Shielding, is an isolation a problem?

Discussion in 'General Electronics Chat' started by DanielK, May 28, 2009.

  1. DanielK

    Thread Starter New Member

    Mar 11, 2009
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    Hello everyone :) I'm in the process of making my audio interconnects (home recording), for balanced/unbalanced connections. There will be three conductors (two of them twisted with each other) in each cable, obviously, and the third conductor will be wrapped around those two twisted ones, so it will act as a shield/earth ground. I have a doubt about shielding, though. Does the third conductor need to be completely stripped of isolation (thus exposing bare copper), in other words, can its isolation defy the shielding purpose/effect to any significant level? But, something inside me tells me that if that was the case then the shield would be unnecessary. I'm asking this question, because I'm slightly worried about fragility of the exposed shield wires that will be wrapped around those 2 other (isolated) conductors. Cheers!
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    You should use shielded audio cable instead of trying to make a Mickey Mouse cable that has poor shielding.
    A shielded audio cable has a shield that completely covers the pair of signal wires and has an overall insulation.
     
  3. DanielK

    Thread Starter New Member

    Mar 11, 2009
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    Excuse me, but your reply shows this is exactly how our unfortunate today's society works. Discourage, set and put off people from DIY and simply get them buy everything and do nothing. This is -not- how we have been evolving. All this when I simply asked a question. Buying something does not always guarantee you a decent job done, as pointed out here: <snip>
     
    Last edited by a moderator: May 28, 2009
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    There are two different type of cables to consider:

    There is a shielded twisted pair, where there is a mesh-like braid or tape-like wire surrounding the entire twisted pair. These cables, when the shield is properly grounded at one end or both which is always up for debate, provide shielding from outside signals.

    Then there is a three wire cable. If you use a cable if a third wire, that has its insulation removed or not, it is typically used to provide a ground down to the other end of the wire. It does not shield the signals of the twisted pair because it does not totally surround the twisted pair.

    In either case, insulation is needed around the entire wire combination to insulate either the shield or ground wire from outside contact.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Shielding is not going to shield unless it gives very good coverage by completely enclosing the conductor/s to be shielded, and unless it makes good electrical contact with circuit ground. While an ohmeter can't show % of coverage, it can show continuity of the shield.

    If you use some standard three pin connector like Switchcraft makes, and shielded microphone cable like Carol C1228, making homemade cabling is no problem.
     
  6. DanielK

    Thread Starter New Member

    Mar 11, 2009
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    This is what I tried to explain in my first post, this is the way I'm going to go.
    The interconnects are going to be made out of 3 pieces of AWG 19 stranded
    speaker cables, where first two "hot" (out of phase) ones will be twisted
    together, and then a third one will be wrapped around those two,
    for shielding/grounding.

    That's why I asked if it is really necessary to strip off the insulation from the
    third shielding wire (because it is originally a speaker stranded wire, with PVC insulation on it),
    for optimal shielding, or can it be simply wound around
    those two "hot" wires, with insulation kept on it and then hot shrinking tubing
    applied on top of it all?

    Each interconnect will be 14 feet long.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Wrapping stranded wire around two pieces of wire is not quite the same as applying shielding. As long as the wires are twisted, some noise cancellation will take place. Look up shielding braid.
     
  8. radiohead

    Active Member

    May 28, 2009
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    shielding is crucial in RF TX/RX and RF amplifier circuits. Chances are low that you will receive any significant stray RF signals in a standard audio circuit, but the chance is still there...
     
  9. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    If you are going to use the third wire as a ground, then the insulation need not be removed.

    If you are going to try to use it as a shield, the insulation would definitely have to be removed in order to get nearly full coverage of the other wires. The insulation would not allow the wire to get close enough to the last loop to work as any kind of shield. For the wire to work as a shield, each loop around the wire pair would need to be in full contact with the last one. Not an easy task.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Some audio shielded cables have foil as a complete shield covering the audio signal wires.

    Just using a 3rd wire wrapped around audio signal wires makes an extremely poor shield.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I've got a reel of twisted pair cable here for making MIDI leads, it has full foil screen AND mesh weave stranded wire screen AND cotton fibre for physical strength. It's still cheap. Even in small lemgths at the local electronics store it's only about $1.10 USD per yard.

    I can't see why someone making interconnects for home recording where you already have thousands of $$ in equipment would mess around for the sake of a few bucks worth of cable??
     
  12. radiohead

    Active Member

    May 28, 2009
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    the "cotton" fiber is the rip cord. It isn't advisable to put un necessary stress on cables. The fiber in the cable doesn't add to the tensile strength of the cable.
     
  13. b.shahvir

    Active Member

    Jan 6, 2009
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    In connection with the ongoing discussion I wish to add a query of my own. At my place of work I've several CCTV co-axial shielded cables running along perforated cable trays.

    However, they are travelling thru a loft which houses 'Air Handling Units' (AHUs). These AHUs use VFDs for speed control. The moment the VFDs come on, it introduces disturbances in the CCTV visuals thus hampering our security systems. The disturbance stops as soon as the VFDs shut off.

    Is there a shielding problem? Do I need to connect the cable shield to system ground? Is it possible to develop a perfect shield to eliminate disurbance 100 per cent? Any kind of guidance will be highly appreciated. Thanx :)

    Best regards,
    Shahvir
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Well normally I'd agree that the cotton or hemp fibre in a multicore cable is just packing to fill up the space...

    Unless it's a quality balanced-pair cable specifically meant for live performance microphone work etc etc.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Audio/video cables are never in the same cable tray as electrical wires. They are always separated.

    The shield stops radio and electrostatic interference. It does not stop magnetic induced signals.
     
  16. b.shahvir

    Active Member

    Jan 6, 2009
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    Can someone plz guide on this particular problem? :(

    Thanx & Best regards,
    Shahvir
     
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