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Re: [micbuilders] A "half-Linkwitz" mod

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  • Scott Helmke
    Here s what I m confused about... and please point me to relevant FET theory if available online. My memory of FETs is a bit fuzzy. If the gate is essentially
    Message 1 of 24 , Mar 6, 2004
      Here's what I'm confused about... and please point me to relevant FET theory
      if available online. My memory of FETs is a bit fuzzy.

      If the gate is essentially only connected to the actual mic capsule and
      through a parasitic "auto magic bias" diode (to what?), how much of a
      feedback loop can actually be accomplished? I've yet to find a schematic of
      the capsule assembly.

      -Scott

      On Saturday 06 March 2004 12:07, Bob Cain wrote:
      > This design does not effect source follower operation which
      > places the gate circuit in a feedback loop enclosing the
      > load resistor so as to reduce FET harmonic distortion. It
      > remains a circuit that is purely transconductance controled
      > and subject to any non-linearity in that function.
      >
      >
      > Bob
      >
      > Scott Helmke wrote:
      > > Hello, all... I'm new here, but I did spend a while last night
      > > combing through the micbuilders and micdiyer groups to see if this
      > > had come up already.
      > >
      > > Here's my take on the Linkwitz mod, the Panasonic omni capsule, and
      > > the usual phantom circuit:
      > >
      > > +---------------------------330R--------+
      > >
      > > | 2k2 +---10k----+ |
      > > |+ | | | |
      > >
      > > 10u | | E--o---|--------------- COLD
      > >
      > > |- o---------||----o--o-------B |
      > > |
      > > | | 1u0 | C |
      > > | |1 + +-100k-+ | |
      > > |
      > > | CAPSULE |---o------o
      > > |3 |2 - +-100k-+ | |
      > > |
      > > | | | C |
      > > |
      > > | o---------||----o--o-------B |
      > > |
      > > | | 1u0 | E--o---|--------------- HOT
      > > | |
      > > | | +---10k----+ |
      > > | | o----+
      > > |
      > > | 2k2 +| |+
      > > |
      > > | | 12V 10u
      > > | | -| |-
      > >
      > > --o-----o---------------------------------o----o---------- GROUND
      > >
      > > This approach provides a true balanced output, but hopefully derives
      > > some benefit from separating the ground. Sort of a "half-Linkwitz"
      > > mod. It does work, but since I haven't been able to induce clipping
      > > in any of my mics (with and without the mod) I'm unable to verify
      > > it. I have been able to observe a balanced output, and the theory
      > > seems solid, at least.
      > >
      > > Comments?
      > >
      > >
      > >
      > >
      > > Yahoo! Groups Links

      --
      ---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
      And you'll visualize not taking any chances
      But meet them halfway with love, peace, and persuasion
      And expect them to rise to the occasion
      (from "Glad Tidings", by Van Morrison)
    • Bob Cain
      ... The direct gate/source path is very high impedence. It is leakage that allows them to reach the same potential. For purposes of puting the source
      Message 2 of 24 , Mar 6, 2004
        Scott Helmke wrote:

        > Here's what I'm confused about... and please point me to relevant FET theory
        > if available online. My memory of FETs is a bit fuzzy.
        >
        > If the gate is essentially only connected to the actual mic capsule and
        > through a parasitic "auto magic bias" diode (to what?), how much of a
        > feedback loop can actually be accomplished? I've yet to find a schematic of
        > the capsule assembly.

        The direct gate/source path is very high impedence. It is
        leakage that allows them to reach the same potential. For
        purposes of puting the source resistor in a feedback loop
        with the gate, that direct path can be considered an open
        circuit.


        Bob
        --

        "Things should be described as simply as possible, but no
        simpler."

