SBAF DAC Talk II

Discussion in 'Digital: DACs, USB converters, decrapifiers' started by Maxx134, Jul 22, 2018.

  1. murray

    murray Friend

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    Yep. I have the next model DAC, the GDA-700. It has 10k pull-down resistors to bias the op-amp outputs. That is nominally 1.6 mA. The chips are Adcom 6AA (AD711).
     
  2. gurubhai

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    Stock opamp in Adcom GDA-600 is Adcom 7AA which is either LT1058 or AD712 depending on where you look at the internet. Regardless, it is a very fine sounding opamp IME.

    The OPA1656 works very well in gda600 and sounds great. I do prefer the OPA1612 and THS4032 over 1656, however these opamps are more finicky and require more care to decoupling/ bypassing. Even with opa1656, I would advise 100-220uf Panasonic FM in parallel to stock 0.1uf ceramic caps.
     
  3. iFi audio

    iFi audio MOT iFi Audio

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    Wow, it's been a while since the last time I've seen PCM63P DAC on the job. Nice!
     
  4. Scott Kramer

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    Yeah, they're cranked into Class A; toasty but not hot. It's using the simple resistor method, measured at ~8.25kΩ.

    An interesting thing looking at the original schematic they even roughed in the "Lone JFET method" as an alternate option.

    I have this in my notes => Adcom 7AA op-amps => rebadged AD712 or LT1057 ??
    ref: https://tangentsoft.net/audio/opamp-bias.html
     

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    • Agreed, ditto, +1 Agreed, ditto, +1 x 1
    • List
    Last edited: Jun 16, 2022
  5. Scott Kramer

    Scott Kramer Friend

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    Done. This is how I did it to avoid messing with the mainboard, seem OK? It's still a delight to listen to!
     

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    Last edited: Jun 16, 2022
  6. gurubhai

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    I prefer to install them on the bottom of the pcb. This way the inductive leads of cap are kept to a minimum.
    20220119_174403_HDR~2.jpg
     
  7. Scott Kramer

    Scott Kramer Friend

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    At least it's under the board! I didn't do it properly before... was thinking about it wrong. Anyway, let's see if it affects the sound (I might even undo this, it has such a unique fast sound as is). Thanks!!!
     

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    Last edited: Jul 1, 2022
  8. Scott Kramer

    Scott Kramer Friend

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    Adcom GDA-600 update. Still can't get over what a delight this is to listen to, new metric for me: SPEED.
    Wrapping it up, completed a full electrolytic cap replace for reliability, and splurged on two Vishay foil resistors for fun, only part the final signal goes through. Under a hundred bucks jelly bean parts unleashed it!

    IMG_0269.jpg
     

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    Last edited: Jul 5, 2022
  9. iFi audio

    iFi audio MOT iFi Audio

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    What a lovely sight, that PCM63P. That Adcom has to be a reaaly nice piece of kit considering its price now.
     
  10. Scott Kramer

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    Gorgeous expressive tone, can't get enough of it! Yeah, crazy prices on the chip & you need 2! Bet it's hard to tell which are fakes on top of that.
     

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    Last edited: Jul 3, 2022
  11. JeremiahS

    JeremiahS Almost "Made"

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    What do you guys think of the Mark Levinson 360 or 360S vintage DAC? Seems it has the PCM1704 DAC chip, a bit unusual since I only know Audio-gd who focused on this particular chip.
     
  12. iFi audio

    iFi audio MOT iFi Audio

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  13. PsiPhi

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    I missed a great deal on a Parasound DAC-1500 (4x PCM63) last week. Still kicking myself because it sold for less than cost of 2 PCM63's on the secondary market.

    Probably for the best because, for me, acquiring one would mean dumping a bunch of parts and time I don't have into mods... Might be a fun retirement project...!

    I kind of rediscovered the PCM63 in a Denon DCD-1520 I recently spun up. What a treat! I have been listening to almost exclusively D/S DACs for years. Taking a few CD's down memory lane, I couldn't help but realize how tired I have become of plastic/artificial sounding transients.

    Ruminating on that, I've become intolerably curious about these 'Modern R2R' DAC's, and how far we have really come in the 30-odd years of digital audio?

    While I'm not an expert on vintage DACs, I acknowledge that the chipset just a fraction of the equation. Digital receiver, Filtering, I/V, and Output Stage all contribute to the sonic solution coming out of the thing.

    It seems to me few can speak on the subject with much nuance - as a result of a lot of generalizations get made...

    I have little of value to add in this department, so I can only hope the above serves as a segue to my question;

    In the 'Modern R2R' category, I find myself between the Yggdrasil LIM and the Holo Spring 3.

    There are some really good comparative reviews on this site, so not looking to compare sonics, per se.

    Yiggy is cool due to the 'flavours' available but my interest is primarily in A2/LIM.

