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Avondale NCC200 mod

Discussion in 'd.i.y.' started by Mynamemynaim, Jan 10, 2020.

  1. timH

    timH pfm Member

    Yes there is an equivalent resistor on nap boards. Have a look at the schematic. Plus think about parallel output transistors - even more worthwhile ( there’s a thread on here somewhere)
     
  2. nuit73

    nuit73 nuit73

    yes, the 22 K is near TR1 (like in the Avondale NCC200) on the NAP schematic.
    Can i say the mod is the same on NAP Board ? ... and change the resistor value with 1K (in place of 22K)

    I think about the parallel output transistors, but i want a clear schematic to do this...
     
  3. GWM

    GWM Lost in the 70's

    Yes the NCC is a derivative of the NAP and that part is the same.
     
  4. nuit73

    nuit73 nuit73

    And for the resistor value? what is the sweet spot, 1K or another value like 3K3 or 5K or another?
     
  5. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    That is of course down to personal taste!
    I can only say I found 1k to be a big improvement

    You can of course try different values by soldering "bypass" resistors across the 22k to bring it down to the required value...and several useful on line calculators available to get those values

    Eg

    https://www.digikey.co.uk/en/resour...rsion-calculator-parallel-and-series-resistor


    But a 6500 over the 22000 gives about 5k as a starter ....or put in a variable resistor pair and play about to you hearts content!!
     
  6. nuit73

    nuit73 nuit73

    Thanks for the calculator on line.

    I hope you prefer the 1k instead of the 22k
    I can begin to try 1K first in place off 22K and listen to the improvement....and if it's good in all area
     
  7. nuit73

    nuit73 nuit73

  8. nuit73

    nuit73 nuit73

    On the Naim Nap amp board schematic, the 22 k is in direct link with "TR2"
     
  9. nuit73

    nuit73 nuit73

    Is there any feedback from people who made the mod.
    What is you musical feeling with 1K instead of 22K from Avondale NCC 200 or Naim nap Board?
    Is there a lack of body?
     
  10. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner


    On NCC 200
    I found all round improvement...that's why I started the tread!!
    Clarity.... improved
    Bass.....firmer and slightly deeper
    Vocals... biggest improvement

    And of course...it's Les W recommend!!
     
  11. nuit73

    nuit73 nuit73

    Thanks for you feed-back.
    Some people report that it lacks body on the "Qudos tread" with the resistor mod
    What du you think of it?
     
  12. GWM

    GWM Lost in the 70's

    It's a matter of preference and overall system sound,if yours tends to be lean sounding then 1K will make it more so, if it's thicker for want of a better description then it will make it clearer.I also found it makes poor recordings sound worse so you can only decide what's best by trying it.I didn't like 1K but going back to 22K was not good either so tried out 5K and 10K, 5K gave me the best result.What speakers are you using if it's old Linn with those awfull Bextreen cones then 1K will probably be good.
     
  13. Shadders

    Shadders pfm Member

    Hi,
    I just read the proposed change which is the resistor based current mirror for the input stage. By changing the resistor, you are in effect increasing 2nd order distortion - which is pleasing to the ear.

    It seems from the description of lean, clearer etc., then these are just examples of interpretation of distortion in the system.

    Regards,
    Shadders.
     
  14. OldSkool

    OldSkool pfm Member

    I assume higher value is higher distortion?

    Having recently gone from NCC200 (with 22k) to Qudos with 1k, the Qudos board is audibly less coloured and lower in distortion than the former. How much of this is due to the output stage change versus the LTP resistor I can't say.

    I'm quite tempted to change the old NCC200's to 1k and pop them back in to compare, and then maybe find if there's "sweet spot" with a view to updating the Qudos to that value as well.

    Tonally the Qudos is a bit leaner than the NCC200 with 22k but also less forward in the mids, which is better. My speakers tend towards the leaner side of things anyway so the Qudos is probably just highlighting that at present.
     
  15. Shadders

    Shadders pfm Member

    Hi,
    Yes, the difference in the LTP collector currents will increase 2nd harmonic distortion, and the greater the difference then the greater the distortion.

    Doug Self's book Power Amplifier Design details this (6th edition page 131).

    Regards,
    Shadders.
     
  16. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    I've no idea how much is the resistor and how much is the change in architecture
    But if someone changed the resistors without me knowing... and said they were a new and improved board ...have a listen...I would probably believe them!
     
  17. 337alant

    337alant Negatively Biased

    The NCC200 amp does have the 2 x 100ohm degeneration resistors in the LTP which helps.
    IMO instead of experimenting with this resistive load, installing a proper Current mirror and matching TR1&2 would be the best way to go
    Changing the output transistors to fully complementary like the NCC220 is also an excellent mod suggested by LesW

    Alan
     
  18. S-Man

    S-Man Kinkless Tetrode Admirer

    The resistor change doesn't change the balance of the currents in the LTP (I have measured this). It changes the voltage and the voltage swing on the collector of the "feedback" transistor, which with higher resistor values increases the Early effect on that transistor.
     
  19. Shadders

    Shadders pfm Member

    Hi,
    I just ran a simulation, and set one resistor to 68ohms, and the other to 1kohm. The DC operating point simulation for the resistors shows the 68ohm resistor has 3.1mA, and the 1kohm resistor has 0.21uA.

    This is not with the LTP input stage used in isolation, but connected to a VAS and output stage with DC offset servo.

    Can you run a simulation and confirm ?.

    Thanks and regards,
    Shadders.
     
  20. MJS

    MJS Technical Tinkerer

    I've not tried the current mirror approach - it's been on the to do list for ever though. As I understand it people have tried it, and Self recommends it for low distortion, but the result is a clean and analytical amp that loses much of the signature that this circuit has.
     
    misterc6 likes this.

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