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On a recent road trip we wanted to have a laptop to play movies while on the road. We brought an Apple MacBook Air, a power inverter (to plug in the laptop's charger into the cigarette lighter), and a 3.5mm audio cable to connect the laptop's audio out to the car's audio in.

We noticed that when we played a movie (and thus audio), this "cycle" created a lot of noise in the car that nearly drowned out the movie's own audio. You could hear the movie, but only just. You had to crank the volume way, way up to hear much (but then the noise was too loud as well). The noise wasn't "static" noise per se - more like a deep hum.

However, when we unplugged the power inverter, the audio played normally!

My questions are:

  • What was it about this cycle that caused this problem?

  • And how can I avoid this problem?


More details:

  • Car: 2014 BMW 3-Series
  • Power inverter: Plugs into the car's cigarette lighter (in the center console) and has a 110-120V plug (and some other unused connectors, such as USB, etc.)
  • Laptop: Apple MacBook Air (2012 probably)
  • Audio cable: Normal male-to-male 3.5mm cable
  • I'm no electrical expert, but I'm betting the inverter-to-power supply is causing a small feedback loop or something. You could try a different power inverter, or just use a different device to plug into your head unit besides the computer (like a smart phone). – Pᴀᴜʟsᴛᴇʀ2 Sep 1 '15 at 0:05
  • @Paulster2 yeah a feedback loop is what I figured. However, the "requirement" here is that I'm able to keep the laptop charged and thus plugged in to AC power (playing movies uses the battery quickly), and to feed audio from the laptop to the car. The only audio output on the laptop is the 3.5mm jack, though the car accepts 3.5mm, Bluetooth, and USB. – Eilon Sep 1 '15 at 2:39
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Your problem is probably a ground loop. This means that the ground path from the laptop to radio has less resistance than the ground path from the laptop to the cigarette liter. This causes the laptop to try and ground through the audio cord which causes the problem.

This can be caused by a bad/poor inverter or by a bad/poor power supply. The suggested solution is a ground loop isloator. Something like this our similar would work

http://www.ebay.com/itm/like/151561106209?ul_noapp=true&chn=ps&lpid=82

  • Nice answer ... confirms what I was thinking (in my non-electrified brain :-) +1 – Pᴀᴜʟsᴛᴇʀ2 Sep 1 '15 at 18:25

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