6AS7 OTL three amp comparison technical measurements

Discussion in 'Headphone Amplifier Measurements' started by atomicbob, Jan 30, 2025.

  1. atomicbob

    atomicbob dScope Yoda

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    Three very similar 6AS7 based OTL design headphone amplifiers technical measurements will be compared. Measurement methods and notes can be found in this example post

    Index:
    Post 1: Introduction, typical schematic, output impedances
    Post 1: distortion surfaces for THD+N
    Post 2: distortion surfaces for D2, D3,
    Post 3: distortion surfaces for 4+HD+N
    Post 3: frequency response, bandwidth (transient response)
    Post 4: distortion FFT vs level, residual noise
    Post 4: attached pdf measurement suite reports
    Post 5: reserved for additional data and/or corrections

    Amps:
    Woo Audio WA3+
    Bottlehead Crack,
    see this post, post 1 and 4 for modifications
    xDuoo TA-66

    Tubes:
    WA3+ 6922 drivers, RCA 6080 output
    Crack: e80cc driver, Phillips JAN 6080wc output
    TA-66: 6n2 driver, 6n5pj output

    The three amplifiers:
    01 WA3 - BH Crack - TA-66.jpg
    Woo WA3+ on the left
    BH Crack in the middle
    Xduoo TA-66 on the right

    I began my journey into tube headphone amplifiers with the WA3+ and Sennheiser HD600 headphones. After years of enduring the headphone output of a Benchmark DAC1 which I used for dialog editing I found I could actually enjoy recreational listening when changing to a Violectric V800 and the WA3+. This was a revelation as I would not engage in recreational listening after work for many years. Too much listening fatigue from the DAC1.

    The WA3 rendered music in a way I found enjoyable but had two issues that bothered me. 60 Hz hum was sufficiently audible to cause distraction. I am particularly sensitive to hum, noise and tonal balance. WA3 also had a particularly high output impedance causing bass to sound out of balance to me.

    Intrigued by the 6AS7 (6080) OTL design I went to a meet at Bottlehead headquarters and heard a Crack. DIY with plenty of customization possibilities Doc convinced me to buy one. Glad I did. Mods are described in the link mentioned above. Choke mod dropped hum considerably below threshold of audibility, bass sounded much better than WA3+ and overall tonality very pleasing. This amp remains in regular rotation in my listening lab for good reason.

    Enter the xDuoo TA-66. An amazing accomplishment in design tradeoffs achieving a great sound at an entry level price point. But this is not, in my opinion, an entry level amp. Combine with a decent DAC and high impedance headphones such as HD6x0 and HD800 results in an impressive auditory experience. My modified Crack betters the TA-66 in some technicalities, but at greater expense and many modifications.


    02 6as7 typical cathode follower.png
    All three amps have a 6AS7 (6080, 6n5pj, 5998, 421A, etc.) OTL output stage as shown in the example schematic above. WA3+ may be a white cathode follower topology with three triode elements per channel, two provided by a 6922 for each channel.


    03 6AS7 OTL amps output impedance comparison.png
    Output Impedance (nominal approximation):
    WA3+: 180 Ω
    Crack: 125 Ω
    TA-66: 125 Ω


    Crack and TA-66 are essentially the same. WA3+ with a substantially higher output impedance will definitely have an interesting effect on low frequencies reproduced by headphones.


    Distortion Surfaces

    When comparing the surfaces presented pay attention to the overall shape of the surfaces. For example note all three THD+N share falling to a dip and then rising again. The overall surfaces vary in the x-axis (Dist dB) and the dip varies in the z-axis (dBu) but the overall shape is remarkably similar. This shape similarity repeats in D2, D3 and 4+HD+N distortion surfaces.

    THD+N

    04 WA3 RCA 6080 THD+N surface 300R - annotated.png
    Observe WA3+ is over the top, literally., due to high 60 Hz hum in the measurement.
    But the shape is remarkably similar to the other two amps.

    05 BH Crack Phillips 6080WC THD+N surface 300R - annotated.png

    06 TA-66 6n5pj THD+N surface 300R - annotated.png

    07 6AS7 OTL THD+N surface comparison.gif

    Animation for enhanced visualization.
     
