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Thread: Ground loop with an external USB card?? Help! (LONG)

  1. #11
    Constant Bitrate accentsound's Avatar
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    There is a problem we come across every once in a while. Blown ground or grounds on the head units. What leads me to believe you may have this problem is that you get a loud hum when both your RCA plugs are not connected.

    Blown grounds on source units are caused when the power grounding on the amplifier is poor or gets interupted for some reason. At this point the amp gets it's power ground through the RCA sheilds and the source unit. Now since the current draw from the amp can be hundreds of amps the foil on the printed circuit boards in the source unit can not take that kind of load and what happens is that the copper foil on the pc board of the source unit burns and creates a open circuit. If the RCA outs on the source unit have seperate paths to ground on the Printed circuit board then lift one or the other and you get a loud hum. If both are blown you get low output or very loud hum or whine, all depending on what ground paths are left available.

    You can tes this by using a multimeter and checking for continuity first between the grounds on the RCA jacks on the source unit. You should get a short circuit indication. Next check the continuity of each RCA jack ground to a known good chasis ground. Even if the RCA ground on the source unit is isolated from chasis ground most amplifier audio grounds are not. Again you should have a near short circuit indication between RCA and chasis ground. If you do not have this indication in any of these tests it means that the grounds on the source unit have opened up and that needs to be corrected.

    Another way to test this is to use a different source unit, such as a walk man or such. If the wnine goes away then the problem is in the sound card.

    If you find that the grounds are blown then before replacing or repairing the unit, you need to thoroughly check the ground path from the amp to the battery and make sure that it is and remains a solid connection.

  2. #12
    Variable Bitrate gameboy's Avatar
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    on my system for what ever reason, when i went from 8 AWG to 2 AWG for the main power to the trunk, i got rid of my alt wine,
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  3. #13
    Constant Bitrate
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    Quote Originally Posted by accentsound
    There is a problem we come across every once in a while. Blown ground or grounds on the head units. What leads me to believe you may have this problem is that you get a loud hum when both your RCA plugs are not connected.

    Blown grounds on source units are caused when the power grounding on the amplifier is poor or gets interupted for some reason. At this point the amp gets it's power ground through the RCA sheilds and the source unit. Now since the current draw from the amp can be hundreds of amps the foil on the printed circuit boards in the source unit can not take that kind of load and what happens is that the copper foil on the pc board of the source unit burns and creates a open circuit. If the RCA outs on the source unit have seperate paths to ground on the Printed circuit board then lift one or the other and you get a loud hum. If both are blown you get low output or very loud hum or whine, all depending on what ground paths are left available.

    If you find that the grounds are blown then before replacing or repairing the unit, you need to thoroughly check the ground path from the amp to the battery and make sure that it is and remains a solid connection.
    I think you may be on to something here. Today I finally got a chance to do more testing. I had unscrewed the amp from the carpeted trunk floor, and I took the PC off as well to try to eliminate the mounts as the ground problem. Noise still there. I reassembled, using rubber washers whenever I could - noise still there. So, I did the headphone test and - I cannot hear the whine. It is tough with the headphones because at any given time I can only get center/sub, front l/r, or rear l/r, so it is not as "loud", but still, I think it was gone.

    Then I plugged the MP3 walkman in again. Again - it was gone....but wait, maybe not! Seems like I can faintly hear the noise in this scenario....again, it is harder to tell because of the whole 2/channel limitation, but it does seem like it is there.

    If so, does that mean that basically a) the amp ground is bad or b) it has the short condition you mentioned? It seems to work "fine", but the very first time I turned it on the amp shop which had installed it hadn't connected the ground (they didn't tell me, they were waiting for me to decide where exactly I was going to screw the amp down). Not good, but I powered off and got it connected securely to an unpainted portion of the chassis.

    Would most custom/high-end sound shops have the piece of equipment you mentioned? I know most of the Circuit City-type shops say their multimeters can't help them with this. I am ready to pay a shop to tell me exactly how to fix this.......everything else works beautifully but this high-pitched whine is killing my hearing.

    Thanks for this very valuable info!

  4. #14
    Low Bitrate Daley's Avatar
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    I used to work for a high-end audio shop, and therefore have seen my fair share of customers coming in with the dreaded "alternator whine". From a very basic standpoint, first ensure that the power and ground cables to the amplifier are of adequate capacity. With an amp if this size, you should be using *at least* an 8 AWG conductor on *both* power and ground. They should both have good connections at each endpoint, and the power wire should not be run near any vehicle wiring, if at all possible. If I remember correctly, your 300 has the battery in the trunk, no? This would obviously work to your advantage here. Also, try to avoid running the signal cables (RCA's, RCA-to-1/8" adaptor) near any vehicle wiring. Your 2-channel tests make all the sense in the world - it's hard to determine how much whine you've got when you have only 33% of the system running. Although I've never worked with the Audiobahn equipment, I don't know that it's known for sound quality - it may not have sufficient noise-cancelling and grounding as the higher-end amps do.

