DIY sheilded cable

Discussion in 'Archived Threads 2001-2004' started by Richard cash, Jan 27, 2002.

  1. Richard cash

    Richard cash Stunt Coordinator

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    Is there any way to sheild my ixos subwoofer cable myself. I think i have a ground loop problem and using this cable makes a humming sound rom my sub. if i use a sheilded cable then there is no hum. I dont really want to have to buy another cable as this one cost me £40, and as a student, £40 is a lot of money. I have tried connecting the inner and outer conductors at the phono plug using a piece of tin foil and this seens to stop the humming completley. Is it ok to keep the piece across the positive and negative or is it dangerous
     
  2. Ken Shiring

    Ken Shiring Agent

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    Richard, based on my knowledge of cables, a ground loop problem cannot be fixed by shielding cables. However, you have managed to isolate the problem to your sub connection, which is important.
    Make sure that your sub power cable (if it's a powered sub) is using a 3 connector plug. For that matter, make sure your amp is using a grounded plug as well. I know that you live in the UK, and my recollection tells me you should have a three wire system as well. You want to make sure the sub is connected to the same ground wire as your amp. This is very important.
    I have another suggestion, but to protect myself, I need to put a big fat disclaimer in front of this: When you work around mains power sources, be very careful. Voltages present there can kill you. Be sure you know exactly what you are doing for this suggestion.
    Now that that is out of the way, you could try measuring the resistance between the ground of the outlet used by the sub, and the ground of the outlet used by the amp. If it is more than a few ohms, the may be connected to different ground points, which would definitely cause ground loops, and subsequently, humming.
    I hope this helps.
     
  3. Graeme Shiomi

    Graeme Shiomi Stunt Coordinator

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    Very true that a ground loop isn't affected by shielding on a cable, but if the humm goes away when using a shielded cable, it is probably not a ground loop causing the hum. Maybe your line is picking up intereference from another source (flourescent light bulb/halogens/dimmer/etc.).
    If this is the case, then it is possible to shield an unshielded cable. You can go out and buy a cheap coaxial cable with copper braid shielding where the diameter of the shield is greater than the diameter of your IXOS cable. You remove the shield from the cheap coax, and slide it over your IXOS cable, and then you solder one end of the braid to the connector on the source (i.e. Receiver/Pre-amp) side. You can cover the shield by using heatshrink tubing or tech-flex.
    Now, this technique is described at Jon Risch's website, http://www.geocities.com/jonrisch/ but, as you can see, it isn't a quick and easy process. It will, however, be cheaper than buying a new shielded cable. The main risk is that you'll go to all the trouble, and the humm will still be there, or even worse, you'll ruin your IXOS cable (during the soldering process).
    Graeme
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    It's not the shielding thats the issue. It's how it is connected.

    There are 2 kinds of nearly identical cables used for subs:

    A) Normal Interconnect where the shield is connected to the RCA plugs on both sides.

    B) Un-Connected Interconnect - One RCA plug does not have the shield connected to it. The cable usually has little arrows pointing to the sub to tell you which one is un-connected.

    If connecting a sub causes a humm, use the other kind of cable. They are both shielded.
     
  5. Jason Watson

    Jason Watson Stunt Coordinator

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    I gotta step in here and elaborate on Bobs correct response. Coax type interconnects use the shield as a conductor and it must be connected at both ends to work. This can present a problem because the "shield" is no longer functioning as a shield, but rather just another conductor. In fact the shield can act as an antenna and cause the cable to be even noisier than an unshielded,two wire, interconnect. A properly shielded interconnect will have at least two conductors AND a foil or braided shield. The shield is connected at only one end,(the source end, and therefore,usually the reason for the directional arrows) to the "ground" conductor to serve as a drain. If your cable is causing the noise try a properly shielded cable. Another thing to look out for is running the interconnect paralel to, and touching a power cable. This often is the case with powered subs since these two cables seem to start and stop at the same place[​IMG]. If this is so with your system, try to separate the two a little and see if that takes care of the problem. If they must cross,try to cross them at right angles.
    Jason
     
  6. Richard cash

    Richard cash Stunt Coordinator

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    Thanks for your replies, i will address each of your replies seperatly so it is clear who i'm talking to:

    KEN: My power cables for both my sub and my amp are not earthed. At the plug one of the prongs are plastic. What do you mean connect the sub to the same ground wire, do you mean the same ring? if so, it is, all of my room is connected on one ring

    GRAEME: I have tried using the cables on another system in another room so it can't be to do with a piece of equiptment other than in my home cinema. Also all the lights in my house are connected on a different ring. Thanks for the idea about using cheap coax cable, if all else fails i will give it a go. Also where would i soldier the braided copper to? there are two components to the phono, should i solder to the inside or the outside or both

    JASON: my cable does have arrows on it and i have tried connecting it both ways (into the sub and out of it). This does nothing, but does it mean that it is sheilded if it has arrows. The salesperson in the shop said it wasn't. You mentioned that the sheild was connected at the source end and the source can act as a 'drain'. Does this mean though that i would have to put an earth on it?

    I am still a bit puzzled about the fact that when i put a piece of foil across the positive and negative terminals of the phono that it stops all the buzzing. Is this the same as shielding it, if i knew which piece of the phono i had to solder the shield to it may make me understand a little better
     
  7. Bob McElfresh

    Bob McElfresh Producer

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    Richard: You have just explained your problem: "My power cables are not earthed".

    When you send a signal to another device, you really need 2 wires:

    Reference Wire: held at some agreed-upon voltage

    Signal Wire: carries a voltage that is different from the reference.

