Help picking a tube amp for HD800/650s Maybe some Planars

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by Judeus, Sep 28, 2015.

  1. dubiousmike

    dubiousmike Friend

    Pyrate
    Joined:
    Sep 27, 2015
    Likes Received:
    269
    Trophy Points:
    63
    Location:
    Los Angeles
    My understanding from Pass' articles was that this is true only for SuSy designs - and that absent SuSy, more active channels equals more noise and distortion, excluding common mode. I believe Benchmark has been arguing the same, including in that sort of nasty exchange with amb, for years. But I tend to take their positions with many grains of salt.

    [Edit to add that I am merely parroting what I have read here and if I am off base, I am happy to be corrected. Just wanted to clarify the bases for my comment above. Doug knows an immeasurable amount more than I do about amps, and I would love to see this point developed in greater detail since it seems there is a lot of misunderstanding about the pros and cons of se vs bridged vs differential etc designs.]
     
    Last edited: Oct 4, 2015
  2. dsavitsk

    dsavitsk Friend

    Pyrate
    Joined:
    Oct 2, 2015
    Likes Received:
    1,616
    Trophy Points:
    93
    Home Page:
    Basically, yes, a differential stage rejects common mode noise. But for a differential stage, all noise is common mode noise meaning that noise that influences the positive signal will also influence the negative -- it is common to both. That means noise picked up on the input cable and noise from the power supply. So a differential stage allows you to relax the power supply considerably whereas a single ended design needs a very quiet power supply. Now, in some sense, that gives one the option of a differential stage with a poor quality power supply or a single ended stage with a great power supply being about equal, but it also gives the option of a differential stage with a great power supply being quieter overall.

    Additionally, it is possible to make a single ended input that is just as quiet as a differential one, but it is not as easy. The differential input is basically always quieter given the same amount of work. One odd consequence if this is that that single ended amps generally need to be larger in order to create more separation from noisy components (power supplies) than do differential. They also need very careful attention to grounding, etc. So while differential circuits may require more components, single ended ones often require larger cases and more care in construction. And for manufacturers, the case and the labor are the most expensive parts of an amp (unless there are unusually expensive transformers or tubes) so a single ended amp requiring more space may well cost more to make.

    As for distortion, differential circuits cancel even order harmonics. Again, a single ended circuit can also cancel harmonics through the use of feedback*, but that has its own issues -- feedback creates high harmonic distortion while cancelling low harmonic distortion which may well sound worse as our brains amplify dissonant sounds.

    Finally, this is with regard to purpose designed differential circuits. The "balanced" circuits where one simply uses a couple of single ended amps back to back are much less effective and such arrangements may well do more harm than good.

    And also finally, the definitions of balanced, single ended, differential, push pull, Class A, Class B, etc are so amorphous and overlapping that it is difficult to make generalizations and different people use these terms differently. Often time, the arguments are more about semantics than circuits.

    * a few people, including Pete Millett have done work with SE circuits where the driver and output tubes cancel each other's distortion. This is a little less predictable and probably loses its efficacy with tube aging. But it is another cool option.
     
  3. dubiousmike

    dubiousmike Friend

    Pyrate
    Joined:
    Sep 27, 2015
    Likes Received:
    269
    Trophy Points:
    63
    Location:
    Los Angeles
    Thanks so much Doug - this is exactly the sort of clear explanation I was hoping for! Really appreciate your taking the time to break it down.

    If any mods are following this discussion, I'd vote that the above be stickied at the top of the amp forum.
     
  4. ultrabike

    ultrabike Measurbator - Admin

    Staff Member Pyrate MZR
    Joined:
    Sep 25, 2015
    Likes Received:
    8,960
    Trophy Points:
    113
    Location:
    Irvine CA
    Awesome post Doug. Thank you. :)
     

Share This Page