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Re: [micbuilders] Digest Number 27

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  • Tom McGee
    ... It s extremely rare for resistors from the late 1950 s onward to need replacement for reasons other than heat stress. Garden variety resistors in most gear
    Message 1 of 5 , Jan 1, 2004
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      Walt sez:

      >Wire wound resisters do have the advantage of being very stable over
      >time. Which could be why they were used. There were a number of
      >varieties of resistors available in the 60's.


      It's extremely rare for resistors from the late 1950's onward to need
      replacement for reasons other than heat stress. Garden variety
      resistors in most gear from the 1960's are also quite stable.
      Capacitors are quite another story.

      In a tuned RF circuit, the inductance (and capacitance) of a wire
      wound resistor has to be considered, along with circuit layout, lead
      spacing, etc.
    • Rich Peet
      Here we are dealing with HF RF. Lead length is not going to mean much but tightly packed wire wound resisters do bother me as the inductance can pick up the rf
      Message 2 of 5 , Jan 1, 2004
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        Here we are dealing with HF RF. Lead length is not going to mean much
        but tightly packed wire wound resisters do bother me as the
        inductance can pick up the rf by increased inductance and transmit it
        to other parts of the circuit.

        In the unbalanced models the RF filter on the output is the weak part
        of the design. This causes problems only when working with more than
        one of these mics in stereo or multi-track.

        For now I will leave the resisters alone but will replace the
        polarized capacitors with tantilums and the caps on the RF filter.

        I will post the component values when I have them all determined and
        will watch for changes in the design.

        Rich

        --- In micbuilders@yahoogroups.com, Tom McGee <tfm@1...> wrote:
        >
        > Walt sez:
        >
        > >Wire wound resisters do have the advantage of being very stable
        over
        > >time. Which could be why they were used. There were a number of
        > >varieties of resistors available in the 60's.
        >
        >
        > It's extremely rare for resistors from the late 1950's onward to
        need
        > replacement for reasons other than heat stress. Garden variety
        > resistors in most gear from the 1960's are also quite stable.
        > Capacitors are quite another story.
        >
        > In a tuned RF circuit, the inductance (and capacitance) of a wire
        > wound resistor has to be considered, along with circuit layout,
        lead
        > spacing, etc.
      • Walter Knapp
        From: Rich Peet ... I would start by assuming that some of the top designers of mics in the world (Sennheiser) knew what they were
        Message 3 of 5 , Jan 3, 2004
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          From: "Rich Peet" <richpeet@...>
          >
          > Here we are dealing with HF RF. Lead length is not going to mean much
          > but tightly packed wire wound resisters do bother me as the
          > inductance can pick up the rf by increased inductance and transmit it
          > to other parts of the circuit.
          >
          > In the unbalanced models the RF filter on the output is the weak part
          > of the design. This causes problems only when working with more than
          > one of these mics in stereo or multi-track.
          >
          > For now I will leave the resisters alone but will replace the
          > polarized capacitors with tantilums and the caps on the RF filter.
          >
          > I will post the component values when I have them all determined and
          > will watch for changes in the design.

          I would start by assuming that some of the top designers of mics in the
          world (Sennheiser) knew what they were doing and took into account the
          characteristics of the components they choose. That would certainly
          include the inductance of the wire wound resistors. Remove that
          inductance and you would need to be ready to retune the circuits, if
          that's possible. We are mere dabblers in this sort of stuff, they are
          the experts, particularly on their own design. Quality mics are
          designed, not just thrown together out of components dictated by the
          accounting department.

          As some have noted there were choices as to resistor type in the 60's.

          I'll be interested to see how you do. I will not mess with my MKH-110's
          without a lot more reason than liking more modern components. I'd like
          to know a lot more about the design first, and that what I do has a very
          good chance of improving the already impressive specs. You may find that
          changing any component sends you into a complete redesign to get back
          where the mic started. I do think if any component would have aged and
          needed replacing it would be the caps.

          Note that the oscillation I observed when using a stereo pair was not
          RF, but modest audio frequencies. It would have been much nicer if it
          had of been RF. I checked this with sonograms and with a oscilloscope,
          they are not leaking RF in mine. A common shield between the pair
          provided the feedback path to cause oscillation. Making it necessary to
          leave the mic housings floating, not connected to cable shield. At least
          in my SASS. Disconnecting the mic circuits from shield would be a
          project as it includes the capsule and the metal frame holding it all
          together.

          Does the unbalanced circuit agree with what you have traced for a
          circuit in the MKH-110? Or is the MKH-110 a variant? I'd like to see a
          true circuit for the MKH-110. When I tried checking mine against the
          diagrams I posted it appeared to not be the same. Though I did not spend
          a lot of time on that.

          Walt
          wwknapp@...
        • Rich Peet
          ok I will agree the old resisters are benign. The oscillation problems we both experienced are not and need a better understanding. The 110 is very similar and
          Message 4 of 5 , Jan 3, 2004
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            ok I will agree the old resisters are benign.
            The oscillation problems we both experienced are not and need a
            better understanding.

            The 110 is very similar and I almost need a microscope to plot the
            parts. I put a photo of the mic without case in "photos area" within
            the MKH diagrams directory. This is an older model. The one easy to
            see difference so far is a capacitor across the mic element to bring
            the total capacitance higher. I will try to do a drawing but am
            working on the mics balanced line driver today as I need to be able
            to have distance between my mics.

