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Balanced vs. Unbalanced Cables

Discussion in 'Live Sound [BG]' started by PNS128, Apr 14, 2009.


  1. PNS128

    PNS128

    Dec 28, 2007
    Rhode Island
    i have read up and THOUGHT i understood balanced vs. unbalanced cables. we have a behringer pmp3000 powered mixer, and with the exception of a headphone out, all the main outs and monitor outs etc are unbalanced. when plugging headphones (obviously balanced) into them, we only get a signal in the left ear. so my question is, if we were to hook up powered mains through these outputs (using balanced 1/4-XLR) would the signal be full??
    thanks for any input,
    -ryan
     
  2. JTE

    JTE Supporting Member

    Mar 12, 2008
    Central Illinois, USA
    Um, I think you don't understand balanced and unbalanced. Headphones are not balanced. They're stereo. That means the sleeve is the common ground, the tip is the one signal from the stereo source, and the ring is a different signal from the stereo source.

    Balanced lines create a mirror image of the signal (positive pulse in one is matched by a negative pulse in the other). They run exactly 90 degrees out of phase with each other so when they're combined anything that's identical in both signals is cancelled out. The real signal was out of phase to begin with so it gets through. But noise induced into the cables gets canceled.

    When you plug your 'phones into an unbalanced output, the tip of the headphone plug connects with the hot of the output jack. The sleeve of the 'phone's plug connects with the ground of the unbalanced output jack. There's no connection to the sleeve of the headphone plug so there's no signal there.

    jte
     
  3. PNS128

    PNS128

    Dec 28, 2007
    Rhode Island
    thank you very much, i knew i had something wrong there. so, if i were to connect two powered mains, would it work? im going to use the XLR in's on the PA's with a mic cable, and go out of the unbalanced 1/4 outputs on the mixer. i would use a 1/4-XLR adapter to connect it, but will everything sound right? and should it be a stereo adapter?? either is available
    as in this
    http://www.music123.com/American-Re...-Male-Stereo-Adapter-339519-i1129378.Music123

    vs. this
    http://www.music123.com/American-Re...4--Male-Mono-Adapter-339518-i1129377.Music123
    thanks,
    ryan
     
  4. Bassgrinder77

    Bassgrinder77 Banned

    Jan 23, 2009
    Close - it's 180 degrees. Not 90.
    Balanced is a differential signal where the + and - (180 out of phase) each have the same impedence relative to each other and ground so they'll theoretically pick up noise equally.
    The receiver essentially subtracts the - from the + so the result is 2x the + signal without the noise (which got canceled in the subtraction).
    Also, no current flows in the ground shield (which helps its effectiveness).
     
  5. Bassgrinder77

    Bassgrinder77 Banned

    Jan 23, 2009
    If you're going to use a 1/4 to XLR adapter, make sure the adapter has an in-line transformer to make it truly balanced. They're not expensive. Hosa brand sells them for (I believe) less than $10 - available at Guitar Center in the Pro Audio dept.
     
  6. PNS128

    PNS128

    Dec 28, 2007
    Rhode Island
    how would i know if it has said transformer
     
  7. You said your mixer was powered, if this is the case then you don't want to hook up powered speakers to the powered outputs, you want to hook up unpowered speakers, using a speaker cable and not a balanced cable.

    That is why all of the powered outputs are unbalanced, because speaker cables are unbalanced.

    You will be sad if you hook powered speakers up to the powered outputs :bawl:
     
  8. JTE

    JTE Supporting Member

    Mar 12, 2008
    Central Illinois, USA
    DOH- I knew it was 180, not 90 degrees out of phase...

    Thanks for correcting that!

    jte
     
  9. PNS128

    PNS128

    Dec 28, 2007
    Rhode Island
    the power outputs are speakons....that was never an issue here
    -Ryan
     
  10. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The signals on the two legs of a balanced line are in-phase but typically in opposite polarity—i.e., mirror images of each other. They don't actually have to be, though. Some balanced outputs have one side driven with a signal, and the other side is terminated to ground through an impedance equal to the output impedance ofthe driven leg. What gives the balanced connection its immunity to noise is that the two legs see equal impedances to ground, and that the input it feeds is differential. This way, noise induced will be equal in both legs and in the same polarity, and thus will cancel out in the input circuitry.
     

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