HD650 technical measurements

Discussion in 'Headphone Measurements' started by atomicbob, Jul 5, 2021.

  1. atomicbob

    atomicbob dScope Yoda

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    HD650 technical measurements

    And how single number metrics provide extremely limited information

    Measurement setup
    DSC_0650_small.jpg
    Klippel QC analyzer
    Furman SP-20A headphone amp
    ACO Pacific 7052SYS microphone
    Brüel & Kjaer 4231 mic calibrator
    Mr BlockHead™ flat plate coupler
    density similar to average human head 1.08 g/cc
    width consistent with average human head 15.25 cm


    DSC_0651_small.jpg
    Mr BlockHead is mounted on a high mass lighting stand to minimize externally induced vibrations.


    DSC_0664_small.jpg
    View of ACO Pacific 7052SYS in Mr BlockHead flat plat coupler
    Alert assistant is nearby ready to help.


    01 20210701 HD650 1V FR Dist Imp L - single point THD 1 KHz _05 pct.png
    HD650 is specified to have 0.05% THD+N at 1 KHz with 1Vrms stimulus

    Historically a low distortion 1 KHz sine generator would have stimulated the electro-acoustic transducer using a low distortion amplifier. Then a steep 1 KHz notch filter removes the stimulus and the measured level of what remains is compared to the stimulus level. That ratio results in THD+N.

    Now consider the capability of a modern acoustic analyzer.

    Klippel generator is adjusted to 1Vrms (circled in red.)
    Low distortion amplifier is set for 0 dB gain to follow the generator settings displayed.
    Headphones carefully positioned on flat plate coupler.

    Sound level observed is 100.6 dB SPL which is consistent with published sensitivity.
    THD+N at 1 KHz is about 0.05% THD+N (the red dot on the graph.)

    Looking at this graphic limited to the single number data point offers minimal information.
    SINAD at 1 KHz provides similarly very limited information about the characteristics of an amplifier. A single dot on a graph.


    02 20210701 HD650 1V FR Dist Imp L - annotated.png
    Expanding the view to include the entire analyzer 1 Vrms sweep from 20 Hz to 20 KHz vastly more information is observed

    Upper left graph: Frequency response, rub+buzz
    Upper right graph: THD+N, H2 (offset by -4% for visual clarity) and H3 (offset by -2% for visual clarity)
    Lower graph: Impedance

    Why would we want to limit our view of an electro-acoustic transducer or amplifier to a single number representing an infinitesimal aspect of performance?

    Single number metrics used for audio component performance rankings are absurd when one realizes how much information is missing.

    02a 20210701 HD650 1V FR Dist Imp L - single point vs entire graphs animation.gif

    Notes about the analyzer graphs

    1) frequency response is not compensated in any way. Data can be exported such that others may use their favorite compensation methods.
    2) rub+buzz is a measure of all that remains when fundamental and harmonics are removed. It is sort of an acoustic crap factor.
    3) impedance measurements are often noisy and unreliable below 0 dBu in normal environments outside an anechoic chamber or very well controlled sound space. Every electro-acoustic transducer can transform electricity to sound (speaker) and sound to electricity (microphone.) They are usually optimized for one or the other but still can function in reverse. To avoid outside influences impedance is typically measured with a sine sweep of between 0 and +10 dBu. It is a balancing act between outside influence and analyzer input level of which there are more considerations than will be discussed here.
    4) rub+buzz below 40 dB SPL is inconsequential

    Note the rub+buzz bump at 6 KHz corresponding to a small THD+N and H2 bump at 6 KHz. Watch these points in the following graphs.
     
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    Last edited: Jul 5, 2021
  2. atomicbob

    atomicbob dScope Yoda

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    Klippel rub+buzz orientation
    03 20210702 rub+buzz residual.png
    With generator off the room residual noise is measured in the upper left graph.
    Room ambient SPL, blue trace, is 40 dB SPL below 80 Hz.
    rub+buzz noise floor is the black trace.
    Keep these measurement noise floors in mind when viewing the following data.

