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Why aren't speaker cables Coaxial?

  1. Sep 26, 2011 #1
    Just out of interest I was wondering and couldn't work out...

    Why are (even high end) speaker cables not coaxial? We always use coaxial cable in the lab to get rid of noise on our measurements.

    I figure that 50/75 ohm standard coax would create an impedance mismatch with the typically low ohmic speakers, and output of the audio amplifier. However I calculate that with an outer diameter to inner diameter radius of 1.1, the impedance would be 2.5 Ohms as per below


    [itex]Z_0 =\frac{138\Omega}{\epsilon_r}log_{10}(1.1)=2.5 \Omega[/itex]


    [itex] \epsilon_r=2.33[/itex] which I think is typical.


    This ratio seems feasible to me, to be made into a thin flexible wire.

    Come to think of it my guitar cable is coax, and so are some headphones cables.


    look forward to hearing your comments on this.

    Ben
     
    Last edited: Sep 26, 2011
  2. jcsd
  3. Sep 26, 2011 #2
    Good afternoon Ben.

    Noise is only a problem if its level is comparable to that of the signal. This is measured by the parameter 'signal to noise ratio'.

    Since the noise in a given situation is pretty fixed thae only variable is the signal level.

    Thus noise is only significant for low level signals like microphones and pickups.

    For signals at loudpeaker levels the nosie necomes an insignificant proportion of the total signal.



    go well
     
  4. Sep 26, 2011 #3
    Thanks Studiot I hadn't thought about that. I guess speakers need a fair amount of signal to get them to move!

    Hence the excuse behind the enormousness speaker wires audio buffs spend vast sums of money on.
     
  5. Sep 26, 2011 #4
    Also coax has shunt capacitance also. As small as it is, still in the range of 50 to 100 pF. Not to mention it would be very stiff.

    The important thing about speaker cable is the size of the conductor. I am into audiophile, I actually experiment with speaker cables. I ended up using 4 pairs of 12 gauge Monster type of cable for each speaker. I add one pair at a time and amazingly it make a difference in the sound.

    Small little capacitance at audio freq don't seems to be important, so are 2 pair or 4 pair of those big cables!!! I ended up spending close to $1000 on all the inter connects in my stereo system and they are not of top quality. Human ears are funny!!! It is not about the highs and the lows of the sound, it open up the perceived sound stage. It make the sound more three dimensional and more airy.
     
  6. Sep 26, 2011 #5

    f95toli

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    The answer is actually a bit subtler than that. Which type of cable is best depends on the source and load impedance. For situations where you are primarily interested in the "current" (low impedances) one should -in general- use twisted pairs; whereas in high(ish) impedance ("voltage") situations (and at high frequencies) coax is better.
    Note that microphone cables are also (usually) twisted pairs.


    Note that this does not only apply to speakers (or audio). In the lab one should use a mixture of twisted pairs (preferably shielded) and coaxial cables; depending on the application (hence, if you are ONLY using coaxial cables in your lab, you are either working with high impedances or you are doing something wrong).

    There are also all sorts of ifs and buts to this; which is one reason why there are also other types of cables (notably triax).
     
  7. Sep 26, 2011 #6
    Hi yungman,

    Thanks for the interesting comment, I can't say I every actually tried playing with big cables but it is interesting to know that you find it makes a difference. I think that the highs and low frequencies make the harmonics and resonances which makes a sounds "open up " . Though I imagine that it less down to transmission of specific frequencies, more to do with a temporal response perhaps.... BTW just out of interest did you try a blind test? the 'placebo effect' can be very strong
     
  8. Sep 26, 2011 #7
    Hi f95toli,

    I think I agree with you on most things. However one 'but' that I think might be worth mentioning . For low frequency, the characteristic impedance of coax makes no difference, but it's shielding is much better than twisted pair. That's why we like to use it, also it radiates much less than twisted pair at high frequencies.


    If I'm wrong please tell me !
     
  9. Sep 26, 2011 #8

    f95toli

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    No, its not. It really depends on what kind of interference you are trying to shield from (inductive/capacitive pickup, RF interference etc). Also, for low-level signals one should of course use shielded twisted pairs (grounded at one or both ends, depending on the situation). Coax would be pretty useless if one was for example to try to do a simple 4-point resistance measurement of a low-ohmic load (a pretty common thing to do).

    See e.g. Ott's "Noise Reduction Techniques in Electronic Systems" or a similar textbook.

    But, yes. For high frequencies one would generally speaking use coax (but not always, network cables are twisted pairs).
     
  10. Sep 26, 2011 #9
    I don't remember ever having seen shielded twisted pair...

    Anyway I'll have a look at the book you recommend next time I'm at work.
     
