your wiring and not your equipment. Look closely, does your equipment setup in this room in any way electrically connect the components?

Speaker wires, cable connections and other low voltage connections count. What is happening is that you are creating a low voltage connection between components thru the ground system of each device-----in other words a ground loop between circuits. If everything is on the same circuit the loop is avoided.

bernie963 03-20-2013 08:25 PM

yes to all of the suggestions. I have seen factory made power cords made with hot and neutral reversed. it happens. have you checked the grounds in the cables you pulled through the conduit?


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