Shortest Way 51+ technical measurements

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

  1. atomicbob

    atomicbob dScope Yoda

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    Shortest Way 51+ (sw51+) technical measurements

    Standard Prologue
    If you are unfamiliar with audio measurements please use a search engine with the query:
    "audio measurements" or "audio measurement handbook"
    Look for publications by Richard C. Cabot and also by Bob Metzler, both from Audio Precision. There are other useful publications as well. These will provide basic knowledge.
    Interpretation of the following measurements is beyond the scope of technical measurements posts.


    The data presented were collected as follows:

    1. PrismSound dScope III, picoscope 5243B, Keysight 34465A,
    Cal Test CT2593-2 balanced probe (if used)
    2. Balanced XLR cables Belden 1800F with Neutrik 110R AES connectors (if used)
    3. Single Ended cables Audioblast HQ-1 1.5 ft with Rean RCA connectors
    4. 30 and 300 ohm loads used for measurements
    5. dScope analyzer sample rate 48 KHz unless otherwise noted
    6. 0dBu level used for testing unless otherwise noted
    7. Amplifier input to output gain set to achieve 0 dB unless otherwise noted
    8. Audioquest Forest and Schiit Pyst USB cables used with measurement equipment
    9. Vaunix Lab Brick USB hub
    10. Shielded 14AWG and 16AWG power cables

    Measurements made in accordance with AES17:2015

    Sensitivity data for two headphones to keep in mind while viewing these measurements:
    HD 650 impedance 300R, sensitivity 98 dB/mW
    HE-500 impedance 38R, sensitivity 89 dB/mW

    SPL levels for above headphones for reference:
    0 dBu 300R 2.00 mW - 101 dBSPL @ 98dB/mW
    0 dBu 30R 20.00 mW - 102 dBSPL @ 89dB/mW

    All testing performed at 0 dBu unless otherwise noted.
    This level is consistent with listening to headphones (referenced above) at 90 dBSPL average with peaks to 100 dBSPL, if the music has 10 dB Peak to Avg ratio. That is LOUD for long listening sessions.



    Shortest Way 51+ (sw51+)
    Measurements commenced after 1 hour of warmup.
    Measurements were performed over a period of several days.

    Index
    Post 1 - measurement setup description, highlights
    Post 2 - 300 ohm load SE input HiZ SE output part A
    Post 3 - 30 ohm load SE input LoZ SE output part B
    Post 4 - 300 ohm load SE input LoZ SE output part C
    Post 5 - distortion at various output levels part D
    Post 6 - IMD sweeps part E
    Post 7 - possibly more sweeps
    Post 8 - reserved for corrections or additional data


    Setup picture:
    DSC_0255_small.jpg


    Output Impedance:
    HiZ: 54 ohm (54R)
    LoZ: 27 ohm (27R)


    Notable highlights:
    Due to some interesting attributes of this amp I will provide additional commentary and interpretation from which I would normally refrain.

    Those frightened by high distortion measurements will have the bejesus scared out of them by sw51+. This very special sounding amp offers an opportunity to demonstrate how chasing low distortion numbers out of context is the very definition of foolishness. It is noted that both THD+N and IMD measure rather high on this amp. Those bothered by such high levels and regard measurements as some sort of diety should bail out here and not waste time with what follows.


    20190930-0 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu.png
    The above complex display contains considerable information. The most informative are the FFT spectrums but many want to see numbers, of which there are plenty on this display. The following series will break the complexity into smaller increments and explain what is represented.


    20190930-1 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - THD+N.png
    Beginning with THD+N we note what appears to be a rather high number of 0.5% THD+N which is an oversimplification. THD+N was one of the first audio measurements and historically easy to measure. Think back 60 years and imagine how to make a measurement without computers. THD+N is simply the insertion of a signal to a component, removing that stimulus with a notch filter and measuring what remains. The remainder comprises everything including mains hum and harmonics, harmonic distortion, residual noise, inharmonic distortion, etc. So let's break the THD+N down into components.


