• PC board available - KevinC 16:16:55 07/02/03 (6)
  In Reply to: Kevin's analog output stage for current output dacs. posted by AnandR on July 02, 2003 at 07:31:03:

  Hi Anand!

  Things are good. So good, in fact, that I already have the PC board that you are asking about. The board houses everything except the power transformer and output RCA connectors. A small 30vAC transformer is fine. I have used the 10VA and 7.5VA PC mount Talema toroidals sold by Digi-Key with success. I will be happy to sell you just the PC board and output transformers, if that suits you.

  I can't say that this is the best output stage in the world for current output DACs, because I haven't heard them all, but it sure is great-sounding.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • Re: PC board available - akltam 20:45:11 07/21/03 (1)
    In Reply to: PC board available posted by KevinC on July 02, 2003 at 16:16:55:

    I am very interested in what you have got here. I don't find further info about this board such as the price. (may be I have not look deep enough).

    Please provide more info or should I email you directly.

    Alan

    
    
    

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    • Re: PC board available - KevinC 16:26:39 07/22/03 (0)
      In Reply to: Re: PC board available posted by akltam on July 21, 2003 at 20:45:11:

      Alan,

      This was never intended to be a kit as such, because it requires a bit of detective work to apply to some CD players / DACs, depending on the DAC chip configuration. I developed it more out of personal curiosity and the needs of a few of the guys here locally. However, for experienced modders it's a nice circuit. I'd be happy to sell the circuit boards at about my cost, $25. The board includes a place for everything except the power transformer.

      As it turns out, I also have a tube circuit that is somewhat similar in concept, but haven't cut a batch of boards.

      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
      
      

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  • about the actual design... - AnandR 18:25:25 07/02/03 (2)
    In Reply to: PC board available posted by KevinC on July 02, 2003 at 16:16:55:

    I was looking at your schematic once again. Coming from tubes, I'm not very good at deciphering solid state so here goes. It looks like you have paralleled two fets (to drop the output impedance and get some good gain), on top of which looks like a current source, whose output you cap couple to the LL1674's. I take it that 3uf (parafeed) cap should be of high quality. I think this is the first time I have seen a parafeed based design using solid state instead of tubes...correct me if I am wrong.

    What is the output impedance of this setup?

    Anand.
    
    

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    • Re: about the actual design... - KevinC 19:09:31 07/02/03 (1)
      In Reply to: about the actual design... posted by AnandR on July 02, 2003 at 18:25:25:

      Well, I suppose that it's not really parafeed in the sense that we are used to seeing it. The solid state stuff is arranged as a cascode amplifier, which gives relatively high gain with relatively low output impedance. The transformer is connected via the cap in a parallel feed arrangement. For one thing it sounds better this way than just with the cap coupled output, for another thing the transformer step-down lowers the output impedance to about 300 ohms, for yet another thing the transformer provides the option of a balanced output connection, and finally, serves as a part of a modest effort at anti-alias filtering.

      I discovered, after devising this using an old textbook that discusses FETs at length, that Nelson Pass uses a similar gain block throughout his prior generation of phono preamp and line level designs. There is so little that is new!
      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
      
      

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      • My bad... - AnandR 20:21:49 07/02/03 (0)
        In Reply to: Re: about the actual design... posted by KevinC on July 02, 2003 at 19:09:31:

        You are indeed correct. That is a cascode. I had to read up on Allen Wright's preamp cookbook and Valley&Wallman's texts to see that. Essentially, as Allen says, a common cathode gain stage (in this case a parallel set of FETS), with a grounded grid stage on top (I see you grounded the 'grid' section with a 10K resistor). This circuit essentially provides very high gain well above the device's amplification factor but with very wide bandwidth (thanks to the GG stage) and..parafeed outputs thrown in for good measure! Output impedance from a cascode is high but coupled with a transformer at 4:1 stepdown...simple and very nice Kevin.

        AnandR.
        
        

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  • Re: PC board available - AnandR 17:58:58 07/02/03 (0)
    In Reply to: PC board available posted by KevinC on July 02, 2003 at 16:16:55:

    Thanks...I'll get it as soon as I secure a Phillips SACD player. Gotta listen to Thriller! in SACD!

    Anand.
    
    

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