Pass B1 volume control

Discussion in 'DIY' started by skem, Dec 24, 2018.

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  1. skem

    skem Friend

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    Trying to build a suitable volume control for Gungnir Multibit & Soekris.

    In the spirit of simpler is better, here is a high-quality build of the original Pass B1 buffer pre-amp that I’ll be testing against known-to-be excellent transformer-based volume controls. More on that shootout perhaps later this week.

    I chose the capacitor-coupled version rather than the more complex capacitor-free variant for circuit-simplicity reasons. After listening to a lot of fancy circuits that have virtually no measurable distortion but sound dead, I’m starting to think the phase distortion from many active components is a good reason to keep it simple.

    I used Audyn reference coupling caps, which I have found very good in prior projects. Resistors are low-noise metal film 1%. Pot is Alps. Power supply is a low-noise switcher in a faraday cage (and is dead silent). Also has an IR-remote control circuit on a dedicated power supply and board. Signal, circuit, and earth grounds are separately managed.

    Listening test results here.
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    Last edited: Dec 28, 2018
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  2. pedalhead

    pedalhead Friend

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    Looks great. Will be very interested to hear your thoughts.
     
  3. dBel84

    dBel84 Friend

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    Papa Pass is particularly excited by his nutube version of the B1. Might be worth looking into too.. dB
     
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  4. skem

    skem Friend

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    Thanks! I read the 6Moons review of it. Seems like a fun thing to play with. I have gone from solid-state, to tube, and back to solid-state in the past year. But specs on nutube seem mighty fine.
     
  5. Cspirou

    Cspirou They call me Sparky

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    Nice! How are you grounding the front panel?
     
  6. skem

    skem Friend

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    Panel is on earth. Connectors are isolated. You can see the little plastic discs.
     
  7. skem

    skem Friend

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    Listening tests

    Today, @porkfriedpork and I listened to the Pass-designed B1. We compared it to the digital volume control on Soekris and the Django-built Silk Audio transformer-based volume control (TVC). Chain was Soekris 1541-> volume control under test->Pass XA30.8->HE6 and HD800. We did not have a proper A/B box, so had to unplug cables to switch volume controls, adding seconds of delay between listening. That undermines our ability to hear subtle differences.

    Let me start by saying these are all VERY close. Porkfriedpork says they were the same within the margin of error. I felt there were differences, but most times I heard a difference it would disappear on my next A/B. Overall, I’m fairly confident the Django TVC had a little dip in the mid treble. The B1 felt like it was brighter to me, with crisper treble than the TVC. Porkfriedpork said B1 was warmer. Our observations seem in contradiction, at least superficially, but I agree that the B1 sounded warmer for particular instruments (maybe some 2nd order distortion?). Overall I felt there was some loss of subbass amplitude with both B1 and TVC—maybe 2dB @ 20 Hz. The capacitor-free version would probably fix that but at what cost is unclear. All of these effects are pretty darn small—overall presentation changes were much smaller than the differences between DACs, for example.

    We noticed an odd effect where the stage on the Soekris appeared to get bigger when the volume was around -30. This larger stage survived passing through the B1, suggesting the B1 does not kill stage. This was not a small effect, but why the Soekris should do this is beyond our comprehension. We did not test for this specifically with the TVC, but we also did not notice it, suggesting either (1) TVC does hurt stage or (2) B1 hurts stage only when the volume is turned down. Given how the B1 operates, however, I seriously doubt theory 2: everything is always getting the exact same gain all the time in B1. I’m inclined to think it’s a Soekris effect that we failed to notice on TVC.

    In separate listening tests carried out with a non-SBAF member who owns PS Audio Direct Stream (snowmass) to Pass XA30.8 to BW 801 series III, he said, “I don’t hear any difference” with the B1. The 801s are not as resolving, however.

    The B1 does have a tiny bit of gain as built, giving you a bit more room to control signals. I like that the TVC is fully passive, but of course it doesn’t have a remote control.

    Conclusion—both make the volume adjustable, neither will f*ck your sound.
     
    Last edited: Dec 26, 2018
  8. skem

    skem Friend

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    Informational update for DIYers. I swapped out the 10uF Audyn Reference (white) caps for Jantzen Alumen-Z. I felt the Audyn were a bit bright, but the Alumen-Z just sound muddy. Not recommend.

    Will put the Audyn back.
     
  9. Taverius

    Taverius Smells like sausages

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    Would jupiters work? Maybe try to borrow/extort a pair from someone.
     
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  10. murphythecat

    murphythecat Friend

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    I had a pass b1 with mundorf zn caps.
    its a very good pre amp, but I did notice a very slight loss of detail and transients, maybe my impressions were placebo. it will not ruin a system, but i prefer using a integrated amp/headamp with already built in VC just to save a IC's.
     
  11. dBel84

    dBel84 Friend

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  12. skem

    skem Friend

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    That would be nice. I’m sort of sick of spending $100+ for a pair of capacitors just to try. If anybody wants to lend a pair of 10uF caps, I’m up for it! Likewise, 10 and 3uF Alumen-Z are available to borrow for anyone having glare issues in their crossovers. I don’t recommend them as coupling caps.

    Interesting, dB. If you’d like to do a temporary exchange just to hear what’s what, I’d be happy to swap for a week. Especially this month since I have another pre-amp in the house.
     
  13. dBel84

    dBel84 Friend

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    I just have a volume pot dangling out of the front of it, so it needs a little more lovin before it is respectable . I built up a muses volume controller but it needs dual 15v rails which necessitates a new chassis anyway. Perhaps if I can make it functional before the major overhaul I can send it over.
     
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  14. skem

    skem Friend

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    I think that’s accurate. I also feel transients are softer. The extent to which does seem capacitor specific, howeve. I admidt to finding this a theoretical puzzle.
     
  15. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    Not sure if I'm missing it, but how do these compare with the digital volume control on the 1541?
     
  16. murphythecat

    murphythecat Friend

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    I think the cap is the culprit as well as the additional Ic's.
    I think the pass b1 without output caps is the DCB1
     
  17. skem

    skem Friend

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    I skipped DCB1 since it has more active components—but I would be open to hearing it. Not sure if I’m up to building it just because I have a lot of projects going at once.
     
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