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I'm part of a church and we are looking to record sermons using a laptop and our sound desk. Everything works fine until we connect the laptop to mains power. Searching on Google I found this post which explains the problem is to do with some sort of voltage issue. Is there anything except for running the laptop off batteries that could fix the problem? I am connecting using a standard RCA to 3.5" cable running from the monitor out on the sound desk to the microphone connector on the laptop.

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That's a well-known problem.

First of all, you shouldn't be using the microphone connector on the laptop. These inputs are usually not only particularly susceptible to buzzing noises, but also to all kinds of distortion and aliasing. Use an external audio interface, there are very affordable USB ones available.

Using an interface with XLR inputs, connected to the mixer with balanced (!) microphone or TRS cables, the problem will probably be solved already. If not or there are no balanced outputs available on the mixer, you can use a DI-box to both balance and ground-lift an unbalanced pair of outputs, or you can try to use an unbalanced connection directly (some interfaces somehow manage to get even that working with hardly any of the buzzing).

Of course, it would be better to fix the problem right at the source, which is the laptop's switching power supply. I tried this once. The main trouble would appear to be that the ground connection runs straight past the switching circuitry,

                             ╭┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄╮          
      Phase ─────────────────┆──Switching circ.──┆────────────────── +
   Mains                     ┆   Power supply    ┆                       Laptop
     Ground ─────────────────┆─────────┴─────────┆────────────────── Ground
                             ╰┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄╯

there picking up a lot of the transients. This can be avoided by putting the power supply in a "loop", like

                             ╭┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄╮      
                  ┌──────────┆──Switching circ.──┆───────────┐
                  │          ┆   Power supply    ┆           │
                  │  ┌───────┆─────────┴─────────┆───        │
                  │  │       ╰┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄╯           │                       
                  │  │                                       │
                  │  │                                       │
      Phase ──────┘  │                                       └────────── +
   Mains             │                                                      Laptop
     Ground ─────────┴────────────────────────────────────────────────── Ground

which does however not quite suffice, some noise still seems to be introduced to the ground loop back from the DC output. This should however be fixable with a fat inductor in that line, but I haven't tried that yet.

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    Thanks for the info. We are using balanced microphones to the sound desk. The only output option we have is the RCA connection. The Balanced outputs are being used to go to the amp/speakers and to the foldback speakers. The only other option is the headphone jack which is a 3.5" jack. I have tried that one and it still produces the buzzing and a fair bit of distortion. The RCA gives a really good recording when the laptop isn't plugged in. We have a spare DI box so I will give that a try before mucking around with power supplies.
    – Greg
    Jul 8, 2012 at 20:43
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    The DI box seems to have fixed the problem for us
    – Greg
    Jul 9, 2012 at 4:20
  • Had the same problem on a different sound desk and discovered that it had a USB port which we could both play through and record from. It means we don't need the DI Box
    – Greg
    Dec 4, 2012 at 22:20
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Everything that user leftaroundabout said is correct. Simpler solution: try putting a three prong to two prong adapter on the laptops power supply. This should break the ground loop that is causing the noise.

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  • Right; however I would hesitate to recommend the two-prong fix. It should indeed work ok for the OP's problem, but can in general cause troubles ranging from increased noise over device failure to electrocution. Unlikely of course, but still...
    – leftaroundabout
    Jul 8, 2012 at 21:04
  • Thanks @JPollock for the suggestion, but the spare DI box seems to have fixed the problem
    – Greg
    Jul 9, 2012 at 4:19
  • @greg, glad to hear the noise is gone.
    – JPollock
    Jul 10, 2012 at 4:19
  • I would not follow this particular tip as it's bypassing the earth which is one of the protection mechanisms against electrical shock! Instead, use a DI which will have the same effect but at signal rather mains levels of voltage!
    – back_ache
    Mar 18, 2014 at 12:47

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