        A. Einstein
      • Bob Cain
        ... Scott, look at this skeleton of a modification to your circuit that was posted in our files section:
        Message 3 of 24 , Mar 6, 2004
          Scott Helmke wrote:

          > Hello, all... I'm new here, but I did spend a while last night
          > combing through the micbuilders and micdiyer groups to see if this
          > had come up already.
          >
          > Here's my take on the Linkwitz mod, the Panasonic omni capsule, and
          > the usual phantom circuit:

          Scott, look at this skeleton of a modification to your
          circuit that was posted in our files section:


          http://f1.grp.yahoofs.com/v1/cMhKQCYgN6s6zuvjLDyCMBchD6m71ZlQrB4VOfSye83n9rmhZSJyQ7yFk2e74fPXIAT7I3SzZT4u12HHj4TUamsu6mXg9TQH2w/Schematics/electret_linkw_bal.PDF

          What transistors do you intend for yours?


          Bob
          --

          "Things should be described as simply as possible, but no
          simpler."

          A. Einstein
        • Scott Helmke
          Bob, the link doesn t work. Is it the impedance-balanced version posted by ljudatervinning? At present I m using 2N5087 transistors. -Scott ... -- ... And
          Message 4 of 24 , Mar 7, 2004
            Bob, the link doesn't work. Is it the impedance-balanced version posted by
            ljudatervinning?

            At present I'm using 2N5087 transistors.

            -Scott

            On Sunday 07 March 2004 02:23, Bob Cain wrote:
            > Scott Helmke wrote:
            > > Hello, all... I'm new here, but I did spend a while last night
            > > combing through the micbuilders and micdiyer groups to see if this
            > > had come up already.
            > >
            > > Here's my take on the Linkwitz mod, the Panasonic omni capsule, and
            > > the usual phantom circuit:
            >
            > Scott, look at this skeleton of a modification to your
            > circuit that was posted in our files section:
            >
            >
            > http://f1.grp.yahoofs.com/v1/cMhKQCYgN6s6zuvjLDyCMBchD6m71ZlQrB4VOfSye83n9
            >rmhZSJyQ7yFk2e74fPXIAT7I3SzZT4u12HHj4TUamsu6mXg9TQH2w/Schematics/electret_l
            >inkw_bal.PDF
            >
            > What transistors do you intend for yours?
            >
            >
            > Bob

            --
            ---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
            And you'll visualize not taking any chances
            But meet them halfway with love, peace, and persuasion
            And expect them to rise to the occasion
            (from "Glad Tidings", by Van Morrison)
          • Bob Cain
            ... That s the one. Bob -- Things should be described as simply as possible, but no simpler. A. Einstein
            Message 5 of 24 , Mar 7, 2004
              Scott Helmke wrote:

              > Bob, the link doesn't work. Is it the impedance-balanced version posted by
              > ljudatervinning?

              That's the one.


              Bob
              --

              "Things should be described as simply as possible, but no
              simpler."

              A. Einstein
            • userno232000
              The mike biases at the point where the forward bias of the gate to source junction just balances the drain to gate leakage. For instance if the gate to source
              Message 6 of 24 , Mar 7, 2004
                The mike biases at the point where the forward bias of the gate to
                source junction just balances the drain to gate leakage. For
                instance if the gate to source diode has .6V at 1mA it will be .122V
                or so at 10e-12A no magic involved. The effective resistance of the
                gate to source is still very large even though it is forward biased
                admittedly by VERY little. It's amazing how nice and repeatable and
                stable this biasing is in practice.

                --- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                >
                >
                > Scott Helmke wrote:
                >
                > > Here's what I'm confused about... and please point me to
                relevant FET theory
                > > if available online. My memory of FETs is a bit fuzzy.
                > >
                > > If the gate is essentially only connected to the actual mic
                capsule and
                > > through a parasitic "auto magic bias" diode (to what?), how much
                of a
                > > feedback loop can actually be accomplished? I've yet to find a
                schematic of
                > > the capsule assembly.
                >
                > The direct gate/source path is very high impedence. It is
                > leakage that allows them to reach the same potential. For
                > purposes of puting the source resistor in a feedback loop
                > with the gate, that direct path can be considered an open
                > circuit.
                >
                >
                > Bob
                > --
                >
                > "Things should be described as simply as possible, but no
                > simpler."
                >
                > A. Einstein
              • Bob Cain
                ... How do you calculate that? Bob -- Things should be described as simply as possible, but no simpler. A. Einstein
                Message 7 of 24 , Mar 7, 2004
                  userno232000 wrote:

                  > The mike biases at the point where the forward bias of the gate to
                  > source junction just balances the drain to gate leakage. For
                  > instance if the gate to source diode has .6V at 1mA it will be .122V
                  > or so at 10e-12A no magic involved.

