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Thread: Poor audio - signal noise. Need a little help

  1. #1
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    Poor audio - signal noise. Need a little help

    About 3 years ago, I bought a Carnetix P1900 power supply, a Xenarc fold-out 7" touch screen and a second-hand Mac Mini to fit into my VW camper van. Finally, having let it all collect dust for all that time, I have fitted it!

    I originally set it in the house, running it off an old leisure battery (as in this pic: http://www.flickr.com/photos/ianlloyd/4046770024/). Once I was happy with it, I set about installing it.

    Everything is working, but there is a lot of noise coming from the audio cable plugged into the back of the Mini. I have unplugged it and then placed the tip of the cable against the Carnetix supply, and the noise is apparently from there.

    The weird thing is that in all the time I was bench testing it, I didn't notice *any* noise while running the audio through a set of desktop computer speakers. In essence, the only thing that has changed is that now I'm running the audio into the line-in on the car stereo.

    So I'm after some advice about what to look for here. Could I have dislodged something along the way? I have read that it might be a grounding thing. I did have the power for the Carnetix properly grounded ... however, if that noise is because the audio cable is acting as a ground, then presumably an easy test would be to remove the cable that it's supposed to be grounding through off the bodywork. If the Carnetix is still powered ... audio cable to blame, possibly.

    Does that make sense? Or should I be looking elsewhere for this?

    All ideas appreciated. Am trying to avoid pulling it all out again - very tight space and fiddly - without getting a pointer or two first.

    Thanks all

  2. #2
    Variable Bitrate natedawgg's Avatar
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    it is almost certainly a grounding issue. I have this same problem in my car at the moment, and the only way i could rid the noise was through an in-line filter. As best i could tell the reason that you werent experiencing it while it was outside of the vehicle, is because it was not all being powered directly from the source. for example:
    In my car i have a carnetix p1900, mac mini, amp, and a factory amp.
    when i hook the mini up to the stereo i get noise. Now if i hook my iphone up i get no noise whatsoever. However, if i have my iphone hooked up and then plug it in to a power charger in my car, i get noise again. essentially when the iphone was running separate from the rest of the systems power/ground circuit there was no noise. In my case i am skeptical of the p1900's ground somewhere but have been unable to locate a good enough one to eliminate the noise.

    i have read several articles on the best grounding practices. Some say to ground everything in the same spot others say ground independently of each other. I would try both and see if you can come up with anything better.

    good luck!
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  3. #3
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    Quote Originally Posted by natedawgg View Post
    it is almost certainly a grounding issue. I have this same problem in my car at the moment, and the only way i could rid the noise was through an in-line filter. As best i could tell the reason that you werent experiencing it while it was outside of the vehicle, is because it was not all being powered directly from the source.
    When it was bench tested, the screen and Mac were both powered off the P1900, via 12v ... but of course the audio was going through a different source.

    Anyway, I just spent an hour and a half pulling everything out and trying to locate the fault. Now I just have an explosion of wires again ... and no solution yet!

    The noise that I get i very loud. If the vehicle were moving, it would cause all sorts of noise as if I wiggle the audio cable in the line in on the stereo, it pops and rumbles.

    I worked out that if I removed *any* of the connection points below, the problem went:

    Line in on stereo to Mac headphone out
    Mac monitor (DVI) out to Xenarc monitor in
    Xenarc power cord to Carnetix P1900 power out
    P1900 power in to 12v supply

    Of course, removing any of these means it won't work either ;-)

    I've taken some video which I'll upload and link to soon. Will hopefully explain just how bad we're talking about here!

  4. #4
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    OK, video time!

    This is the first video I took. Everything is still plumbed in and nicely tucked away at this point:

    [media]http://vimeo.com/7424460[/media]

    Then I started up the Mac Mini so you can hear the noise coming from when it's running:

    [media]http://vimeo.com/7424661[/media]

    Finally, I pulled it all back out again and tried to trace source of problem (no joy yet). This one is the one I'd really like some ideas about. It seems like the cause of the noise is channeling all the way through the car stereo head unit from the line in via the touchscreen monitor and and then on to the power supply. The video shows what I mean.

    [media]http://vimeo.com/7424695[/media]

    I would really like some help on this. I thought I finally had it done. It was looking good, powering up, everything fit ... but the noise/feedback makes it unusable.

  5. #5
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    Yes this could potentially be a grounding issue. What you want to make sure is that if whether the grounds are all in one spot or all over that they are actually grounded to the frame and that any paint primer or other material has been stripped away. Essentially you want to tap into bare metal. Now in some cars there are metal frames in the dash or under the steering column but those are not actually part of the chassis and alhtough they do act as ground they aren't always the best places. Also try using what I call a star washer. basically its just a washer that causes your ground terminals to stick into the metal better. Now the 2nd issue it could be is that the aux cable you have ran from your radio to the mac mini is possibly run too close to either a ground or a power cable. Just like with RCA's when installing amps you need to keep that wire away from power cables as you can get noise of power running thru the cable. Most of the time this is obvious when you accelerate and is called "alternator whine" but its not always the case. Another idea is start with the basics try a new aux cable. Then if your able try a different radio. I know its time consuming and isn't always the easiest but my theory is always check not only the wiring but the devices as well.It would surprise me if its the mac mini but I would def. check the radio and aux cable and go from there. Let me know if any of this helps.

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