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Thread: solving hiss

  1. #1
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    solving hiss

    Hi guys,i have a problem we our sound system in church,when we connect a laptop with the system, there is a hiss ( noise).We have to disconnect the power chord to stop it.
    How can we solve this problem by still using the power chord with the laptop?

    Thank you.

  2. #2
    When you say you have to disconnect the power cord - can you run the laptop on batteries then? To track this down, it's necessary that the computer actually be running.

    If that's the case - if you can run the computer on batteries and it makes no noise - then the likely cause is that noise from the external power supply is leaking into the system. Most computer power supplies use high-frequency switching which creates a lot of electronic noise.

    This could be due to direct radiation of crud from power supply to audio system. Move the power supply and cord around and you may hear the noise increase and decrease. In this case the cure is to physically reposition things so the power supply and cord are far from the audio gear.

    If moving the power supply around does not change things, then you've got a ground loop. Most computer power supplies have grounded electrical plugs. You can take a 3-to-2-wire ground lift adapter and temporarily open the AC ground circuit. If this cures the noise, you've found the problem.

    For safety reasons, you can't leave the system in this condition. That ground pin has to be connected. So you need to break the circuit on the other end; you need to isolate the audio feed. This can be done by inserting an audio isolation transformer between the computer output and the sound system input.

    The people who have this problem most often are car stereo guys, so that's where you're going to find a source of transformers. Radio Shack sells a decent one; if you haven't got a RS nearby you'll have to track one down on the web.

  3. #3
    Something I hadn't thought of - you're not feeding into an input with phantom power on it, are you? That would cause noise.

    The solution then is the same, a transformer, but you need one with a balanced out. The Radio Shack one won't help here, you need a direct box which will be more expensive.

  4. #4

    Dim it

    While its plugged in try dimming the screen on the laptop. LCD screens suck a lot of power and can cause interference from the power caps. The reason it goes away when its unplugged is because of the power saving settings for battery life auto dim the screen.

    Lextone

  5. #5
    Like mentioned in a previous post, the most likely fix for this problem is to purchase a 'ground loop isolator'. A better quality audio cable would make no difference because the problem is being caused before the sound has even been outputted. Sometimes a good surge protector with an RFI filter can help to solve problems. The RFI filter stops fluctuations in the electricity signal and produces a consistent output.

  6. #6
    If you read his post its only when they are on cord power to the laptop. I am going out on a limb, but they probably are feeding into the board via the headphone jack. They can simply dim the screen or purchase a usb sound card.

    If you look at the age of his post and his non reply I think he figured it out.

  7. #7
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    Quote: ThatCable
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    Like mentioned in a previous post, the most likely fix for this problem is to purchase a 'ground loop isolator'. A better quality audio cable would make no difference because the problem is being caused before the sound has even been outputted. Sometimes a good surge protector with an RFI filter can help to solve problems. The RFI filter stops fluctuations in the electricity signal and produces a consistent output.
    Yeah, the PCDI by whirlwind is a great example. takes the headphone output, transformer isolates/balances the left and right signals and sends them out of XLR's to mate properly with mixer inputs.

    I've seen plenty of good productions ruined by poor laptop/PC sound into a board. People figure that if it gets sound at all, it works and is going to be fine, but it can be very nice and clean, they just need to get the right interface/gear.

  8. #8
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    Sorry for not replying earlier,i have used a DI BOX and it works.Dimming the screen increase the noise more.
    thanks everybody.

  9. #9
    Hi Davis, I discovered this forum today and I read your problem is solved usong DI box; I encounter the same problems as you did; can you explain shortly where exactly you put the DI box in the chain? and what kind of connectors you use from...to.... (XLR, jack, RCA,....). Thanks for your reply. Kriskras

  10. #10
    The DI box is put between the output of the computer and the input of the mixer. There's a transformer inside the box. The signal is converted from electricity to magnetism to electricity, so there is no direct electrical connection between one end of the cable and the other. Thus any ground-related problems are eliminated.

    From an interference standpoint, with a DI box it doesn't matter what connectors you use. You may still have to deal with issues of balanced vs. unbalanced, and what is grounded to what inside the cable - you can accidentally short out the signal, for example. But this is a signal issue, not an interference issue. The general answer is, use whatever connectors are required to interface with the equipment you're using.

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