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Review: NwAvGuy's O2 DIY Amplifier

Discussion in 'Sound Science' started by shike, Aug 25, 2011.
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  1. mikeaj
    Quote:
     
    Okay, so these headphone amplifiers are real-world systems and thus not 100% textbook LTI systems.  We can even quantify how "off" it is by looking at symptoms like THD given a certain input signal, load, and output level.  The less "off" it is in various senses, the more we expect the behavior to be like the ideal "wire with gain" or whatever you want to call it.  If you hypothetically had two different "wire with gain" then they should sound identical, right?  The signal being sent to the headphones would be exactly the same for either.  So if we had two "wire with gain plus no more than a really small disturbance like 0.00000001% added in" then surely those would sound the same too.
     
    One question is then about the level of disturbance that is still so small it can't be detected.  Intuitively this seems like this should depend on the listener, the kind of music, the listening volume, the headphones, the type of disturbance (for example, 2nd order harmonics are of course much harder to detect than something else), and more.  There's probably not a one-size-fits-all perfectly exact answer, but some studies have established some kind of safe estimates, with a lower estimate of the threshold being safer.
     
    The other question is about how indicative the typical audio benchmarks are of real-world performance with music into headphones.  Usually bench tests are with resistive loads using combinations of test tones.  With a fairly low-output-impedance amp that's stable, performance into non-resistive loads like headphones should be pretty close to the performance into resistive loads.  Music is just a combination of tones, but it will in general contain more of them at once than you'll see in a typical IMD test.  Anyway, keep in mind again that the more ideal the response is, the less distortion we should expect of all different kinds.  The THD and IMD tests can be thought of as some kind of predictors of behavior.  If the values are high, then it's hard to say. If the values are low, then we probably expect performance to follow the ideal fairly tightly.  Has anybody ever seen an amp that performs really well at IMD CCIF and SMPTE into different loads and output levels, have high distortion with any other kind of input signal?  It doesn't seem likely that this should happen.
     
     
    Also a trip down to InnerFidelity will show that the difference between headphone drivers (left and right side), not to mention sonic differences from slight differences in headphone positioning, are all way way higher in terms of FR and THD and so on, than between headphone amps that are considered to be probably sufficiently close to "wire with gain."
     
    edit: on a side note, I'm waiting for headphone reviews where they go into the night and day differences between the left and right earcups.
     
    "When paired with my überAmp, the velvety midrange of the left earcup is unmatched, such as when retrieving the low-level details of Lilliputian feet scampering about in the morning dew.  But when combined with the authority of the right earcup in resolving every dynamic, the result I feel is a sound stage that is engaging with some music, yet muddled with certain passages.  Rather than placing you in the second or third row, you are unceremoniously dumped in the second-and-a-half row, hardly a comfortable experience."
     
    maverickronin and LizardKing1 like this.
  2. LizardKing1
    zzffnn I wasn't implying 3 variables are enough to describe an amp's behavior. "distortion" isn't even a point, it's a category of points I think. What I mean is if we assume/know that we know, and are able to measure, every one of the variables that an amp introduces to sound, I think it's ok to assume that 2 amps with all variables below the audibility treshold (regardless of one having these values much lower than the other) will indeed sound the same. And from there we cross a line into assuming that all subjective impressions that contradict this theory come from imagination, and not actual amplifier characteristics. So do we know and are we able to measure every variation an amp is capable of producing in an audio signal? Because if so, and all of these check well below this limit for both the O2 and the B22, I don't see why it's not ok to say they are equal in sound, let alone compare them.
     
  3. zzffnn

     
    Quote:
     
    I somewhat agree with your statement above. Although you are assuming lot of things there. I would love to know whether or not we can measure EVERY variation of an amp and how they affect human perception of amp performance. However, I would have a hard time to believe someone's statement unless that person has a doctoral degree in Audio Engineering with concentration in amplifier design. 
     
  4. fishski13


    Quote:

    i see a lot of numbers thrown around for the price of a B22.  if you throw it in a stock Par-Metal chassis with a similar aesthetic as the O2, and stick with an inexpensive pot, a DIYer can build a 2 channel for around $500.  builders often spend the extra cash for a stepped attenuator or more expensive pot and other chassis hardware.  you could easily spend the $80 cost of an O2 on a pair of feet and a volume knob alone.
     
    zzffnn likes this.
  5. zzffnn


    Quote:

    @ mikeaj,
     
    I was talking about human preference, while you are talking about signal transparency. We are not talking about about the exact same thing. I was saying that you should hear 2 amps side by side to determine your preference. Signal transparency =/= preference.
     
    It is debatable whether all variations shown on Innerfidelity are truly audible. Headphone positioning, for example, affects high frequency range wherein not much music is present. We are talking about limited variation there, not purposely introduced variation such as headphone pad change (e.g., from very thin pad to very thick pads) or using tube amplifier.
     
    In the realm of audiophile headphones, left vs. right driver should not differ that much (< 3db) to affect music enjoyment (as long as the headphones in question are produced by well-established companies).
     
