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Balanced and Unbalanced Inputs?

Discussion in 'AV Receivers & Amplifiers' started by Apocalypse, Sep 4, 2002.

  1. Apocalypse

    Apocalypse
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    On a power amplifier what is the difference between balanced and unbalanced inputs. At this point my only guess is that the output levels of each channel aren't equal on an unbalanced input amp, please enlighten me someone.
     
  2. Charlie Whitehouse

    Charlie Whitehouse
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    In the more usual unbalanced connection (aka single-ended), the signal is carried on a single conductor surrounded by an earthed shield.

    With a balanced connection, there are actually two signal carrying conductors in the cable again surrounded by an earthed shield.

    In both cases, the earthed shield is there to reduce the effect of noise being introduced into the cable by stray magnetic fields from other nearby cables/equipment. But this is only ever partially effective. Over long cable runs, for example in recording studios and the like, normal unbalanced cables would be too noisy.

    So the balanced connection was developed, which as I said has two conductors carrying an identical signal, except one of them is inverted (i.e., converted to its mirror image) before it is sent down the cable. Any noise that makes it through the shield will induce an identical 'spike' in both signal cables.

    The clever bit, is that when it gets to the receiving end, the power amplifier re-inverts the inverted leg and adds it to the normal leg and by magic any noise spikes get eliminated since they are now equal and opposite.

    It also has the advantage that by adding the two signals together you double the voltage which is equivalent to a 6dB increase in gain.

    For home use, it is not usually necessary to use balanced connections unless you, for example put your power amplifier right next to your speakers (short speaker cables are a good idea) and run long interconnects from your pre-amp/processor.

    Does that help? :)
     
  3. fastedd

    fastedd
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    On an unbalanced input you have two connections. The -ve is connected to the screen and is usually held at a fixed voltage- usually zero volts w.r.t the amplifer case. The voltage on the +ve terminal swings either side of the negative terminal. They are therefore not really +ve and -ve terminals, I'm just calling them that by convention.

    Two probs with unbalanced inputs. Ground loops, its possible that the -ve on the source is at a slightly different voltage to the -ve on the amp and so you may get a small current flowing which can cause noise. Also any interference that gets picked up by one of cables is amplified with the sam gain as the signal

    With a balanced connection you have three connections, 2 signal wires and a shield. The shield is just that, it just protects the conductors from interference. The 2 signal wires are twisted together and feed a differential amplifier. Basically as one wire becomes more +ve the other cable becomes more -ve. The amp sees the difference between the two cables and amplifies it. Balanced connections have very good common mode rejection.

    Because the wires are twisted together they pick up the same interference, and because that is common to both cables the amp doesn't amplify it, only the differences remember. This makes balanced connections very good at rejecting interference. Even if only one signal wire sees the interference the amp will reduce the effect of the interference by half, due to it not being a differential signal.

    Balanced cables are usefull if you like running power amps at the speakers and runnng preamp signals to them.

    HTH

    Edd

    Charlie, you beat me to it but I'm glad its a similar reply :)
     
  4. Apocalypse

    Apocalypse
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    Both of you, thanks a lot for that I appreciate it.

    I'll be ok with unbalanced inputs then as my speaker cable runs are about 2 metres, well that's a weight off my mind :)

    How did I exist without this forum before I wonder :blush:
     
  5. Orac

    Orac
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    I was expecting someone to start talking about directional interconnects being 'pseudo-balanced' and thus better.

    Which is of course a load of crap.
     

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