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Why not use shielded cable????

Discussion in 'Amps [BG]' started by gojirin, Dec 7, 2004.

  1. gojirin

    gojirin

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    My new 2001 SWR 350 manual(thanks Matthew) says not to use shielded instument cable to connect amp and cabs. Why?

    I can understand using 18 - 12 gauge cable, but what is wrong with shielded cable for this purpose?

    How did I miss this one!?
  2. Tim__x

    Tim__x

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    First reason: most shielded cable is 20-26 gauge, way too small

    Second reason: it's more expensive and provides no benefit
  3. Lyle Caldwell

    Lyle Caldwell

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    What they meant was don't use shielded single conductor cable (instrument cable). You have to use two conductor cable for speaker connections.
  4. Stinsok

    Stinsok Supporting Member

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    Won't the power running through the cable eat the insulation away?
  5. Lyle Caldwell

    Lyle Caldwell

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    That's not the danger. The problem with single conductor shielded cable is the shield is not rated for the voltages present, and is failure prone anyway. Once that shield fails, you'll have a short and your $$$ amp has no load, followed by a fried transformer.

    What's merely a noisy crackling cable when used with a bass is death to your amp when used as a speaker cable.

    Plus, the small guages don't make efficient speaker cables anyway, for reasons I'm too lazy to go into. Short version: you'll lose highs and lose level.
  6. gojirin

    gojirin

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    Okay, gauge makes sense.

    And single conductor cable makes sense, too. In fact that is kind of a "duh" on my part. I hate it when that happens in public, but I asked...

    Thanks all - I'm going to go play my new SWR 350 with my new MIM P and my new Aggie 12's. All bought through B Talk.
  7. Tim__x

    Tim__x

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    Some clarification, the shield is not rated for such current. A short is an infinite load, an open circuit is no load and only harmful to transformer coupled amps (read: tube amps).
    You probably won't lose highs, you will lose power and will likely overheat the cables and so destroy them.
  8. Lyle Caldwell

    Lyle Caldwell

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    See, I was too lazy to even be accurate. Sorry about that.
  9. notanaggie

    notanaggie Guest

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    It once was true that amps were "unhappy" with capacitive loads, and a shielded cable has more capacitance than regular two-wire cable. Some amps still have that problem.

    That is just another reason to add to the others above, which are also true.
  10. xyllion

    xyllion

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    As was stated nearly correctly, instrument cable is not designed to carry the current that can flow between the amplifier and the speaker cabinets. This is what the manual is warning against because there is a risk of damage or even fire (in the extreme case of extremely high wattages).

    As for why speaker cable is not shielded, it is because there is no benefit to shielding cable where both the load and the source are low impedance. In this situation, the amount of noise that will be introduced into the system will be so small that it cannot be heard. Shielding would add expense without providing a noticeable benefit.
  11. Tim__x

    Tim__x

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    Yes, that is what I meant, Lyle Caldwell just happened to say shield and so I did too. But if you look above you'll see that I did imply the cable as a whole is not capable of carrying such currents.
  12. xyllion

    xyllion

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    It is all good. Not trying to offend you. I was just trying to clarify to avoid someone from passing around misinterpretted information. The previous posts definitely were on the right track.

    The bottom line is that instrument cable is built for one purpose and speaker cable is designed for another. They really aren't interchangeable although I have met people that have used the wrong type of cable and have survived.
  13. Doug Parent

    Doug Parent Gold Supporting Member

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    I'm not an electronics tech but I'll pass on something that I heard many years ago from a "know it all" in the audio industry. Basically in addition to the other reasons stated above, the other reason its bad for the amp has to do with an imbalance of impedance between the positive and negative due to difference in guage with a shielded cable. Thats not supposed to be good for the output transformer. And the shielding has some magnetic effect thats bad as well. Thats it. I don't understand any of it. Thats just what he said. Bad for your amp. I have seen shielded cables used for speakers and saw them almost melt. Common sense I guess if you have any idea of the basics of electronics.Many people dont.
    I guess its the terminology and the confusion about what the terms mean. From my expirience many people misunderstand the words "impedance" "current" "voltage" "polarity" Maybe someone could publish a book that helped musicans understand thier tools of the trade better.
  14. xyllion

    xyllion

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    I could easily see a shielded cable causing problems for an old tube amp. I haven't done the math, but I could imagine the cable characteristics causing trouble for a design that uses an output transformer.

    Fortunately, modern solid state amplifiers don't use an output transformer. So, this would only be an issue for tube amps.

    Either way, use the right tool for the job.
  15. slinkp

    slinkp

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    no, a short is by definition zero ohms, not infinite.

    With a short, Ohm's Law says that the current source (the amplifier) will try to deliver infinite current, maybe that's what you meant. If you're lucky you will blow a fuse before anything is damaged.
  16. Mark Reccord

    Mark Reccord

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    A short is indeed zero ohms but it is also an infinite load. An open circuit is no load. Tim_x is correct. The load on a system increases as the impedance decreases. You guys are saying the same thing, it's just terminology.
  17. gojirin

    gojirin

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    Very good information.
    I was confused in part because the wire I was using was stereo, so the + and - were equal in gauge, though not a large enough gauge to run for long current wise.
    My tiny brain was not thinking about what would happen in a "mono" cable with differiing + and - gauges.
    Thanks again, all, for offering clear answers. I like coming out of posts knowing more...
  18. nashvillebill

    nashvillebill

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    This is an AC signal. A small amount of difference in resistance in one leg of the cable doesn't affect the signal, so even if you ran 16 ga for the + and 12 ga for the - ....there wouldn't be a problem at all.

    Also, for cabs with a crossover, there is an inductor on the + leg. This inductor has a DC resistance far greater than you'll experience with such an unequal cable. A typical inductor in such a crossover will have a DC resistance somewhere around 0.5 ohms, give or take a lot depending on the gauge of wire and the value of the inductor. A few feet of wire will have a resistance much less than a tenth of an ohm.

    Bottom line: just buy a few feet of 16 gauge or heavier two-conductor "zip" cord--the kind where the two wires are joined together--and make your own speaker cable. For even better life, buy something like the 12 gauge 2 conductor Carol brand SJ/SJO wire that Parts Express sells. Tough jacket, it'll last for years.
  19. Tim__x

    Tim__x

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    Quite right, symmetry or lack there of, is not a problem. In a series circuit, the order of components has no effect.

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