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Mains quality

Discussion in 'Electronic Design' started by Eeyore, Apr 5, 2009.

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  1. Eeyore

    Eeyore Guest

    A friend and colleague of mine is investigating with me methods of
    eliminating audible clicks in high-end audio caused by the swiching of
    other loads in the vicinity.

    Some of it may be radiated from 'loopy' wiring but I reckon a lot comes
    up the mains as common-mode noise in the audio band. So An MOV etc won't
    and doesn't help.

    As a diagnostic, I'v suggested building a diff amp that can check both
    common and differential mode noise by examining the mains and notch
    filtering the fundamental in DSP. But the waveform is also distorted
    (typically 'flat-topped'), so harmonics would need to be filtered too.

    Does anyone have any numbers for the harmonic content of a 'typical'
    mains supply ? Voltage, not current.

    Graham
     
  2. Tim Williams

    Tim Williams Guest

    5%.

    Tim
     
  3. Eeyore

    Eeyore Guest

    I've never seen it that high myself, only about 2.5% but I was thinking in
    terms of the harmonic distribution, as all those of any magnitude will
    have to be filtered as well as the fundamental.

    Graham
     
  4. Eeyore

    Eeyore Guest

    No I wouldn't ! I've seen classic BAD EMC tactics used by supposed 'experts'
    who people respect.

    You're not kidding.

    One of the more intriguiging examples I came across of EMC was a very quiet
    mixing desk with an external power supply. One of my own design but the
    'manager' designed the simple PSU with an E-I . Just the routing of the earth
    / ground wire inside the PSU would affect the noise floor. The trick was to
    'tack it' to the steel case so as to minimise radiated pickup. You could wave
    it about and see the 50 / 150 Hz go up and down. Mind you we were operating at
    a very low noise floor, yet such effects may explain other audible differences
    where it's not so obvious.

    I like to use chassis as earth / ground. That's pretty much the EMC rule.

    Graham
     
  5. You mean circuit "zero volts" too, right?
     
  6. Eeyore

    Eeyore Guest

    LOL.

    Don't even get me started !

    Graham
     
  7. GregS

    GregS Guest

    As a rule of building equipment, you ground the chassis for protection,
    and shielding for the circuit.I don't ground the circuit usually, except
    for a static discharge resistor. My old Hitachi preamp used
    100 ohm resistor from the inputs jack common to the chassis ground
    which was not really grounded since it had a 2 wire cord. Keeping circuit
    impedance high to ground helps keep ground currents low.

    greg
     
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