  11. Sep 26, 2011 #10
    50 ohms or less coax is high impedance?

    Whereas 'figure of eight' or ribbon is 300 ohms.
     
  12. Sep 26, 2011 #11

    f95toli

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    Shielded twisted pairs are very common. Standard LAN (network) cable is a good example; shielded CAT5e is very common (contains 4 pairs in a foil shield)
     
  13. Sep 26, 2011 #12
    I have always been curious about this and I wonder if you've ever done any blind testing. What kinds of sounds is it most noticeable with? How much power are you delivering to the speakers? Which characteristics of the monster cables do you think are responsible for the difference in sound, less resistance, less inductance or what? Don't the wires inside the speaker cabinet from the terminals to the speaker itself have enough resistance and inductance to nullify the effect of monster cables?

    As you refer to cable pairs, I assume you have a stereo system rather than a 4 channel system. Since stereo and our ears are at best a two dimensional system, how is the third dimension perceived?
     
  14. Sep 26, 2011 #13

    f95toli

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    I was refering to the impedance of the source/load; the cable impedance is more or less irrelevant unless you are measuring at high freqiencies (although the cable capacitance/inductance can matter, so the impedance has an indirect effect).
    And yes, 50 ohm is a high(ish) impedance in many situations.
     
  15. Sep 26, 2011 #14
    No of course the cable impedance is not irrelevant.

    For the record.

    The standards for audio and telephone work are set at 600 ohms

    Most coaxial laboratory instrumentation uses 50 ohms, as does computer coaxial connections.

    Radio (including TV) uses 75 or 150 or 300 ohms.

    In all cases it is important to ensure the cable and loads are matched to avoid reflections.
     
  16. Sep 26, 2011 #15

    f95toli

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    But reflections are only an issue if your measuring at a frequncy where the wavelength is of the same order of magnitude (say lambda/4) as the length of the cables. In the lab this is usually only an issue RF/MW measurements (over say 10 MHz). It is complettely irrelevant if you are for example measuring a reistance with a multimeter.
    Low-noise measurements are much, much trickier near DC than at high frequencies.
     
  17. Sep 26, 2011 #16
    I was doing the test together with my wife. It was quite obvious. Believe me, I was a absolute non believer before. The reason I even tried this was because I bought a pair of JM Lab to replace the pair of Kef and I did not hear any significant improvement and I was getting desperate and looked into it. The better the quality the system, the more critical it is. Yes we went back a fore to verify.

    I was absolutely surprised.
     
  18. Sep 26, 2011 #17

    AlephZero

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    If anybody can give a rational scientific explanation as to what is so good about speaker cable that costs $1000 per meter, and which must for best performance be connected "the right way round" (it is marked to show which end to connect to the amp and which end to the speakers), go right ahead.

    Personally, I find flat 30-amp mains power cable works just fine. I don't really need more power than 1800 watts per speaker in my house, so I'm not going to exceed the rating of the cable!

    Cheap and nasty connectors with high and/or inconsistent internal resistance are a differnet matter. Fractions of an ohm are significant for high power low impedance wiring like loudspeakers. But copper wire is just copper wire, whether it comes from your local electrical store or insulated with snake oil and costing 10,000 times as much, IMHO.
     
  19. Sep 26, 2011 #18

    AlephZero

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    That has no relevance to speaker cables.

    Why would you want to connect an amplifier with an output impedance of say 0.01 ohms (if it's a good quality amp) to a speaker with nominal impedance 4 ohms, with a 600 ohm impedance cable?
     
  20. Sep 26, 2011 #19
    Have you ever heard of 100V line audio distribution systems?
     
  21. Sep 26, 2011 #20
    How long are your speaker cables? Have you checked the size wire used inside the speaker cabinet? Do your speakers have crossover networks? I was about to consider that an extremely low source impedance may have a damping effect on the speaker but wondered if the internal wiring of the speaker, including the crossover network provide enough impedance to negate any effect a low output impedance amp and low resistance wires might provide.

    Have you ever driven your speakers with a square wave to see how much overshoot and ringing you have?
     
  22. Sep 26, 2011 #21
    Feels like I'm beating on a dead horse here, but I had to look up this directional audio (i.e., AC signals -- should be obvious why the cable can't be directional!) cable nonsense. FWIW, this is from the FAQ of one of the most well-known manufacturers of such cables:

    Q: Will I notice a difference if I hook up my cables in the wrong direction?
    A: You probably will not, but if you think that you are experiencing noise problems, check that they are. Cables are directional for shielding purposes only. Should you not see these arrows, remember that the signal flow should go in the same direction of the print on the cable jacket, reading left to right away from the source (source to destination)

    Q: Why do the cables have directional arrows?
    A: This is done for shielding purposes only, a design where we do not solder the shield on the signal's destination. This ensures that any noise picked up by the shield will not be transmitted into your signal path.
    Yungman, the test you did with your wife produces very little valuable information IMO. As an absolute minimum, you should do a blind test where--among many other cognitive biases--the emotional pain of having wasted a lot of money on cheap copper wire is not allowed to impair your judgment. And you're also testing cables, interconnects and everything else in between your source and destination at the same time, so how do you know what contributed to what?

