Audio noise problem

Discussion in 'The Projects Forum' started by jeep642, Nov 2, 2012.

  1. jeep642

    Thread Starter New Member

    Nov 2, 2012
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    Hi, I'm new to this forum and after months of searching I figured may be someone with knowledge in electronics might have the answer.

    I'm a film maker and I'm putting together a camera system based on the Canon 5D mark II camera.
    It's a still camera that shoots high definition video.

    The problem is that, by itself, it's far from perfect. The footage is great but sound recording is bad and it's not practical in terms of handling and battery life.
    To make it work well, you need a few things:
    - a sound preamp to feed a signal strong enough to the camera audio input so as not to use the camera pre-amp (the trick is to bring the recording level in the camera to zero + one click that way the built in preamp doesn't affect the sound)
    - an external monitor or viewfinder.
    - one main battery to power the whole thing.

    The problem comes when I try to power everything from the main battery. I use SWADJ3 25w switching regulators to go from the 14-16 volts of the Anton Bauer main battery to go down to 7.2v for the camera, 12v for the monitor and 9v for the external preamp.

    If all the elements are powered separately, everything works fine.
    If I use a common battery for everything, it generates a huge amount of noise.

    It doesn't sound like a ground loop. I used ground loop isolators, no luck.
    The noise fluctuates depending on what the camera does. If it's idle, less noise, if it's computing a lot (recording, using video out), there's more noise.
    Someone told me it could be oscillation because my power source is not stable enough, so I built a circuit based on a LM317 (may be more stable than the SWADJ3) no luck.

    I'm still convinced that I need some kind of circuit that would create two isolated completely independent power sources from one battery but I can't find anything like that on the web and my knowledge in electronic is too limited to be able to come up with one.

    Any ideas would be greatly appreciated.

    Thanks,
    JP
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    What is wrong with the mic preamp in the camera?
    What is wrong with connecting an external mic to the camera?

    Did you connect the output of your preamp to the audio input of the camera with shielded audio cable?

    Post the schematic of your mic preamp so we can see why it is sensitive to power supply noise. Adding a filter is easy.
     
  3. jeep642

    Thread Starter New Member

    Nov 2, 2012
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    The 5D mark II makes great pictures (videos and stills) but it wasn't conceived as a video camera, the preamp is extremely noisy. The fix is to use a preamp that will feed a strong signal to the camera so as to virtually not use the camera's own amp.

    Here is the preamp I used. I will switch to this one (better ergonomic) soon. They both work fine if powered separately and the same noise is generated if they are powered with the same battery as everything else.

    The idea with this rig is to be able to power all of the elements with one single battery. When you shoot news, documentary or fiction, it's very practical to not have to check a whole bunch of batteries constantly (wireless receiver, camera, preamp, monitor...).

    I have attached a recording of the noise:
    @0s: all off
    @2s: preamp on
    @8s: 5D on
    @13s: video out on
    @18s: monitor on
    @24s: 5D recording

    Yes, the output of the preamp is connected to the input of the camera. The cable is shielded but I don't think it would be the problem even if it wasn't since it's about 3 inches long...
     
  4. jeep642

    Thread Starter New Member

    Nov 2, 2012
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    Any suggestions?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    If I see their schematics then I might know why they produce the noise when they use the same power supply.
     
  6. nigelwright7557

    Senior Member

    May 10, 2008
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    71
    Sounds like a classic ground loop to me.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    Deleted reply.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The noise is caused by the digital recording circuits, not from power supply or ground loop mains hum.
     
  9. jeep642

    Thread Starter New Member

    Nov 2, 2012
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    No, it's not a ground loop. I tried that route.

    Thanks though.
     
  10. jeep642

    Thread Starter New Member

    Nov 2, 2012
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    0
    I called the company about the problem.
    First, they had no idea what I was talking about. You're supposed to power their product only with a 9v battery.
    Second, they were not exactly interested or willing to help.

    I don't have the schematics for this preamp, since it's a copyrighted product, they are not available.

    I don't think the schematics would help since the problem is the same with every single mixer/preamp I tried. (like Sound Devices 302, Shure FP 33)
     
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