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cathode follower calculations

Discussion in 'Electronic Design' started by William Weinstein, Mar 26, 2013.

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  1. Hello All,

    I realize this may be outside the current design Architecture, I was hoping
    someone could lend assistance with some math. I'm using a 5840 miniature Pentode
    in a triode configuration for noise reasons, further its application is cathode
    follower in a True capicator microphone preamp. The Plate voltage is 105
    regulated at ~3mA, the grid cap is [email protected](its what I had on hand) the
    Cathode resistor is 3.3K into 100K to Ground and the Grid leak resistor is 10M.
    my output capacitor is 4.4uF (2X 2.2uF) into a 3:1 transformer (other options
    are 7:1 and 10:1 on same transformer). Polarization works (41.2V derived from
    plate voltage), my output is too low and has almost no bottom end, My best guess
    is the output impedance matching is wrong, and or my input cap is too small.
    Some quick measurements yeilded these results: with primary loaded ar 1.2K (.25w
    MF resistor), secondary L=16.8H measurement taken with B&K L-C-R meter portable
    model not bench model; with same load secondary Z=~70K, measurement taken with
    TOA portable Impedance meter. All assistance is welcome.

  2. [...]
    A low signal could be due to a number of causes, but lack of bottom end
    suggests that there is too short a time constant formed by the
    microphone capacitance and the resistance effectively across it. Is the
    grid leak going directly to earth or is it bootstrapped to the cathode?
    How are you feeding in the polarising voltage?

    For a capacitor mic you need hundred of meghohms between grid and earth
    (and the mic and HT+). Either you need special high-value grid leak
    and feed resistors or you need to bootstrap them so that the 'earthy'
    ends follow the cathode signal and there is effectively no signal
    voltage change across them, so they draw no signal current. Even with a
    bootstrap. 10 megohms is a bit low.

    Circuit 1 at:

    ....shows bootstrapping for the grid and the HT+ with some typical
    component values.

    An alternative system which uses fewer components is shown in Circuit 2;
    but this has the disadvantage that the microphone backplate (and
    possibly casing) will be live. It also demands a very well smoothed and
    decoupled HT+ supply.

    If you have plenty of HT to spare and you only need 40v across the
    microphone (which seems very low to me, as I would have expected about
    300v), Circuit 3 shows a way of polarising the mic with the backplate
    earthed, by increasing the voltage dropped across the cathode 'tail'.
    (I should add that I have never seen it done this way before.)
  3. Robert Baer

    Robert Baer Guest

    Practice indicates at least 30V and not much more than 100V across
    the capacitive mike element.
    However, theory indicates that a large voltage bias will cause more
    non-linearity than a smaller bias; in the limit (zero bias) no
    non-linearity - but then no signal either!

    Most capacitive mikes i have seen use 300 megs, and i think i still
    have a lot of those, so if you need one or two i would need a name and
  4. Mark

    Mark Guest

    check the transformer, if you drive a transformer from a source Z that
    is too high, you will loose the bottom end. What Z is the transformer
    designed for?

  5. josephkk

    josephkk Guest

    It wouldn't be a follower if you did that. Just pull a smidgen of grid
    current in U2 and it works fine.

  6. josephkk

    josephkk Guest

    I suspect that it rolled off.

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