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Naim Nac 42 mods into 42.5

Discussion in 'd.i.y.' started by ricardo007, Aug 19, 2017.

  1. ricardo007

    ricardo007 Member

    I have a snaps modified into snap2.
    And a nac 42 preamp which i would .like to transform into a 42.5 so that i can take advantage of the dual supply rails feature of the snaps2.

    Question 1
    What does each rail supply power to?
    Inside the 42 you have 4 circuits :
    1- the relay common to both channels
    2- the phono cards
    3- the gain circuit
    4- the buffer stage
    So rail 1 would supply phono stàge plus buffer stage?
    And rail 2 would supply gain stage and relay?
    What is the advantage of this?

    Question 2
    How to realize this? I see that the 42.5 has a different main PCB with a different layout but schematics seem to be the same. There is a chassis 5 pin connector on the 42.5 to accomodate the extra rail instead of the 4 pin connector on the 42 but what has to be done inside in terms of wiring and grounding

    Question 3
    After the mods should settings be changed?

    Need help to go forward
  2. snowman_al

    snowman_al pfm Member

    Re question 1: Yes you are correct on all points.
    ''What is the advantage of this?'' you will have many opinions, I will leave it to others to answer.

    Question 2: This I how I have done them in the past.

    On that one I have kept the power and signal wires separate. SNAPS or Hi-Cap goes to the new 5 pin socket and powers the NAC. The old 4 pin socket now only has the signal wires. Connect the 4 pin SNAIC directly from the NAC to your power amp. (No risk of getting the power amp 24 voly supply, if it has one, mixed with the SNAPS supply.)
    You can reproduce the NAIM standard NAC 42.5 way if you wish of course.

    Question 3: What 'settings'? Might be worth changing the tantalums and ROEs when you have the board out. Have a look at too for ideas.

  3. MJS

    MJS Technical Tinkerer


    Great writeup and I exactly how we do them here.
    Gain boards and relay get their own rail, everything else runs off the second rail.
  4. ricardo007

    ricardo007 Member

    Very interesting! Thanks a lot. I am interested to know the various options for using these 2 rails!

    Option 1, yours, separating signal from power. I understand it detracts from Naim orthodoxy (which itself is not catholic...) whose advantage is to put all grounds planes of snaps2 nap110 and nac 42 to a common reference in the preamp. What are the advantages you perceive? If i follow your way there is not anymore advantages to socalled Linnik Xmas mod (direct connection of earth of key components like feedback cap to starearth point)?

    Option 2, the original Naim way of making a 42.5 : how to do that one and the rationale for it?

    Option 3? , i was thinking of bypassing the relay (how can this be done?) and supplying separately the right and left channels like dual mono but can this be done with this board easily?

    My main source is a revised vintage LP12 on that system with a new ortofon blue quintet cartridge

    Other options may be?

    For the change of components i was thinking changing all the tantalum beads to axial wet tantalum and all the power rail decoupling caps to oscon SP and keep marroon ROE feedback caps to keep vintage sound

  5. snowman_al

    snowman_al pfm Member

    I'm not sure I fully understand your questions. But I'll try these.

    Doing the mod as shown gives the same division of buffer and gain stages as a real Naim 42.5 board.

    You can do a left / right split, but there is a bit more involved.
    The easy way to try is to lift the 24 volt side of the 4 x 27 ohm resistors and run 24 volt flying leads to them as you want. If you go that route you could link out the relay, but I would keep it and use one 'rail' to run the relay and the other to run the LED...

    Using the 4 pin to power amp and 5 pin to SNAPS system does reference the 'star point' on the NAC board. There are 2 benefits, one: the power amp connects straight to the star without having to go through the SNAPS and two: the signal ground wire does not carry any power supply current.

    None of the options you list changes the Christmas Mod, by the way.
  6. KiwiMike

    KiwiMike Member

    This is how i am converting over to dual 24v rails keeping all left circuits on one rail and right circuits on the other .
  7. misterc6

    misterc6 Wasted and wounded, it ain’t what the moon did

    Your photo link is not working.
  8. misterc6

    misterc6 Wasted and wounded, it ain’t what the moon did

    Very good advice so far. If you have no intention of using the phono boards you could remove them and link the input and output pins to make an additional line-level input. In this case, I'd suggest splitting the power rails into left and right channels.
    Tom Bonjour likes this.

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