Help requested on blind cable test

Discussion in 'High End Audio' started by Michael Mossey, Nov 28, 2003.

  1. I'm about to compare interconnect cables blind. I'm trying to
    establish for myself whether cable quality is an important factor, and
    whether interconnects of similar construction can vary in subjective
    sound.

    I'd like to use two cables that are electrically similar. Right now I
    own two cables that might be fit for comparison: a 2M Radio Shack
    "gold" special, and a Transparent 2M Link 300. I thought I would
    measure the capacitance of each cable using this rather cheap meter
    that I own. Turns out the Radio Shack is 400 pF, and the Transparent
    audio is 280 pF. Do these numbers sound reasonable? I'm not sure if
    my meter is working or if I'm doing this correctly.

    And my EE knowledge is rusty. I want to figure out how this
    capacitance might affect the transfer function. This interconnect
    runs between a CAL Sigma II DAC and an Audio Research SP-6. What is
    the impedance of a capacitor again? Something like 1 / ( 2 * pi *
    frequency * capacitance) ?

    Since I suspect that cable quality is real and can be heard blind if
    the test is carried out properly (my theory is that quick switching is
    an improper way to seek out small changes in sound), I'm interested in
    results that would challenge the scientific field of psychoacoustics.
    How small a difference in transfer function is considered inaudible?

    Regards,
    Mike
     
    Michael Mossey, Nov 28, 2003
    #1
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  2. A difference of less than 0.3dB may be considered "pretty inaudible",
    especially at the top end of the audio range. The capacitance values you
    quote are a tad on the high side, unless the interconnects are very long.
    Nevertheless, they're unlikely to amount to a noticeable HF reduction.

    I'm not saying you won't hear a difference, but if you do, the capacitance
    and its attendant HF reduction per se isn't going to be the cause.
     
    Bruno Putzeys, Dec 1, 2003
    #2
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  3. These are reasonable figures for 2 metre cables.
    The important thing is the output resistance of the DAC, which forms a
    low-pass filter in conjunction with the cable capacitance. If it's
    less than 300 ohms, there should be no audible treble droop with
    either cable.
    If that's your theory, then you are indeed challenging a century or
    more of psychoacoustics! :)
    Typically, 0.5 to 1dB level difference is audible. Freequency droops
    are less obvious, you might need as much as 2-3dB at 20kHz to be
    audible.
     
    Stewart Pinkerton, Dec 1, 2003
    #3
  4. Sadly, I suspect that this is true! :-(

    Of course, we must assume young and undamaged ears if we are not to be
    accused of excessive 'lowest common denominator' thinking.
     
    Stewart Pinkerton, Dec 2, 2003
    #4
  5. Michael Mossey

    Svante Guest

    ...and he would be the perfect person to conduct this test! If he
    really beleives in his theory, he will turn every stone to find the
    difference, let us just hope that he does not forget what "blind"
    means. :)
    I have performed blind tests with music, and I could hear a first
    order 47 kHz lowpass filter. Yes, I had a random generator connecting
    the filter for me, and seven correct responses out of seven tries,
    giving me 99% confidence. Need I say that the difference was not
    large? Note that this does not not mean that i hear frequencies at 47
    kHz, sadly the limit is somewhere around 15kHz nowadays, but a 47 kHz
    first order lowpass filter has a drop of about 0.42 dB at 15 kHz and I
    am pretty convinced that this is what I heard.

    So, if we play with the thought that the output resistance and cable
    capacitance form a first order lowpass filter, 400 pF and 47 kHz means
    that the output resistance of the driving amplifier should be 8.5
    kohms. Probably the output resistance is 100 ohms or less, so the
    cutoff frequency should be higher.
     
    Svante, Dec 2, 2003
    #5

  6. What test material were you using? Music or noise?



    --

    -S.

    "They've got God on their side. All we've got is science and reason."
    -- Dawn Hulsey, Talent Director
     
    Steven Sullivan, Dec 2, 2003
    #6
  7. Michael Mossey

    Norbert Hahn Guest

    Most likely you heard changes due to phase shift which start to get
    noticable at 1/10 of the frequency of your filter, in this case at
    about 5 kHz. You may repeat the test but use pink noise rather than
    music and you should no longer be able to detect the presence of the
    filter.
    So I think that you didn't hear the 0.42 dB drop at 15 kHz but the
    phase shift. OTOH, at 7.5 kHz the drop ist 0.2 dB which may not be
    audible as loss of trebble but as a slight veil on the music.

    Norbert
     
    Norbert Hahn, Dec 3, 2003
    #7
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