                  How do you calculate that?


                  Bob
                  --

                  "Things should be described as simply as possible, but no
                  simpler."

                  A. Einstein
                • Scott Helmke
                  I m obviously missing some crucial little concept here, because I can t see where you d have a feedback loop. My theory was that by changing the relative
                  Message 8 of 24 , Mar 7, 2004
                    I'm obviously missing some crucial little concept here, because I can't see
                    where you'd have a feedback loop.

                    My theory was that by changing the relative voltages at the drain and source
                    by moving the current limiting resistor, you'd be changing the relative gate
                    voltage... but that biasing is automatic. Hmm....

                    -Scott

                    On Sunday 07 March 2004 12:27, userno232000 wrote:
                    > The mike biases at the point where the forward bias of the gate to
                    > source junction just balances the drain to gate leakage. For
                    > instance if the gate to source diode has .6V at 1mA it will be .122V
                    > or so at 10e-12A no magic involved. The effective resistance of the
                    > gate to source is still very large even though it is forward biased
                    > admittedly by VERY little. It's amazing how nice and repeatable and
                    > stable this biasing is in practice.
                    >
                    > --- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                    > > Scott Helmke wrote:
                    > > > Here's what I'm confused about... and please point me to
                    >
                    > relevant FET theory
                    >
                    > > > if available online. My memory of FETs is a bit fuzzy.
                    > > >
                    > > > If the gate is essentially only connected to the actual mic
                    >
                    > capsule and
                    >
                    > > > through a parasitic "auto magic bias" diode (to what?), how much
                    >
                    > of a
                    >
                    > > > feedback loop can actually be accomplished? I've yet to find a
                    >
                    > schematic of
                    >
                    > > > the capsule assembly.
                    > >
                    > > The direct gate/source path is very high impedence. It is
                    > > leakage that allows them to reach the same potential. For
                    > > purposes of puting the source resistor in a feedback loop
                    > > with the gate, that direct path can be considered an open
                    > > circuit.
                    > >
                    > >
                    > > Bob
                    > > --
                    > >
                    > > "Things should be described as simply as possible, but no
                    > > simpler."
                    > >
                    > > A. Einstein
                    >
                    > Yahoo! Groups Links
                    >
                    >
                    >

                    --
                    ---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
                    And you'll visualize not taking any chances
                    But meet them halfway with love, peace, and persuasion
                    And expect them to rise to the occasion
                    (from "Glad Tidings", by Van Morrison)
                  • Bob Cain
                    ... In the source follower (Linkwitz mod) the output voltage is fed back from the load resistor to the source of the FET in a negative sense so that the
                    Message 9 of 24 , Mar 7, 2004
                      Scott Helmke wrote:

                      > I'm obviously missing some crucial little concept here, because I can't see
                      > where you'd have a feedback loop.

                      In the source follower (Linkwitz mod) the output voltage is
                      fed back from the load resistor to the source of the FET in
                      a negative sense so that the voltage excursion across the
                      gate/source is smaller. A fraction of the diaphragm voltage
                      appears there instead of all of it. The smaller that
                      excursion, the more linear the operation of the FET is
                      because its transconductance is non-linear, following a
                      square law function I believe.


                      Bob
                      --

                      "Things should be described as simply as possible, but no
                      simpler."

                      A. Einstein
                    • Bob Cain
                      ... BTW, if anybody is handy at PC board layout and fab I d love to buy a handful of small boards that implement a finalized version of that circuit in both
                      Message 10 of 24 , Mar 7, 2004
                        Bob Cain wrote:

                        >
                        > Scott Helmke wrote:
                        >
                        >
                        >>Bob, the link doesn't work. Is it the impedance-balanced version posted by
                        >>ljudatervinning?
                        >
                        >
                        > That's the one.

