Speakers connected to laptop make desktop monitor flicker

I've been wondering about the cause of this phenomenon for a long time and since I haven't managed to find a good explanation, I thought I'd post it here.

My desk is in a corner, there's a radiator running along the wall. I have a desktop computer next to the desk which is connected to a monitor. I also have a laptop with a power brick. The monitor, the computer and the laptop are plugged into the same power strip which is plugged into a non-earthed outlet. The power strip has a switch on it so I can turn everything off at night.

Here's the deal: I have a headset connected to the laptop but I'm only using the microphone. If I hold the top part of the headphone's jack plug to the radiator, I get static noise (ground loop) coming from the left speaker. The middle part gives me static coming from the right speaker. But if I hold the bottom part to the radiator (the ground), the image on the monitor that's connected to the desktop via HDMI disappears.

The same thing happens if I use USB-powered speakers. If I connect the USB cable to the laptop and hold the audio jack to the radiator, I get the same effect.

This only happens if the power brick is plugged in. If I'm running off the battery, it doesn't do this.

Why does this happen and how could it affect a completely separate device?

Reynolds

Posted 2016-03-19T22:22:09.573

Reputation: 3

Maybe this is a stupid question, but have you tried to turn around the powerplug? If its inserted the wrong way, current flows in the wrong direction and static electricity builds up. You can know this is the case if you put your fingers on the case, move and you feel that the case itself is static. – LPChip – 2016-03-19T22:28:26.407

No, this did not solve the issue. I'm pretty sure that there's a ground loop that's causing the static noise if I put the audio jack to the radiator. What I don't get is how it can make a monitor connected to my desktop flicker. – Reynolds – 2016-03-19T22:40:00.933

Which plug did you turn around, the one from your laptop or the one from your desktop? Also try doing this with the plug from the monitor. The monitor flickering means there's power leakage. The monitor doesn't get the power it wants, and flickering is the result. In layman's terms, this is also called "interference" – LPChip – 2016-03-19T23:04:14.893

1@LPChip, that's completely wrong. First, static electricity has nothing at all to do with this: that only happens when you rub two insulators together. You also can not plug in an AC power cable the wrong way: current flows back and forth over both conductors -- there is no positive and negative. You also can't physically turn around a plug with a ground pin. – psusi – 2016-03-20T00:40:05.110

@psusi It really depends on which country you live in. Most countries have sockets that have protection for this, but not everywhere. And I speak from experience. I've tested this so I know I'm right. Maybe for different reasons, but having the powercord plugged in the wrong way can actually cause for interference to occur. – LPChip – 2016-03-20T12:53:08.560

Answers

Get yourself a multimeter and measure the impedance and voltage between the interesting points. Is the ground pin of the headphone jack connected to the ground in the power strip? Is there voltage between the ground of the power strip and the radiator? Are the monitor and the PC both connected to the ground in the power strip? My guess is that the monitor is not and is only weakly grounded via the HDMI cable, and is leaking current to ground. When the power strip has no ground, the monitor and desktop are sufficiently coupled by that HDMI cable that both of their grounds have the same float voltage. When you touch the headphone jack to the radiator though, you ground the desktop through the laptop, allowing current to flow from the monitor through the HDMI cable, to the desktop, then through the power strip and out to the radiator. The ground in that HDMI cable though isn't meant to carry current, and so some voltage builds up between the ground of the monitor and the ground of the desktop.

psusi

Posted 2016-03-19T22:22:09.573

Reputation: 7 195

I did some measurements and you seem to be right. It turned out to be an incredibly elusive problem. Thanks for the help. – Reynolds – 2016-03-20T09:15:11.513

First off, why would you do this? You're creating a path to ground that wasn't meant to be. Chances are the radiator is earth grounded, so touching an ungrounded electrical device to it is asking for trouble (difference in potential). Grounds loops can be extremely confusing and difficult to troubleshoot. They can also damage equipment and people.

Mike Lowery

Posted 2016-03-19T22:22:09.573

Reputation: 121

Obviously, this was not intentional. It was a result of me fiddling around and consequently getting shocked by the radiator while I was talking on Skype. This is also not an apartment and the only people who could have touched the radiator were asleep. I was just really wondering about the cause of all this. – Reynolds – 2016-03-20T09:13:13.147