    20190930-2 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - THD+N THD2.png
    Expanding scope a bit we see THD+N, THD and 2nd harmonic distortion are almost the same value. This is the first clue that THD+N and THD are 2nd harmonic dominant. I believe 2nd harmonic distortion is part of the magic that provides an enriched presentation of the music. Make no mistake, this is a coloration, but in my opinion a very pleasant euphonic distortion.


    20190930-3 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - 3rd harm.png
    Now look at 3rd harmonic distortion. Note this value is a factor of 10 lower than 2nd harmonic distortion. Again, my opinion, I believe 2nd and 3rd harmonic distortion ratio comprise the voicing of a component. So this smaller amount of 3rd harmonic distortion adds just a hint of sparkle to the 2nd harmonic enriched sound.


    20190930-4 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - 4+5 + noise.png
    4th and 5th harmonic distortion are a factor of 100 lower than the 2nd harmonic, an insignificant level. Hum and residual noise likewise are a factor of 100 lower than the second harmonic, also insignificant.


    20190930-5 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - 4+HD+N.png
    This is a measurement I developed, 4+ HD+N, with stimulus, 2nd and 3rd harmonic removed. What remains is a simplification of 4th, 5th and higher harmonics, mains noise, residual noise, in-harmonic noise. This is what I consider the crap factor. A low number here is desirable. sw51+ has 0.005% crap factor which is approximately -86 dBu, well below audibility.


    20190930-6 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - HD slope.png
    Putting the foregoing into perspective the harmonic distortion depicted in the FFT is downward sloping as shown by the Blue line on the Right channel, highly desirable for those who appreciate what 2nd harmonic distortion offers, of which I include myself for many recreational listening situations; not all though.


    20190930-7 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu - slope+noise.png
    Finally, adding another horizontal Blue line on the Right channel we note mains noise and other residual noise are all at or below -85 dBu, relatively inaudible. Consider the following:

    -85 dBu 300R 98 dB/mW HD800 will produce 16 dBSPL
    -85 dBu 55R 104 dB/mW Focal Clear will produce 29 dBSPL
    -85 dBu 27R 106 dB/mW ER4P-T will produce 34.5 dBSPL


    G2_L1T3_1_A.png
    Exceptional gain linearity down to -100 dBu, remarkable for a SET amp, especially considering the price point.


    Bottom line, a minimalist SET amplifier with very special sound, a 2nd harmonic heaven for SET magic and excellent gain linearity.

    Well done @Zampotech !
     
    Last edited: Oct 7, 2019
  2. atomicbob

    atomicbob dScope Yoda

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    300 ohm load SE input HiZ SE output part A


    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output
    20190930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu.png

    sw51+ 50 + 7000 Hz 300 ohm load SE input HiZ SE output - Left Channel

    FFT_2_L1T13_1_A.png

    sw51+ Gain Linearity 300 ohm load SE input HiZ SE output - Left Channel
    G2_L1T3_1_A.png

    sw51+ THD+N vs Frequency 300 ohm load SE input HiZ SE output - Left Channel
    G2_L1T5_1_A.png

    sw51+ Residual Noise 300 ohm load SE input HiZ SE output - Left Channel
    FFT_2_L1T16_1_A.png

    sw51+ Square Wave 2000mV 10mS/div spdif input SE output - Left Channel
    20190930 SigGen SW51+ 6Z51P 20 Hz square 2000mVpp 10mS div 100KHz filter 300R HiZ.png

    Complete 300 ohm load SE input HiZ SE output analysis and square wave reports pdf attached
     

    Attached Files:

    Last edited: Oct 6, 2019
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  3. atomicbob

    atomicbob dScope Yoda

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    30 ohm load SE input LoZ SE output part B


    sw51+ A04 THD+N THD nth-HD FFT 30 ohm load SE input LoZ SE output
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 30R LoZ - 4+HD+N with 60Hz 0dBu.png
    This might be over the top for low impedance listeners when listening LOUD. Distortion levels drop significantly when listening at -10 dBu or lower which will be presented in post 5 below.

