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1G ohm resistors

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  • captainvideo_007
    All, I m starting the build of my first mics and in following the threads about the value of the resistor across the capsule, as the 1G ~5G resistors are
    Message 1 of 4 , Nov 1, 2010
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      All,
      I'm starting the build of my first mics and in following the threads about the value of the resistor across the capsule, as the 1G ~5G resistors are expensive and not in abundance I wonder if snyone hsd tried s jfet as the resistor? It seams to me that the use of a jfet as a variable resistor has been used for other circuits, any thoughts about this? Any and all input is welcome.

      Sincerely,
      Bill
    • edgarbigdog400
      I don t know if this will work in yout application, but I had great results when I used a pair of reverse biased 1N914 s as a 2:1 voltage divider to supply 1/2
      Message 2 of 4 , Nov 1, 2010
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        I don't know if this will work in yout application,
        but I had great results when I used a pair of reverse biased 1N914's as
        a 2:1 voltage divider to supply 1/2 VCC bias to a TL072 in a VLF
        receiver. Best Regards, Edgar
        --- In micbuilders@yahoogroups.com, "captainvideo_007"
        <billssupport@...> wrote:
        >
        > All,
        > I'm starting the build of my first mics and in following the threads
        about the value of the resistor across the capsule, as the 1G ~5G
        resistors are expensive and not in abundance I wonder if snyone hsd
        tried s jfet as the resistor? It seams to me that the use of a jfet as a
        variable resistor has been used for other circuits, any thoughts about
        this? Any and all input is welcome.
        >
        > Sincerely,
        > Bill
        >
      • Zapnspark
        I have tried a PN4117A JFET as a substitute for a 1 Gig resistor in a Schoeps-like microphone. See: Photos ZAPNSPARK PN4117A The cathode of the PN4117A
        Message 3 of 4 , Nov 1, 2010
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          I have tried a PN4117A JFET as a substitute for a 1 Gig resistor in a Schoeps-like microphone.
          See:
          Photos > ZAPNSPARK > PN4117A

          The "cathode" of the PN4117A goes to the gate of the JFET that you use as the trans-impedance converter in the microphone.
          It seemed to work OK but I took no serious performance measurements.
          (I had already stockpiled 1 and 5 gig resistors so I didn't do any more follow-up on this approach)

          The PN4117A JFETs are about 6 cents a piece at Mouser.

          Cheers.

          Jim G.

          --- In micbuilders@yahoogroups.com, "captainvideo_007" <billssupport@...> wrote:
          >
          > All,
          > I'm starting the build of my first mics and in following the threads about the value of the resistor across the capsule, as the 1G ~5G resistors are expensive and not in abundance I wonder if snyone hsd tried s jfet as the resistor? It seams to me that the use of a jfet as a variable resistor has been used for other circuits, any thoughts about this? Any and all input is welcome.
          >
          > Sincerely,
          > Bill
          >
        • userno232000
          ... The zero bias resistance of a diode is related to its Is. The PN4117 s are femto amp FET s the 1N914 and 1N4148 are too noisy for a mic they only measure
          Message 4 of 4 , Nov 2, 2010
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            --- In micbuilders@yahoogroups.com, "Zapnspark" <zapnspark@...> wrote:
            >
            >
            >
            > I have tried a PN4117A JFET as a substitute for a 1 Gig resistor in a Schoeps-like microphone.
            > See:
            > Photos > ZAPNSPARK > PN4117A
            >
            > The "cathode" of the PN4117A goes to the gate of the JFET that you use as the trans-impedance converter in the microphone.
            > It seemed to work OK but I took no serious performance measurements.
            > (I had already stockpiled 1 and 5 gig resistors so I didn't do any more follow-up on this approach)
            >
            > The PN4117A JFETs are about 6 cents a piece at Mouser.
            >
            > Cheers.
            >

            The zero bias resistance of a diode is related to its Is. The PN4117's are femto amp FET's the 1N914 and 1N4148 are too noisy for a mic they only measure 10's or 100's of Meg Ohms. Philips has a femto-amp version of the BAV99 (BAV199?) that should work. Linear Systems is also now making some previously obsolete diodes available again.

            Ohm out a capsule some time and you can see a diode in both directions on the gate to source.(At least on my WM61's there was.)


            > Jim G.
            >
            > --- In micbuilders@yahoogroups.com, "captainvideo_007" <billssupport@> wrote:
            > >
            > > All,
            > > I'm starting the build of my first mics and in following the threads about the value of the resistor across the capsule, as the 1G ~5G resistors are expensive and not in abundance I wonder if snyone hsd tried s jfet as the resistor? It seams to me that the use of a jfet as a variable resistor has been used for other circuits, any thoughts about this? Any and all input is welcome.
            > >
            > > Sincerely,
            > > Bill
            > >
            >
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