Background noise


I’m opening this topic which is not directly linked to Mixxx but, I think this is the best place where I could find some help advices for my problem.
My configuration is the following

  • Screen, mouse and keyboard connected to a Targus «Display Link» dock171
  • My personal laptop (a DELL Latitude 3350) using the «Display Link» tips to get the power supply
  • A sound amplifier (an antic Pioneer SA-506) on which I connect my speakers and my headphone. The sound amplifier can be connected either to the laptop jack output or to the «DisplayLink» jack output.

My problem is this one : I got a general background noise louder enough to be clearly heard in my headphone or the speakers, even at low level. Over the background noise itself, I can hear a kind of regular «beat». All of those parasites are slightly louder either each time I’m moving my mouse or use the trackpad of my laptop or when I’m typing on the keyboard.
But this background noise is disappearing in the following cases

  • If I’m disconnecting the power supply / use the original power supply of my laptop when I’m using the laptop jack output (but, I’m upset to get many power supply everywhere around my desk…)
  • If I’m disconnecting the laptop jack output (and use instead the DisplayLink jack output) when I’m connected with the DisplayLink power supply
  • it is almost disappearing if I insert an «adapter» to bypass the mass between the socket and the DisplayLink power supply. But I always thought the mass was done to protect my equipment…

So, I can’t determine if the problem is coming from the DisplayLink power supply or from my jack / RCA cables or anything else.
I tried with another sound amplifier : same problem.
I tried with other jack / RCA cables : same problem
I tried with another laptop (a DELL too) : same problem
I tried to connect many different power supply (Tips of the DisplayLink, a power supply of some other DELL, and the initial DELL power supply) : the background noise almost disappear with the «original power supply, and have various level with the other power supplies.

Does some of you already experimented such problems ? And how did they solved it ? I heard those background noises are usually pain in the ass, so, thank you for the ones who will accept to spend time to try to help me.


So, If I understand you correctly you say that these work fine:

power → targus adapter → laptop, and targus adapter → jack output → amplifier
power → laptop , and laptop → jack output → amplifier.

Then, what’s the problem? The other setups seem rather problematic.

Most problably, you are getting some ground loop which is what is causing the extra noise. So not having the ground connected also fixes your problem as you indicated.
The problem with ground loop with analog audio is that the components might interfere with each other as ground is changing.
Sometimes, using two different power sockets, or using the same but changing one plug ( i mean rotating 180 degrees one of them) might improve the situation.

Thank you for your answer JosepMa.
Indeed, it look like a ground loop and it disappear when I’m using 2 different power sockets.
I’ll continue with many power sockets to avoid ground loop but the targus solution seems less and less efficient as it have other problems like detecting the analog microphones : a SM58 connected in jack is not connected, unless I connect / disconnect old cheap microphones many times until, with no special explanation, the analog input suddenly appears in the «Pulse audio» (I’m using Linux) configuration…

Balanced connections would not have this problem.

Thank you for your reply Be0.
My jack output from the laptop is not balanced. How can I make it balanced ? With a simple converter jack to XLR ? I heard balanced signals have inverted phases so, a simple passive converter probably won’t do the job, isn’t it ?

You can’t simply make an unbalanced connection balanced. DI boxes can do that, but for the cost of one you may as well get an audio interface with balanced connections. It does no good to have one side of the connection balanced; both the sending and receiving device need balanced circuitry and the cable needs to have two conductors in addition to the ground. The Essential Guide To Audio Cables for DJs - Digital DJ Tips

This is quite common for audio interfaces built into computer motherboards. External audio interfaces do not have this problem.

These devices might be helpful although I haven’t used them myself:

External audio interfaces with balanced outputs. I do have an old ESI U46DJ that only featured RCA jacks and had the same problem.

External audio interfaces with unbalanced outputs can still have ground hum, yes, but they won’t have the problem of picking up interference from the mouse or keyboard.

I’ve disconnected the earth from 2 different laptop power supplies to fix grounding issues. Technically, one shouldn’t do that, as grounds are included in power supplies for safety reasons. However, the power supply bricks have no exposed metal, not even a screw, and only pose a danger should the brick be cracked open, which would eliminate any design safety, anyway. Many power supplies are not grounded. Are they unsafe? Electrically, the output which plugs into the laptop is DC and not grounded, as well.

I agree that balanced audio outputs are better.

So, if I’m using an USB external soundcard with balanced ouptuts, it may solve my problem, isn’t it ? But at home, my sound amplifiier only got RCA (cinch) in entrance. (No, I won’t use a professional 2000 W RMS sound amplifier in my living room… ^^). So, At a time, I’ll have to convert my balanced signal to an unbalanced one. Will I have to use one more «box» to make this conversion ?

Correct. If you are going into an amplifier that only accepts an unbalanced input, an audio interface with a balanced output will not help. Most studio monitors, active PA speakers, and PA amplifiers except some of the cheapest models have balanced inputs (sometimes they also have unbalanced inputs too for convenience). If you are set on keeping your home theater style amplifier that only accepts unbalanced input, you’d have to use workarounds like devices I linked above.