    04 20210701 HD650 -10 dBu FR Dist Imp L.png
    -10 dBu (0.245 Vrms) observe 88.5 dB SPL
    Impedance measurement should be ignored
    Observe the changes in rub+buzz and distortion as stimulus levels are increased
    THD+N is approximately 0.5% at 60 Hz and doesn’t rise above 5% even down to 20 Hz
    All H2. H3 is very low.
    Listening at low levels has many benefits including lower overall distortion.

    05 20210701 HD650 0 dBu FR Dist Imp L.png
    0 dBu, 0.775 Vrms, 98.4 dB SPL
    SPL and Frequency Response plot rise 10 dB as expected
    THD+N is rising, especially at low frequencies.

    06 20210701 HD650 1V FR Dist Imp L.png
    Changing generator reference system from dBu to Vrms
    1 Vrms, 100.6 dB SPL
    1 Vrms is 2.2 dB higher than 0.775 Vrms
    SPL and Frequency Response plot rise by 2.2 dB as expected

    07 20210701 HD650 2V FR Dist Imp L t2.png
    2Vrms, 106.5 dB SPL
    SPL and Frequency Response curve rise 6 dB as expected
    Observe THD+N now above 0.5% below 200 Hz and exceeds 10% at 40 Hz

    08 HD650 distortion animation.gif
    Animation of previous four slides for easy visualization of distortion rise from -10 dBu (0.245Vrms) to 2Vrms

    09 20210701 HD650 -10 dBu FR Dist Imp L THD y-zoom t3.png
    Expanded y-axis view of THD+N
    Even at -10 dBu (0.245 Vrms) distortion is approximately 0.05% above 150 Hz, listening about 89 dB SPL

    10 20210701 HD650 2V FR Dist Imp L t2 THD y-zoom.png
    At 2Vrms distortion below 300 Hz is above 0.15% , and typically above 0.075% at higher frequencies, listening at approximately 107 dB SPL

    As most of the distortion is H2 and is often above 0.1% there will be masking of upstream component low level distortions.

    It is unwise to infer distortion performance of amplifiers and DACs or other source sound sources when an electro-acoustic transducer is producing this much distortion.

    Keep in mind Sennheiser HD6x0 and HD800 headphones are already very low distortion headphones; these measurements demonstrate very good performance.
     
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    Last edited: Jul 5, 2021
  3. atomicbob

    atomicbob dScope Yoda

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    BK891 LCR measurements

    Cal check with SR1 1K laboratory standard reference resistor
    20210710 BK891 LCR SR1 1K standard resistor cal check.jpg

    HD650 measurements
    20210710 BK891 LCR HD650.jpg

    SR1 1K impedance sweep (calibration check) 0.5Vrms
    20210710-01 BK891 LCR SR1 cal 0_5V 20-20K Z.png

    HD650 impedance sweep 0.5Vrms
    20210710-02 BK891 LCR HD650 0_5V 20-20K Z.png

    HD650 capacitance sweep 0.5 Vrms
    20210710-03 BK891 LCR HD650 0_5V 20-20K C.png

    HD650 inductance sweep 0.5Vrms
    20210710-04 BK891 LCR HD650 0_5V 20-20K L.png

    HD650 impedance sweep 1.0Vrms
    20210710-05 BK891 LCR HD650 1_0V 20-20K Z.png

    HD650 capacitance sweep 1.0Vrms
    20210710-06 BK891 LCR HD650 1_0V 20-20K C.png

    HD650 inductance sweep 1.0Vrms
    20210710-07 BK891 LCR HD650 1_0V 20-20K L.png

    Notes:
    Where capacitance goes negative the load is more inductive
    Where inductance goes negative the load is more capacitive

    All data for above measurements in the attached zip file
    Added a csv file with all data converted to a more easily imported format
     

    Attached Files:

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    Last edited: Jul 10, 2021
  4. atomicbob

    atomicbob dScope Yoda

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    reserved for additional data and corrections if necessary.
     
