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I' trying to record guitar using line-in. But there's a noticeable noise in the recordings, and it is present even when guitar is unplugged (only cable is connected).

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As you can see its clearly mains hum. But what I think is strange is that the amplitude of the hum kind of depends on how close I am to the cable.

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At 2 seconds mark i moved away from the cable, then came back at around 20 sec, at 26.5 I laid my hand on the cable (on isolation, not on the plug), which resulted in that huge amplitude change (amplitude had been adjusted by me to show it more clearly).

Is there any way to fix this so I can record my guitar wihtout noise?

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  • \$\begingroup\$ If you are using a front plug on your PC consider switching to the one at the back which probably has less wire (or no wire) from sound chip to plug. \$\endgroup\$ – Octopus Feb 26 '16 at 22:20
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Your body and the cable act as a (poor) antenna that helps get more of the 60hz EMI into the line in amplifier. Assuming you're using the stock line in on the PC, the line in circuitry isn't very good and doesn't have as much 60hz rejection as a higher end system would. Unfortunately, 60hz is all around and usually a great deal of effort is put into the design of studios and studio equipment to minimize it. For more consumer oriented products, perhaps a higher quality sound card would provide better results.

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  • \$\begingroup\$ It's 50hz BTW... \$\endgroup\$ – PlasmaHH Feb 26 '16 at 22:11
  • \$\begingroup\$ So shielding the cable would help? \$\endgroup\$ – Lugi Feb 26 '16 at 22:11
  • \$\begingroup\$ whoops, 50Hz. And a higher quality cable might help, only one way to find out. \$\endgroup\$ – Brendan Simpson Feb 26 '16 at 22:13
  • \$\begingroup\$ A 'higher quality cable' isn´t going to do anything. And the guitar cable should already be shielded. The only solution is to unplug the cable from your sound card/amplifier, etc, not the guitar. Are you using an amplifier, or going straight into a card or preamp? All decent instrument amplifiers will use an input jack that earths the input when the jack is removed, thus avoiding this sort of problem. \$\endgroup\$ – F. Bloggs Feb 28 '16 at 15:29
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An unconnected cable is a large antenna with indefinite source impedance (= your unconnected guitar jack). A cable connected to a guitar is a large antenna with a fairly high input impedance, because your guitars pickups have far more than a "line" source. (typical values are "Line": 5...47 kΩ; guitar pickup: 100...1000 kΩ)

Noise immunity improves with low source impedance values, so you end up with a somewhat conflicting situation. Will the hum become low when you short the open end of your cable, using a paper clip or some other piece of metal?

Also, becuase guitar pickups have such a high impedance, a guitar feels happy when you run it into an amplifer that has a high input impedance (say: 1 MΩ). Using it with a "line" input and its low impedance will likely ruin the sound because the line input is too much load for the guitar pickup and the reactive parts of the impedance will form a filter with the low-Z line input.

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