                        BTW, if anybody is handy at PC board layout and fab I'd love
                        to buy a handful of small boards that implement a finalized
                        version of that circuit in both the 2 wire source follower
                        form and the original common source form.


                        Bob
                        --

                        "Things should be described as simply as possible, but no
                        simpler."

                        A. Einstein
                      • russell_john@hotmail.com
                        You should give serious consideration to building small and one off circuits using the mahhattan Pad technique described on the folling website
                        Message 11 of 24 , Mar 7, 2004
                          You should give serious consideration to building small and 'one off'
                          circuits using the "mahhattan Pad" technique described on the folling
                          website http://www.qsl.net/k7qo/

                          RF circuits built this way are often more stable than the same
                          circuit built using a traditional PC board, thanks to a massive low
                          impedance, low inductance ground plane ... Audio circuits are
                          extremely quiet when laid out properly (about the same rules you'd
                          use for laying out a PC board) ... it is especially effective for
                          discrete transistor circuits ... one thing is for sure, it eliminates
                          ground loops because the ground is one big plane under the
                          circuit ... plus circuits are real easy to modify without having to
                          worry about a copper trace lifting because you soldered/desoldered a
                          component one time too many ...

                          I haven't made a circuit board for anything in well over a year
                          because this technique is faster, easier and just as stable, if not
                          more so ... I even made a stereo mic preamp with a couple of INA103's
                          using one double sided circuit board and building each preamp on
                          opposite sides of the board ....

                          J. R.

                          --- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                          >
                          >
                          > Bob Cain wrote:
                          >
                          > >
                          > > Scott Helmke wrote:
                          > >
                          > >
                          > >>Bob, the link doesn't work. Is it the impedance-balanced version
                          posted by
                          > >>ljudatervinning?
                          > >
                          > >
                          > > That's the one.
                          >
                          > BTW, if anybody is handy at PC board layout and fab I'd love
                          > to buy a handful of small boards that implement a finalized
                          > version of that circuit in both the 2 wire source follower
                          > form and the original common source form.
                          >
                          >
                          > Bob
                          > --
                          >
                          > "Things should be described as simply as possible, but no
                          > simpler."
                          >
                          > A. Einstein
                        • Bob Cain
                          ... That s an idea but this is something that comes up often and there are more and more inexpensive front ends appearing that have phantom power. It would be
                          Message 12 of 24 , Mar 7, 2004
                            russell_john@... wrote:

                            > You should give serious consideration to building small and 'one off'
                            > circuits using the "mahhattan Pad" technique described on the folling
                            > website http://www.qsl.net/k7qo/

                            That's an idea but this is something that comes up often and
                            there are more and more inexpensive front ends appearing
                            that have phantom power. It would be way cool if whenever
                            the question comes up we could refer it to someone who sells
                            them (or a kit.)


                            Bob
                            --

                            "Things should be described as simply as possible, but no
                            simpler."

                            A. Einstein
                          • umashankar mantravadi
                            i believe rick chinn sells a kit, phantom power, with current sources instead of load resist0rs, and the board designed for various configurations. designed as
                            Message 13 of 24 , Mar 8, 2004
                              i believe rick chinn sells a kit, phantom power, with current sources
                              instead of load resist0rs, and the board designed for various
                              configurations. designed as a mod for the radio shack pzm, it should be
                              perfect for those who want a ready made card. comes as a kit with parts,
                              only i do not remember the url.

                              umashankar
                              >
                              >
                              >russell_john@... wrote:
                              >
                              > >

                              _________________________________________________________________
                              Easiest Money Transfer to India. Send Money To 6000 Indian Towns.
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                            • Scott Helmke
                              Bob, thanks for the continuing explanations. I m still having trouble seeing a feedback loop , though. What you describe sounds more like a simple increase
                              Message 14 of 24 , Mar 8, 2004
                                Bob, thanks for the continuing explanations. I'm still having trouble seeing
                                a "feedback loop", though. What you describe sounds more like a simple
                                increase in load resistance.

