Shortest Way 51+ technical measurements

Discussion in 'Headphone Amplifier Measurements' started by atomicbob, Oct 6, 2019.

  1. mdr30

    mdr30 Acquaintance

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    Thanks for added measurements and comments, quite illuminating.
     
  2. atomicbob

    atomicbob dScope Yoda

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    And where else on the internet can one find such extensive, detailed information on a relatively obscure headphone amplifier?

    Support your local SBAF!
     
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  3. atomicbob

    atomicbob dScope Yoda

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    Additional discussion of SW51+ output voltage vs. distortion.

    Maximum output voltage is dependent on load impedance and distortion level tolerated. A much higher Vpp (Voltage peak to peak) may be achieved if greater than 1% distortion is considered.

    Several power measurements are presented in post 20 here Voltage output for a given distortion may be determined by reading the dBu on the x-axis for a given THD on the y-axis.

    Distortion vs. amplitude plots distortion in either percent or dB below the stimulus on the y-axis and amplitude on the x-axis. My power measurements cited above were made with amplitude in dBu which is an absolute: 0 dBu = 0.775 Vrms. Using the formulae below dBu may be converted to Vrms and then Vpp:

    dBu to V conversion formulae - negative.png

    Example conversions:
    20191008-1a SW51+ 6Z51P THD+N vs dBu 1000Hz 300R HiZ 0dB gain - annotated.png
    Example 1:
    For a 1 KHz stimulus the left channel (blue) measures 0.2% THD+N at -10 dBu. Using the formulae above -10 dBu calculates to 0.245 Vrms and 0.693 Vpp.

    Example 2:
    For a 1 KHz stimulus the left channel (blue) measures 2.0% THD+N at +10 dBu. Using the formulae above -10 dBu calculates to 2.45 Vrms and 6.93 Vpp.


    Bifrost-2 + SW51+ system voltage vs distortion oscilloscope measurements at 440 Hz

    Measurement setup

    DAW 440 Hz 0 dBFS sine output to Dante Virtual Soundcard
    Dante RDL spdif output routed to bifrost-2
    bifrost-2 RCA 2Vrms to SW51+
    SW51+ HiZ output with 300R load substituted for HD800-Jmod

    Signal routing diagram:
    20191117-0 RDL SF-ND2 - B2 - SW51+ - HD800.png


    20191117-1 bifrost-2 0dBFS SW51+ 6Z51P max Vpp 1 pct THD 440Hz 300R HiZ small.jpg

    SW51+ volume knob setting (above) for 1% THD when driven by Bifrost-2 440 Hz 0 dBFS 2Vrms - SW51+ output 3Vpp (below):

    20191117-2 SW51+ 6Z51P max Vpp 1 pct THD 440Hz 300R HiZ Vpp.png

    FFT associated with SW51+ 440 Hz 1% THD at 3Vpp:
    20191117-3 SW51+ 6Z51P max Vpp 1 pct THD 440Hz 300R HiZ FFT.png



    20191117-4 bifrost-2 0dBFS SW51+ 6Z51P max Vpp 7_5 pct THD 440Hz 300R HiZ small.jpg

    SW51+ volume knob setting (above) for 7.5% THD when driven by Bifrost-2 440 Hz 0 dBFS 2Vrms - SW51+ output 18.4Vpp (below):

    20191117-5 SW51+ 6Z51P max Vpp 7_5 pct THD 440Hz 300R HiZ Vpp.png

    FFT associated with SW51+ 440 Hz 7.5% THD at 18.4Vpp:
    20191117-6 SW51+ 6Z51P max Vpp 7_5 pct THD 440Hz 300R HiZ FFT.png
     
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  4. atomicbob

    atomicbob dScope Yoda

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    Explanation of corrections from impressions thread.

    How mistakes happen: complicated manual system setup + multiple serious interruptions.

    In the impressions thread I had incorrectly stated the SW51+ gain was 1.72 as measured on the dScope. However the picoscope measured approximately 3.26 as distortion was rising. There should be a change but typically the gain drops as distortion rises. Here is how the error occurred.

    dScope setup for fully balanced measurements but output derived from unbalanced BNC
    Generator has been set for 2Vrms
    Note the analog output mode set to Balanced normal - this panel is not usually displayed - it is displayed here to make the point
    SW51+ L ch output measures 3.43Vrms
    20191117-21 SW51+ 6Z51P 2V in 3_43V out 440Hz 300R HiZ dScope Bal Norm output amplitude.png

    Gain incorrectly determined as 1.715, which is 4.7 dB
    20191117-23 SW51+ 6Z51P 2V in 6_86V out 440Hz 300R HiZ dScope Bal Norm output gain.png