    In most equipment, the Reference wire is tied to power system ground. This tries to make the reference be something like 0.000 volts.

    Since neither of your equipment has a earthing plug, there is no agreed-upon reference voltage, or way for the 2 devices to have a common reference.

    Your cables: Lets take the non-IXOS cable first

    This cable has the shield connected to the outer part of the RCA plug on both ends. When you plug the cable in, you have now tied the ground/reference of the receiver and sub together. Now they both know what the "reference" is.

    Now signals work properly.

    Your IXOS cable:

    One side has the coax shield NOT CONNECTED to the RCA plug. Now your 2 pieces of equipment dont have an agreed-upon reference. It's free-floating. Along comes a signal and your equipment does not know what to do with it. It's free to pick up floating signals (likley 50 hz power signals) and your "reference" now changes at 50 times per second.

    You have 2 choices:

    - Get an ordinary interconect (without arrows) that has the shield connected at both ends.

    - Use your IXOS cable, but run a separate earthing wire between your receiver and sub amp. This allows them to have the same "reference".
     
  8. Richard cash

    Richard cash Stunt Coordinator

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    "Use your IXOS cable, but run a separate earthing wire between your receiver and sub amp. This allows them to have the same "reference". "

    how exactly do i run this earth wire between the components? forgive me for being so dumb.
     
  9. Bob McElfresh

    Bob McElfresh Producer

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    Many receivers actually have a thumb-screw/binding post marked "Ground" that you can attach to.

    The ground in many receivers & amps is usually also connected to the metal body of the amp. This means you can attach a wire to a handy screw.

    Try this (to validate the theory): Just go out and get some insulated, dc "hookup" wire from the hardware store. Turn on your system so that you hear the humm with the IXOS cable.

    Strip the insulation from the ends of a long piece of wire and tape/touch the bare wire ends to the metal on the receiver and the amp. Try to find a un-painted part of the amp. If the humm goes away when you touch things, then you can look for a screw to un-screw and attach the wire to on each device.

    You can even use a spair run of speaker wire to do this test.

    (Personally: I'd give up on the IXOS cable and get a different cable, one without the little arrows. Since you know this will work, it's your easiest solution.)
     
  10. Richard cash

    Richard cash Stunt Coordinator

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    i know that replacing the cable may seem the easiest way but i just can't let it beat me. Anyway, i looked in the manual for my receiver and it has a SIGNAL GND, this is the little ground screw i think your talking about. Then it says at the bottom of the page 'DO NOT USE THE SIGNAL GND TERMINAL FOR GROUNDING THE RECEIVER' then on the next page it says 'if a component has a ground wire, connect it to the SIGNAL GND terminal'it gives an example, a turntable.

    I am a bit wary of the dont connect to receiver warning. I am not actually grounding the amp but the sub, right? (forgot to mention the sub is a paradigm servo-15 and the amp a sony strdb940)
     
  11. Greg_R

    Greg_R Screenwriter

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    Are you sure it's the subwoofer? Keep in mind that the ground loop hum is 60Hz (which happens to play on your subwoofer). Are you sure it's not your poorly grounded cable TV (usual culprit)?
     
  12. Bob McElfresh

    Bob McElfresh Producer

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    Yes Richard. We are talking about a "Signal Ground" here. Your two devices (receiver and subwoofer amp) must have a common "Signal Ground" to be able to transfer signals to each other.

    And the signal ground is sometimes different than the electrical POWER ground.

    Follow the directions, but run the wire to your sub instead of the turntable.
     
  13. Richard cash

    Richard cash Stunt Coordinator

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    BOB, i tried connecting the sub to the signal ground and nothing happened the humming continued. I couldn't see how that would work if the receiver and the sub both dont have earths?

    If i was to put an earth on the amp or sub or both would this make any difference.

    I think the sub is definatly the problem, i took the connection out of the receiver, so only one phono was in the back of the sub, and held the other phono plug (the one i just took out of the receiver) it buzzed when i held the phono on the inside pin but if i held both bits of the plaug the buzzing went away.

    I also tried it with a sheilded cable and the noise was still there, i though i had tried it with a sheilded cable before and it was fine but guess not. Does this shed any ligh? i am starting to get really pissed off!

    one last suggestion, could it be a faulty receiver?

    thanks for everyones input

    ps, i do not have cable tv so this is not the cause
     
  14. Bob McElfresh

    Bob McElfresh Producer

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  15. Richard cash

    Richard cash Stunt Coordinator

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    BOB, how can i earth my receiver? do i have to take the power cord out and replace it with a 3core cable.

    thanks for helping me and i hope we can crack the problem soon.

    'Try this: from the Signal Ground wire from the receiver, touch the outer metal part of the RCA plug where it plugs into the sub. Does the humming stop? '

    no, i tried connecting it but the buzzing is unaffected.

    I also tried taking all the components out of the receiver so that the only thing connected was the sub. I thien turned it on and the humming was near silent, i could still hear it but it was minimas and bearable. While music was playing i couldn't hear anything. Then i put the front 2 speakers into the amp(eltax symphony 8 via cable talk talk 3.1 biwirable cable). Then i played music and it humming came back.

    What does that mean?
     
  16. Richard cash

    Richard cash Stunt Coordinator

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    have just come back from my mates house, i took my sub,receiver and amp round there to see what differences there would be. I couldn't beleive it, i plugged my components into his system and it did the same thing, so then i tried the proces of elimination, first i changed the leads to his, no joy. Then i disconnected my sub and put his there (paradigm pdr-10). When i did this the buzzing was still there. All his components remained, the only change to his system was my amp.

    Doesn't this mean that it is my amp
     

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