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Power cable ...

Discussion in 'Electronic Repair' started by Arfa Daily, Sep 27, 2007.

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  1. Arfa Daily

    Arfa Daily Guest

    I know that there is always furious debate on the audio groups about this,
    but in all the years that I have been mending this stuff, I have never
    actually been faced directly with it ...

    Yesterday, a boat-anchor Yammy turned up from one of the high-end dealers
    that I do work for, and with it was a power cable that the owner wants
    fitting in place of the one that Yamaha saw fit to put on when they designed
    it. This cable comprises a couple of metres of (20A?) three core rubber
    power cable (the sort of stuff that you would use as the flexible 'tail' to
    go from a wall plate to a storage heater, or maybe a hot water immersion
    heater) plus a very ordinary UK 13A power plug on one end, and a reasonable
    quality IEC straight plug on the other. The cost of this lead ? 100 UKP.
    That's about $190 at the current exchange rate !! And he now wants to pay
    the store to get me to fit it.

    Now I'm actually not very happy about modifying anything to do with hot-side
    power wiring, for obvious legal reasons, but my real question is about the
    number of wires. Originally, the amp is fed with a standard 2 core power
    lead. When this three core lead is fitted in its place, should I connect the
    earth lead to the metal chassis ? I can't see that this should lead to any
    potential safety issues, but as the amp was originally designed not to have
    a power ground connected, might not connecting one lead to *more*
    power-conducted noise getting in, actually making the performance *worse*
    than the owner thinks that he is going to achieve, by his dubious mods ?

    I have a contact within Yammy, who has direct access to the design boys back
    in Japan, so I think that I am going to give him a call anyway, but I would
    value the opinions of others on here as well.

    Arfa
     
  2. N Cook

    N Cook Guest

    If the user-accessible part of the chassis is metal then that has to be
    bonded to mains earth.
    2 core is only allowed for double insulated kit in the UK.
    Its not a quality of sound issue but a health and safety issue first and
    foremost.
     
  3. Ron(UK)

    Ron(UK) Guest

    Some of the older 'pro' Yamaha power amps are 'double insulated' in that
    the mains tranny is toroidal and totally isolated from chassis and all
    mains potential is contained within double sleeving and neoprene boots
    over the switches etc. The ground connection is provided via the signal
    leads.

    Ron(UK)
     
  4. **Personally, I won't do those jobs, UNLESS the cable is an approved
    type. Examine the cable and I'll bet you won't find any of the relevant
    electrical authority markings. It has probably not been tested nor
    approved. Some of the leads I've exmamined barely have insulation
    suitable for 12 Volts, let alone 240VAC. As for your earth question,
    yes, I'd earth it.

    Trevor Wilson
     
  5. Eeyore

    Eeyore Guest

    LMAO !

    So, the equipment is Class II (double insulated) and doesn't require an earth.

    It might introduce 'hum loops' for sure.

    It would be just desserts I think for the owner if that happned. It would serve
    him right for being such a crackhead as to believe in magic cables.

    What does the website of the cable manufacturer say about how its product works
    ? They may have some special magic view about the role of grounds !

    Graham
     
  6. Arfa Daily

    Arfa Daily Guest

    It is indeed a 'double insulated' item - as indeed most domestic equipment
    these days is, irrespective of whether it has a metal chassis / case
    accessible to the punter, or not. It of course carries the double insulated
    symbol to show this to be the case. So what I was asking was not primarily a
    health and safety issue, but a sound one, as the owner believes it to be. As
    I said, I don't think that connecting a mains earth to the metalwork of such
    an item - double insulated or not - represents any kind of safety issue, but
    as the equipment was originally designed to operate without a connection
    *directly* from the chassis to the mains power outlet, then it might in fact
    make the noise performance worse than 'as-designed', rather than better, as
    the nutcase who owns it, believes ...

    Arfa
     
  7. Eeyore

    Eeyore Guest

    Good point.

    To attach an unapproved cable would strictly be illegal. You could be held liable
    for any consequences too.

    Graham
     
  8. Wrong class of customer. Such things tend to be purchased by people
    with more money than technical abilities. Did you notice that the
    cord is rather stiff? That's intentional as no self respecting
    audiophile would hide such an expensive purchase behind their
    equipment. It's made to be hung out the front of the rack or shelf,
    where all can admire it.
    Ouch. One of these cables perhaps?
    <http://www.audioadvisor.com/products.asp?dept=25&view_all=true>
    Be sure to check out the $1,080 power cable.

