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Semi-balanced interconnect

Discussion in 'Home Cinema Building DIY' started by eviljohn2, Mar 5, 2004.

  1. eviljohn2

    eviljohn2
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    Assuming my XLS200 ever arrives I'm planning to build a mains cable and interconnect for it.

    Is it worth semi-balancing these, particularly the interconnect as it will be several metres long.

    Looking in your direction Retro as I'm hoping to use Starquad mic cable for it. :)
     
  2. eviljohn2

    eviljohn2
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    Maybe I'd get better results if I connected all of the signal carriers together. ;)
     
  3. Nic Rhodes

    Nic Rhodes
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    What is semi balanced? It is either balanced or unbalanced, i'm confused by your terminology EvilJohn2, can you explanin?
     
  4. eviljohn2

    eviljohn2
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    Not to worry.

    A balanced interconnect has three conductors: signal carrier, return-path carrier and a ground.

    A non-balanced interconnect just has signal and return path.

    A semi-balanced cable uses a signal carrier and a return path carrier in a multicore format, but also has shielding around this which is connected to the return path (usually only at the source end) to provide a form of active active shielding.

    Like this for a mains cable:
    http://www.tnt-audio.com/gif/scheme.jpg

    Obviously the third core wouldn't be present in a standard interconnect.
     
  5. Nic Rhodes

    Nic Rhodes
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    Rubbish jargo then. It is a screened unbalanced wire, nothing to do with balanced what so ever!! Yes I have seen this before and all I would say is urge 'caution' re capacitance of your cable
     
  6. Mr THX

    Mr THX
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    Can you fill us in Beekeeper on the caution to watch out for ?? Is there a right and a wrong..................
     
  7. Nic Rhodes

    Nic Rhodes
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    I am not going to write about the pros and cons of balanced connections. The Tag web site has done this admirably already.

    Balanced is one where two conductors, neither of which are grounded, carry the signal. A balanced line may or may not be shielded but if it is shielded, the shield is not joined to either of the two signal conductors, it is on that third pin. When one of these two conductors is carrying electrons to the source, the other is taking them away (some call this signal and return, inaccurately). The polarity of the two conductors, at any moment, is always opposite.

    Unbalanced still uses two conductors required to work. One of these conductors is attached to an external reference point ground, the other carries the signal.

    The devices are therefore balanced or un balanced and not the cable per se. The difference is in their constructions.

    Balanced cables use a symmetrical design. Examples include twisted pair telephone cable, CAT 5 computer network cable. NOTE CAT 5 / 6 cables ARE BALANCED designs. VERY IMPORTANT GUYS, remember this..

    Professional audio cables (analogue and digital) made for balanced operation, just like telephone cable, are usually composed of twisted pairs of insulated wire, frequently but not always shielded.

    Unbalanced cables generally take the form of coaxial cable. The two conductors in coaxial cable share a common axis (hence coax). In the centre is a signal conductor, coated with an insulating dielectric. The outside of the dielectric is a shield, consisting either of a wire braid, foil, or a combination of braid and foil, and this shield is used as the ground conductor.

    Not all cables are coax for unbalanced audio however and I will try and cover other good ways of doing something.

    I have balanced and unbalanced audio kit, under normal operations there is no difference, but there is one BIG difference. Balanced and unbalanced lines use very different strategies for noise reduction, and this difference accounts for the widespread use of balanced circuits in professional audio gear, as well as for the differences in construction between balanced and unbalanced lines.

    Every cable is exposed to electromagnetic energy, which comes from other cables, from power sources, from fluorescent lights, switches, and a thousand other (mainly electrical) sources. When this energy meets a cable, the cable will absorb some of it, which causes the unwanted phenomenon we call noise.

    In unbalanced circuits you try to keep ALL NOSE OUT. They rely on the screen to block everything and pass it away to ground. The screen does not interact with the signal at all. In our perfect utopia this all works brilliantly, in reality it is a little different. Hence we have noise.

