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Thread: Noise fix for ATX motherboard

  1. #1
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    Noise fix for ATX motherboard

    I have an ATX motherboard and I'm using a modified sine wave inverter to run the system. I'm getting a considerable amount of noise and I've narrowed it down to the inverter. I've read other discussions and tried the things suggested like "Ground Loop Isolators" but have not succeeded in eliminating the noise. I get a slight shock from the computer case when I touch it and the car at the same time so I am hesitant to ground the system. I've also looked at the DC-DC converters that are out there and they seem to be all for the AT motherboards. Does anyone have any suggestions as to how I can eliminate the noise? Or where I can find out how to make/buy a DC-DC converter for an ATX motherboard?

  2. #2
    Variable Bitrate
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    My suggestion would be for you to ground the case to the vehicle's frame. You can do so with a fuse inline or a resistor if you're worried the case may be "live", but chances are it's not.

    My company is working on a 12V input, multiple output DC-DC converter that is fully ATX compatible at the moment, but it won't be available for commercial sale for a couple of months I'll keep you posted...

    All you need for ATX is a +3.3V output in addition to +5V from your power supply. You could use one of the currently available supplies and add the +3.3V output to it, or build your own from scratch. Maxim ( http://www.maxim-ic.com ) and National Semiconductor ( http://www.maxsemi.com ) are excellent sources for switchmode DC-DC converter components.

    You will also need to watch out for the +12V line -- I don't recommend feeding the car's 12V directly to the motherboard & peripherals like many people do, or using a linear regulator (i.e. LM7812) -- the car's 12V can fluctuate from 8-16VDC and is very noisy to a computer. Look into building a flyback converter to convert 8-16V into steady, regulated +12V.

    You will need to be able to solder and follow schematics to build a supply though... they don't offer kits or any details other than schematics and electronics equations for figuring resistor values, etc.

    DC-DC converters are great because they usually operate at several kHz and therefore don't introduce any audible noise into the audio path.


    --Jason
    http://jump.to/m2pc
    Jason Johnson
    Yorba Linda, California
    http://www.m2pc.com

    MPC Phase IV - *** PENDING ***

  3. #3
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    I agree with Jason that you should ground the case to the vehicle. The slight shock you are feeling is probably just the two pieces trying to equalize out their loose electrons. Link them with a wire an they wont have to use your body for that.

  4. #4
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    Thanks for the tips Jason...I'll first try grounding the system and see how that works. Do you recommend a good contact point on the computer? I was thinking the motherboard mounting screws or something near the sound card to give everything a common reference point. The ideal thing would be to develop the DC-DC converter since it's stupid to go from DC to AC and back to DC...I checked out the specs on http://www.jarcom.com/inmotion/power.htm and I figure if I use the 3.3v Maxim Step Down chip I could create the correct voltage for the ATX but I already bought the DC-AC inverter so hopefully grounding it will do the trick! I'll let you know! Thanks for the tips!

  5. #5
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    Update:
    I tried grounding the computer and that reduced the noise from a loud buzz to a less annoying buzz. I also noticed when I touched the wire to the car I get a little spark. I'm using a Whistler PP300AC (300 watt DC-AC inverter)...does anyone else have one and seen similar problems?

  6. #6
    Variable Bitrate
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    I would recommend grounding the case near the power supply. A good way to do this is to use crimp connectors and find a screw on the case and run the ground wire to somewhere on the car where a good ground is. If you can't find a good ground, you can ground it to the trunk latch (unfasten one of the bolts a little and use another crimp connector) or drill a hole in something metal and fasten it with a screw there.

    Yes, doing DC->AC->DC is stupid; a DC-DC supply should draw 2 or 3 amps instead of the 5 or 6 drawn by an inverter. Plus power supply noise wouldn't be a factor anymore.

    If you're still getting noise you could try 1:1 isolation transformers on the output from the sound card to the amplifier or even try bypassing capacitors across the DC input to the inverter. I've even heard of people installing an AC line filter across the output of the inverter.

    --Jason
    http://jump.to/m2pc


    Jason Johnson
    Yorba Linda, California
    http://www.m2pc.com

    MPC Phase IV - *** PENDING ***

  7. #7
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    I'm also waiting on a DC->DC converter. Until I get it, I've been using an inverter w/ an inline RCA ground loop inductor for the audio and a power strip plugged into the inverter. Why a power strip? Well it was $2 at a show and acts as a pretty good filter.

  8. #8
    Variable Bitrate
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    I also get shocked. My system is running a p-75 and Im using my friend power-to-go inverter. I just started getting engine noise, Ill try to ground it. Is it totally safe for the alternator and/or battery?

    Thanks

    Josh

  9. #9
    Variable Bitrate
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    Grounding the case shouldn't cause any problems with your alternator or battery. If anything, there is some sort of voltage potential developing in the case of your car PC, and grounding the case will eliminate that potential. Some audio equipment (i.e. head unit, amp, etc.) may not like the case being live, and grounding serves to neutralize whatever stray voltage you have.


    --Jason
    http://jump.to/m2pc
    Jason Johnson
    Yorba Linda, California
    http://www.m2pc.com

    MPC Phase IV - *** PENDING ***

  10. #10
    Low Bitrate JesTa784's Avatar
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    jjohnson... what is a "flyback converter"? I've been using a linear regulator and it seems that i get about 10v of power instead of a 12 i need. Can you point me to some sites or datasheets? thanks!

    Also, I'm using the max787 and it seems that my hd keeps on spinnig up and down. I have a mobo w/ onboard sound and video and just one hd. I'm using a mmx cpu so it should take less current. Any ideas what may be causing the hd failure?

    Oh, and i used a normal high filter inductor instead of the toroid i see people using, could this be the problem?

    thanks for you time

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