    There was an article in EDN on the topic a while back. Digging...
    "OFC madness: Facts, not fantasy, regarding power cables for high-end
    audio equipment."
    If it was 2 wire, it would need to be double insulated. Grounding the
    chassis is always a good thing (unless it's a 40 year old AC-DC power
    supply). Yeah, I would ground it.
    Only if the AC power line noise were fairly high level (several volts)
    and the power supply design was relying on the common mode rejection
    of the AC power cord to remove the noise. A rather crude power line
    noise filter will remove all of that.
    Don't give them any ideas, or we may be plagued with $200 replacement
    power cords.
     
  9. Meat Plow

    Meat Plow Guest

    Sounds like you're opening up a can of worms.
     
  10. James Sweet

    James Sweet Guest


    I've never seen a piece of consumer A/V gear with a grounded power cord,
    regardless of what the case is made of, but it certainly won't hurt anything
    to ground it.


    Maybe you ought to manufacture some of these obscenely expensive power cords
    yourself, obviously suckers will buy them.
     
  11. Sjouke Burry

    Sjouke Burry Guest

    Hooking up mains plug ground to all audio equipment produces
    very nice ground loops. So I hope the customer is going to enjoy
    the nice 50 or 60 HZ hum and its harmonics.......
    A cluster of audio equipment should be grouned at one point
    only to avoid loops.
     
  12. Eeyore

    Eeyore Guest

    It MAY create problems. A grounded case will almost certainly ground the outers
    of the RCA jacks. That will provide a path to ground for any leakage currents
    and in turn that may result in 'hum' as currents flow where they weren't
    intended to.

    Graham
     
  13. Franc Zabkar

    Franc Zabkar Guest

    This month's Australian Silicon Chip magazine has an article on the
    hum and RF noise produced by 2-pronged DVD players. The author(s)
    tested quite a few of these players by connecting them to an old
    3-pronged amp. They found that audible hum was introduced when earth
    current flowed from a player's switchmode PSU to the amplifier's earth
    via the analogue signal leads. Additional radiated interference was
    attributed to the digital noise produced by the main decoder chip, and
    to the 100kHz switching frequency of the PSU.

    The only solution recommended by the author(s) was to replace the SMPS
    with a linear one. Doing so eliminated the hum and PSU noise.
    - Franc Zabkar
     
  14. Eeyore

    Eeyore Guest

    100% predictable. The usual cuplrit is the Y caps in the SMPS of the 2 wire
    equipment which results is significant (but legal) leakage current of up to
    around 2 mA. Of course, this won't happen if the amplifier is a 2-wire type too
    since there's no earth for the current to flow to.

    So, adding an earth to a Class II amplifier is likely to add 'hum'. Simple
    really.

    Graham
     
  15. Franc Zabkar

    Franc Zabkar Guest

    The same article measured the chassis potentials of the various
    devices. One measured 81VAC with respect to earth, others measured
    115-117VAC. In some devices the A-E and N-E RFI suppression caps were
    not installed. However, all appeared to have a coupling cap between
    the AC and DC sides of the switching transformer. One end of this cap
    appears to be connected to the case, while the other end probably goes
    to the negative side of the bridge.

    I would think that it would still be possible for a noise current to
    flow from one case to another if the chassis potentials were
    significantly different.
    - Franc Zabkar
     
  16. Eeyore

    Eeyore Guest

    That one's a killer. The idea of it is to return switching noise on the output to
    the low impedance of the bulk filter cap. Of course it also results in the high
    frequency content of rectified AC being passed to the secondary side.

    Or positive. Doesn't matter which especially but returning it to the negative side
    can interfere with the regulation circuitry operation.

    I suppose that must be possible but maybe it'll be of a smaller magnitude.

    Graham
     
  17. Not so. If the power supply and mains wiring etc within the amp conforms
    to the standard there is no need to earth the metal chassis. Otherwise
    every single metal cased amp etc would have the casing earthed and hum
    loops would abound.
     
  18. Lots of Far East gear in the early days had grounded cases to pass UK regs
    without going to the cost of double insulation. And when you plugged
    separates from the same maker together using the provided leads you got a
    ground loop...

    Grounding an amp case only may well be ok since that will be the 'master'
    part of any system. But doing the same to other separates will almost
    certainly create hum.
     
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