                                Anyway, I sat down and did some testing. I hooked up a 100 watt amp into a
                                10" guitar speaker, and used a specific spot on the grill cloth as my
                                reference point. I measured both mic output (from a mixer channel direct
                                out) and speaker drive signal, both in dBV. Distortion by visual
                                observation on oscilloscope.

                                I tried all three setups: unmodded, half-modded (my version), and the fully
                                modded, impedance balanced version posted by ljudatervinning. I did two
                                sets of tests, one with 2.2k load resistors and one with 10k load resistors.
                                Panasonic WM60-AY capsule (same capsule for all tests).

                                2.2k load resistors:
                                unmodded mic output -10, speaker at 2.3, mic almost distorted
                                half-modded: mic output -3.5, speaker at 10.5, mic almost distorted
                                fully-modded: mic output -6, speaker at 10.5, mic almost distorted

                                10k load resistors:
                                unmodded mic output +5.1, speaker at 8, mic almost distorted
                                half-modded: mic output 0, speaker at 12, amp at limit
                                fully-modded: mic output -4.3, speaker at 12, amp at limit

                                10k load resistors, capsule almost touching speaker dust cap:
                                half-modded: mic output +4, no visible distortion, amp at limit
                                fully-modded: mic output -2.5, no visible distortion, amp at limit

                                Results? I think I'm right. Wish I could get it loud enough to see the two
                                different mods into mic distortion, though. It also looks like the classic
                                PZM phantom power circuit would better match the Panasonic capsules if
                                implemented with a higher load resistance. It's worth pointing out that the
                                unmodded half-modded versions have twice the load resistance of the
                                fully-modded version. Hmm...

                                -Scott

                                On Sunday 07 March 2004 17:45, Bob Cain wrote:
                                > Scott Helmke wrote:
                                > > I'm obviously missing some crucial little concept here, because I can't
                                > > see where you'd have a feedback loop.
                                >
                                > In the source follower (Linkwitz mod) the output voltage is
                                > fed back from the load resistor to the source of the FET in
                                > a negative sense so that the voltage excursion across the
                                > gate/source is smaller. A fraction of the diaphragm voltage
                                > appears there instead of all of it. The smaller that
                                > excursion, the more linear the operation of the FET is
                                > because its transconductance is non-linear, following a
                                > square law function I believe.
                                >
                                >
                                > Bob

                                --
                                ---- Scott Helmke ---- scott@... ---- (734) 604-9340 ----
                                And you'll visualize not taking any chances
                                But meet them halfway with love, peace, and persuasion
                                And expect them to rise to the occasion
                                (from "Glad Tidings", by Van Morrison)
                              • Bob Andres
                                thanks for the link --- I m going to give it a try on the next project. -Bob on 3/7/04 7:34 PM, russell_john@hotmail.com at russell_john@hotmail.com wrote:
                                Message 15 of 24 , Mar 8, 2004
                                  thanks for the link --- I'm going to give it a try on the next project.

                                  -Bob


                                  on 3/7/04 7:34 PM, russell_john@... at russell_john@...
                                  wrote:

                                  You should give serious consideration to building small and 'one off'
                                  circuits using the "mahhattan Pad" technique described on the folling
                                  website http://www.qsl.net/k7qo/

                                  RF circuits built this way are often more stable than the same
                                  circuit built using a traditional PC board, thanks to a massive low
                                  impedance, low inductance ground plane ... Audio circuits are
                                  extremely quiet when laid out properly (about the same rules you'd
                                  use for laying out a PC board) ... it is especially effective for
                                  discrete transistor circuits ... one thing is for sure, it eliminates
                                  ground loops because the ground is one big plane under the
                                  circuit ... plus circuits are real easy to modify without having to
                                  worry about a copper trace lifting because you soldered/desoldered a
                                  component one time too many ...

                                  I haven't made a circuit board for anything in well over a year
                                  because this technique is faster, easier and just as stable, if not
                                  more so ... I even made a stereo mic preamp with a couple of INA103's
                                  using one double sided circuit board and building each preamp on
                                  opposite sides of the board ....

