Noise, caused by the laptop, perhaps?

Discussion in 'Computing' started by Ellegaard, Feb 22, 2005.

  1. Ellegaard

    Ellegaard Active Member

    Feb 17, 2004
    Central Copenhagen
    Hi guys,

    I have this simple mobile setup which consists of a Dell laptop (slightly old version, I think - 1GHz processor, 256MB RAM and 10GB harddisk, but enough for tracking on the road), a M-Audio FireWire 410 connected via a firewire CardBus adapter and a couple of ADK large diaphragm microphones. Not the most powerful quality setup, but the budget isn't overwhelming either.

    Now, I have a strange problem with noise. Let me just post a couple of examples on what this noise sounds like, with microphones connected and phantom power added:

    And the normalized version (beware!):

    I've discussed this back and forth on a Danish audio discussion board, but noone really has a clue what the exact problem is. The thing is, when using a dynamic microphone (the Shure SM58), it is absolutely clean. This noise only appears on condenser microphones.

    Strangely, the noise is not present when using the same soundcard on my home PC with the same microphones!

    So somehow it must be related to the laptop, or perhaps the CardBus interface. The noise is also present when using external pres connected directly to the FireWire 410, strangely, although not to the same extent. Someone also pointed out the noise was especially present at 50hz and 100hz, which is the speed of our alternating current for power...

    Have any of you ever seen or heard anything like this before? I'm not technical enough to be able to conclude anything, but I wish there was something that could be done about it.
  2. DavidSpearritt

    DavidSpearritt Well-Known Member

    Jan 9, 2005
    Brisbane, Australia
    Home Page:
    Laptops have god-awful switch mode psu's which usually cause problems. Can you run off batteries?
  3. Ellegaard

    Ellegaard Active Member

    Feb 17, 2004
    Central Copenhagen
    Yeah, for a limited period. Not the most powerful batteries in this laptop, and I think recording drains a lot of power. But perhaps I should try and see if it gets better if I do so. Anyway, I thought balanced cables were pretty resistant to that kind of stuff...
  4. ghellquist

    ghellquist Member

    May 25, 2004
    Hi ellegard,
    mightily annoying noise.

    I can only do guesses here, but there are probably a few things you can try.

    First, I would expect the system to not be grounded. Generally laptop power supply units does not carry ground over to the laptop side. So find something grounded, say a water tap in the kitchen and draw a cable from there. Put the cable to the enclosure of the 410 and see if it makes a difference.

    Secondly, the laptop or the power supply unit could be radiating strong radio signals that are somehow picked up. Try moving things around and see what difference it makes.

    Maybe you could try with a different firewire cable, you cannot really be sure what helps.

  5. MasonMedia

    MasonMedia Guest

    Hi Ellegard,

    It sounds like interference from a switching power supply being coupled into the analog front end of the A-D. By chance when you are out in the field, is the Motu interface placed under the PC?
    If so, try separating these units to see if it goes away.

    Just a thought.

  6. Ellegaard

    Ellegaard Active Member

    Feb 17, 2004
    Central Copenhagen
    Gunnar, thanks for your suggestions. I will try your water tap tip and see if it helps. Moving the FireWire unit around doesn't make any difference - same noise everywhere, and it's regardless if I'm at home, in the conservatory, or at any other remote place. I think I can exclude the firewire cable since the same cable works fine on my home computer.

    Peter, I guess you're right about power interference in the AD converter (except this happens to be a M-Audio unit). I usually keep the laptop and the firewire interface apart. I will try, as suggested earlier, to record while running off batteries. If that solves the problem, I might be able to get rid of the noise by buying a longer firewire cable and place the laptop and the M-Audio well apart...
  7. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Dec 12, 2001
    Oberlin, OH
    Home Page:
    I have not had the 410 apart but I think it uses a switching type of power supply to keep the weight down. The power supply has to provide +/- 15 volts for the opamps plus 48 volts for the phantom. It could be a bad supply. One good way to check is to hold an AM radio tuned off a station close to the 410 and see if it is putting out a lot of hash. If it is the closer you come to it the more noise the radio will make. Not sure about grounding to the water pipe since in some communities there is a rule that you have to have a piece of plastic pipe in between the street feed and the rest of the house so their is no glavantic action happening to the pipes. The other problem could be the computer itself putting out a lot of hash. Again check with an AM radio (some countries call AM Long wave if that makes a difference)

    As to battery power. Most computers run off 12 volts DC which can be taken from a motorcycle battery. I made up a small case that has a gell cell motorcycle battery (actually a lawn mower battery) and a connector for my computer. I also have a small charger that goes into the case. I can run my laptop for about 8 hours on the external battery without problems if I am in the field. Maybe this would work for you as well.

    Best of luck!
  8. sigir

    sigir Guest

    I had the same noise with another, like old vynils. The answer is very simple : unplug the ground of the DC supply of the computer.

    To be sure, I asked the builder of my laptop, he told me that there is no problem to do that.
  9. anonymous

    anonymous Guests

    Feb 10, 2001

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