Massively overbuilt 45 headphone amp all noise no signal

Discussion in 'DIY' started by johnjen, Aug 4, 2018.

  1. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    So we have initiated a build project of a 1.5w/ch headphone amp using the venerable 45 output tube with 6J5 driver tubes.
    It will also be easily convertible to run 2A3 output tubes should the vintage (1938 RCA Radiotron globe/ballon) 45's fail.

    This thing will be monstrously massive with LOTS of BIG iron.
    At last count there are 10 transformers of various types and sizes and a few more may be added depending upon our final configuration.

    In a word this is OLD Skewl tube design, pushed to work on high impedance headphones instead of low impedance speakers.

    We have already breadboarded the basic circuit and aside from the noisy makeshift power supply the 'magic' of tubes was obvious from the very start.

    This is the schematic we are basing this build upon and it will give you an idea of the analog section, but we have already considerably modified it.
    From here, https://www.dhtrob.com/schemas/#PowerAmps

    2A3 or 45 SE AMP.GIF

    The 'matching' power supply schematic will not be used, as a more 'robust' psu will be designed using 5 chokes, 5 filter caps, and separate filament psu's.
    We were originally thinking of monoblocks but the size of this thing was already getting out of hand :), so we 'scaled' back a bit, and we'll use just one mains xfmr with 2 secondary filter sections, one for each channel.

    One of the design goals is to be able to match the authority of The Rok amp in the extreme bottom end (below 20 Hz) with the same (or better) focus and finesse and LEDI (Leading Edge Dynamic Impact). And even in pre-prototype breadboard form, the bass was near the equal of the prodigious subsonic bass The Rok is capable of.

    This is why this will be a BIG iron amp, the plate chokes alone are rated at 64H which should enable the bottom end to reach down to the mid teens.

    But the Magic in the Mids (MitM) was clearly evident even from the crude breadboard, wired up in one day, pre-prototype.

    Once we get a 'proper' PS operating and use the oh so few tweako passive (non-iron) parts in the direct audio path (4/ch with 9/ch total, see the schemo above), things should start to blossom like crazy in terms of the overall SQ, especially with the MitM's.

    And yeah I know, boys and their toys and too much time on my hands etc, etc.

    This should be a kickass amp for my 800-jmod's and Yggdrasil A2.

    JJ
     
    Last edited: Aug 5, 2018
  2. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    So ordering BIG iron is a PIA, especially when there is, say one or 2 pieces that are not 'in stock' and must be built (can you say up to 6 weeks).
    And granted this build is a tad unusual in how much BIG iron is being used given there are only 4 tubes, and that some of these transformers are build to order - only.

    Still the delay in actually being able to get all of the big ticket items together to be able to wire it all up will give us a chance to do layout and chassis work with less pressure to git'r done, at least quickly.

    And some of the resisterators and capacitators etc. are good to have on hand first so we can factor in their sizes etc.
    For instance, the 4.7µf 800volt caps are ≈75% the size of a hockey puck, not exactly a dainty item.

    But ordering them means that the selection of the big, heavy, large chunks needed for this amp are somewhat fixed now.
    Which in turn means we have the sizes of the transformers and can begin their layout, which in turn will determine the chassis size etc.

    It's sorta like dominoes all falling into place.

    JJ
     
  3. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    So here is an early component layout.
    And know it will change, especially during the build stage.

    JJ

    45 amp layout-6.pdf

    you'll probably have to zoom down the size, as it’s a big layout.
     

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  4. highflyin9

    highflyin9 Facebook Friend

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    That's a big boy alright. I bet it'll be pushing 60-70 lbs. What AWG metal are you using for chassis?
     
  5. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Thus far I figure I've got about 30lbs in BIG iron, so I figure, at least thus far, the amp will weigh in at ≈ 50lbs.

    Which is a bit over the top for 1.5watts/ch no matter how you slice it.

    JJ
     
  6. sodacose

    sodacose MOT: WTFAmps

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    Why the Lundahl 2743 plate load instead of the Sowter 8982 (or why not a LL2765 OPT)? Parts on hand?

    Looks like a monster in the works.
     
  7. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Say there.
    We went with Lundahl because they are ≈ 1/2 the price, are (mostly) available from stock (faster delivery).
    Even so the Sowter parafeed transformers (8983) and the Lundahl Plate Chokes (LL2743) are still up to 6 weeks out.

    The LL2765 "headphone" output xfmr is a bit of an unknown and I figure a lowered output impedance will help provide more tightly defined bass.
    And the freq response of the parafeeds is -3dB at 10Hz which is more in keeping with our design goals.

    And those 100H 8982 plate chokes are roughly twice the inductance we figure we need, so we stuck with the design goals, instead of going completely off the deep end.