    sw51+ 50 + 7000 Hz 30 ohm load SE input LoZ SE output - Left Channel

    FFT_2_L1T13_1_A.png

    sw51+ Gain Linearity 30 ohm load SE input LoZ SE output - Left Channel

    G2_L1T3_1_A.png

    sw51+ THD+N vs Frequency 30 ohm load SE input LoZ SE output - Left Channel

    G2_L1T5_1_A.png

    sw51+ Residual Noise 30 ohm load SE input LoZ SE output - Left Channel

    FFT_2_L1T16_1_A.png

    sw51+ Square Wave 2000mV 10mS/div spdif input SE output - Left Channel
    20190930 SigGen SW51+ 6Z51P 20 Hz square 2000mVpp 10mS div 100KHz filter 30R LoZ.png

    Complete 30 ohm load SE input LoZ SE output analysis and square wave reports pdf attached
     

    Attached Files:

    Last edited: Oct 7, 2019
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  4. atomicbob

    atomicbob dScope Yoda

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    300 ohm load SE input LoZ SE output part C

    This section is for those wondering about high impedance headphones connected to the low impedance output.


    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input LoZ SE output
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R LoZ - 4+HD+N with 60Hz 0dBu.png

    sw51+ 50 + 7000 Hz 300 ohm load SE input LoZ SE output - Left Channel

    FFT_2_L1T13_1_A.png

    sw51+ Gain Linearity 300 ohm load SE input LoZ SE output - Left Channel

    G2_L1T3_1_A.png

    sw51+ THD+N vs Frequency 300 ohm load SE input LoZ SE output - Left Channel

    G2_L1T5_1_A.png

    sw51+ Residual Noise 300 ohm load SE input LoZ SE output - Left Channel

    FFT_2_L1T16_1_A.png

    sw51+ Square Wave 2000mV 10mS/div spdif input SE output - Left Channel
    20190930 SigGen SW51+ 6Z51P 20 Hz square 2000mVpp 10mS div 100KHz filter 300R LoZ.png
    A peculiar transition develops on the rising and falling edge. A better view of this is available in the pdf.

    Complete 300 ohm load SE input LoZ SE output analysis and square wave reports pdf attached
     

    Attached Files:

    Last edited: Oct 7, 2019
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  5. atomicbob

    atomicbob dScope Yoda

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    distortion at various output levels part D


    300R load HiZ output

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at 0 dBu
    20190930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at -10 dBu
    20190930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz -10dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at -20 dBu
    20190930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R HiZ - 4+HD+N with 60Hz -20dBu.png



    30R load LoZ output

    sw51+ A04 THD+N THD nth-HD FFT 30 ohm load SE input LoZ SE output at 0 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 30R LoZ - 4+HD+N with 60Hz 0dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 30 ohm load SE input LoZ SE output at -10 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 30R LoZ - 4+HD+N with 60Hz -10dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 30 ohm load SE input LoZ SE output at -20 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 30R LoZ - 4+HD+N with 60Hz -20dBu.png



    300R load LoZ output

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input LoZ SE output at 0 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R LoZ - 4+HD+N with 60Hz 0dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input LoZ SE output at -10 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R LoZ - 4+HD+N with 60Hz -10dBu.png

    sw51+ A04 THD+N THD nth-HD FFT 300 ohm load SE input LoZ SE output at -20 dBu
    201900930 SW51+ 6Z51P A04 THD+N  THD  nth-HD FFT 300R LoZ - 4+HD+N with 60Hz -20dBu.png

    Distortion lowers with lower operational levels, common to all three measurement sets. The sweet spot to me is -10 dBu and -20 dBu with 300 ohm headphones.
     