                                  J. R.

                                  --- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                                  >
                                  >
                                  > Bob Cain wrote:
                                  >
                                  > >
                                  > > Scott Helmke wrote:
                                  > >
                                  > >
                                  > >>Bob, the link doesn't work. Is it the impedance-balanced version
                                  posted by
                                  > >>ljudatervinning?
                                  > >
                                  > >
                                  > > That's the one.
                                  >
                                  > BTW, if anybody is handy at PC board layout and fab I'd love
                                  > to buy a handful of small boards that implement a finalized
                                  > version of that circuit in both the 2 wire source follower
                                  > form and the original common source form.
                                  >
                                  >
                                  > Bob
                                  > --
                                  >
                                  > "Things should be described as simply as possible, but no
                                  > simpler."
                                  >
                                  > A. Einstein





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                                • russell_john@hotmail.com
                                  You could contact these people, they ll do low quantities about a cheap as you ll find and if you can convince them they can sell a fair number, they may even
                                  Message 16 of 24 , Mar 8, 2004
                                    You could contact these people, they'll do low quantities about a
                                    cheap as you'll find and if you can convince them they can sell a
                                    fair number, they may even carry the board themselves .... I built a
                                    PIC uProcessor controlled RF Direct Digital synth using one of their
                                    boards and they do nice work as it contained a 28-lead SSOP surface
                                    mount IC which is an extremely small package with closely spaced
                                    leads, we are talking 14 pins on a side in the space 4 leads takes up
                                    in a DIP package so they can definitely do detailed work

                                    http://www.farcircuits.net/
                                    http://www.farcircuits.net/terms1.htm

                                    "Custom boards may be made for $.50 square inch or $4.00 minimum per
                                    board for single sided, etched, drilled and solder coated. Custom
                                    double sided non-plated through holes, etched, drilled and solder
                                    coated boards may be made for $.75 per square inch or $6.00 minimum
                                    per board. A clear film negative is required for custom boards. Film
                                    from camera ready artwork is $8.00 per shot based on 8" x 10" film
                                    per shot. Quantity pricing available. Far Circuits reserves the right
                                    to change the pricing based on the complexity of the board.

                                    FAR Circuits offers layout and prototype services. Please call for
                                    these details."




                                    --- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                                    >
                                    >
                                    > russell_john@h... wrote:
                                    >
                                    > > You should give serious consideration to building small and 'one
                                    off'
                                    > > circuits using the "mahhattan Pad" technique described on the
                                    folling
                                    > > website http://www.qsl.net/k7qo/
                                    >
                                    > That's an idea but this is something that comes up often and
                                    > there are more and more inexpensive front ends appearing
                                    > that have phantom power. It would be way cool if whenever
                                    > the question comes up we could refer it to someone who sells
                                    > them (or a kit.)
                                    >
                                    >
                                    > Bob
                                    > --
                                    >
                                    > "Things should be described as simply as possible, but no
                                    > simpler."
                                    >
                                    > A. Einstein
                                  • Bob Cain
                                    ... Great resource, thanks. I ll follow up on it tomorrow. BTW, this is my latest stab at the simplest 2 wire Linkwitz moded source follower circuit powered
                                    Message 17 of 24 , Mar 9, 2004
                                      russell_john@... wrote:

                                      > You could contact these people,

                                      Great resource, thanks. I'll follow up on it tomorrow.

                                      BTW, this is my latest stab at the simplest 2 wire Linkwitz
                                      moded source follower circuit powered by phantom and almost
                                      balanced.

                                      +---------------------------------------+
                                      | |
                                      | +---10k----+ |
                                      | | | |
                                      | + - | E--o---|--------------- HOT (2)
                                      o----???--------||----o--o-------B |
                                      | 1uF | C |
                                      |1,3 + +-100k-+ | |
                                      CAPSULE |---o------o
                                      |2 - +-100k-+ | |
                                      | + - | C |
                                      +---------------||----o--o-------B |
                                      | 1uF | E--o---|--------------- COLD (3)
                                      | | | |
                                      | +---10k----+ |
                                      | o----o
                                      12k2 +| |+
                                      | 12V 10u
                                      | -| |-
                                      +---------------------------------------o----o---------- GROUND (1)


                                      It will be impedance matched on the differential out when I
                                      figure out or get a suggestion for what resistor ??? should
                                      be. It should be the same as the impedence at the source of
                                      the capsule's FET. Help! In the meantime, 12k would get
                                      something going.

