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10 metres audio cable going into PC = too long?

Discussion in 'Electrical Engineering' started by Andy, Apr 18, 2006.

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  1. Different resistance/impendence between different parts of the
    equipment and "ground". Whatever that is.
  2. tony sayer

    tony sayer Guest

    Thats not strange as such, just the general lack of 3 phase

    Why do you still use 115-120 supplies what with the extra current
    demands, or is there still a perceived electric shock issue?.....
  3. tony sayer

    tony sayer Guest

    Thanks for that enlightening post. It may well be that we don't have the
    centre tapped supply arrangement. In the local substation Y arranged,
    the centre is earth connected and referred to as the Neutral return and
    each phase is then carried on a three conductor cable with the wire
    armouring used as the neutral return and its also connected as the
    safety earth.

    as strange as that may seem;).....
  4. Jim Lesurf

    Jim Lesurf Guest

    To be able to give a complete answer we would need to know the output
    impedance values for your stereo and TV.

    The output impedance (resistance) will tend to combine with the cable
    capacitance to make an RC low-pass filter. This may or may not matter, but
    to estimate the effect we'd need to relevant values.


  5. Arny Krueger

    Arny Krueger Guest

    All true. It's called "legacy technology". Unlike Europe, the US missed
    out on the cleansing benefits of being the site of a world war.
  6. Arny Krueger

    Arny Krueger Guest

    Most US factories that use substantial amounts of power use 3 phase. It's
    mostly just the residential areas and isolated light industrial areas that
    lack it.

    Electric power use per capita in the US is closer to being uniform or
    decreasing, as opposed to there being extra current demands.

    "Total primary energy use per capita in the United States in 2000 was almost
    identical to that in 1973. Over the same 27-year period economic output
    (GDP) per capita increased 74 percent"

    Since 2001 or so, there has been a major conversion of existing residential
    lighting to compact fluorescent bulbs which produce about 3 times as much
    light per watt.
  7. Jim Lesurf

    Jim Lesurf Guest

    Since I am reading (and replying) on an 'audio' newsgroup, my assumption is
    that people may be concerned up to around 20kHz, and want to be aware of
    any changes that might be considered worthy of note in the context of using
    good audio equipment in a domestic situation.

    If I assume a cable capacitance of around 100pF/m, then a 5kOhm source
    would, I think, give a roll-off of the order of 1dB in the 15-20kHz region.
    (Assuming I managed to push the right buttons on my guess-box. :) )

    On this newsgroup I'd normally expect people to regard that as being large
    enough to be worth at least being aware of it.

    You are probably correct. Alas, consumer equipment sometimes dissapoints
    out expectations - particularly when the makers have failed to provide the
    relevant data for the users. ;->
    Once one becomes aware of the problem, and knows what adjustment to make
    to correct for it appropriately. Hence my initial comment. :)


  8. Jim Lesurf

    Jim Lesurf Guest

    I would agree that modern circuits that use IC amps with feedback should be
    able to provide nominally low o/p impedances.

    However, although in general you are probably correct, the snag is that
    life may not always be that simple. :) For example, some outputs may
    have a low nominal output impedance, but have a relatively low current
    limit. Hence they may act like a low impedance source for low currents, but
    show slew-rate limiting when asked to drive a high capacitance load (long
    cable). For this reason the designer may even have included o/p series
    resistors to protect against this and avoid the IC being overloaded.

    As I indicated, all we have so far is that the source is a 'stereo or TV'
    with no details of their actual outputs. Hence it will probably be OK, but
    as I pointed out, we can't really be sure in the absence of the relevant

    IIRC the old IHFA-707 specs required audio signal sources to be tested with
    loads of 1000pF//10kOhms as the 'worst case'. I don't know what standards
    (if any!) makers of domestic TVs routinely adopt these days, but 10 meters
    of cable does seem to me to bring us into the area where the 1000pF limit
    might be an issue.