    Last edited: Oct 6, 2019
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  6. atomicbob

    atomicbob dScope Yoda

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    IMD sweeps part E

    sw51+ 300 ohm HiZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to 0 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 300R HiZ 0dB gain.png

    sw51+ 300 ohm HiZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to -10 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 300R HiZ -10dB gain.png

    sw51+ 300 ohm HiZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to -20 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 300R HiZ -20dB gain.png


    sw51+ 30 ohm LoZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to 0 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 30R LoZ 0dB gain.png

    sw51+ 30 ohm LoZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to -10 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 30R LoZ -10dB gain.png

    sw51+ 30 ohm LoZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to -20 dB gain
    20191007 SW51+ 6Z51P 19+20KHz IMD sweep 20KHz to 5KHz 30R LoZ -20dB gain.png


    sw51+ 300 ohm HiZ 19+20 KHz IMD vs amplitude - amp gain set to 0 dB
    20191007 SW51+ 6Z51P 19+20KHz IMD vs Amplitude 300R HiZ 0dB gain.png

    sw51+ 30 ohm LoZ 19+20 KHz IMD vs amplitude - amp gain set to 0 dB
    20191007 SW51+ 6Z51P 19+20KHz IMD vs Amplitude 30R LoZ 0dB gain.png
     
    Last edited: Oct 7, 2019
  7. atomicbob

    atomicbob dScope Yoda

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    reserved 7/8.
     
  8. atomicbob

    atomicbob dScope Yoda

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    Corrections:

    19+20 KHz IMD measurements in the pdf files previously attached have a non-compliant IMD measurement due the signal generator set incorrectly for the twin-tone second frequency. Pdf files containing the non-compliant measurement have been deleted and replaced with versions containing the compliant measurement.

    Here is the ramification summary of IMD CCIF % measurements, rounded to 2 places:
    20191007 sw51+ IMD correction.png

    This is how the signal generator should be set for a compliant measurement and the resulting analyzer readings (sw51+ 300R HiZ):
    20191007 SW51+ 6Z51P A15 19+20KHz IMD 300R HiZ L ch  -16dBu compliant.png

    Here is the non-compliant measurement setting and resulting analyzer readings (sw51+ 300R HiZ):
    20191007 SW51+ 6Z51P A15 19+20KHz IMD 300R HiZ L ch  -16dBu non-compliant - annotated.png
    Signal generator second tone (20 KHz) has been set for an offset of +18 dB with respect to the first tone (19 KHz). RMS amplitudes reported are at approximately +3 to +4 dBu. Normally they should report between approximately -10 and -16 dBu. In the FFT this ratio imbalance is apparent.

    For those who already downloaded analyzer pdf files, please download and replace with the corrected analyzer pdf files.
     
    Last edited: Oct 7, 2019
  9. purr1n

    purr1n Finding his inner redneck

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    I love all the grass and will await the other IMD measurements (which I expect to be not good). What I love about tube amps how badly they tend to measure in terms of grass and IMD, yet how certain ones actually sound less grey and veiled (particularly in the mids and highs) compared to many far superior measuring solid-state amps.
     
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  10. Hands

    Hands Overzealous Auto Flusher - Measurbator

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    I imagine the original/prototype Shortest Way that I have would measure similarly. I mean, it's a SET amp without caps in the signal path. Very, very simple design overall. Looks like input, pot, tubes, a few resistors, then output transformers (ignoring power supply).

    So, while I haven't yet heard the production model, what struck me about the original was how freakin' clear it sounded. None of the fake, over-sharpened BS, nor overly fast (or slow!), etc. Just zero grain or other messiness. And while the transformers in that had noticeable bass roll-off, it was not a bloomy or what I'd call warm sounding amp. Straight to the point with everything.

    Funny how people would have this preconceived notion in their head about how the amp would sound based on measurements.

    You know, the Crack measures somewhat similarly. And while, yeah, it has a much higher output impedance, it sounds wildly different than the Shortest Way.
     
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  11. atomicbob

    atomicbob dScope Yoda

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    IMD sweeps added. Corrections to IMD analysis made and pdf files updated.
     