                                      The output is not balanced from a signal perspective in that
                                      it all appears between XLR pins 3 and 1 and is inverted. Pin
                                      2 is at AC ground but this is not an uncommon configuration
                                      I've found from both the Oktava MC012 and the Earthworks TC30K.
                                      It's still differential and will be impedence balanced to
                                      minimize common mode noise pickup as soon as ??? can be
                                      determined. Unless I'm all wet, of course. :-)


                                      Bob
                                      --

                                      "Things should be described as simply as possible, but no
                                      simpler."

                                      A. Einstein
                                    • russell_john@hotmail.com
                                      I m on web read and Yahoo really mucks up an ASCII schematic ... Funny thing is if I reply then it looks correct in this editing window .. go figure ... Anyway
                                      Message 18 of 24 , Mar 9, 2004
                                        I'm on web read and Yahoo really mucks up an ASCII schematic ...
                                        Funny thing is if I reply then it looks correct in this editing
                                        window .. go figure ...

                                        Anyway I found a like to the Rick Chinn kit Umashankar was talking
                                        about, $30 for the kit but it is a real nice design using a LM394
                                        Supermatched Pair IC, ferrite beads of RF bypassing ... of course the
                                        guy used to be an engineer for Mackey so he knows what he's doing ...
                                        further up the page he also has a series of articles on phantom power

                                        http://www.uneeda-audio.com/#pzm

                                        J. R.

                                        -- In micbuilders@yahoogroups.com, Bob Cain <arcane@a...> wrote:
                                        >
                                        >
                                        > russell_john@h... wrote:
                                        >
                                        > > You could contact these people,
                                        >
                                        > Great resource, thanks. I'll follow up on it tomorrow.
                                        >
                                        > BTW, this is my latest stab at the simplest 2 wire Linkwitz
                                        > moded source follower circuit powered by phantom and almost
                                        > balanced.
                                        >
                                      • umashankar mantravadi
                                        i think the ??? should be a resistor of 12.2 k, connected to ground. you are then balanced throughout. there is no signal on pin 2, but that hardly matters.
                                        Message 19 of 24 , Mar 9, 2004
                                          i think the ??? should be a resistor of 12.2 k, connected to ground. you are
                                          then balanced throughout. there is no signal on pin 2, but that hardly
                                          matters.

                                          umashankar

                                          >
                                          >
                                          >russell_john@... wrote:
                                          >
                                          > > You could contact these people,
                                          >
                                          >Great resource, thanks. I'll follow up on it tomorrow.
                                          >
                                          >BTW, this is my latest stab at the simplest 2 wire Linkwitz
                                          >moded source follower circuit powered by phantom and almost
                                          >balanced.
                                          >
                                          > +---------------------------------------+
                                          > | |
                                          > | +---10k----+ |
                                          > | | | |
                                          > | + - | E--o---|--------------- HOT (2)
                                          > o----???--------||----o--o-------B |
                                          > | 1uF | C |
                                          > |1,3 + +-100k-+ | |
                                          > CAPSULE |---o------o
                                          > |2 - +-100k-+ | |
                                          > | + - | C |
                                          > +---------------||----o--o-------B |
                                          > | 1uF | E--o---|--------------- COLD (3)
                                          > | | | |
                                          > | +---10k----+ |
                                          > | o----o
                                          > 12k2 +| |+
                                          > | 12V 10u
                                          > | -| |-
                                          > +---------------------------------------o----o---------- GROUND (1)
                                          >
                                          >
                                          >It will be impedance matched on the differential out when I
                                          >figure out or get a suggestion for what resistor ??? should
                                          >be. It should be the same as the impedence at the source of
                                          >the capsule's FET. Help! In the meantime, 12k would get
                                          >something going.
                                          >
                                          >The output is not balanced from a signal perspective in that
                                          >it all appears between XLR pins 3 and 1 and is inverted. Pin
                                          >2 is at AC ground but this is not an uncommon configuration
                                          >I've found from both the Oktava MC012 and the Earthworks TC30K.
                                          >It's still differential and will be impedence balanced to
                                          >minimize common mode noise pickup as soon as ??? can be
                                          >determined. Unless I'm all wet, of course. :-)
                                          >
                                          >
                                          >Bob
                                          >--
                                          >
                                          >"Things should be described as simply as possible, but no
                                          >simpler."
                                          >
                                          > A. Einstein
                                          >
                                          >

