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Non-Phool jellybean audio-frequency JFET

Discussion in 'Electronic Basics' started by Ecnerwal, Aug 20, 2012.

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  1. Ecnerwal

    Ecnerwal Guest

    Trying to pick a JFET that's still being made with reasonable low noise
    characteristics for simple audio preamp type duties. No golden-ear BS,
    just things that can be measured. Mic preamps, instrument pickup
    preamps, a FET to have in the junkbox for unknown things yet to be
    cobbled in the small-signal audio range. Cheap is also good. Jeorg cheap
    would be even better ;-) Old-fashioned packaging would be nice, but its
    unlikely these days, I think.

    Mouser has one item that comes up with Audio JFET (that's not a jfet
    input something else...) Audio FET gets a few more, but most are class D
    power devices.

    Toshiba 2SK880 in "irritatingly tiny" package. Par for the course these
    days and I have adapted to soldering irritatingly tiny if I have no
    other choice in packaging. 43 cents for 1, $29.50 for 100 Looks to be 5
    years old judging by the datasheet date.

    One that is mentioned in some older web circuits that's still marginally
    available (in the 150% larger SOT23 only) is the J201, which seems to
    have somewhat worse noise numbers. 23 cents for 1, $21.90 for 100, and
    all of its relatives in other packages are already obsolete, so it may
    not be long for this world, either?

    Digi-key's search is as usual near useless (or it and I search
    differently), and Newark comes up with a bunch of class-D power fets
    that probably won't like non-switching use (If I have even a vaguely
    correct recollection of what "class D audio amp" means. Looks like I do
    per 5 seconds of checking my memory)

    There are of course lots of RF parts that have no specs below 100Khz, or
    1 Mhz, or 1 Ghz, depending on part. Perhaps some of them work fine for
    audio. Anyone care to clue me in?
  2. amdx

    amdx Guest

    Just because I was curious I searched BF862 preamp and found a
    high performance phono stage that uses 8 parallel BF862s.

    3/4 downpage

    I'm clueless as to whether it is a good circuit.

  3. amdx

    amdx Guest

    Hey Phil,
    Do you have a compliment to the BF862?
  4. amdx

    amdx Guest

    Ok, ok, Hello BF862, you've got some really nice parameters there.
    Do you have a complement for the BF862?
  5. amdx

    amdx Guest

    Ya, most of this is over my head, I was just thinking about
    replacements for an AM radio antenna preamp circuit I know of.
    Is this a physics problem building a P type to match an N type?
    Thanks, Mikek
  6. Ecnerwal

    Ecnerwal Guest

    They fit the mold of what I was thinking might be the case - an "RF"
    part with application elsewhere. I had actually found Phil's earlier
    postings praising them (and verified that they were still available)
    before I posted, but was not at all clear after looking at a datasheet
    that cut off frequency specs at 0.1 Mhz on the low end if they'd work
    reasonably for audio, given that they were being touted for RF in car
    radios and a lot of what Phil does goes up there a ways.

    Quite a price variance between digikey/newark/mouser, with mouser
    winning strongly (27 cents for 1, 23.3 for 100.)

    Given a lack of directly comparable graphs I have a hard time telling if
    the slightly more expensive (43/29.9) and smaller (smaller not being an
    advantage for me as a one-off tinkerer) 2SK880 has a _slightly_ lower
    corner frequency or not really, but I suspect it's not enough of a
    difference to actually matter (not going phoolish), given that I'm
    mostly looking for a reasonable part to have on hand and play with in
    the same way I have a pile of 3904/6, LM833, etc.

    Now to see how I do with Larkin's x-acto/copper foil prototyping
    technique, since I can't see getting boards made for every iteration of
    goofing around I want to do, and these things are seriously small...
  7. Phil Allison

    Phil Allison Guest


    ** JFETs have many desirable properties and one HUGE drawback.

    The sample to sample parameter spread is massive - so much so that it is
    normal to select devices for a given circuit so that bias / operating point
    conditions will be met.

    If you need diff pairs with low input offsets - then be prepared to waste
    a lot of FETS.

    FET input op-amps and matched FETs on a chip are the way to go.

    ..... Phil
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