  12. Zampotech

    Zampotech Friend

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    I completely agree with you. In amplifiers, subjective perception and objective evaluation can be completely opposite.
    Probably there are still some important parameters that we do not yet know and do not know how to measure.

    Of course, objective measurement is very important. This gives us most of the information. Objective measurement allows to avoid or to correct errors in the design of the amplifier. The measurement results of dear @atomicbob confirmed that I did not make mistakes in the design. I do not possess such precision equipment, so I was very interested in getting the result.


    The results of the THD measurements are most accurately interpreted by the method proposed by BBC engineers.

    It will probably be interesting for SBAF users to know.
    The essence of this technique is that it takes into account not only the magnitude of the harmonic, but also its number.

    Standard formula for calculating THD:

    [​IMG]

    This is the mean square sum of the first 25 harmonics, except for the first one.
    k-harmonic number, Uk-the magnitude of the harmonics

    BBC calculation formula:

    [​IMG]

    Conversion example:
    We measure three amplifiers, "green", "blue" and "red", with different harmonics tail.
    All have THD=0.1% when calculated according to the standard method.
    When recalculated by the "BBC method" we get different THD

    [​IMG]

    Subjectively, the " red "amplifier will sound worse than the "blue" and noticeably worse than the "green" amplifier. Although the standard THD is the same for everyone. But even in this case, it is not the value of THD, measured in any way, but the value of the length of the tail of the harmonics that will be important.

    Of course, this method is not a panacea. But this method is closest to our subjective perception.

    IMHO: Evaluating the performance of an amplifier by THD magnitude is like evaluating a woman's beauty by knowing only her weight.Woman's weight probably important, but it does not determine of its other merits [​IMG]
    Although there was a period in my life when I was guided only by the value of the THD when designing an amplifier.
     
    Last edited: Oct 8, 2019
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  13. MisterRogers

    MisterRogers Ethernet Nervosa

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    Very interesting - I wasn't aware of the BBC THD formula. I can see objectivists push against this formula, but it does seem to make THD more subjectively meaningful.
     
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  14. purr1n

    purr1n Finding his inner redneck

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    Hahaha. I love it. Punishment of higher order distortion at exponential levels as we go up! Could use optional modifier: x2 additional punishment for odd order harmonics.

    The moral of the story is that simplistic figures do not work, and what works for one person might not work for another. Also measurement methods that tend to work for one type of gear might not apply so well for other types. For example, discrete R2R DACs tend to have ongoing harmonics, yet punishing the higher orders with DACs may not make as much sense subjectively when compared to amps.

    Folks like Amir should be shot and quartered (figuratively).
     
    Last edited: Oct 8, 2019
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  15. atomicbob

    atomicbob dScope Yoda

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    THD with BBC weighting of harmonics is interesting to know. However I still maintain all the distortion measurement numeric methods still represent oversimplification. The FFT spectrum display yields far more insight. In the following examples I have increased FFT size to 128K rather than usual 32K for added visual clarity to discriminate several distortions.

    20191008-0 SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  0dBu LR tube swap.png
    Here is the 1 KHz sine at 0 dBu input and 0 dB amp gain distortion test with a proliferation of numbers as presented in the main technical measurement posts above and the FFT spectrum. Plenty of numbers. If I must look at numbers I prefer 2nd and 3rd harmonic combined with 4+HD+N on the right. Voicing + crap factor.


    20191008-1 SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  0dBu LR tube swap no numbers.png
    Dispensing with numbers other than amplifier output level, I prefer to concentrate on the FFT spectrum.


    20191008-1b SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  0dBu LR swap - annotated.png
    In the lower left box annotated "source stimulus" the signal generator has been set to sine 1000 Hz at 0 dBu output.
    The box annotated "output level" measures the amplifier output and displays the system gain.
    Green line annotates the Harmonic Distortion series with respect to the 1000 Hz 0 dBu stimulus.
    Red line annotates the power supply noise series with respect to a 1000 Hz 0 dBu stimulus.