    What you stated under "edit" is a dramatization, I take that you were trying to be funny? There are indeed reviews like that on Headfi, but nobody would take that seriously. Respectable reviewers would not dramatize small variations in that way. I respect a good review as much as machine measurements.    
     
     
     
  6. mikeaj
    Quote:
     
    Well it should be pretty clear that you can measure the output of the amp with extremely high accuracy, in response to any kind of input you want (where the input signal has very high accuracy as well, compared to what you're intending to send as the input).  This includes running music through the amp and measuring how it handles that, when loaded by whatever headphones you want.  However, there are an infinite number of possible inputs you could be looking at.  If you were looking at a nonlinear system with a very odd response, it might be difficult to get a decent model of the system with a relatively limited number of measurements.  With a very close to linear system, it becomes pretty easy.  If it were completely linear and time invariant, all we would have to do is measure the impulse response (or equivalently the frequency / phase response) and then we'd know the output for any possible input.
     
    How that affects human perception of amp performance can be measured too.  Ideally you want to measure every human's perception, but that takes a lot of time.  But again, we do know that extremely small differences are imperceptible and can be shown not to have any impact on perception, at least for the average listener as well as highly-trained and exceptional listeners of different kinds.
     
     

    Quote:
     
    the point is:  to have a preference based on the sound signature, there has to be a perceptible difference in sound.
     
    We're talking about amplifiers that should have no perceptible difference in sound.  Two different 0.5% THD (for a given load, output level, etc.) amplifiers could sound way different.  Two different 0.000001% THD amplifiers should sound the same, and thus there would be no way to have a different preference for one opposed to another, based on how it actually sounds.
     
    The argument is that, if you think the comparatively HUGE differences between headphone drivers are insignificant (in the grand scheme of things, they are kind of small), what about the differences between "sufficiently good" amplifiers, which can be orders of magnitude less?
     
  7. zzffnn


    Quote:

    Exactly. Some people are assuming things without building equipments or listening them.
     
    My Beta22 is a budget build 2 channel that costs around $550. My JDS O2 costs $150. If I can save $400 by selling Beta22, I would have after hearing them side by side. I did sell my M^3 without regret, because O2 is good enough as a replacement (this replacement saved me $150-200 and kept me happy).
     
    I enjoy both O2 and Beta22 in different ways, although I prefer Beta22 if I can sit down with LCD2s.
     
  8. zzffnn


    Quote:

    >>>>> Comment: Is this "perceptible difference in sound" by machine or by human? Again, for a human to perceive a sound difference, he/she should at least have 2 amps side by side to compare/"perceive", correct? Or is this "human perception of sound" predicted by machine again?
     
    Re "We're talking about amplifiers that should have no perceptible difference in sound".
    The key word here is "should", have you compare O2 vs Beta22 in reality? And, are you aware that at least 3 actual owners in this thread (who has both O2 and Beta22) perceived some sonic difference? 
     
    Of course, people here are free to make their own judgement with machine prediction vs. actual owner "perception" of an amp's sound.
     
  9. upstateguy


    Quote:

    I wonder if someone could itemize these differences so we could have a look at them in one place?
     
     
  10. zzffnn
    ^ mikeaj's point is that there should not be any sonic difference,
    because both Beta22 and O2 are both designed to be flat/neutral/with extremely low distortion.
     
    I respectfully disagree and I think there is sonic difference between O2 and Beta22, based on my own ears, not based on machine prediction. I was arguing that we should listen with ears and look at specifications at the same time. We should not tell people "those 2 amps will sound the same" based on spec alone.
     
    @ upstateguy,
    You are welcome to go back in this thread and itemize those sonic differences, if you like to. Then we can ask mikeaj to provide an explanation on each item.
    As an example, I heard larger soundstage from Beta22 (compared to O2) with LCD2s, even when I matched volume by decibel (Edit: both were fed by AMB Gamma2). I would love to hear mikeaj's comment on that.
     
  11. xnor
    How about using a simple null difference testing setup with both the O2 and b22? :wink:
     
    (Carver claims to have achieved a -70 dB (0.032%) null in his challenge and the stereophile listeners gave up after two days because they couldn't hear a difference.)
    (btw: "In the twenty or so years that have elapsed since the emergence of the Subjectivist Tendency, no hitherto unsuspected parameters of audio quality have emerged." - Douglas Self)
     
    upstateguy and LizardKing1 like this.
  12. Nebby Contributor
    I don't believe a Beta22 has been tested/measured in the same rig as an Objective2 yet, so making any argument based on the assumption that they should be audibly identical is rather moot. There simply isn't enough information to really make any sort of objective decision on the differences between the amps.
     
  13. upstateguy


    Quote:


     
    Quote:

    Wondering what your testing protocol was?
     
     
     
     
  14. stv014
    Quote:

    For correct level matching, you need to measure the voltage on the headphones playing a test tone, and match it within at most 0.1 dB or 1%. Setting the volume by ear is not enough.
    Was you comparison sighted, by the way ?
     
     
  15. Negakinu
     
    o2.jpg
    My O2 on top of his equally awesome bigger brother. 
     
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