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Thread: Noise/static problem

  1. #1
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    Noise/static problem

    Hi,

    Still finalizing this new carPC and I am having some problems.

    First, I know this is far from ideal but I am using onboard audio and a pretty cheap 2x2 amp. I've used similar hardware before and never ran in this type of trouble though.

    Anyway, I have 2 problems:

    * static on the line - which I can only really hear when the engine is idling and when there is no music (in between songs)
    * a high pitch noise (pitch is amusingly linked to the engine RPM...). same thing I can mainly hear that in between songs and when the engine is ramping up RPM.

    I think #2 might be due to lack of shielding on my cables?
    what about #1? I am using cheap 3.5mm to RCA cables.

    I made a custom junction cable in between the car harness and the amp. I used 16AWG cable but I just realized I used AL based cable not copper. Could this be my problem?

    the whole PC is installed in the place of the glove box in my Tacoma.


    Any idea? or recommendation on the cables/shielding I should be using?

    thanks

  2. #2
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    forgot to mention that there is a filter switch on my amp and tried using it but it didn't reduce or get rid of the noise/static.

  3. #3
    Neither darque nor pervert DarquePervert's Avatar
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    A few things...

    - The whine is a ground loop. If you're using the ground on the radio harness, that's likely the problem. In my Tacoma, the radio is grounded through the antenna, which is a crappy grounding point. Find another ground, preferably to the body.

    - The cheap 3.5mm-to-RCA cables are a possible cause of the static. It could also come from a number of other sources, though. I suggest connecting another audio source (MP3 player or something) to your amp. If there is still static, then move one step closer to the speakers, although this gets difficult.
    Have you looked in the FAQ yet?
    How about the Wiki?



    Under normal circumstances, a signature would go here.

  4. #4
    North of the land of Hey Huns
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    Cheap 3.5mm to RCA cables are often not shielded properly, if at all. I had noise issues with mine until I replaced them with a decent set ($15 set rather than a $5 set). As Darque said, start eliminating components and try to track down piece by piece where the noise is coming from.
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  5. #5
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    that sounds like a good starting point, thanks guys.

    I spent some time last night making more google searches and found about ground loops. What confuses me though, is that in this project, the mini-ITX aluminum chassis that I am using is attached directly to the metal bracket that is in the area of the glove compartment.

    Basically, it didn't fit inside the glove compartment so I god rid of it and make a couple of brackets out of aluminum to attach the computer chassis directly to the truck's frame (I don't know if that metal part is really attached to the frame but I suppose it is).

    I am getting the ground out of the harness which goes directly to the DC-ATX PSU. the MB is grounded to the computer chassis which is made of aluminum so I think I should have a decent ground here.

    Funny thing, on my infill G4 I didn't have this problem, though I was using the antenna and you said that on Tacoma's the antenna is grounded.

    That makes me realize I kind of let the antenna jack loose in there.




    If you know a place where I can find decent 3.5mm to RCA cables(shielded) with a right angle on the 3.5mm please let me know. I have some clearance issues in this setup and I can't use any standard size straight 3.5mm jacks.

  6. #6
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    Get out your Multimeter, and set it to the Ohm setting. If you have an auto one great, if you have one that you need to select the range on, select something around 0-10ohms. Now touch one end to your ground bolt, and touch the other end to the batteries negative terminal. I try to aim for 1ohm or less for a good ground. If you see to high of a resistance, your ground probably isn't that great. Even if it looks like a good place to ground, if it goes through multiple welds and joints before reaching the battery, it can sometimes be a very poor ground. An easy clue can sometimes be to look around and see where everything else is grounded. Did they ground anything from factory to where you are grounding to, or did they ground everything to the piece of steel 6" away? That can be a clue as to what is the better ground point, before even busting out the multimeter.

    Don't just check the ground of your computer, but also your amps ground, as well as any components you may have attached to the computer. If a component has an easier time grounding itself by going through your computer/audio cables/etc than going through the ground it is connected to, it will do it, so you'll want to check every ground.

    If you have a tough time getting a reasonable resistance reading, you may need to run everything to one central ground distribution, and run a wire back to the battery directly.

  7. #7
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    Yeah well it doesn't seem I have a decent ground. I have no way of measuring it directly between the battery and the carPC because it's just too far. But I checked the resistance between my PSU ground and the metal bracket on which the PC attaches to and it's several KOhm. That seemed weird and after checking I realized that the piece of aluminum I used to machine a support frame for the chassis is actually insulated on each side.

    I have checked around the passenger side area and under the glove compartment and I don't seem to any obvious ground.

    I am guessing the best thing to do would be to run a wire (what gauge btw?) between my ground and the battery or any good ground that would be closer.

    Any idea where I could find a decent ground behind the radio front end? Since I had no problem before (with my old infill G4) should try to connect my PSU ground to the antenna ground?


    Thanks

  8. #8
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    Here's a quick update. Maybe someone will have an idea.

    With the help of a ohmeter I found a few pretty good grounds. I make a hook-up ground cable between one of these points and my ground cable (that is used by both the Amp and PC).

    That didn't fix the problem.

    But let me describe how the computer is physically attached to the car:

    * it's an aluminum mini ITX case. the aluminum body is grounded.
    * I made (out of aluminum also) several parts to attach the PC directly to a metal bracket that is attached to the car's body. I couldn't use the glove compartment which was too small and found this brackets with holes.
    * for some reason I measure 50 Ohm between my ground and these aluminum brackets.

    It doesn't make much sense as they're made out of aluminum but the raw aluminum I used was coated on both sides. So it's possible the coating is insulating these brackets from the ground.

    I still have a hook up ground cable that connects the AMP and PC ground to a pretty good ground, so I don't understand why I still get this alternator humming noise.

    Could the bad ground between computer chassis and body (50Ohm) be a problem? In my mind, current should go wherever the resistance is the smallest (i.e. the hook up cable and not the chassis/bracket/body).

    Any idea?

  9. #9
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    i think i know the problem, are your RCA's and power running close together? sometimes the interference is from that.you should also check anywhere that RCA is running sometimes if u have a neon light or somethinging thats powered going across that RCA it'll produce noise

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