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I have an auxiliary input on my car stereo, which I plugged my laptop into to play music with. It worked fine until I plugged my laptop into an inverter so the battery didn't die. An extremely noticeable hum started coming through the speakers which corresponded to the speed of the engine. Is there any way to prevent this?

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  • \$\begingroup\$ Try an "audio isolation transformer". \$\endgroup\$ – Juancho Jul 6 '12 at 0:10
  • \$\begingroup\$ Probably introducing some loop via the power supply. Can you describe what the power supply to the laptop looks like? Start from the point where you 'plugged your laptop' until it enters the laptop. \$\endgroup\$ – jippie Jul 6 '12 at 20:41
  • \$\begingroup\$ It just has a two-prong plug and cord that goes into a 19V transformer, and the transformer has a cord that terminates to a coaxial connector that plugs into the laptop. Pretty standard. I think this is it. \$\endgroup\$ – jdickson Jul 6 '12 at 20:54
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The hum is caused by ground loop, a common issue with audio equipment that shares inputs with various devices within the same DC circuit (sometimes in more complex floating negative circuits too)

This hum is always there! It is just filtered out by the audio equipment using Ground as a reference point for removing this "noise", always.

This noise can include clicking from indicators(relays and high impedance), speed (caused by high voltage discharge on spark plugs) other mechanical/electrical related equipment.

The reason it starts to "pick up" this noise once you plug your charger into the inverter(which makes things worse because it "amplifies" all the noise) is that because the ground loops over the filtered audio and causes a double refernce which confuses the filters and assuemes this is the correct audio.

You need a simple ground loop isolotor between the inputs. or avoid creating ground loops. ie ( ground creates a full circle via the audio cable)

enter image description here

Just for fun

Professional (or highly addicted) car audio enthusiasts go to crazy extends in trying to reduce noise in audio to be able to amplify the signal into thousands of watts and over 120decidbels. Noise can cause speakers to distorts which technically makes the loudness go away and also damages the speakers core. Some methods include pre amplifiers, special and very expensive alternators-sometimes even several alternators that go through very expensive current stabilizers, and a variety of other highly priced goods. But unless you build something like this.. a simlple 3quid ground loop isolator is your best bet.

enter image description here

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    \$\begingroup\$ Wow, I wouldn't want to live in that guy's neighborhood. The phrase get a life takes on a whole new magnitude. \$\endgroup\$ – Olin Lathrop Jul 17 '12 at 14:48
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    \$\begingroup\$ @Olin - Neighborhood meaning a range of 10 miles/ 16 km I presume. \$\endgroup\$ – stevenvh Jul 17 '12 at 16:35
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No, I don't think the hum has anything to do with the switching of your switching power supply for two reasons:

  • The hum is proportional to engine speed. The switcher wouldn't know nor care what the engine speed is.

  • The hum is in the audible range. Switching frequencies are generally much higher.

What is most likely happening is that the connection to the laptop back thru the power supply causes a ground loop. The audio signal coming out of the laptop is referenced to ground in a different place than the audio amp's input. The difference between the two grounds shows up as signal at the amp.

Such ground differentials are very common in a car. Since this is proportional to engine speed, it is probably the pulses caused from firing the sparkplugs. Each spark plug firing is a multiple-kV event, so a little bit of that causing ground differential between two separate points isn't surprising at all.

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Probably the humming is related with the power supply of your laptop.

Laptop power supplies are SMPSs, that means they switch on and off the transformer in order to regulate the output voltage. This switching is made at a much higher frequency than we have on our outlets (50Hz in Europe or 60Hz in US).

The humming must be generated on the power supply (in this case the frequency should be high) or by the mains itself (in the case the frequency should be low).

In both cases this can be caused for the filtering used on the power supply or by a defective component (usually a capacitor).

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This Hum is caused by the inverter. I use my inverter for my sub causing a light hum. if you look on the back of the inverter case (or you could google it.) it says that it can causes a hum or buzz noise to speakers.

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