  5. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Stellar job, Bob! My experience is that systems with multiple significant distortion sources subjectively start to sound noisy. It's like signal correlated noise. I've often encountered cases like these in clubs and concerts where it's impossible to converse, despite there not being much info in the vocal range. With clean systems one could easily hear and understand speech despite ridiculously loud music.
     
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  6. Pancakes

    Pancakes Almost "Made"

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    Epic X2 for animation.
     
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  7. Armaegis

    Armaegis Friend

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    Wow I haven't seen one of those in decades... did you specifically pull that one out to make a point of not using a modern ultra low sinad amp?
     
  8. Aklegal

    Aklegal Almost "Made"

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    I get that we can't judge the performance of an amp measured with real-world loads in isolation. But what about in comparison to other amps? I found the Topping vs Jotunheim measurements interesting. What's wrong with finding an amp that is known to be well built with low distortion and using that as a baseline of comparison with tests like these? I'm sure I am missing something.

    EDIT: I re-read your posts again. I think I get it but I'll wait for your follow-ups.
     
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  9. Marvey

    Marvey Super Friend

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    I have an FFT (recorded at audio output of HD650 driver, not amp) of a 37Hz and 130Hz two tone signal using Schiit Jotunheim 1 and Topping L30. They look the same. The reason is, and what @atomicbob was saying, is that the distortion of headphones is simply way way too high for the distortion from an amp to matter. A amp needs to be really bad, like within 10db (or even less) of the distortion of an headphone, to really make dent in the measured output. This is if you believe that a THD / SINAD type measurement rules all (we all know it doesn't).

    I will post the result later tonight.
     
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    Last edited: Jul 6, 2021
  10. atomicbob

    atomicbob dScope Yoda

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    The two major points of my post are:

    1) in evaluating measurements for an audio component would you rather have

    THD+N at 1 KHz = 0.05%

    or

    02 20210701 HD650 1V FR Dist Imp L - annotated.png

    Ranking components with highly reduced information is a terrible idea.

    2) attempting to hear amplifier distortion of less than 0.01% while listening at 107 dBSPL where the headphone has between 0.075% ~ 0.25% distortion across the frequency range, greater than 1% below 150Hz and rising exponentially at lower frequencies, is a categorically Bad Idea™

    AB and ABX comparison of level matched amps is useful and discrimination achievable at 75 to 85 dBSPL where the headphone distortion will be significantly lower than HD650 at 2Vrms, 107 dBSPL.
     
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  11. atomicbob

    atomicbob dScope Yoda

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    Good eye. I acquired the Klippel QC in 2012 when I had an idea to create an improved driver that would be part of a system to improve speech intelligibility for medical devices used in noisy environments. See US Patent US9713728B2, which is one of many of my ideas for which I obtained patents. The driver constraints included mylar diaphragm due to IP65 rating for the device including sound system. The driver was designed to have a specific distortion profile which changed in a desired way at specific amplifier drive levels.

    Also about this time I needed to develop a mobile system which could demonstrate sound lab psycho-acoustic experiences. It is expensive to fly people to my campus acoustic lab. Enter the need to measure headphones and know how well they could perform when driven to 115 dBC SPL. The user was required to wear hearing protection if wishing to have more than a minute of psycho-acoustic video demonstration on the mobile system. HD650 was one of very few that made the cut.

    The SP20A was also used for cue mix during recording sessions. I have three of them and with the companion distribution boxes could handle cans for approximately 20 musicians in the studio.
     
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  12. Marvey

    Marvey Super Friend

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  13. atomicbob

    atomicbob dScope Yoda

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    Added LCR measurements in post 3
     
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