    The level and slope of those two lines convey far more information with better visual discrimination when compared to other amplifiers measured the same way.

    Now lets examine power supply noise behavior at several stimulus levels; 0 dBu, -60 dBu, and generator off.
    My long held contention is the power supply is 50% of an amplifier design.
    20191008-2 SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  0dBu LR tube swap.png
    In the measurement above L channel 60 Hz spike is -88.936 dBu and R ch -87.468 dBu. Threshold of perception for Sennheiser HD6x0 and HD800 headphones is approximately -75 dBu as noted in the ZDT Jr thread discussing audibility of hum. So this level is both well below perceptual threshold and balanced between channels. In the following graphs dBu will be rounded as 3 places past the decimal point is a ridiculous level of precision given how multiple repeated measurements vary easily by 1 to 2 dB or more.


    20191008-3 SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  -60dBu LR tube swap.png
    Lowering the 1 KHz sine from 0dBu to -60 dBu the power supply 60 Hz spike changes. L channel drops to -90 dBu but the R channel actually rises to -83..6dBu. This is interesting. Still below perceptual threshold though.


    20191008-4 SW51+ 6Z51P A04 4+HD+N suite 128K FFT 300R HiZ -  Off dBu LR tube swap.png
    Turning the stimulus generator off the result is again interesting. L channel 60 Hz spike continues to drop to -98 dBu while R channel rises again to -81 dBu. Given this is the residual noise limit, the power supply noise in the R channel won't rise to the perceptual level for the headphones listed above, which is 6 dB higher.


    20191008-5 sw51+ 6Z51P 1KHz THD 128K FFT 0dBu -.mains noise removed.png
    Now imagine we could create a perfect power supply free from noise. The resulting spectrum shown above then would be clearly harmonic distortion dominant. 2nd harmonic at approximately -43 dBu, 3rd harmonics at -68 dBu and everything else at or below -100 dBu. I contend this provides information that has better correlation to the audible experience than any bunch of numbers concocted.
     
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  16. Zampotech

    Zampotech Friend

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    I was very pleased with this schedule. You have inspired me with optimism and pride in my work.
    It turns out that the amplifier, in fact assembled from three parts produces only two harmonics, the second and a little third.
    For clarity, I cut out from your chart is the area accessible to our ears.

    [​IMG]



    But of course this is a great merit of the designers of the prototype tube 6Z51P. Tube fantastic!

    Using your measurement algorithm and using different tubes of the 6Z51P/EF184 family it is possible to determine the most useful tube in this amplifier.
     
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  17. atomicbob

    atomicbob dScope Yoda

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    This contributes greatly to the magic of the sw51+

    A great tube indeed! So glad you introduced me to this tube.

    While I must return my attention to corporate measurements for awhile, measuring a few select parameters with other tubes is in my queue.
     
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  18. mdr30

    mdr30 Rando

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    Any idea of power output capacity of the 6Z51P (6J51P) in this amp?
     
  19. purr1n

    purr1n Finding his inner redneck

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    The better question to ask is what is the amp's performance in terms of the spectrum analysis (or THD if something more simple is desired) for a particular headphone and listening levels.

    Also, asking for power (typically where THD reaches 1% around the knee when distortion starts to rapidly rise) without specifying a load is insufficient when it comes to headphones where loads can vary widely from 10-ohms to 1000-ohms

    All too common, manufacturers will say numbers like 12W. Which are utterly meaningless at best and nefarious at worst.

    Yes, more power is often better, but with SET amps, I find it more beneficial to find the sweet spots of operation.

    FWIW, this amp seems ideally suited to Sennheisers or Beyers of around 300-ohms nominal impedance, where typically listening levels are around -20 dbU into the that load with peaks 0dbU.

    It will also do very well with the moderate and efficient planars down to 30-ohms from the low Z output.
     
