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A question of Cables

Discussion in 'Amps and Cabs [BG]' started by Brendan, Feb 25, 2001.


  1. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    Reading back posts, I come upon the fact that speaker cables and instrument cables are completly different, and can/should not be interchanged. is this correct? And why not? is it the sheilding? Just wondering, beccause I'm going from combo, where I don't need a speaker cable, to a actual rig with seperate head/cab.
     
  2. Bob Gollihur

    Bob Gollihur GollihurMusic.com

    Mar 22, 2000
    New Joisey Shore
    Big Cheese Emeritus: Gollihur Music
    Your conclusion is correct. There are two differences that make them unsuitable for interchanging.

    Instrument cables must be shielded - easily recognized by unscrewing the phone plug and seeing that there is a center conductor surrounded by a ground shield, usually, but not always, woven. That configuration helps to prevent hum - an unshielded cable will give you a 60hz hum.

    While they COULD be used for speaker cables in a pinch, it is unwise because the gauge (thickness) of the wire is for the low level of instruments, preamps, etc., and is insufficient for quality conduction of the higher output of an amplifier.

    Speaker cables are generally just two conductors, unshielded, and of lower gauge (lower the number, thicker the wire) conductors. For proper phase, they should be wired so + is + at the other end (you can't just randomly solder wires, polarity is important).

    Of course, it depends on the power of the amp. 16 gauge is usually adequate for a couple hundred watts, 14 for more, and 12 for high powered rigs.
     
  3. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    Thanks Bob. That helps a lot.
     
  4. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    OK, I guess instrument cables COULD be used for speaker cables in a pinch, it's still a REALLY bad idea. Don't do it. An instrument cable is meant to carry about 0.1 to 1.0 volts at low current. It may have something like 22-gauge wire in it. An amp can produce in excess of 100 volts at high current. This could literally cook your instrument cable. Also, at the higher voltage, you're probably going to lose some highs due to capacitance of the cable. Now, a speaker cable should be at least 16 gauge, 12 is better, 8 is better yet (remember, thickness goes up as the gauge number goes down). It really doesn't need to be shielded. You're better off using ordinary 2-conductor power cord than an instrument cable. Heed these words, and, as they say in New Hampshire, "Live Free or DIE!"
     
  5. Munji, just want to set a tiny detail straight.

    By using a too thin cable for speaker connection, you loose LOW end, not high end. The thin conductor has too high a resistance, reducing the damping factor of the amp. The amp loses its grip on the speaker, and because low frequencies use far more current than high frequencies, it shows up at the lowest end. And because the speaker starts to act as a bit of a "loose canon" (with the amp losing its grip) it will resonate sooner, giving a pronounced mid-bass peak.

    And of course you'll just lose power inside the cable. Hence it runs hot.
     
  6. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Joris -

    Of course, you are correct, but you really didn't address the capacitance issue. I suppose that the attenuated frequency would be a function of resistance and capacitance combined, resulting in some sort of errant RC ciruit. Would this effect necessarily attenuate lows?
     
  7. In my opinion (please note that) the capacitance of a speaker cable is of negligible effect. Because the output impedance of a power amp is close to zero (maybe .01 ohms on a good amp) the high frequency roll-off due to "RC'ing" of the cable with the amp output is in the Megahertz range. [ f-3db = 1/(2*pi*R*C) ].
    Most amps even have an RC damper circuit on the output (behind the ouput transistors) to suppress oscillations. The value of the capacitor used in those circuits exceeds the capacitance of the speaker cable by far.