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I have two 5.1 speaker sets, one in the front and one in the back of my living room. I have connected them together with an "audio wire" (non-shielded unfortunately) that's 12 meters in length.

Since I did this, the speakers started buzzing/humming when turned on. This however stops when I connect the audio wire (through an Y splitter) to my laptop. This got me curious: What happens if I connect the COMM wire on from the Y audio split to the COMM on my laptop charger? The buzzing/humming stops!

There exists some circuit in the laptop charger that is being able to cancel/even out the noise.

Why does the humming/buzzing happen? And Why does it stop when I connect audio COMM to charger COMM?

Can I fix the buzzing/humming noise by making some circuit using passive components I have lying around from DYI kits (Arduino etc)?

This is how the setup looks like in diagram form:

enter image description here

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The reason for the humming noise is improper grounding. Audio signals are low voltage level AC signals (over simplified). An audio signal could be for example 1V, 1 KHz signal. When there is improper grounding, a low level noise signal example 10mV 50Hz can get coupled to the audio signal causing humming. (Values are made up)

I would suggest installing shielded cabling which will provide good grounding, that essentially is the passive circuit that you are seeking.

An audio signal at specific frequency with improper grounding (with noise) would look something like the signal shown below.

Audio Signal with Noise

enter image description here

An audio signal at specific frequency with proper grounding (without noise) would look something like the signal again shown below.

Noiseless Audio Signal

enter image description here

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  • $\begingroup$ Out of curiosity: Do you maybe know what could be in that laptop charger that makes the noise go away (what kind of circuit / filter)? If the cables really do pick-up other signals, then shouldn't leaving L and R channels just connected between the speakers also create noise? (so: not connected to an audio output) $\endgroup$ – Gizmo Jun 5 '20 at 7:54
  • $\begingroup$ Some or most modern laptop chargers have a passive filter built into the design. It is difficult to explain details in a comment section. If you post this as new question and linking to this question I can provide a detail answer. $\endgroup$ – Mahendra Gunawardena Jun 5 '20 at 11:28
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That long audio cable is acting like an aerial.

Does it run close to any mains wires - in the wall perhaps?

You will be better off getting a quality shielded audio cable as a first step. If that does not work then you have to consider some filtering.

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As mentioned the cable is acting like an aerial. A ferrite core to wrap the cable around a couple of times might help. The comm to the charger comm is grounding it with less resistance as the other answer said.

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    $\begingroup$ This should perhaps be a comment on one (or both) of the other answers, since it doesn't add any new information? $\endgroup$ – Jonathan R Swift Jun 3 '20 at 17:03

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