Topping L30 Review and Measurements

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by Marvey, Jun 28, 2021.

  1. Marvey

    Marvey Super Friend

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    Dynamic (voice coil) headphone loads are inductive, rising impedance with frequency. They also exhibit an impedance peak at their resonant frequency. The amplitude and frequency of this will be affected by quality of seal.

    What we are looking at is likely the effect of back EMF. Headphone drivers are like a microphone too. If we take a headphone and talk into it, we will see voltage at the phono jack. The idea behind back EMF has been discussed much, but we haven't seen evidence, at least I haven't seen evidence of it until now.

    There were two hypotheses presented here:
    1. Current will be the thing that can fight against back EMF. This makes sense from a thought experiment point of view. Voltage is potential. The height of the water tank at the top of the water tower. Current is the actual stuff of water moving through the pipe. We can have all the potential we want, the water tower can be 5 miles high, but it won't do jack if there isn't enough water to push back against back EMF.
    2. Global feedback loops muck with the fight against back EMF. I will explore more this later. However I will present the following (tentative, I need to reconfirm results)
    Note how high-gain has overall slightly worse THD numbers (as would expected because of less feedback), except in the lows where back-EMF could possibly play a larger role.
    upload_2021-7-2_0-55-36.png

    [​IMG]
     
    Last edited: Jul 2, 2021
  2. dsavitsk

    dsavitsk Friend

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    Another thing that might be interesting is to take a close look at the noise floor with no signal vs the noise floor when there is a 1K (or any other) signal. The theory is as follows - a signal generates harmonics. When an amp has feedback, you feed the inverse of the signal, plus the inverse of the harmonics, back to the amp. The harmonics cancel and voila, low distortion (and low impedance, a win-win).

    However, the cancellation is probably not perfect. For example, with a 1K signal the 2nd harmonic, 2KHz, will cancel almost all of the 2K distortion, but a small amount will theoretically just be reamplified where it will generate its own distortion. It's 2nd harmonic is the 4th harmonic of the original signal, it's 3rd is the original's 6th, etc. The original's 3rd, 4th, nth have the same issue which would presumably lead to a bunch of orderly stuff that might be buried in the noise floor and cause no issues, or that might appear buried but that might account for the dry, analytical sound we often associate with feedback.

    Interestingly, if you drop an insufficient amount of voltage across a transistor, in some cases it will generate a bunch of high harmonics. The sound from that is that voices feel really large and clear, the sound is dry and analytical, it feels more real than real, and it is very fatiguing after a little while. Similar, to my ear, to feedback.
     
  3. nishan99

    nishan99 Friend

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    Isn't those current reservoirs out of the signal path for both of those amps? And also the current delivery of both amps (at least for 300 ohm loads) is essentially the same right? How is it affecting the signal exactly? . (three question marks because I may be too dumb for this discussion :D )
     
  4. Marvey

    Marvey Super Friend

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    Everything is in the signal path, especially the power supply. The transistors (or tubes) are just valves. Impedance is the same though, so that's a headscratcher.

    Another possibility is that the 37Hz is below the resonant frequency of the HD650. Strange things happen when we try to power stuff below this. Many more questions than answers.
     
  5. Psalmanazar

    Psalmanazar Most improved member; A+

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    @Marvey Amir includes thd graphs at volume for speakers now but his subjective impressions that lead to
    The thumbs down or thumbs up are all past the port of massive distortion. He bottomed out a passive ATC with a nutty bass shelf and hit the limiters on the Genelec 8351, suggesting he’s listening at over 90 db rms with popular music and insane peak volume. Most of the half decent popular ones are pretty clean in the midrange at 80 db except for cab and crossover issues Metal tweeters will have some imd issues but nothing like pushing the speakers.

    Ears and small speakers won’t last long at those volumes. Either the amp is distorting which can literally set paper drivers on fire if it oscillates or square waves enough or he’s using a high powered amp and any transient can nuke the woofer or tweeter. We know he’s using a shitty crown pa amp to measure and an unrepairable mark levinson so the outlook is not good.

    He hated on Barefoots, which are pretty much made as an all in one solution for guys like him. He would hate on the huge Quested 10” woofer active that is too with 700w of class d on the woofer.
     
  6. Ox Cart

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    That would suggest that feed forward error correcting designs should be more resistant to this. Would be interesting to compare to a THX789 for example.
     
  7. GoldenOne

    GoldenOne Friend

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    This is great stuff!
    Neurochrome makes a headphone dummy load with both resistive and reactive/capacitive load settings which can also be controlled by APx.
    Not suuper cheap or anything but might be well worth the investment to make looking into this stuff super easy.

    Awesome stuff!
     
