Question: Line out and long cables

Discussion in 'Digital Audio' started by Blue Velvet, May 12, 2006.

  1. Blue Velvet Moderator emeritus

    Jul 4, 2004
    A friend of mine has an oldish G4 tower which we're putting a new Sonnet processor card in this weekend. He's extremely reluctant to throw more money at the machine in the form of a wireless card/airport but he would like to get an audio signal out from the Mac to the living room stereo which is about 6-7 metres (20-23 feet) away.

    Is it possible to run a good quality shielded cable over this length for a standard line-level 600 ohm (?) signal and get acceptable results without too much treble roll-off, hum or other interference?

    Moving the Mac or amplifier closer to each are not options, I'm afraid.

    Thanks for all words of wisdom.
  2. WinterMute Moderator emeritus


    Jan 19, 2003
    London, England
    The standard 1/8" jack output is unbalanced and there lies your (possible) problem, the unbalanced system uses a common negative and shield connection, which renders long cable runs liable to RF interference (mains hum, taxis etc) and increases in impedance which can affect the audio signal.

    Good quality cable helps in fact anything with a hi copper mass will do, 30 amp mains cable for B&Q is fine, but it won't solve the problem.

    There are "line drivers" which are precisely for situations like this, but they are extra money.

    The distance you describe is right on the edge of being too far (a guitar cable for instance is normally less than 20 feet long), but you may find that it works out ok. The possible problems are decreased levels from the input (turn the amp up) increased noise levels (hmmmm....) and a slight possibility of nuclear explosion....;).

    I'd be inclined to give it a go, make sure you don't run the cable near mains cables.
  3. Blue Velvet thread starter Moderator emeritus

    Jul 4, 2004
    Hell. Nothing to lose then...

    Will explore different cable options and ask a handy person to solder the plugs on.

    Thanks WM. :)
  4. drlunanerd macrumors 68000


    Feb 14, 2004
    I made a cable up like you describe, from parts bought at Maplins, of around 6 metres. No problems using it on both PowerMacs G4 and G5.

    One word of advice though - using the on-board line-out/headphone jack won't give you the best sound, and in come cases delivers horrible hum (really bad on the iMac G5 in my experience). Buying an external DAC eliminates this. The cheap Griffin iMic does the job.
  5. zimv20 macrumors 601


    Jul 18, 2002
    BV -- your other option is to get a direct box. they perform several functions, including balancing an unbalanced signal. they can be had starting at $20USD.

    you'd connect it like this:

    mac 1/8" out --unbalanced--> direct box --balanced--> receiver

    the unbalanced line should be short, and the balanced one would take you to the other room.

    note that you'd still be using the d/a converter in the mac. i do not find this d/a converter suitable for real work (tracking and mixing), but i think it's fine for casual listening.

    for your stated purpose, i think you should get a passive direct box (i.e. not active).
  6. Mr Skills macrumors 6502a

    Mr Skills

    Nov 21, 2005
    As far as treble roll-off is concernced, you'll be fine. Someone else mentioned guitar cable but guitar signals are really high impedance - it is not going to be an issue at that length with a line level signal.

    As far as interference is concerned, you'll be more susceptable because the cable is unbalanced, but you'll probably still be fine as long as you minimise risks - for example, by not running it alongside a power cable.

    So... yeah, you'll be fine :)

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