    Balanced cables may have a shield to carry noise away to ground, just like above. But when noise does reach the inside, it strikes the two conductors. Because the conductors are closely physically associated with one another, the noise reaches them more or less simultaneously, and affects them in the same direction. The polarity of the intended signal on these two conductors is opposite in balanced operation, when one conductor is negative the other is positive, at any moment the polarity of the noise is always the same in both. This is WHY people prefer the longer leads to be balanced or the professionals prefer balanced approaches. The noise simply cancels itself out and is called common mode noise rejection.

    This is the advantage of balanced audio, it is only an advantage in noisey environments or ones that are long (and thus more susceptible to noise pick up).

    For a well screened unbalanced coax cable there should be little difference but you already know not all coax’s are well screened don’t you.

    Cable Design: Coax or twisted pairs

    Remember common mode noise rejection?

    It is important to use balanced cable for balanced circuits, and unbalanced cable for unbalanced circuits. Now what do I mean by this. Coax cable is designed to focus as much of the noise as possible onto the shield conductor and as little as possible onto the centre conductor. Common mode noise rejection (balanced) only works if noise affects the two conductors equally. We therefore need very different cables for these two jobs to be done effectively. This is the crucial point here.

    Thus if we used coaxial cable in a balanced audio circuit, we would get plenty of noise because coax is designed to focus as much of the noise as possible onto the shield conductor and as little as possible onto the centre conductor. Common mode noise rejection only works if noise affects the two conductors equally.

    If an unshielded twisted pair was used to convey signals in an unbalanced circuit, figuring that twisted pair cables provide common mode noise rejection, we will find it doesn’t work. COMMON MODE NOISE REJECTION WON'T WORK IN AN UNBALANCED CIRCUIT, because there is ‘minimal’ current flow (noise) in the ground conductor. (There are tiny amounts there but twisted pairs are is not designed to maximise this in the screen as opposed to the coax cable!).

    Now how many twisted pairs do you see in analogue leads? The circuit accounts for common mode noise rejection and not the cable per se.

    Many ‘audiophiles’ use shielded twisted pair balanced audio cable as an unbalanced interconnects. They ground one of the two signal wires at both ends of the cable, and then ground the shield and sometimes at both ends, but sometimes only at one end, causing a loss of shield effectiveness. This increases capacitance! Something, which you will have realised by now, is the last thing we need in any cable. I found this out the hard way years ago by using un buffered passive pots as volume controls which emphasise the C problems on audio leads. Coaxial cable, not twisted pair audio cable, is the right choice when connecting unbalanced components. It has significant technical advantages.

    Coax in 'semi balanced' is asking for trouble
     
  8. eviljohn2

    eviljohn2
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    Thanks Beekeeper :clap:

    Needless to say my physics course hasn't covered cable geometry in that kind of detail yet :)
     
  9. Nic Rhodes

    Nic Rhodes
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    Well buffer outputs you shouldn't have a problem (or X 10D), but passive pots, big problems, other stuff in the grey area in the middle.
     
  10. eviljohn2

    eviljohn2
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    Sounds like I'd be better off getting some decent 75ohm video coax or similar then. :)
     
  11. Nic Rhodes

    Nic Rhodes
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    :)
     
  12. eviljohn2

    eviljohn2
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  13. Nic Rhodes

    Nic Rhodes
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    I always use it :)
     
  14. eviljohn2

    eviljohn2
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    I find that cables in general can make a huge improvement to my system. It's just not the same without ;)

    Would connecting a shield to the earth pin make a difference?
     
  15. Nic Rhodes

    Nic Rhodes
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    Again it depends on the quality of you mains (variable in the UK) and the ability of the power supply in your equipment. If done well it should make no difference but reality check....VERY few are in the if done well category. Have a search of Mark G stuff, some hints there and he can supply if you want
     
  16. eviljohn2

    eviljohn2
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    Thanks. In that case, considering the difficulties in getting short runs of shielded mains cable I'll just braid some decent gauge equipment wire together and stick a ferrite round it :smashin:
     
  17. Nic Rhodes

    Nic Rhodes
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    Mark G will I think sell you what you need :) Ask him nicely :)
     

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