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DIY NAP250

Discussion in 'd.i.y.' started by Jeff Young, Jan 10, 2019.

  1. Jeff Young

    Jeff Young pfm Member

    Here's a first draft of the amplifier board. Comments appreciated.

    Cheers,
    Jeff.

    [​IMG]
    [​IMG][​IMG][​IMG]
     
  2. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    Zobel ground shouldn't go to the input 0V, separate connection to the PSU is best.

    Neil McBride has board layouts for 135 clones.

    Pete
     
  3. Jeff Young

    Jeff Young pfm Member

    Thanks, Pete. I added a second GND pad for the Zobel.
     
  4. orangeart

    orangeart KJF Audio Ltd.

    You know there are Gerber's out the already right? With better grounding and separate front end power options like the HackerNap or boards ready made from Avondale.
     
  5. Jeff Young

    Jeff Young pfm Member

    Sure, but this is DIY, right?
     
  6. orangeart

    orangeart KJF Audio Ltd.

    Absolutely, just checking

    Nice to see someone who is on such a mission to get the job done
     
  7. martin clark

    martin clark pinko bodger

    +1 to that!
    (Willhave a think about the pcb layout - PeteMBD made an excellent point, now addressed.)
     
  8. Jeff Young

    Jeff Young pfm Member

    I've corrected another issue: I had removed the input coupling cap because my preamps all have them on the output. But that allowed the non-inverted LTP input to "see" the 100K resistor leaving it with a different impedance from the inverted input.

    So I've added provisions for coupling cap options: an above-board 10V tant or an under-board polyprop. That should afford for another good comparison.

    I'll post updated pics tomorrow.
     
    orangeart likes this.
  9. Jeff Young

    Jeff Young pfm Member

    Draft 2:

    1) separated IPS/VAS ground from OS ground
    2) more copper for OS
    3) replaced input coupling cap (with options for above-board tant or below-board polyprop)
    4) added more info on schematic

    [​IMG][​IMG]
     
  10. Jeff Young

    Jeff Young pfm Member

    Moved Zobel ground to keep it from coupling with FE traces.
    Fixed input resistor value and input coupling cap polarity.
    Added jumpers so a single-supply version could easily be built.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  11. orangeart

    orangeart KJF Audio Ltd.

    Looking good. Which layout package do you use?

    Might be worth running group buy, there might be quite a few interested in a build.
     
  12. Jeff Young

    Jeff Young pfm Member

    I use Kicad.

    I'd be willing to do a group buy if there's interest. Either way I'll post the source files once I have something that works.

    Cheers,
    Jeff.
     
  13. orangeart

    orangeart KJF Audio Ltd.

    Never used Kicad myself, I use Eagle. Your renders look nice. Eagle works well with Fusion for renders but is a bit of a pain to get your head around.
     
  14. Jeff Young

    Jeff Young pfm Member

    I do a lot of Pass designs which while being simple, don't fit within the free version of Eagle's board limits. This one, for instance, is 14" x 1-7/8":

    [​IMG]
     
    orangeart likes this.
  15. Jeff Young

    Jeff Young pfm Member

    An update on the board designs. Main changes are to shorten paths in CFP and to join +/- outputs before feedback take-off point.

    Regulators (both Naim and inverted-Naim) also get shroter paths in CFP along with more compact compensation networks.

    Compensation schemes remain traditional Naim: Miller + lag for amp and Miller/dominant pole hybrid for regulators.

    [​IMG]
    [​IMG]
    [​IMG]
     
    Rosewind and orangeart like this.
  16. Jeff Young

    Jeff Young pfm Member

    I bought 25 BC239Cs from Langrex. The theory is that Naim selected these for Vec (since the BC239 datasheet only claims 25V), and then for matching.

    So I avalanche tested mine with a 0 to +86V supply through a 33K resistor to the collector, and the emitter to ground.

    With the base also tied to ground, they all held tight all the way up to 86V.

    With the base floating, about 1/3 avalanched just above 50V, 1/3 avalanched between 50V and 60V, and 1/3 had not yet avalanched at 61V (I didn't run them all the way up to 86V in this test).

    So I don't know that there was a lot of selecting to be done....
     
  17. MJS

    MJS Trade: Consultant at WH Audio

    For many years, pre CAT testing, the transistors will selected by careful (and time consuming) checking of an amp's DC offset by putting the device under test as one of the matched pair. All with the same offset were deemed matched.
     
  18. Jeff Young

    Jeff Young pfm Member

    I still buy the matching part; just not sure much selecting for voltage was needed....
     

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