Ground loop, RFI, or what?

[rtadams89]

Hey guys,

I've been reading these forums for a while now and have gotten a lot of good info. I've stumble upon a problem which has turned from a reader into a poster:

In my home office I have a desktop computer + LCD monitor. Next to it, i have a second LCD monitor which connects to a dock for my laptop. I have one set of speakers, and so, recently installed a Rolls MX41 passive stereo mixer to be able to share the speakers between both computers. The speaker system itself is ungrounded (it has only a 2 prong power plug). The laptop/docking station is also natively ungrounded. The LCD monitors are grounded, and because one LCD monitor connected to the laptop dock, the dock/laptop is then grounded when ever the LCD is connected. All the devices plug into one UPS/battery backup.

When I connect my laptop and desktop to the passive mixer, I get a high pitched whine and static over the speakers. If one of the computers does something (like hard drive activity) or if I move the mouse, the noise changes. I've found that the only way to eliminate this is to ungrounded one of the computers (either ungrounded the desktop power supply or ungrounded the LCD monitor that connects to the laptops dock). Once I lift one of the grounds, the noise disappears.

So it sounds like I have a ground loop problem. (Correct?)

Very shortly I will be installing a second desktop computer which I will hook up to the passive mixer as well. While I'm OK not having my one LCD monitor grounded, I'm not sure what to do when the new desktop is installed as I would prefer both desktops be grounded. What are my options? Will a ground loop isolator device work in my setup?

 

[TLS Guy]

Hey guys,

I've been reading these forums for a while now and have gotten a lot of good info. I've stumble upon a problem which has turned from a reader into a poster:

In my home office I have a desktop computer + LCD monitor. Next to it, i have a second LCD monitor which connects to a dock for my laptop. I have one set of speakers, and so, recently installed a Rolls MX41 passive stereo mixer to be able to share the speakers between both computers. The speaker system itself is ungrounded (it has only a 2 prong power plug). The laptop/docking station is also natively ungrounded. The LCD monitors are grounded, and because one LCD monitor connected to the laptop dock, the dock/laptop is then grounded when ever the LCD is connected. All the devices plug into one UPS/battery backup.

When I connect my laptop and desktop to the passive mixer, I get a high pitched whine and static over the speakers. If one of the computers does something (like hard drive activity) or if I move the mouse, the noise changes. I've found that the only way to eliminate this is to ungrounded one of the computers (either ungrounded the desktop power supply or ungrounded the LCD monitor that connects to the laptops dock). Once I lift one of the grounds, the noise disappears.

So it sounds like I have a ground loop problem. (Correct?)

Very shortly I will be installing a second desktop computer which I will hook up to the passive mixer as well. While I'm OK not having my one LCD monitor grounded, I'm not sure what to do when the new desktop is installed as I would prefer both desktops be grounded. What are my options? Will a ground loop isolator device work in my setup?

What happens when you disconnect your phone or cable system from the system?

 

[rtadams89]

"Phone or cable system" -- Not sure what you mean. The only devices involved are a laptop, a desktop, a PC speaker set, a passive mixer, and the LCD monitors.

 

[TLS Guy]

"Phone or cable system" -- Not sure what you mean. The only devices involved are a laptop, a desktop, a PC speaker set, a passive mixer, and the LCD monitors.

You just need one ground. What I'm trying to find out is if any of your devices, such as your desk top are hard wired to a router, which is connected to a cable or phone system. What is your Internet set up.

Some where you have a potential between grounds, which needs eliminating. You get trouble when grounds are circular. Grounds need to be low resistance all going to one point.

 

[rtadams89]

Oh, I see where you are going.

I already know where the two grounds are. One is the LCD monitor (which is grounded by it's 3-prong power plug) which because it is connected to the laptop dock and the laptop, grounds the laptop. The other ground is the desktop PC.

As I mentioned, I can eliminate the noise by ungrounding either of these devices (using a cheater plug as I believe they are called), but I obviously don't want my desktop PC or the LCD monitor to be ungrounded, so this is not a permanent solution.

 

[TLS Guy]

Oh, I see where you are going.

I already know where the two grounds are. One is the LCD monitor (which is grounded by it's 3-prong power plug) which because it is connected to the laptop dock and the laptop, grounds the laptop. The other ground is the desktop PC.

As I mentioned, I can eliminate the noise by ungrounding either of these devices (using a cheater plug as I believe they are called), but I obviously don't want my desktop PC or the LCD monitor to be ungrounded, so this is not a permanent solution.

I assume then both the PC and laptop have internet wireless connections.

Laptops don't like being grounded. Just lift he ground to that LCD connected to the laptop. You don't have another option.

You can try bonding the chassis of all units with No 4 copper and having one ground, but that will be a lot of work. I would just lift the ground of that LCD monitor and be done with it. Everything will ground via the connecting cables anyway. Just make sure everything is off when you unplug or plug in cables.

More than one ground often gives trouble unless you create a professional grounding buss. That is not easy with consumer equipment.