  9. Stacia

    Stacia Guest

    I'm a complete newbie when it comes to this stuff, so I'm jumping in
    with a question and hoping I'm in the right place.
    We have always hooked up our computer (in one room) to our stereo (in
    another) so we can play audio from the computer through the stereo
    speakers. We recently moved and in this new house we're getting a hum
    or buzz on the line when we play music from the computer. The cable
    from computer to stereo is 30 feet and there was no hum in the audio
    when we did this in our previous house.
    What can we do to get rid of the hum? We've tried physically moving
    the cable but that hasn't changed anything.

  10. tony sayer

    tony sayer Guest

    Yep, well thats balanced operation which as you say will go for miles
    over telephone copper lines without humm..

    Oddly enough in the UK they don't as a rule use screened cable, the
    twisted balanced pair has very good rejection.

    I seemed to think we were talking about domestic unbalanced lines
  11. GregS

    GregS Guest

    Ground loop isolator.

  12. tony sayer

    tony sayer Guest

    Over here it seems to, well the half a dozen or so I've looked at!. We
    we're involved in a short term radio broadcast some years ago and the
    cable co supplied free of charge a few circuits about 3 odd miles to
    link Two studios together, and apart from a small amount of HF
    hum at all or other noise for that matter and all that cable was
    Balanced working.. ever read up about it or used it in practice?...
  13. Karl Uppiano

    Karl Uppiano Guest

    Virtually all power is generated as 3-phase...
    Really? I'm not aware of any large scale generating facilities that generate
    single phase, since you can always extract single phase from 3-phase.
    No doubt 3-phase delivery is a problem in areas where they run just a single
    phase. But using the original 3-phase rather than reconstructing 3-phase
    from single phase has got to be more efficient. It also seems more reliable,
    since the converter is a piece of rotating equipment (basically a
    motor/generator set, IINM). Yuck. I bet the energy conversion efficiency is
    less than 70%.
    It would be extremely expensive to retrofit large areas of legacy 1-phase
    distribution with 3-phase today. However, I'll bet that the main substation
    gets 3-phase feeds, which it parcels out as single phase in some
    load-balanced arrangement from each of the 3 phases.
  14. Arny Krueger

    Arny Krueger Guest

    When the power company wants you to cough up $35,000 or so for running 3
    phase from where it is, up to your building, you've got to be talking pretty
    heavy use to cost-justify the new lines.
  15. tony sayer

    tony sayer Guest

    Doesn't matter either way as long as its balanced working and in any
    case telephone bandwidth isn't that responsive to 'ummmm...

    Anyways these days in the UK the copper part isn't that long in new
    cable co installations, the fibre to copper conversion is done very
    locally to a subs premises and in the BT system the copper is longer but
    doesn't humm..
    Yep but they don't use shielding on a lot of phone multicore in the UK
    and it wouldn't matter anyway..
    Yes.. Thats got some good points but they don't seem to be very savvy on
    some matters about EMC and RF and you can pick a few holes in that but
    yes their correct in screening or shielding earthing at both ends
    provided that the balance in the sending and receiving ends is what it
    should be, injecting current into the shield won't affect what's carried
    in the encased conductors. However in practice the final result is and
    can be affected by transformer and electronic balanced inputs and how
    "floating" they are.

    I think we could all agree that balanced working isn't really a problem.

    Now they mention unbalanced working, but haven't given it much

    Now ASCII art permitting are we agreed that the following isn't going to
    cause too much upset?..

    A ________________________________________________________________ M


    Poxy ASCII!. Now consider A is an amp input and M is a source microphone

    The dotted lines are the shield on a lump of single cored microphone
    cable. Now the amp is connected A to the centre conductor at the amp end
    the screen to the earthed side of the amp input, at the other end the
    microphone has say a phono type connector, and the mic is a dynamic
    moving coil type with one end connected to the inner shielded conductor
    of the cable, the other end is connected to the outer shielded
    conductor, the mic is in a metal case and is connected to the shield of
    the cable too.

