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Musical Fidelity 3A preamp.

Discussion in 'd.i.y.' started by Chops54, Apr 7, 2015.

  1. Chops54

    Chops54 pfm Member

    I gave my son a 3A preamp years ago and I thought he would have been done with it by now but he still has it and asked me to service it. The main issue I think is that it's a bit noisy due to the quad N channel mosfets it uses. I can't find any sort of replacement for these ( ZVN4206E) but I did read of someone replacing theirs with four individual 2SK170s. It worked, the noise had gone but there were some instability issues. Can anyone suggest something that might do the job?
     
  2. davidsrsb

    davidsrsb pfm Member

    2sk170 is a jfet, that requires negative voltage on the gate to turn off. The ZVN4406 was a nmos device, so something like 2n7002 is closer
     
  3. Chops54

    Chops54 pfm Member

    Thanks for the suggestion David. Your right of course that the 2sk is a jfet so I googled small signal mosfets and it came up with BS170. I think the chap made a typing error. I found another reference to the BS170 in a discrete preamp build also. I think I'll download the specs for the ZVN4206E and see if I find any single devices with similar specs. I'm wondering if all similar small signal mosfets are noisy?
     
  4. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    These pre amps are indeed fairly noisy due to the mosfets. It's intrinsic to the design and there is nothing you can do about it. VQ1000J was also used and was switched to the ZVN device later.... maybe because of supply problems with the VQ1000J.
    The basic topology was also used in the mega bucks MVX pre amp.
     
  5. Chops54

    Chops54 pfm Member

    Thanks Jez. I was hoping to find suitable single better spec mosfets and insert the leads straight into the fourteen pin holders as a bit of an experiment. It's a shame that the hiss spoils an otherwise nice sounding preamp.
     
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I guess it depends on how sensitive to hiss one is.... although audible with no music playing I never found it troublesome personally... wow and flutter is a bete noire of mine and I seem sensitive to the tiniest bit!
     
  7. Chops54

    Chops54 pfm Member

    My son hasn't complained about the background noise, it was just something that I thought could be improved upon.
    I know what you mean about wow and flutter. Maybe that's why I don't use a particular turntable of mine any more ;)
     

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