                                          _________________________________________________________________
                                          INDIA TODAY @ Rs. 5 + a free gift !
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                                        • Bob Cain
                                          ... That should do to get it to basically work but if the goal is a balanced impedence drive then it needs to be what is seen at the source of the capsule FET.
                                          Message 20 of 24 , Mar 9, 2004
                                            umashankar mantravadi wrote:

                                            > i think the ??? should be a resistor of 12.2 k, connected to ground. you are
                                            > then balanced throughout. there is no signal on pin 2, but that hardly
                                            > matters.

                                            That should do to get it to basically work but if the goal
                                            is a balanced impedence drive then it needs to be what is
                                            seen at the source of the capsule FET. I'm not sure how
                                            much the use of 12k actually takes it out of balance though.

                                            It depends on how far that is from the source impedance and
                                            how much the base circuit of the output transistor affects
                                            the impedence seen at the emitter. Figuring impedances at
                                            points of active circuits is not something I'm very facile at.


                                            Bob
                                            --

                                            "Things should be described as simply as possible, but no
                                            simpler."

                                            A. Einstein
                                          • Bob Cain
                                            ... That s quite a circuit! For my immediate purpose the cost is in line with the importance of the application so I think I ll get a couple kits. Thanks.
                                            Message 21 of 24 , Mar 9, 2004
                                              russell_john@... wrote:

                                              > Anyway I found a like to the Rick Chinn kit Umashankar was talking
                                              > about, $30 for the kit but it is a real nice design using a LM394
                                              > Supermatched Pair IC, ferrite beads of RF bypassing ... of course the
                                              > guy used to be an engineer for Mackey so he knows what he's doing ...
                                              > further up the page he also has a series of articles on phantom power
                                              >
                                              > http://www.uneeda-audio.com/#pzm

                                              That's quite a circuit! For my immediate purpose the cost
                                              is in line with the importance of the application so I think
                                              I'll get a couple kits. Thanks.


                                              Bob
                                              --

                                              "Things should be described as simply as possible, but no
                                              simpler."

                                              A. Einstein
                                            • Jon Pike
                                              ... Hmm.. Don t want to commit yet, but I ve been meaning to get the rust off by doing some layout work again.. this should be easy enough. I d like to talk
                                              Message 22 of 24 , Mar 9, 2004
                                                >
                                                >
                                                >Bob Cain wrote:
                                                >
                                                >
                                                >
                                                >>>>>Bob, the link doesn't work. Is it the impedance-balanced version posted by ljudatervinning?
                                                >>>
                                                >>>
                                                >>>
                                                >>>
                                                >>> That's the one.
                                                >>
                                                >>
                                                >
                                                >BTW, if anybody is handy at PC board layout and fab I'd love
                                                >to buy a handful of small boards that implement a finalized
                                                >version of that circuit in both the 2 wire source follower
                                                >form and the original common source form.
                                                >
                                                >
                                                >Bob
                                                >

                                                Hmm.. Don't want to commit yet, but I've been meaning to get the rust
                                                off by doing some layout work again.. this should be easy enough.

                                                I'd like to talk to you Bob, and whoever others, about really finalizing
                                                the circuit, desired size and shape of the boards, etc. If size isn't a
                                                major issue, should be possible to make a multipurpose one that has pads
                                                and traces for either style of circuit, just stuff it the way you want.

                                                Jon
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