    The mic case is not connected to any earth, other than the outer shield
    of the connecting cable, and lets say thats 10 meters long or 12
    yards;) The mic is suspended in free space by a lump of nylon cord and
    isn't connected to anything else at all...

    Now are we agreed that that arrangement will or won't hum?......
  16. tony sayer

    tony sayer Guest

    Yes they do, in fact we've got a broadcast transmitter site which is fed
    by a bit of BT, (British Telecom, the national Telco), overhead wire
    for some miles and no hum at all!. And that is on the same pole set as
    240 volt mains wiring and I've actually seen 11 kV lines with phone
    lines near them. Not that advisable owing to the safety factor!.

    Yes of course you can get leakage via induction and capacitance into the
    telecom lines but this does not matter as it will inevitably be induced
    in both conductors and cancelled out by common mode rejection. Doesn't
    matter providing the insulation in the line and transformers will stand
    it to have some kilovolts actually on the line as such...
    Can you explain how your measuring or have that configured please?..
    Often less than in ntl or telewest installations but longer in BT ones.
    Ntl care the cableco in the UK but that name is to disappear and their
    to be called Virgin!...
    Nope;!, just a way of putting that, see above,...
    In a central office most all of it here is twisted pair. I think some
    terminology things betwixt the UK and USA are showing up here. All the
    cable co Telco multicores I've seen, though not all, are unshielded.

    What do you define shielding as, just a wrap of aluminium foil with a
    drain wire or a fully woven copper mesh?..
    OK then, part 2 "On the other hand cable shields which are bonded at one
    end etc". Read that thorough carefully, doesn't make sense. Then take a
    lump of Andrews 4-50 Heliax and see what a good radiator that is even
    greater number of wavelengths . They didn't even state if it were open
    circuit or terminated on a load...

    Actually we've had a lot of EMC experience over the years in radio,
    audio and automotive environments and what's made by far and away the
    biggest effect is bypassing of transistor junctions at RF
    Were is this noise coming from then?...
    Well think about that, Say we have a cable the inner pairs are wrapped
    around one of the power lines that you describe, and there are a LOT of
    volts induced on that wiring. OK now into a transformer there will be
    galvanic isolation i.e. the ends or centre tap of that transformer isn't
    connected to anything. Now take a electronically balanced input. At some
    point that will be connected to say an input IC which will have supply
    rails etc, and that IC will be coupled through to the output of that
    line receiving amplifier now don't you think that if there were some
    matter of kilovolts on said line, then that will break down the
    transistor junctions ?..
    I think you have that wrong. Provided that the rejection is what it
    should be then whatever is induced on the pairs will cancel out.
    What we've been discussing. Take a signal source and connect a
    transformer thereto and connect that to a pair of wires twisted together
    and then connect that to another transformer and the out put winding of
    that to a load. That do?..
    Yes agreed and you wouldn't do that, well not in a pro environment

    Now if say you ground that to the local mains earth at one end, and say
    10 meters away at the microphone case end earth that to a driven rod
    earth, will it or wont it hummmmmmmmmmm?.....
  17. Ah. THAT'S why America is picking fights with everyone! Fair
    enough. Where would you like bombed first? :)
  18. Arny Krueger

    Arny Krueger Guest

    Oh come on, we're very selective with the fights we pick. Our targets are
    always very weak.
    Someone already did a big number on downtown Manhattan, but it was not
    sufficient to get a new electrical code written.
  19. World Trade Center?

    Oh, wait a minute... :)
  20. You don't ("you" being the generic "Usenet user"). "He" on the other
    hand does. On many occasions he's quoted back several pages and added
    one or two lines to the bottom... the word "trim" doesn't exist in his
    vocabulary. One wonders if he uses AOL.

    Still, it could be worse. At least it's not top-posted HTML.
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