  8. Biodegraded

    Biodegraded Friend

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    Which leads me to yet another request: To investigate inductive vs resistive loads, try a dynamic with a reasonably flat impedance profile vs a planar of similar impedance and sensitivity and see if the differences in the bass persist.

    Two possible candidates: AKG K7XX (or other K7--), which has a pretty flat impedance through the mids & bass including the likely driver resonance point; and Fostex T50 RP3.
     
  9. Beefy

    Beefy Almost "Made"

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    Neat point that I had never considered. A planar should have little-to-no back EMF, for the same reason a planar can be so fast - because there is virtually no mass/inertia in the membrane. Whatever little kinetic energy there is an a vibrating membrane is dissipated incredibly quickly, vs the mass of a dynamic voice coil and diaphragm.
     
  10. Armaegis

    Armaegis Friend

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    I dunno if there would be any value in putting the headphone drivers right in front of a speaker and blasting a few tones?... not even sure how we'd measure any effect that would have feeding back into the amp.
     
  11. crenca

    crenca Friend

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    Is current (as opposed to amount of feedback, or something else) the likely explanation for the difference (i.e. at 37hz) between the Jot and the Magni/Topping?
     
  12. Marvey

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    I believe that IEMagni and Atom uses TI OPA parts that implement feed-forward. Also the THX789 still uses negative feedback, a ton of it. They are not mutually exclusive and it's important not to get hung on up terminology such as Feed-Forward, North County Fire Authority, Bend-Over-Backwards, etc.

    It's easy enough to build a network that simulates a headphone's impedance curve complete with the Fs bump. However, I'm not convinced that artificial loads simulated by L, C, and R parts will suffice. Headphone drivers are highly non-linear. Magnetic force, mechanical compliance, stiffness, all these things change depending upon where the voice-coil is in magnetic cap of the motor.
     
  13. Marvey

    Marvey Super Friend

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    Want to keep this thread more focused on L30. I'm gonna start a new thread and itemize a list of to do / to explore items. Science, learning, and investigation is a lot more fun than dogma.
     
  14. Ox Cart

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    I'm aware of that, I just don't know how much feedback the 789 uses, whether it's global, local or both, and how much that even matters for your experiment.
     
  15. Marvey

    Marvey Super Friend

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    789 uses nested opamps and has a x1 gain setting. That means a ton of gain, which means a ton of global and local feedback. I believe the global feedback loop is described in their patent, although their patent has transistors in the output stage instead of the OPA564.

    What I'm saying is that FF isn't some special tech that changes or revolutionizes things. It's a method that been around - forever - that fits into existing methods. Don't get fooled by fancy terms. It's not even worth a patent. That would be like me patenting a algorithm for Quicksort with a tweak for small lists.
     
    Last edited: Jul 2, 2021
  16. Degru

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    Very nice to see some figures for real loads; would be nice to try with other headphones. I'm curious to see what kind of correlation there is between the loaded amp measurement results and the commonly perceived improvement described on various cans with amplifier upgrades.
     
  17. Marvey

    Marvey Super Friend

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  18. Marvey

    Marvey Super Friend

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  19. Marvey

    Marvey Super Friend

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    Here is the potentiometer tracking measurement for this specific Topping L30 amplifier.
    upload_2021-7-5_21-33-45.png

    I would have liked to see better tracking to -60db. Seems things fall apart quickly below -50db where the tracking between L and R start to totally get out of whack. -60 to -40db is where the volume knob will be most of the time, depending upon source and headphone. At worst, the pot is off 0.5db from -50db and up. Personally I wouldn't worry about it for headphones like the HD650. However if numbers like SINAD rule your world, I'd do a double-take.

    Knowing what I do about potentiometers, their variablity, especially inexpensive ones that go into starter amps, I had always been suspicious of the ASR L30 review results below. Note the X-axis is reversed and ASR uses "time" for the x-axis instead of attenuation. I suspect this unit was cherry-picked because that pot below is too good to be true. I'd actually pay a $100 for a pot like this. $200 even. I'd kill for this pot.

    [​IMG]

    This wouldn't be the only occurrence ASR received a cherry-picked unit.
     
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    Last edited: Jul 5, 2021
  20. Marvey

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    @LetMeBeFrank: Can I send this to you so you can put a bullet in it? I initially wanted to sell it to recoup costs, but this is a bad unit that shouldn't be sold to anyone. One of the channels starts to fart when being asked for more current, the potentiometer sucks, and the static electricity that kills headphones fix is not in place.

    If anyone wants to send me a "good" L30, I'd be happy to redo the THD vs power/voltage and potentiometer measurements. This could have just been a bad luck of the draw. Or unscrupulous retailers off-loading older static-sensitive units to gray party sellers. Topping should have made an earnest effort to recall these units sitting on retailers shelves.
     

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