    We needed to show SOME kind of restraint… hahahahahahahahahahahahaha :)

    JJ
     
    Last edited: Aug 15, 2018
  8. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Oh yeah I forgot the top plate gauge question.
    Really, I'm not sure yet.

    That part of the design process is still missing a few 'data points' to be able to 'properly' answer that question.
    But as a initial guess I'd say at least 3mm, (≈1/8") and probably thicker just due the span length of the entire top plate and how many holes and their size, like those 100µfd/440v caps need a 60+mm (2.35") hole, and there are 6 of them.

    JJ
     
  9. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    A short update.
    It looks like the xfmr count is now up to 14 and probably will climb even higher as the design 'fills out'.
    We have added plate chokes to the 6J5 input tube (voltage gain stage) to the design.

    JJ
     
  10. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Status report.
    In one case, the 'back ordered' transformers are being readied for shipment sooner than expected, so I'll have all the iron here in 1-2 weeks, ready for prototyping the placement and arrangement of the layout which is still morphing as I type this.
    We have come up with a 'novel' layout scheme we are mulling over, that mightily tugs at my imagination.
    I just have to figure out some 'technical details', like how do I actually build it and be able to easily modify the the innerds as needed during the refinement stages.

    It tickles my muse.

    JJ
     
  11. Cspirou

    Cspirou They call me Sparky

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    14 transformers! You sure you don't want to make it monoblocs? Sounds like wherever you build it will be the final resting place.
     
  12. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Going with monoblocks would add at least 2 more (bigish) transformers and 2 more big caps (5" long and 2.5" in diameter), and take up even more desktop real estate.

    And while it will have a large (20"x12" or bigger) foot print, it will be designed to be refined and dialed in, which means easy access to all of the wiring is a must.

    I'm close to the next layout being done.
    That is until I actually start laying out the actual parts and figuring where all the wyres gotta go.
    Then of course the layout will change yet again.

    I'll be using Jupiter 14awg solid copper with a cotton braid as insulation to wire everything together, with a few exceptions like the ac heater feeds, which will use twisted and shielded 20awg wire.

    All just to keep the 'over the top' theme, firmly embedded in all aspects of this build.
    Bwwwhhhhahahahahahaha.

    JJ
     
  13. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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  14. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Here is a picture with most of the pieces needed and arranged, in a sorta like way, as it'll wind up being arranged. IMG_1863.jpg
    Only it already has morphed.
    And drilling out 8 2.5" holes was a major PIA.
    We'll see about using a water jet or similar for the 'final' version.

    Yeah we upped count of the giant 2.5" x 5" PSU filter caps.

    I'll add more pics as we go.

    JJ
     
  15. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    It's ALIVE, Alive I tell you…

    IMG_1896.jpg

    And now the dialing in the operating parameters and tweaking commences.

    JJ
     
  16. Cspirou

    Cspirou They call me Sparky

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    Nice work. What are the parafeed capacitors you are using?
     
  17. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    Thanks!

    There are no parfeed caps per se.
    Those 8 filter caps are non-electrolytic 100µfd 440vac paper/oil Temco caps.

    And the output from the 45 tube is capacitor coupled to the parafeed xfmr via 4.7µfd 800vdc Mundorf "supreme evo", (I call it the hockey puck, for a reason).

    And one of the potential goals, down the road, is to not use ANY electrolytic caps anywhere, especially directly in the audio path.
    This is a major undertaking because the bias circuits for the 45 and 6J5 tubes both have bypass caps in the hundreds of µfd's which would add 12 more of those huge non electrolytic caps for a total of 20 of them, which would add ≈9" of length to the 24" of the existing length just for those 20 caps.
    Of course other suitable lower voltage non-electrolytic caps could be used but they ALL tend to be 'large'.

    And fortunately there might be a away to to use interstage xfrms with the parafeed output transformers, but that is all for another day.

    Thus far the SQ is about on par with my ROK amp and fortunately has the extreme bottom end reach that the ROK's has WAY down low.
    And at this point in the dev cycle (tweaking the operating parameters) reaching this goal so early is a major confirmation of the designs capabilites.

    Now to get the rest of the circuit dialed in and run in balance, so the full magic can come thru.

    JJ
     
    Last edited: Sep 16, 2018
  18. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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  19. zerodeefex

    zerodeefex SBAF's Imelda Marcos

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    I'm incredibly glad you stopped spelling iron like you were pushing out a huge shit in every post. I know @Hands and @jexby were concerned after the first few posts in this thread.
     
  20. highflyin9

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    I love the "8 pack" of oilers all lined up. The look is definitely old-school muscle :)
     

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