    Last edited: Oct 8, 2019
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  20. atomicbob

    atomicbob dScope Yoda

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    To expand on @purr1n discussion here are yet some more measurements to consider.

    1000 Hz 300R load measurements

    sw51+ 6Z51P THD+N vs amplitude at 1000Hz - 300R load HiZ:
    20191008-1 SW51+ 6Z51P THD+N vs dBu 1000Hz 300R HiZ 0dB gain.png

    sw51+ 6Z51P THD+N vs amplitude at 1000Hz - 300R load HiZ: with annotations:
    20191008-1a SW51+ 6Z51P THD+N vs dBu 1000Hz 300R HiZ 0dB gain - annotated.png
    Power levels and approximate SPL levels for HD6x0 and HD800 added. Listening to music at 90 dB SPL with peak to average ratio of 10 dB will have peaks hitting 100 dB SPL. That is LOUD. In that range the sw51+ is running between 0.2 and 0.6 % THD+N. But remember in the posts above sw51+ THD+N is 2nd harmonic dominant. So really the amp is presenting between 0.2 and 0.6 % 2nd harmonic distortion. Sweet! Even better for those listeners that prefer 80 dB SPL which is still pretty loud.


    sw51+ 6Z51P THD+N and 4+HD+N vs amplitude at 1000Hz - 300R load HiZ:
    20191008-1b SW51+ 6Z51P THD+N 4+HD+N vs dBu 1000Hz 300R HiZ 0dB gain.png
    Another set of curves have been added. These are 4+HD+N, which is similar to THD+N but with 2nd and 3rd harmonics removed. Note these curves are substantially lower above -15 dBu.

    sw51+ 6Z51P THD+N and 4+HD+N vs amplitude at 1000Hz - 300R load HiZ: annotated 1:
    20191008-1c SW51+ 6Z51P THD+N 4+HD+N vs dBu 1000Hz 300R HiZ 0dB gain - annotated.png
    This sweet spot occurs between approximately -13 and -3 dBu, consistent with listening level peaks of 87 to 97 dB SPL

    sw51+ 6Z51P THD+N and 4+HD+N vs amplitude at 1000Hz - 300R load HiZ: annotated 2:
    20191008-1d SW51+ 6Z51P THD+N 4+HD+N vs dBu 1000Hz 300R HiZ 0dB gain - annotated 2.png
    Now if the music has a peak to average ratio of 10 dB, then the amplifier operating are is approximately bounded by the green rectangle above.


    100 Hz 300R load measurements

    sw51+ 6Z51P THD+N vs amplitude at 100Hz - 300R load HiZ:
    20191008-2 SW51+ 6Z51P THD+N vs dBu 100Hz 300R HiZ 0dB gain.png
    For completeness 100 Hz performance should be considered, especially for a transformer coupled amplifier.


    1000 Hz 30R load measurements

    sw51+ 6Z51P THD+N vs amplitude at 1000Hz - 30R load LoZ:
    20191008-3 SW51+ 6Z51P THD+N vs dBu 1000Hz 30R LoZ 0dB gain.png

    sw51+ 6Z51P THD+N vs amplitude at 1000Hz - 30R load LoZ with annotations:
    20191008-3a SW51+ 6Z51P THD+N vs dBu 1000Hz 30R LoZ 0dB gain - annotated.png
    Again, approximating for a low impedance headphone. The HE-500 is actually 38R instead of 30R used for testing. But the concept remains close enough.


    100 Hz 30R load measurements

    sw51+ 6Z51P THD+N vs amplitude at 100Hz - 30R load LoZ:
    20191008-4 SW51+ 6Z51P THD+N vs dBu 100Hz 30R LoZ 0dB gain.png

    So how much power does one need?
    Depends on the listening level, distortion level tolerated, type of distortion produced by amp, etc, but for many headphones and listeners 20mW at 300R and 200 mW at 30R is more than enough; Keep in mind the sw51+ is 2nd harmonic dominant at those levels.
     
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