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Finding good ground to prevent Ground Loop

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  • Finding good ground to prevent Ground Loop

    Today I was working on my car audio system and decided to use a meter to measure all the different ground points in my car. I put my meter into OHMs mode and started measuring between the ground points in my car and the negative post of the battery.

    First I measured the ground point of my headunit and it shows 0.3 ohm.

    Then I measured the ground point of the OPUS and power inverter which shows 0.4 ohm.

    I also measured a few other factory ground points and they are all around 0.3 to 0.5 ohm.

    Are my ground points good enough to not have any ground loop?

    Lastly I compared the resistence between the car's frame metal with paint and without paint. The difference is HUGE. This shows that when making a ground it is VERY important that the painted is being sanded off.
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  • #2
    How accurate is that multimeter? Put it on ohms again, and touch the two leads. Many ohmmeters will read .3 or so ohm even when directly connected.

    ColdPhreze
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    • #3
      I thought ground loop is the cause for engine and electrical noise?

      Anyways I am not too sure how accurate my meter is, but I guess it woudln't be too far off. I believe in order to be a good ground there should be as little resistance as possible.
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      • #4
        Yes, you are correct, ground loop is the cause of most electrical noise. I did a little reading on car audio systems, and they recommend that you should ground everything to the frame, not the body. This is because on vehicles (especially newer ones), body panels no longer have as good of an electrical connection to the vehicle due to glues and only spot-welding of parts.

        ColdPhreze
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        • #5
          spot welds

          i very much doubt the use of "spot welds" are the cause of poor grounding issues... do you know how many amps you can run thru a single weld line? or even the copper wire used during mig welding

          anyways, alot has to do with the way cars are made these days (in that respect, its right, but not cause of using spot welds) and that is cause most cars these days are a monoque chassis, older cars have chassis "rails" which are actually two really large metal beams that run the length of the car in parrallel, and provide an admirable source for grounding, monoque cars dont have these chassis rails, and in my opinion are little more than pressed sheets of metal stuck together at the sills (a sill is where the floor-pan meets the quater panels and stuff, its the little lip under the door, go on, get down on the ground and take a look).

          oh, and spot welding in car manufacturing is no knew invention, it was discovered when welding was first invented that using short weld lines on metal is a must, cause using a full length weld, on sheet metal, over a long distance will warp the two sheets (panels) thru heat, which is the biggest product of welding.

          just a bit of FYI.
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          • #6
            I never said that spot welding is a new invention. What I said (and maybe should have been clearer) was "ONLY spot welding". I understand about not using full length welds, I have done a bit of welding myself. However, the point being made is that due to spot welds, there is less electrical connection between body panels and chassis, therefore more resistance, therefore, higher chance for ground loop.

            I also never said you couldn't run much current through a spot weld. What I said was body panels do not have "as good of an electrical connection", which is true. Granted, a spot weld can handle hugh amounts of current, but because it is a small juncture, it has a higher resistance, thus ground noise can be more prevalent, thus what I said is correct.

            I also know new vehicles do not have a "frame" per say, but actually are pressed metal. The POINT was that it is a solid piece of metal and is, overall, the best connection to ground.

            I also stated that I was reading on car audio systems... That site was trying to give readers the BEST way of grounding their hardware, not simply an adequate method, as many car audio systems are going to pull in excess of 100 amps (mine, including pc, pulls about 150 to 175 Continous amps during moderate to high volume levels, up to 400 amps at peaks (and when I say moderate, realize that is in relation to 1800wrms).

            I didn't think I needed to spell it all out...
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            • #7
              Granted, a spot weld can handle hugh amounts of current, but because it is a small juncture, it has a higher resistance
              This statement contradicts itself. The very idea that somethin can handle huge amounts of current tends to lend itself to lower resistance. If it weren't lower then we would be talking about huge amounts of power loss and that just isn't the case.

              the point being made is that due to spot welds, there is less electrical connection between body panels and chassis, therefore more resistance, therefore, higher chance for ground loop
              This simply isn't correct. You are crazy to think the welds within the body are the cause of the problem and that they can't support the type of current being sent through them. Now, if you were to make the point that the actual connection from the body back to the battery (which doesn't imply going through the welds, which may or may not be the case) then you would be correct.

              fishy was right.