    EDIT: I'm not sure it matters much though whether or not you can prove objectively that one cable is better than another, since the listening experience is subjective in any case (i.e. you'll bring your personal biases to the table every time you use the system). But surely this topic has been discussed to death already so I probably haven't added anything new here!
     
    Last edited: Sep 26, 2011
  23. Sep 26, 2011 #22
    Actually this got me thinking this morning. I did the experiment in 97 when I first bought my speakers. I did not do a blind test, it was very obvious for the first two pairs of 12 gauge. When I said 4 pairs, I meant 4 pairs of 12 gauge for EACH speaker!!!:surprised!!

    Now I have been studying EM, I just did a calculation of the skin effect at 10KHz:

    [tex]\delta=\frac 1 {\sqrt{\pi f \mu \sigma}} =\frac 1 {\sqrt{3.14\times 10^4\times 4\pi\times 10^{-7}\times 5.8\times 10^7}}=6.6\times 10^{-4}m[/tex]

    As you can see, the current is not evenly distributed in the cable, most are still on the surface. So up to a certain point, the size of a single cable is not as important as the surface area of the conductor.

    I don't think you can quantify with just by the highs and the base of the sound. When the quality of the system is at certain standard, it is more the perceived sound stage and the three dimension quality of the system. You cannot really hear the high frequency, but you can feel it, the sharp attack of the sound. It is like if you put the tv sound into the system, every time the door open or the phone rings, you turn around thinking that's your phone or the door.

    I had surround sound, but I no longer hook it up because it never sound as good as stereo.

    Regarding to the volume, you'll notice a not as good system only sound good when you crank to volume up. A good system sound good in low volume. This is very noticeable. I don't even turn the volume up. I double the average power is more than a watt or so, the instantaneous power might be high, that's why you need a good power amp.
     
  24. Sep 26, 2011 #23
    All my speakers have cross over. I never touch the JM Lab, but I did experiment with the Kef. I really beef up one speaker I even put big wires parallel with the pcb trace in the cross over network. I compare with the other one, it only make little difference. Not even close to the difference with the external cable.

    The JM Lab and the true high end speakers have separate input for the woofer and the mid/treble speakers. So they beef up by default. I use two pair of 12 gauge to drive the woofer, two pairs to drive the mid/treble speakers. I don't think it make as much difference on the woofer, it's the mid/high that is more critical.
     
  25. Sep 26, 2011 #24
    Do you have a good pair of speaker to try before? If you speaker is under $1000 a pair, I don't think you need to worry about this. I am talking about $5000+ a pair.

    I am actually very skeptical about all this, it made a believer out of me. Value or no value is in the ear of the beholder. I don't even buy the uni-direction thing as I did compare and don't hear the difference. BUT I do know in RF pcb layout, we do specify the direction of the copper cladding. The difference between one pair and two pairs are so distinct that I never thought of blind test. I stopped at 4 pairs because I don't notice much difference between 3 pairs and 4 pairs. Psychosomatic never work on me particularly if I don't believe it and I never did until that time. I still don't believe in buying expensive cables, I just bough 12 gauge speaker cables from hardware store, the ones that has very fine strands. I don't even believe in buy expensive cable from audiophile store.

    Maybe if I buy a pair of $20K+ speaker, I might buy into the more expensive stuff, for now, the more cable the better.
     
    Last edited: Sep 26, 2011
  26. Sep 26, 2011 #25
    One side is about 10 feet, the other side is about 5 feet. I would make it shorter if I could at the time. No, I never test with square wave.

    There is still some black magic in audiophile and guitar amps that I don't quite understand. Human ears are more sensitive than any of the instruments. You can never explain why some of the extreme high end tube power amp is less than 5W single end tube only, brands like Cary. A lot of the high end power amp are still using tubes.

    You would think two power amp with the same rated power should be very close since they are all electronics and if you can make it to same minimal distortion, they should not be to far apart!!!! They are a world apart. This is another subject all together. My favorite amp is a European brand call YBL. I am just too cheap as I made the mistake of buying one that match the Kef and then I bought the JM Lab. I am too cheap to put the Acurus aside and buy that. In fact, the YBL was only 70W per side and the Acurus is 200W per side.
     
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