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Help please - Naim phono RF mod?

Discussion in 'd.i.y.' started by YNWOAN, Apr 4, 2012.

  1. YNWOAN

    YNWOAN 100% Analogue

    Figured it out - Rf breakthrough now resolved
     
  2. PigletsDad

    PigletsDad pfm Member

    Earth loop.

    They are tied to earth at the board, so must be kept isolated everywhere else.
     
  3. sq225917

    sq225917 situation engineer

    What was the solution, bigger cap on input, or fiddling with earth on deck/arm?
     
  4. YNWOAN

    YNWOAN 100% Analogue

    That's interesting. Before I read your advice I had already had them wired up precisely as you subsequently wrote not to do it; I didn't experience any obvious earth loop issues. I then read your comments and altered the arrangement as you advised - no earth loop issues either. To be honest, I've tried quite a lot of earth connection options and the system, as a whole, seems quite insensitive to earth loop problems. That's not to say that different configurations don't seem to sound different, just that earth noise doesn't alter.
     
  5. YNWOAN

    YNWOAN 100% Analogue

    Fiddling with the deck, arm and phonostage wiring made no difference. Fiddling with the deck and arm wiring could be made to introduce 50Hz noise, but had no impact on radio breakthrough.

    I've got other 323S and K boards and experimented with them (my normal boards have K loading). The S boards did not suffer from RF breakthrough and the (unmodified) K boards suffered exactly the same as my modified boards. As a result I took out the 1000pF loading capacitors and replaced them with some 4700pF (though the actual S value is 6800pF I believe) caps I had, I left the 560 ohm resistor in place). Changing the cap value has cured the RF breakthrough problem, but I do think it actually sounded a bit more 'open' with the 1000pF cap. It may be that I can get away with a value closer to 1000pF (2200 or 1500pF, but I don't have any). If anybody has any experience of this please share?

    Edit: I have read somewhere on the net that Naim altered the K loading on the Prefix to use a 560 ohm resistor and a 4700pF cap (I wonder if this is specifically because of the RF issue?). Can anybody confirm what the loading cap is on a K Prefix (I can't make it out on the pics I have found)?
     
  6. sq225917

    sq225917 situation engineer

    I have 4n6 in mine though I may not have replaced exactly like for like. I never have any RF breakthrough from the Prefix, though I did get some from the external arm wire when the armtube and arm base were electrically decoupled in the unipivot due to the presence of silicon in the arm pivot cup.
     
  7. YNWOAN

    YNWOAN 100% Analogue

    Well the Aro pivot is also decoupled electrically as the cup is Sapphire - but the arm-tube is independently earth wired through the multi connector.

    I think I will just have to buy some styrene 2n2 and 1n5 and try them.
     
  8. sq225917

    sq225917 situation engineer

    Yeh, the Kuzma tube isn't wired separately, just the arm base, so continuity is expected. I might have some values in styrene, they might be the small signal sizes though, 6n8, 22n and 68n. I'll check.
     
  9. LesW

    LesW Making electronics make music

    I tend to cure this problem by fitting 3n3 or 4n7 polystyrenes as cartridge loading. Let me have an address and I'll snail-mail some to you.

    Cheers,

    Les
     
  10. YNWOAN

    YNWOAN 100% Analogue

    Very kind. Actually, it is 4n7 that I have fitted now and I have already ordered some 'posh' styrene ones. I hadn't considered 3n3 though - may well be worth trying (do you think lower than this will fail to block RF pickup?).
     
  11. LesW

    LesW Making electronics make music

    It'll be fine, stop worrying.
     
  12. YNWOAN

    YNWOAN 100% Analogue

    I'm not worried as such, but I do think it sounded better with the 1000pF cap; so I suppose I would like to get as close as possible without allowing the ingress of RF (which, unfortunately, 1000pF definately does).
     
  13. YNWOAN

    YNWOAN 100% Analogue

    Well Ive ordered some 2000pF polystyrene caps; so will see if I get any RF with them - if I do, it's back to the 4n7's.
     
  14. _webwasher

    _webwasher pfm Member

    so for my understanding the rf mod consists of
    - the two moved resistors and
    - the tone arm earth connected to the incoming 0v star via 10nF capacitor.
    the 0v star is connected to the black snaic wire and the prefix case earth point.

    but where is the separate black ground wire from the nac connected to? 0v star?
     
  15. Darran.H

    Darran.H pfm Member

    There are a couple of other capacitors that are changed to a different value as well, but I am away from work right now though so cannot check the values...
     
  16. _webwasher

    _webwasher pfm Member

    thank you for this additional information, Darran.

    so you think it makes no sense just to change the resistors positions and the tone arm earthing via cap?
    would be great if you can check the values and give me an hint which caps are changed.

    what about the separate black ground wire from the nac? is it really connceted to 0v star and then to the earth screw of the preamp? sounds like creating an ground loop and hum...
     
  17. sq225917

    sq225917 situation engineer

    2n2 to 5n6
     
  18. YNWOAN

    YNWOAN 100% Analogue

    I just phoned Naim and they were kind enough to explain the required mods.
     
  19. _webwasher

    _webwasher pfm Member

    ynwoan, would you be so kind and share the information here or by pm? thank you in advance :)
     

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