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              • #8
                This magical "good ground" doesn't exist. What you need to do is use the SAME ground for everything. (Well, that doesn't always help but it's about the best bit of advice you'll get).

                Run the ground cables from your headunit, amps, power supplies etc to the same point and you're far less likely to get ground loop.

                This forum really needs a Sticky about ground loop.

                Ground Loop occurs because any audio equipment in a car uses the vehicle ground as a reference point for the AC audio signal. If the ground in one part of the car is different from the ground in another part of the car you have a different reference point for the signal and this manifests itself as your annoying noises in the speakers. The ignition systems of cars often cause different grounds and that's why you hear the buzzing sound changing pitch with the engine.

                Ground Loop Isolators (excellent bits of kit) are actually 1 to 1 transformers. For anyone that's studied electronics you'll know that only AC can pass over a transformer, not DC. That's how the isolator filters the noise back out.

                NB. Don't forget that all audio signals are AC.

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                • #9
                  Originally posted by Engram
                  This magical "good ground" doesn't exist. What you need to do is use the SAME ground for everything. (Well, that doesn't always help but it's about the best bit of advice you'll get).

                  Run the ground cables from your headunit, amps, power supplies etc to the same point and you're far less likely to get ground loop.

                  .
                  I tried running everything to the HU ground, neither that or a Isolator solved my problem.

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                  • #10
                    Since ground loop is caused by difference of resistence, then if I measure different ground points and get similar resistence measurments, therotically the ground points should be good to not cause any ground loop..right?
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                    • #11
                      Originally posted by Matthew230
                      I tried running everything to the HU ground, neither that or a Isolator solved my problem.
                      I'm surprised the Isolator didn't work.

                      Ground Loop is a total B1TCH to get rid off. So glad it hasn't been on a problem on my current car. (Although we'll see what happens when I bung this laptop in!)

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                      • #12
                        Originally posted by lookinco
                        Since ground loop is caused by difference of resistence, then if I measure different ground points and get similar resistence measurments, therotically the ground points should be good to not cause any ground loop..right?
                        No. That's about the most unscientific thing I've ever heard!!

                        Connect the grounds of all your audio equipment to the same place on your chassis. (And if possible make a new ground point).

                        In my car I bolted my ground cable to part of the boot. A short 4 gauge cable leads to a distribution block and everything audio related in the vehicle takes it's ground from that distro block. My headunit is powered by cable that runs to the back off the car from the dashboard.

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                        • #13
                          So would you recommend me to run a big fat cable from the dash to the trunk and ground the headunit and everything together as well as computer and amp? But I thought the ground cable should be as short as possible?

                          I thought in order for a ground to be good there should be as little resistence between the grount point and the negative post of the battery. Let's say i have 2 ground points A and B. When I measure the resistence between battery and point A i get 0.3 ohm, when I measure between point B and battery i get 0.4ohm. When I measure between point A and B, I get around 0.3ohm as well. So would I have ground loop trouble if I hook up headunit to point A and computer to point B.
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                          • #14
                            Ground loops.... What a pain in the ***....

                            I also have a ground loop problem. Fortunately for me I have used a ground loop isolator which has fixed most of my problems. I still have background noise from within the pc though.

                            Originally posted by Engram
                            Ground Loop Isolators (excellent bits of kit) are actually 1 to 1 transformers. For anyone that's studied electronics you'll know that only AC can pass over a transformer, not DC. That's how the isolator filters the noise back out.
                            But just to point out that any coil has a frequency response range and you WILL loose top (treble) and bottom (bass) frequency response (to some degree...). Some people who run top end audio equipment will tell you that an isolator is the worst thing you can do. But I believe that it is a case of horses for courses. I haven't got the 'be all and end all' of audio systems so an isolator suits me fine.

                            In my quest to find my ground loop problem I went to the extent of running my carpc on one battery and my amp on another battery (totally isolated). Everything was fine. I then connected the grounds of the two batteries together which then caused the wretched noise to come back.

                            I am therefore left with one conclusion:- Somewhere there is a ground loop in my pc......

                            NOW WHAT DO I DO???? Back to the isolator.

                            As for resistance testing ground points. What is more interesting is measuring the voltages (usually mV) between ground points.

                            Ground loops.... What a pain in the ***....

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                            • #15
                              You never said what you get when you hold the probes together?

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