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Efficacy of Balanced XLR Cables for Noise Reduction

  1. Apr 11, 2016 #1
    When attempting to find an answer to the "quality" of sound one attains from various audio cables, most of the research leads to the very subjective answers of audio/sound engineers. I was hoping to gain some insight from a physics/engineering perspective.

    I'm curious if an expensive or "professional" cable is somehow more capable of reducing interference noise, or conveying the transduced signal from a microphone more faithfully than an inexpensive cable. My suspicion is that there would be exactly no difference between, say, a $10 dollar 20' cable and a $100 20' cable simply because, in both cases, the same type of circuit is employed to convey two relatively inverted signals, and thus eliminate RF interference through phase cancellation.

    Of course, more expensive cables are more durable/rugged, but I am finding very little satisfying arguments that suggest that two cables will sound differently.

    I'll leave the question intentionally vague for now in the hopes that other considerations will be brought into the discussion. Thanks in advance for any responses!!!
     
  2. jcsd
  3. Apr 11, 2016 #2

    Averagesupernova

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    Try asking that on an audiophile forum. :smile: You are basically correct. I will admit that there can be some shortcuts made which can cause problems but that aside they would be about the same.
     
  4. Apr 11, 2016 #3

    rbelli1

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    The capacitance of the cable will also affect the signal. Higher capacitance will attenuate high frequencies more.

    The better cables use thicker and/or more exotic material for the insulation. Whether better equates to more expensive is uncertain.

    BoB
     
  5. Apr 12, 2016 #4
    Balanced is better at rejecting interference from magnetic fields than single-ended is. That's because the equipment on the receiving end, the mixer or amplifier, looks at the voltage difference between the two signal wires. An interfering magnetic field will induce almost identical signals in both wires, so it is ignored at the receiving end.

    The truly professional grade cables are more flexible due to using more strands of finer wire for the conductors, more flexible insulation and jacket material, and a more durable jacket. They are also more effectively shielded from electric field pickup. The shielding is many strands of fine bare wire braided together surrounding the signal leads. Sometimes there is more than one layer. There may also be a layer of aluminum foil to give 100% coverage. The lower cost cables will use a loosely woven braid with much less coverage, letting more interfering electric field thru to the signal line. I've also seen, sadly, some low-cost, consumer grade, imported audio cables that didn't have any shield. There was just a few strands of bare wire under the jacket, enough to carry the signal ground thru to the RCA phono plugs on each end.

    The other thing to watch out for is microphonics in the cable itself. The insulation, being plastic, can generate a voltage when struck or bent. This of course comes thru as noise or static. You can see it happen if you have access to an oscilloscope. Even some 'scope probes have that problem!

    For the average home use, none of that is of much concern, it's not a very noisy electrical environment. There is an awful lot of misleading marketing in the consumer and pro-sumer market though. The high priced cable may or may not be 'good.' The highly advertised high priced stuff in your local department store doesn't qualify. Find an outlet that sells to the entertainers, they don't put up with the junk.
     
  6. Apr 12, 2016 #5

    davenn

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    great post, @Tom.G :smile:

    covered the significant points well

    Balanced lines also important for negating earthloop hum problems

    That cannot be stressed enough ... mic cables get a "hell of a life" on and off stage during use and packing/unpacking from storage
    one of the big reasons many bands/other organisations do away with them completely and use radio mic's and transmitter units for guitars etc

    The joy of cheap and crappy cables !!!!!


    Dave
     
    Last edited: Apr 12, 2016
  7. Apr 12, 2016 #6

    jim hardy

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    Nice job, Tom !

    Is most audio gear still 1Kohm inout impedance ? I guess not..

    I see Amazon bragging about low capacitance cables that won't attenuate highs.....38pf per foot.
    Run a quick calculation and see how much capacitance it takes to be 1Kohm at 20khz
    then for a 20 ft cable, what would be the capacitance per foot to give that?


    Myself - i'd spend just enough to get a nice flexible cable with robust connectors and strain reliefs.. The only "Monster" brand wires i own i found at a yard sale....
     
  8. Apr 12, 2016 #7
    Yes, an excellent example of excellent marketing. Their ads at least used to reference a US Patent on the cables. When I investigated, the patent was for a cable with the center of the conductor filled with a non-conductive material. The stated rationale was due to skin effect, the center part wasn't being used anyhow so there was no point in paying for the unused Copper. And the patent had long expired when those ads were running. Caveat emptor!
     
  9. Apr 12, 2016 #8
    Great answer, Tom. Here's where I'm getting lost. Why is "shielding" important for a balanced line? Are not all transient signals eliminated when both signals are combined at the end of the signal chain? Am I simply expecting theoretical results in a non-theoretical world? I'm thinking: (Signal + Noise) - (-Signal + Noise) = 2xSignal.

    Also, distance between two conductive materials will affect capacitance, but what other conditions will increase or reduce the cables capacitance? @jim hardy and @rbelli1 , since you both brought it up.
     
  10. Apr 12, 2016 #9
    The last time I looked (several years), balanced lines and output impedances were 600 Ohms, nominal; sometimes cables 150 Ohms. The 150 Ohms is probably more realistic, it's tough to get above 150 in a small cable. Recall the spacing of 300 Ohm TV twinlead. For the output impedance, of course 'lower is better' from a bandwidth standpoint. At audio frequencies you don't have to worry about reflections from impedance mismatching; after all 20KHz wavelength is over 6 miles. (That's an AWFULLY big stage.)

    Yes and... without the shielding you get common mode noise pickup which may exceed the common mode rejection capability of the receiving mixer/amplifier.
     
  11. Apr 12, 2016 #10

    jim hardy

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    capacitance is area/distance. It's the size and spacing of the conductors.

    As Tom said, big stage.
    Transmission line effects , ie "Impedance" of a pair of wires , becomes significant only when the length is a goodly fraction of a quarter wavelength ,
    which at 20 khz is something like two miles.

    Shielding reduces currents induced into signal lines from external sources.
    In a perfectly balanced system you're right, they'd cancel and that's why balanced works so well .
    In the real world there's always some unbalance and that's what turns common mode interference (into both wires equal and in phase)
    into normal mode interference(unequal in the two wires so looks just like signal) .

    CM_to_Normal.jpg

    Are internal Z's perfectly balanced ? How about stray capacitances ?
    Minimize induced currents and you reduce their effect through unbalanced Z's.

    A high quality instrument system will specify along with its common mode rejection ratio just how much unbalance it will tolerate and still perform within that spec.
    My old plant computer system's "front end" specified 140db with 1Kohm unbalance
    which was stellar for 1973 and is pretty doggone good today.
     
    Last edited: Apr 12, 2016
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