    Changing analog output mode to unbalanced with the same generator level of 2Vrms
    Now SW51+ L ch amplitude measures 6.6556 Vrms
    20191117-22 SW51+ 6Z51P 2V in 3_43V out 440Hz 300R HiZ dScope unBal output amplitude.png

    Gain correctly determined as 3.32, 10.4 dB
    20191117-24 SW51+ 6Z51P 2V in 6_86V out 440Hz 300R HiZ dScope unBal output gain.png

    Here is the gain linearity sweep for -60 to +20 dBu
    The gain drops as expected, but only significantly so above approximately +10 dBu.
    20191117-25 SW51+ 6Z51P gain linearity 440Hz 300R HiZ dScope unBal output gain.png

    The dScope output mode either derives unbalanced BNC as half of balanced XLR output or corrects by 6 dB depending on the mode setting. I thought I could pull off a quick manual mode measurement. Bad assumption when the day is likely to be filled with interruptions.
     
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  5. gixxerwimp

    gixxerwimp Professional tricycle rider

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    Any chance this has made it up towards the top of your queue ...?

    [​IMG]

    Edit: above photo needs Asian representation.
     
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    Last edited: May 23, 2020
  6. atomicbob

    atomicbob dScope Yoda

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    Not yet. At this time I only have two types of pentodes for SW51+: the stock 6Z51P and Mullard EF184. I should probably acquire a few other models to make the comparisons worthwhile.
     
  7. atomicbob

    atomicbob dScope Yoda

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    It appears multi-tone measurements are in-vogue FOTM metrics of discussion across the intenet. Here are some SW51+ multi-tone measurements for consideration.

    First up is a dScope analyzer loopback through a Goldpoint SA2 for reference at approximately -5 dBu.
    00 20201026 dScope SA2 loopback multitone 31 -5 dBu 300R v2.png
    The above represents ideal response with no extraneous spurs between the 31 tones (AES standard)

    SW51+ with CGE6EJ7 tubes adjusted for output at -5 dBu with 300R load - 12 tone analysis for simplified visualization
    00 20201026 SW51+ CGE6EJ7 multitone -20 dBu 300R.png
    In the graph above the gold line depicts frequency response for the 12 tones while the blue line highlights spur peaks for intermodulation distortion.

    SW51+ with CGE6EJ7 tubes adjusted for output at -5 dBu with 300R load
    03 20201026 SW51+ CGE6EJ7 multitone 31 -5 dBu 300R v2.png
    -5 dBu will produce approximately 95 dB SPL in HD6x0, HD800, HE-500 headphones. Listening at that level for long sessions will invite early adoption of hearing aids. Intermodulation distortion above 1000 Hz is approximately 65 dB below the stimulus tones.

    SW51+ with CGE6EJ7 tubes adjusted for output at -10 dBu with 300R load
    04 20201026 SW51+ CGE6EJ7 multitone 31 -10 dBu 300R v2.png
    -10 dBu is approximately 90 dB SPL for the three headphones mentioned. Note how IMD is dropping.

    SW51+ with CGE6EJ7 tubes adjusted for output at -20 dBu with 300R load
    05 20201026 SW51+ CGE6EJ7 multitone 31 -20 dBu 300R v2.png
    Above 1000 Hz IMD is below the graph lower limit. If 0 dBu relates to 100 dB SPL for the three headphones mentioned, then -100 dBu relates to 0 dB SPL. Keep that in mind when evaluating this and other measurements. Relating distortion measurements to use case sound levels is extremely important to maintain useful perspective rather than auger into the ozone on performance metrics that are interesting, but not likely to be perceived in actual use. As mentioned earlier distortion sweet spot for this amp is about 80 dB SPL, which is still very loud for long listening sessions.

    Same measurement as above but I have added the 80 phon line from the ISO contours of equal loudness, also for perspective.
    05 20201026 SW51+ CGE6EJ7 multitone 31 -20 dBu 300R v2 - 80 phon line.png
    If calibrating to 80 dB SPL headphone loudness, then the 80 phon line is applicable to this use case. Greatest Human Auditory System (HAS) sensitivity is between 2 and 4.5 KHz. Now notice where the IMD appears on the graph. Hint: it's not there. Also note how HAS sensitivity is reducing as frequency lowers from 1000 Hz down. Those IMD spikes below 1000 Hz are likely to be masked, though they may contribute to a perceived slight fuzziness when compared directly to one of the EC or DNA DHT amps. Without that direct A/B comparison, it is highly unlikely most listeners will notice.

    CGE6EJ7 tubes appear courtesy of @gixxerwimp - more distortion measurements coming but I don't have a firm schedule at the moment.
     
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    Last edited: Oct 26, 2020
  8. Changeling

    Changeling Tube Slut

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    Hey @atomicbob I’ve got a pair of Telefunken EF184 that I can send.
     
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