Tisbury Audio Domino - Initial Measurement Oddity

Discussion in 'Source Measurements' started by Azimuth, Mar 1, 2021.

  1. Azimuth

    Azimuth FKA rtaylor76, Friend

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    This is not a full measurement review of the Tisbury Audio Domino Phono Preamp, but only something odd I stumbled across.

    Setup:
    Presonus Audiobox 44VSL
    MCM custom audio reverse RIAA for MM input from line input
    ARTA software

    Using the ASIO, I was just doing some basic sweeps and saw something funny. Here is a frequency sweep of all four gain modes:

    Tisbury Audio - FR - initial 001.png

    Notice the sharp roll-off at 15k. This looks like a notch filter or something. You can even see the ringing wavy lines in the lower frequencies and then a roll-off in the low end below 100hz.

    The 49db gain also has a slight roll-off around the same point, just not as sharp. Which is the the first gain set, which would make sense if is before the filter, which is why the input impedance to the filter section is throwing the filter off.

    Anyway, things get even more weird when I switch to Periodic Noise and MLS in measuring 67db gain. Note, 67db requires gain 1 and 2.

    Tisbury Audio - 67db.PNG

    The yellow is of course the sweep, the violet is the Periodic Noise, and the red is MLS. Note, I did not change the gain setting on my interface. Normally, the noise or MLS follows nearly exactly the sweep. It does on the other gain settings, although on the 58db, the roll-off is flattened out on Periodic Noise and MLS.

    And yes, I am in the 47kohm input like I should.

    Now I am sampling at 48KHz. When I move up to 96K, the roll-off is still there, just no lift and no oddities below 100Hz:

    Tisbury Audio - sweep 96k.PNG
     
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    Last edited: Mar 1, 2021
  2. Azimuth

    Azimuth FKA rtaylor76, Friend

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    For comparison, here is just a frequency sweep in 96k sampling:

    Sweep - 96k.PNG
     
  3. purr1n

    purr1n Super Friend

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    You could be overloading at the highest gain. Try reducing input signal level feeding the phono pre or reducing gain / padding down input level at the ADC.

    As for the treble roll-off at the second to lowest gain, beats me.
     
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  4. Azimuth

    Azimuth FKA rtaylor76, Friend

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    I think you are correct. I think the sweep was getting to close to the anti-aliasing filter.

    Let me back it off.
     
  5. Azimuth

    Azimuth FKA rtaylor76, Friend

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    Ok...here is is again, this time input is -2db and input is also down about the same. Note, the software compesates for the lower gain structure, so if I run -2 input, it shows 0db on the graph. So I bumped the input down just a touch to make sure nothing strange going on with 96k A/D sampling:

    Sweep - 96k down2.PNG

    It still shows a slope at the second lowest gain, although not quite at 15k.

    It also still shows downward slope on bass on highest gain, as well has sharp cutoff on high frequency. My guess is that it is running out of steam on high gain. You can also still see the wavyness.

    Ok...now I am getting somewhere. Full report on this and a few other phono preamps later.
     
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  6. purr1n

    purr1n Super Friend

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    LOL, if you were Amir / Audio Science Review, you would just post and crap on the product! Especially, if they had not yet paid you protection money. Just saying.
     
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  7. Azimuth

    Azimuth FKA rtaylor76, Friend

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    LOL, true. At least now it does not look as bad as the first set of measurements.

    Oh, and subjectively sounds like a concrete wall. Bland and dull.
     
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