understanding cable loss

Discussion in 'General Electronics Chat' started by veenife, Jan 21, 2014.

  1. veenife

    Thread Starter Member

    Jan 16, 2014
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    Hi,

    some people say cable impedance are so small that one doesnt need to worry about it... but that seems to be a bg issue among guitar players...

    ive heard of cable capacitance in parallel and inductance in series...
    but im still not quite sure of how high frequencies can get lost with cable length... and i mean how these also applies for unbalanced cables??
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    capacitive losses and inductive losses as well as resistive losses increase with length. audio cables are not usually rated for impedance, the braid is mostly for shielding.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The key thing here is the capacitance of the cable. A guitar without a preamp inbuilt has a high output impedance usually above 100kΩ, which combined with the capacitance of the cable will act as a low pass filter and attenuate high frequencies. If the cable has 100pF and the output impedance is 100kΩ then the -3dB point is at 16kHz.
     
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  4. #12

    Expert

    Nov 30, 2010
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    Some cables are a lot worse than that. That's why I designed a pre-amp that fits inside the 1/4 inch connector on the end of the cable. The sensor is plugged right into the guitar and the driver is at the other end. I can only claim flat response from 7 Hz to 200 KHz because that's as far as my signal generator goes (200 KHz). I left the bottom 7 Hz for the driver to use for self centering the DC level.

    This amp has about 500 Meg input impedance and 600 ohm output impedance. Great cable driver! I suspect it would drive a lamp cord, but I haven't tried that.

    Edit: This design is calculated for 50 feet of the worst coax guitar cable I could find in my junque box.
     
    Last edited: Jan 21, 2014
  5. Papabravo

    Expert

    Feb 24, 2006
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    A cable with distributed series inductance will attenuate high frequencies. A cable with distributed shunt capacitance will pass low frequencies and short high frequencies to the return. That is basic component behavior.
     
  6. veenife

    Thread Starter Member

    Jan 16, 2014
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    okay... but how does this theory of inductance and capacitance of a two wire cable applys for an unbalanced cable???
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    doesn´t matter if the cable is balanced or unbalanced. The same therory still works.
     
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  8. #12

    Expert

    Nov 30, 2010
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    Any wire with a length more than zero has inductance. Any 2 conductors have capacitance between them which depends on the distance apart and the length.
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    ..., and the dielectric constant of the material in between. Insulation in cables of all types is chosen for dielectric properties and velocity of propagation.
     
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  10. veenife

    Thread Starter Member

    Jan 16, 2014
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    so you mean even if a cable is single-wire, capacitance can happen between the wire and the isulation material?
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Not exactly.

    There is always capacitance between a single conductor and earth, since we can't get away from it.

    You need two conductors and an insulator (also called a dielectric) between them to form a capacitor. The earth is a special type of conductor and so can form one of these. But the earth is complicated since it can also act as the dielectric so performing both the role of one of the conductors and dielectric simultaneously.

    #12 said it very well and very shortly.

     
  12. veenife

    Thread Starter Member

    Jan 16, 2014
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    alles klar! thanks that helped a bunch!!!
     
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