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    Last edited: Feb 7, 2025 at 11:06 AM
  2. atomicbob

    atomicbob dScope Yoda

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    2nd harmonic distortion D2

    11 WA3 RCA 6080 D2 surface 300R - annotated.png

    12 BH Crack Phillips 6080WC D2 surface 300R - annotated.png

    13 TA-66 6n5pj D2 surface 300R - annotated.png

    14 6AS7 OTL D2 surface comparison.gif


    3rd harmonic distortion D3

    15 WA3 RCA 6080 D3 surface 300R - annotated.png

    16 BH Crack Phillips 6080WC D3 surface 300R - annotated.png

    17 TA-66 6n5pj D3 surface 300R - annotated.png

    18 6AS7 OTL D3 surface comparison.gif

    Animation for enhanced visualization
     
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  3. atomicbob

    atomicbob dScope Yoda

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    4+HD+N distortion (crap factor)

    21 WA3 RCA 6080 4+HD+N surface 300R - annotated.png

    22 BH Crack Phillips 6080WC 4+HD+N surface 300R - annotated.png

    23 TA-66 6n5pj 4+HD+N surface 300R - annotated.png

    24 6AS7 OTL 4+HD+N surface comparison.gif

    Animation for enhanced visualization.


    Frequency Response comparisons

    Frequency Response WA3+
    25 20250129 WA3 RCA 6080 frequency response 5Hz - 96KHz 300R 0dBu.png
    Greatest high frequency extension of the three.

    Frequency Response modified Crack
    26 20250128 BH Crack Phillips 6080WC frequency response 5Hz - 96KHz 300R 0dBu.png
    Lowest bass, less high frequency extension

    Frequency Response TA-66
    27 20250126 TA-66 6n5pj frequency response 5Hz - 96KHz 300R 0dBu.png
    Least low frequency extension of the three.


    Square wave response comparisons

    Square wave response WA3+
    28 20250129 SigGen WA3 RCA 6080 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 222.8 KHz fastest of the three amps

    Square wave response modified Crack
    29 20250129 SigGen BH Crack Phillips 6080WC 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 109.2 KHz slowest of the three amps

    Square wave response TA-66
    30 20250126 SigGen TA-66 6n5pj 100 Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 125.8 KHz
     
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    Last edited: Jan 30, 2025
  4. atomicbob

    atomicbob dScope Yoda

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    Distortion vs level animations

    WA3+
    31 20250129 WA3 RCA 6080 A04 Distortion 300R 0 to -30dBu.gif

    modified Crack
    32 20250128 BH Crack Phillips 6080WC A04 Distortion 300R 0 to -30dBu.gif

    TA-66
    33 20250126 TA-66 6n5pj A04 Distortion 300R 0 to -30dBu.gif


    Residual Noise

    WA3+
    34 20250129 WA3 RCA 6080 Residual Noise 300R.png
    Observe 60 Hz at approximately -62 ~ -64 dBu. 13 and 11 dB above -75 dBu audibility threshold for HD6x0 and HD800 headphones. This bothered me but many listeners will not find this a problem.

    modified Crack
    35 20250128 BH Crack Phillips 6080WC Residual Noise 300R t2.png
    Very quiet as per my modification goals

    TA-66
    36 20250128 TA-66 6n5pj Residual Noise 300R t2.png
    Not as quiet as the modified Crack but levels are below threshold of audibility and I am not bothered by hum from the TA-66 with HD800 headphones.

    We have been regaled with stupefying SINAD headphone amps by many Chinese designs. xDuoo has gone against the trend with a design rendering music so immersive that I find worth putting in rotation in my listening lab.

    Well done, xDuoo designers!



    Standard measurement suites pdf files for the amps attached

    Please note WA3 shows a FAIL result. Three tests results failed due to the high 60 Hz hum being above -75 dBu or 0.0178%.
     

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    Last edited: Jan 30, 2025
  5. atomicbob

    atomicbob dScope Yoda

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    reserved for additional data and/or corrections.
     
  6. chesebert

    chesebert Friend

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    TA-66 looks remarkably good for $250! I bought one because it’s too good to be true and I just had to try it. Fingers crossed on good QC.
     

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