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75 ohm coax for audio. Possible?

Discussion in 'Electronic Design' started by Peter Nielsen, Oct 29, 2003.

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  1. I need to run cable to some 150W speakers for a one-off peformance
    installation. I have a big roll of top quality 75 Ohm coax on hand.

    Rather than buy new speaker cable, is there any way I can make use of
    the coax?

    Is it just a matter of cranking up the volume, or do I risk blowing
    the power stage.

    The cable run is 15-20 metres.

    Thanks for your reply.

    Peter Nielsen
  2. Dave VanHorn

    Dave VanHorn Guest

    Use two runs, and only connect the shields. IOW, each leg is a single
    The center conductor may be rather resistive over that distance.

    Audiophile delusions not withstanding, only the resistance of speaker wires
    really matters, in any real world situation.
  3. Mike Page

    Mike Page Guest

    And even resistance doesn't matter that much as an absolute quantity,
    but you should pay attention to current handling (the two are related).
    At 150W and 4 Ohms, you'll be pushing through 6A. Do you have some 6A
    mains cable instead ? At least then you don't have to worry about it.
  4. Ban

    Ban Guest

    Peter, do not use the cable with the signal on center and shield, because
    this will have a substantial capacity (90pF/m), which might make the amp
    unstable or even blow it. What you can do is to connect shield and inner
    connector together and use two runs of that to power one speaker. Still the
    resistance will eat a lot of your amplifier power, so I advise you to rather
    get at least 2*4mm^2 cable.

    ciao Ban
  5. GPG

    GPG Guest

    Get a drum of heavy twin flex, we call it trurip here. Waste using
    co-ax outer for this
  6. Tim Shoppa

    Tim Shoppa Guest

    "Top quality 75 Ohm Coax" doesn't tell us much. If it's RG-59, that
    means the center conductor is 22 gauge wire. There is 75 Ohm Coax with
    substantially thicker center conductors (some are thicker than my wrist),
    but it's probably not what you have.
    20 meters of 2-conductor 22 gauge wire is probably a few bucks and will
    give similar performance. 20 meters of 16 gauge zipcord will have a fraction
    of the resistance and cost a buck or two more.

  7. Chris Hodges

    Chris Hodges Guest

  8. Fred Bloggs

    Fred Bloggs Guest

    OMG- another top quality utterance from Australia! Lessee the power
    attenuation will mean the power stage should supply (75+8)/8>=1500W to
    drive that setup- probably can be done with scrap car batteries running
    off kangaroo urine electrolyte. The cable should be buried for heat sink
  9. Notwithstanding the fact that 75 ohm coax does not have a series impedance
    of 75 ohms to audio signals, you might be right.
  10. John Larkin

    John Larkin Guest

    Fine, no problem. Just do it.

  11. Yes, and not hardly noticably, the series impedance is probably only
    an ohm or two.

    Best regards,
    Spehro Pefhany
  12. Mac

    Mac Guest

    What exactly are you talking about? What is it you think you are

    The load, properly speaking, can be viewed as 8 Ohms, at the end of a
    75-Ohm transmission line. If we take 20kHz to be the upper limit worst
    case frequency, then the wavelength is 15,000 meters, so the cable is
    still so short that we can ignore transmission line effects. Which means
    that the load is the sum of the DC resistance of the cable plus the
    impedance of the speakers. Presumably 8 Ohms.

    And the power delivered to the 8-Ohm speaker load, then is total power times
    8/(8+R) where R is the cable resistance. So it IS just a matter of turning
    up the power, most likely.


  13. Mac

    Mac Guest

    What are you talking about? The capacitance of the cable is immaterial.
    What we have is a transmission line whose length is a tiny fraction of a
    wavelength of the highest frequency of interest. It is terminated in a
    mismatch (8 Ohms vs 75), but since it is so short, we don't care.

  14. Ban

    Ban Guest

    Really Mac,
    you do not explain anything. The cable certainly has a capacity, we are at
    near DC (so to say) and then there are no more transmission lines...
    In fact the 1.5nF of that cable against maybe 500p of a normal speakercable
    shouldn't make much difference, but if the amp is already at a low phase
    margin and there might be additional crossover capacity, this might result
    in oscillations or instability.
    Put this 20m coax on the output of an opamp and you will see what happens

    Also subwoofer speakers have usually 4 ohms nominal 3.5 ohms at DC, so a
    1.5-2 Ohms cable resistance will consume around 30% of the power, which is
    more of a concern than the capacity.

    BTW: You should study that bit with transmission lines and lumped elements
    before you write statements like above.

    ciao Ban
  15. Mike Page

    Mike Page Guest

    Any amp that can't drive several tens of nF is a dud. What do you think
    a piezo horn looks like ?
  16. Fred

    Fred Guest

    Have you made the calculation of how many meters you need to have a marked
    effect? I think the resistive losses will kick far earlier than the 1-2km
    of to get the odd fraction of a microfarad. LOL

    I would suggest the use of good mains cable. 2 core but in a round sheath
    because the conductor have a slight twist and so help the audiophiles in
    respect of stray magnetic fields etc.
  17. Mac

    Mac Guest

    Well, I wasn't very eloquent. But my whole point is that because the
    transmission line is a tiny fraction of a wavelength, we can ignore
    transmission line effects. This means that we can ignore both the
    capacitance and inductance of the cable. (The capacitance of the cable is
    one portion of the lumped-element transmission line approximation. I don't
    know why you are ignoring the inductance which is a series element and
    precedes the capacitance)
    Not likely. As you know, the impedance seen by the amp is what determines
    if it will be stable. Transmission line theory tells us that when the
    transmission line is short with respect to the wavelength, the impedance
    at the input will essentially be the load impedance.

    Furthermore, audio amps tend to be pretty stable because there's no
    telling what kind of a load a speaker may present.

    But my point, really, is that if you use the full lumped element
    approximation, there is an inductor, followed by a capacitor in parallel
    with the load. The inductor impedance and capacitor admittance are very
    small, so the net effect is nill.

    There is an equation for the impedance looking in to a terminated
    transmission line which is as follows:

    Zin = Zo * (ZL + jZo * tan(Bl))/(Zo + jZL * tan(Bl))

    Zin is the input impedance, Zo is the characteristic impedance of the
    transmission line, ZL is the load (termination) impedance, and Bl is the
    length of the transmission line, in degrees (or radians) of the frequency
    of interest. In our case, Bl is so small that we can assume the tangent
    term is zero. So the whole thing reduces to:

    Zin = ZL

    My whole point is that as long as the cable is connected to a
    load, we can ignore the effects of the cable (except DC resistance)
    because the impedance of the load itself will dominate. This follows
    directly from transmission line theory.
    There is no problem for an opamp to drive 20m (or 1000m) of coax as long
    as the coax is terminated with the characteristic impedance of the coax.
    All the opamp sees is a resistive load. I just recently designed an amp
    board which works up to UHF frequencies, and the opamp is very sensitive
    to capacitive loads. But it works fine with 10 or 20 feet of coax, as long
    as the coax is terminated in 50 Ohms. I should add that not all opamps can
    drive 50- or 75-Ohm loads, although this one can.
    Well, the OP didn't say anything about subwoofers, but he clearly
    understood that he would be losing power in the cable, and he seemed to be
    OK with that.
    I hope I have convinced you that I have a reasonably good understanding of
    transmission line theory. ;-)

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