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Is it difficult to dual rail a Nac 42?

Discussion in 'd.i.y.' started by Maxbertola, Feb 17, 2013.

  1. Maxbertola

    Maxbertola pfm Member

    Before I open the preamp and venture into cutting and soldering, can someone tell me please if it's as relatively simple as it was to dual rail a Snaps?

  2. MJS

    MJS Technical Tinkerer

    It should be. You'll see the power track going to both sides of the board from the solder pin. Naim simply chopped that so that each side got it's own rail. I've done it before.
  3. Maxbertola

    Maxbertola pfm Member

    thanks. My Nac is a 42, not a 42.5, yet it has a 5-pin, 270° DIN socket labelled 'HiCap', although I suspect it only uses a single rail of the HiCap until I do the mentioned operation.
    Can you confirm it's so?

  4. mudlark

    mudlark nearly half a clue

    This appears to be a 42 with 5pin din. Don't ask me about the internal wiring.....

    I suppose the conversion would be very easy, and may well have been done. You should have the blanking plug in any case.

    I would post a picture of the insides if you want any further advice. It would give you access to the people who know what they are doing.
  5. timH

    timH pfm Member

    Yep picture would help. Does it also have a four pin socket for output like the one in the link above?
  6. MJS

    MJS Technical Tinkerer

    It has a 5 pin DIN with a link plug. It already is dual rail.
  7. mudlark

    mudlark nearly half a clue

    One suspected all may not be as it seems. It would appear that anyone should make sure they understand the contents of a box notwithstanding what it seems to be. Just because it is labelled as a 42 it could be a 42.5 or modified in various other ways.

    It would seem that some 42s are very, very easily dual railed.
  8. Maxbertola

    Maxbertola pfm Member


    yes it has one, as is normal when the 42 is powered by the power amp and feeds the stereo signal to it.

    thanks for the link. I know it should be obvious that a 42 having a 5-pin 270° socket should already be a .5, but I have read somewhere that apparently a sort of intermediate generation of 42s exist that can be powered by a HiCap but only use a single rail of it, so are not yet dual railed.

    I don't know enough of it. I'll simply open the pre and take a look. If 5 different cables run from the 5-pin socket and no two are bridged, it's already dual railed.

  9. davidsrsb

    davidsrsb pfm Member

    There is no reason why a WELL DESIGNED single rail preamp should be in any way inferior to dual rail. They have the advantage of having well defined dc bias on inter stage electrolytics
  10. bivalve

    bivalve pfm Member

    This is a novel viewpoint for the DIY forum given the evolution of experience and ideas here over the last 10 years. Is there anything else you can add?

    We generally discuss the naim preamp circuit wrt this venerable schematic

  11. davidsrsb

    davidsrsb pfm Member

    Split rail has advantages for opamp based designs, mainly ease of biasing. Then you get into trouble with dc blocking between stages. Either you use high impedances and plastic capacitors or you end up with back to back electrolytics with no dc bias

    Single rail designs suit discrete circuits. Careless designers do not ensure that each block has good power supply rejection. As an example in the linked schematic, R10 injects supply noise directly into the current source on Q1. If R10 was split into two resistors with the centre tap decoupled to 0V, the psu rejection would be improved
  12. mudlark

    mudlark nearly half a clue

    I suspect some confusion here. Dual rail in naim preamp means 2x24V not -24 - 0 - +24.

    If I've got the wrong end of the stick, sorry.
  13. davidsrsb

    davidsrsb pfm Member

    If you mean running separate 24V for each channel, then I can see why inadequate supply rejection could make this an improvement.
    The input buffer shown on Neil McBrides page is an odd design. If the source is low impedance, then it is just a over complex low pass. If the source is about 1k or more (Quad 44 tuner etc), then the low pass filter cut off will start to reduce and the supply rejection at low frequencies becomes very poor, due to the two 330k resistors setting the 12V bias. A far better biasing circuit, with good supply rejection would only have required a couple more cheap components
  14. mudlark

    mudlark nearly half a clue

  15. Maxbertola

    Maxbertola pfm Member


    thanks for this, but the discussion has gone far beyond a point where I still understand.
    I can only confirm that dual rail means 2x+24v in Naim world, but as soon as it becomes really technical I am left behind.

    I have no doubt that better designs exist. I have this, I am not unhappy with Naim electronics. I might well leave the 42 powered by the 140. Today I have done some comparison between such configuration and the 42 powered by a modern HiCap-2, and the unexpected result is, that the 42 gains a number of things from being powered by a modern HiCap, but none of them of musical interest.
    It gains sharpness, it gains attack, it gains 'voice', but naturalness, balance, even the delicate ambience informations so evident with the lesser configuration, are gone.

    So, a false problem it appears.
    Thanks to all for this sideways enlightenment.

  16. bivalve

    bivalve pfm Member


    Mudlark has cleared up the meaning of dual rail as used here. Your suggestion for filtering noise through R10 has been tried and adopted by many, and a similar treatment on R3 also. This caused much excitement 6-8 years ago (I'm a bit fuzzy here).

    The most evolved version of the Naim preamp circuit became known as the Starfish, for which PCBs are available. It is split rail, has two positive and two negative on-board supplies for each channel, and complete star earthing. It also has on-board switch mode power supplies. The Starfish is 5? years old now and still has its fans.

    The B4 is the latest pfm preamp, designed by pigletsdad (PD) and is split rail with a completely symmetric design. Group buys for the PCB and required constant current diodes have taken place several times.
  17. NRWler04

    NRWler04 pfm Member

    I build my NAC24 with 4 24V supplies.It 's really easy. If you like, I post a pic.

  18. Maxbertola

    Maxbertola pfm Member


    thanks, but I am not sure what a NAC24 is (unless you mean NAC42), but I'd gladly see a pic. What's the benefit of 4 x 24V supplies?

  19. NRWler04

    NRWler04 pfm Member

    yes ... NAC42 ....

    Each amp stage has its own power supply. I'm not an expert sorry .....

    But it sounds quite well.
  20. MJS

    MJS Technical Tinkerer

    Each stage with its own power supply. That'd be a 52

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