    FWIW, I have *never* had to use ground-loop isolators, noise-filters, etc. to rectify a problem - IMHO, they're merely band-aids for the underlying problem. Start with the basics - maybe plug the Audigy into a laptop running on battery. If that's not an option, maybe run it off a PC that's not in any way attached to the car. Maybe pick up a couple 1/8" stereo Y-adaptors so that you can get all six channels plugged in to one device.

    In short, if it were mine to do, I think I'd try swapping out components and substituting things one at a time until the problem goes away (after ensuring the power/RCA requirements first). You seem intelligent enough to tackle this - don't let it get the best of you.

    Alternator whine is a pain in the *** to find in the first place, and the complexity added with a PC just exacerbates the whole scenario.
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  5. #15
    Constant Bitrate
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    Quote Originally Posted by Daley
    I used to work for a high-end audio shop, and therefore have seen my fair share of customers coming in with the dreaded "alternator whine". From a very basic standpoint, first ensure that the power and ground cables to the amplifier are of adequate capacity. With an amp if this size, you should be using *at least* an 8 AWG conductor on *both* power and ground. They should both have good connections at each endpoint, and the power wire should not be run near any vehicle wiring, if at all possible. If I remember correctly, your 300 has the battery in the trunk, no? This would obviously work to your advantage here. Also, try to avoid running the signal cables (RCA's, RCA-to-1/8" adaptor) near any vehicle wiring. Your 2-channel tests make all the sense in the world - it's hard to determine how much whine you've got when you have only 33% of the system running. Although I've never worked with the Audiobahn equipment, I don't know that it's known for sound quality - it may not have sufficient noise-cancelling and grounding as the higher-end amps do.

    FWIW, I have *never* had to use ground-loop isolators, noise-filters, etc. to rectify a problem - IMHO, they're merely band-aids for the underlying problem. Start with the basics - maybe plug the Audigy into a laptop running on battery. If that's not an option, maybe run it off a PC that's not in any way attached to the car. Maybe pick up a couple 1/8" stereo Y-adaptors so that you can get all six channels plugged in to one device.

    In short, if it were mine to do, I think I'd try swapping out components and substituting things one at a time until the problem goes away (after ensuring the power/RCA requirements first). You seem intelligent enough to tackle this - don't let it get the best of you.

    Alternator whine is a pain in the *** to find in the first place, and the complexity added with a PC just exacerbates the whole scenario.
    Sorry I haven't updated this thread in a while. The advice has been helpful, and I'll be buying a multimeter today. I actually took the car back to the Circuit City that did the amp install (they have been very, very cool, but although they get a few car-Xbox installs, a carpc is definitely a step for difficult for them). The guy spent several hours tracking every wire and seperating the signal wires from the power to run tests, and the whine is still there. The gauge is fine, the grounds seem good, but it's still there. I think it must be the amp, so I am going to buy a new one, I guess. If it isn't that, I don't know what to do...

  6. #16
    Low Bitrate VehicrossPC's Avatar
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    I had a ground loop problem with my pci card that wouldn't be fixed by this expensive-azz ground loop isolator from some sound shop.. anyway, I went to radio shack and tried there's.. and it worked... this thing doesn't even have to be grounded to the car.. not sure if it has a resistor or what but it fixed the prob. Hope thismight help
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  7. #17
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    Thanks, but it was actually the Radio Shack model I used that didn't fix my problem. FWIW, I've now replaced the amp and again disconnected everything from the setup except the PC and the RCAs to the amp and I still get whine. It doesn't occur when the RCAs are unplugged from the amp, so I guess it isn't the wiring harness anywhere.... Looks like it is my PC, and I've got a 10 gauge ground wire running from one of the screws on the back to the same point as where the amp grounds - helped quite a bit but STILL have the whine.

    Grrrr....I am about to say fuggedaboutit.....

  8. #18
    Variable Bitrate gameboy's Avatar
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    Quote Originally Posted by vettejock99
    Thanks, but it was actually the Radio Shack model I used that didn't fix my problem. FWIW, I've now replaced the amp and again disconnected everything from the setup except the PC and the RCAs to the amp and I still get whine. It doesn't occur when the RCAs are unplugged from the amp, so I guess it isn't the wiring harness anywhere.... Looks like it is my PC, and I've got a 10 gauge ground wire running from one of the screws on the back to the same point as where the amp grounds - helped quite a bit but STILL have the whine.

    Grrrr....I am about to say fuggedaboutit.....
    u need to be using at least 8 ga wire for both v+ and gnd,

    i went from 8 ga from battery to trunk(where it went to an amp, and computer) to using 2 ga from battery to trunk, to a distroblock, 8 ga to amp, and 8 ga to carpc,

    also am using a 2 ga ground wire to distroblock, and 8 ga to amp and to car pc

    it severaly REDUCED my wine.
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  9. #19
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    I am actually using 4 gauge wire for the power and ground to the amp - but do you really think I need 8 gauge from the PC? Did you ground the PC via an existing screw or did you tap the chassis somewhere? Thanks!

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