KG CFA3 - building big ᛋolid ᛋtate

Discussion in 'DIY' started by Hrodulf, May 19, 2020.

  1. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    The year is 2020. Situation for headphone electronics DIY has never looked more bleak. Whatever you build, some Chinese outfit will be able to built it for cheaper and Schiit will probably do it better than you.

    An exception is e-stat amps due to reasons outside the scope of this post and big friggen transistor amps. Think balanced Beta22, balanced Dynahi, Bryston BHA-1... Hot and heavy Class-A beasts in speaker amp enclosures which cause a hernia if you stare at them for too long.

    Beta22 is old and prone to self-destruct if you unplug the phones at the wrong time. Dynahi is old and boring, and I've already built a Dynalo which is almost the same thing. As for BHA-1... I prefer paying the iron price, rather than the gold price for my amps.

    Why not just take a TOTL opamp, plonk a TOTL buffer and wrap them with NFB? Because it's boring AF and it won't probably be able to play Slayer on Ultra settings. And lately I've been dissatisfied with deep NFB designs.

    So, wat do?

    Enter the CFA3! A design conjured up in the cult of Dr. Gilmore a few years ago. A design so complex and sparsely documented that to this day no one has attempted to build this amp.

    The pitch is simple. The amp consists of two modules:
    1. Input stage/phase splitter - provides voltage gain, phase splits SE signals, "just" amplifies DIFF signals. Has no overall negative feedback, works in Class-A.
    2. Unity gain buffer amp which provides the actual current drive. Each channel has two, which amplify their own phase of the signal. Employs current feedback, works deeply in Class-A, will ignite your headphones before transitioning to Class-B.
    For power I'm using a pair of cloned Golden Reference PSU's which should provide a clean bipolar 23VDC with noise under 1uV.

    Here's the PCB:

    JzgFDeH.jpg

    For enclosing this beast I've chosen a Modushop Dissipante big 2U case. Needless to say I'm not looking forward to metalworking. Designs for both front and back panels are in progress, as I haven't decided on what to use for volume attenuation. A Twisted Pear Joshua Tree would be appropriate, but there might not be enough space for it. I might settle for a decent TKD pot instead.

    Here's the progress so far after 10h of soldering -

    DmxKur0.jpg

    Still waiting for knobs and feet from China. And some clarity regarding the attenuator.
     
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  2. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    The first channel is alive!

    [​IMG]
     
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  3. Armaegis

    Armaegis Friend

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    Is it this thing? https://translate.google.ca/translate?sl=auto&tl=en&u=http://shadowsfalltw.gjisland.net/blog/?p=6019


    That's kinda been my feel on things for a while too. Nevermind amps, even with cables and stuff where you can buy the finished ones cheaper than getting the raw materials yourself.
     
  4. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Sort of, the outputs are the same, but the phase splitter/input amp is different. I use a different design which can be configured as zero feedback. The Uber input board is supersymmetry.

    And half of the time the cheap shit is actually half decent!
     
  5. Armaegis

    Armaegis Friend

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    Or some random box that actually looks decent on the shelf! (compared to my non-existent casework skills)

    Or I can buy a fully braided whatever cable for $40, vs 4 hrs sleeving and braiding and aggravating my arthritis for something that doesn't even look as good. Once upon a time I used to make custom cables and sell a couple here and there, but nowadays I just tell people to buy online because the price differential is so high, it's not worth competing and I'm not going to disillusion people that I can do something magically better.

    Sigh, this reminds me that I have a half-assembled B22 sitting in the basement for a project that never materialized. Come to think of it, I have a full kit for the M3 that's sitting there too. I should just sell those off or something.
     
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  6. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    [​IMG]

    So, it makes music! Preliminary listening very promising, slight AC hum is barely audible when no signal is present, which is to be expected with the rats nest wiring scheme. Still pondering whether to implement a DC protection circuit, which would need a small power source.

    Overall I'm very happy with the build!
     
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  7. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    [​IMG]

    The minimum-viable version is complete!

    Things that still need doing:
    1. Mount the DC protection board, so I can safely power the amp down with headphones connected. Currently it pushes a 2VDC transient on shutdown, which can be needlessly destructive on headphones.
    2. Mount an ON/OFF switch on the bottom plate which can be reached by going under the front panel near the volume knob. I'm kinda scared that AC wires that close to the potentiometer could transmit noise.
    3. Swap out the THAT340 for a pair of 2sk389/2sj109's.
    4. Swap the RK27 to a 4CP-601 pot (the Alps is no slouch, tho).
    5. Tidy up the wiring and figure out the best grounding scheme in a mixed SE/BAL signal environment.
    Initial impressions are extremely good. My maxxxed out Torpedo 3 is going up for sale. I will disclaim right away that I'm using a Yggdrasil, so Torpedo was limited by its SE outputs, while the CFA3 is using BAL. With that said:
    1. The bass control and extension on the CFA3 seems better
    2. The CFA3 seems way more nimble than the T3
    3. Both amps have roughly the same resolution
    4. T3 seem a bit mid-heavy, compared to the CFA3
     
  8. Riotvan

    Riotvan Got lost for three weeks at Delft City Hall

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    That’s a very nicely polished knob sir! And a great build as well ;)

    As for that T3 i might be interested if the pairing with the Clear is any good. Still regret selling mine...
     
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