    It's great that Schiit has been working on USB implementation, and has a series of revisions to show for it. I understand the Unison is pretty good, but based on some jitter measurements I've seen, the Holo Spring 3 really appears to be on the cutting edge in the USB department.

    I'm not really interested in DSD. My goal in the home system is to connect with the music, and not necessarily the 'space' or the 'plankton' or whatever.

    (FWIW, I cut my teeth as an assistant, and later a freelance ME in a multichannel SACD mastering studio some 15 years ago. I am intimately familiar with the format and could at one time speak with some authority on the subject)

    ...but now, I'm just a guy who wants to connect with the music in my home.

    In another life, I fooled around with all kinds of filters, dithers, formats...

    The idea of booting up HQPlayer and listening to different filters through 'True NOS' on the Spring 3 I find strangely intriguing and repulsive at the same time...

    Perhaps with the Yiggy, I'm perhaps more likely to go down the path of music server upgrades... Innuos or something like that.

    With the Holo, can I resist the temptation to enter what I'll jokingly call the PC Nightmare World?

    Is one path more rewarding than another?

    User friendly-ness is a consideration. Mrs.Phi and the little Phi's love music too.

    Present source is USB from a rPi4. PiCorePlayer seems to work well for us. Current DAC is Ayre CODEX.

    To me eye, the Spring looks a little better than the Yig, and I like that the display can be turned off, but just like the Sonics I'm confident either will be an appreciable upgrade from the Codex.

    Sonically, my priorities for the system at this time are Transients and Timbre - I think system synergy comes into play here/hear and I am open to the possibility of comparing these two DACs for myself should the need or opportunity arise.

    Any other boring sounding DAC's that are absolute chameleons in the 'Modern R2R' category?

    Is Multicomboburrito goodness worth the warm up hassle of waiting a couple of days for stability after shutting down during electrical storms (not infrequent out here on the prairies)?

    Thanks for reading. It was helpful to type that all out, even if nobody cares to reply to this Neurotic Rando
     
  14. iFi audio

    iFi audio MOT iFi Audio

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    I don't know many modern R2R devices, only our AMR stuff that didn't sound the same. But if you're curious, you could get a Soekris board and make something out of that I think ;)
     
  15. PsiPhi

    PsiPhi New

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    Thanks for the suggestion!

    We recently relocated to a rural property, and between starting up a small farm and constructing a recording studio I haven't even got a proper tech bench setup yet, so... Looking for a 'plug and play' solution for the present.

    I just noticed an AMR DP-777 pop up in the classifieds this morning - wasn't aware of your involvement - I'll take a closer look, thanks again!
     
  16. Scott Kramer

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    Does anyone know deep into the BB DF1700 (Adcom GDA-600 OS filter) chip? It's 20bit output here for the PCM63Ps and simple to set it to 18 or 16bit. My dumb wishful thinking: could 16bit actually be NOS, since it's 16bit on the input? It'd be killer adding a NOS switch and way too easy!

    Also, I wonder if it really ignores data above 16bits on the input... Thanks!

    Surprising amount of hires that's 44.1/48 24bit (not 96+) with new releases on Qobuz... that sound awesome on the PCM63's.
     

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    Last edited: Jul 20, 2022
  17. iFi audio

    iFi audio MOT iFi Audio

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    Yep, that's ours and actually loaded with two separate very different DACs ;)
     
  18. Scott Kramer

    Scott Kramer Friend

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  19. iFi audio

    iFi audio MOT iFi Audio

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    They sure are, which is why AMR's 77 series products had them ;)
     
  20. Scott Kramer

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    Adcom GDA-600 update haha!

    ref: SBAF DAC Talk II
    ref: https://tangentsoft.net/audio/opamp-bias.html
    ref: https://www.superbestaudiofriends.org/index.php?threads/class-a-bias-opamp.5813

    I researched a bit more on that Adcom GDA-600 alternate Class-A bias... the parts are J555 Current regulator diodes (CRDs) and not available anywhere (need to find a better modern equiv.)... I believe this is ~roughly the 2nd method biasing scheme in the tangentsoft link.

    Screen Shot 2022-07-26 at 12.23.45 AM.jpg Screen Shot 2022-07-24 at 9.08.14 PM.jpg

    Low and behold an old GFP-715 preamp uses the J555 on its op-amps... so I harvested them (put in sockets and replaced with biasing resistors for now) The thing needs work in other areas anyway, or may even get parted out/scrapped. But gonna leave it in a working state, and sockets to change later!

    dhGix.jpeg IMG_0223.JPG IMG_0224 2.JPG

    Also I'm temp using OPA1655s (single circuit version OPA1656s) with dual/single adapter to separate out the two and make adding the J555's easier (see pic) plus they're on hand... anyway want to explore everything, could be better or worse, gotta try it, who knows maybe modern OPA1656s don't need biasing!! Keeping it modular to swap back out etc.


    So far it sounds really good and there's a change, letting it play now- just finished!


    IMG_0312.jpg
     

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    Last edited: Aug 9, 2022

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