            My sass/mkh mod works fine but it also still needs some
            improvements. I like being able to switch between the stock pzm's
            and the mkh-110s. Right now I have it set with separate outputs so I
            can record with two separate recorders and take my pick of the
            better. I will switch this to shared balanced outputs and be
            switchable. Once I am happy with the end result I will finish my
            second sass to match and then I will have four channel back to back
            or Front room / Back room for a compact 5.1 surround recording mic
            package.

            Rich


            Rich


            --- In micbuilders@yahoogroups.com, Walter Knapp <wwknapp@m...> wrote:
            > From: "Rich Peet" <richpeet@c...>
            > >
            > > Here we are dealing with HF RF. Lead length is not going to mean
            much
            > > but tightly packed wire wound resisters do bother me as the
            > > inductance can pick up the rf by increased inductance and
            transmit it
            > > to other parts of the circuit.
            > >
            > > In the unbalanced models the RF filter on the output is the weak
            part
            > > of the design. This causes problems only when working with more
            than
            > > one of these mics in stereo or multi-track.
            > >
            > > For now I will leave the resisters alone but will replace the
            > > polarized capacitors with tantilums and the caps on the RF filter.
            > >
            > > I will post the component values when I have them all determined
            and
            > > will watch for changes in the design.
            >
            > I would start by assuming that some of the top designers of mics in
            the
            > world (Sennheiser) knew what they were doing and took into account
            the
            > characteristics of the components they choose. That would certainly
            > include the inductance of the wire wound resistors. Remove that
            > inductance and you would need to be ready to retune the circuits,
            if
            > that's possible. We are mere dabblers in this sort of stuff, they
            are
            > the experts, particularly on their own design. Quality mics are
            > designed, not just thrown together out of components dictated by
            the
            > accounting department.
            >
            > As some have noted there were choices as to resistor type in the
            60's.
            >
            > I'll be interested to see how you do. I will not mess with my MKH-
            110's
            > without a lot more reason than liking more modern components. I'd
            like
            > to know a lot more about the design first, and that what I do has a
            very
            > good chance of improving the already impressive specs. You may find
            that
            > changing any component sends you into a complete redesign to get
            back
            > where the mic started. I do think if any component would have aged
            and
            > needed replacing it would be the caps.
            >
            > Note that the oscillation I observed when using a stereo pair was
            not
            > RF, but modest audio frequencies. It would have been much nicer if
            it
            > had of been RF. I checked this with sonograms and with a
            oscilloscope,
            > they are not leaking RF in mine. A common shield between the pair
            > provided the feedback path to cause oscillation. Making it
            necessary to
            > leave the mic housings floating, not connected to cable shield. At
            least
            > in my SASS. Disconnecting the mic circuits from shield would be a
            > project as it includes the capsule and the metal frame holding it
            all
            > together.
            >
            > Does the unbalanced circuit agree with what you have traced for a
            > circuit in the MKH-110? Or is the MKH-110 a variant? I'd like to
            see a
            > true circuit for the MKH-110. When I tried checking mine against
            the
            > diagrams I posted it appeared to not be the same. Though I did not
            spend
            > a lot of time on that.
            >
            > Walt
            > wwknapp@m...
          • Walter Knapp
            From: Rich Peet ... I would agree on the oscillation. I can live with it primarily because I m nature recording and a good ways from
            Message 5 of 5 , Jan 4, 2004
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              From: "Rich Peet" <richpeet@...>
              >
              > ok I will agree the old resisters are benign.
              > The oscillation problems we both experienced are not and need a
              > better understanding.

              I would agree on the oscillation. I can live with it primarily because
              I'm nature recording and a good ways from sources of interference that
              could get at unshielded mics. It definitely needs more study.

              Been a long time since I fiddled with oscillator design. But, I'm
              assuming that the shield is a feedback path via the capsule. The
              frequency is determined by the delay of the path. I did poke around
              inside with a extra cap to see if I could change the frequency. It was
              pretty resistant to change. I did not, by any means, exhaust all
              possible options inside as I was being cautious. I took the practical
              approach of buttoning it all up once I found a method that got rid of
              the oscillation that I could live with.

              Should you find a better method that does not blow the specs of the mic,
              I'm very definitely interested. I don't have enough spare mics to risk.

              > The 110 is very similar and I almost need a microscope to plot the
              > parts. I put a photo of the mic without case in "photos area" within
              > the MKH diagrams directory. This is an older model. The one easy to
              > see difference so far is a capacitor across the mic element to bring
              > the total capacitance higher. I will try to do a drawing but am
              > working on the mics balanced line driver today as I need to be able
              > to have distance between my mics.

              It's about this time of year I realize how many off season projects are
              still not done. Priority shifts to the most critical. In my case, more
              windscreening to sew. Building furry covers for my SASS. Crown's wind
              cover is not enough. Particularly with the high tripod technique I've
              been using with the SASS wind can be a problem.

              I've used my SASS/MKH-110 on as much as 50' of cable without problem.
              Got the impression I could go much more.

              The photo does not look quite like mine, but real close. If you say this
              is one of the older ones, then mine must be a little newer.

              > My sass/mkh mod works fine but it also still needs some
              > improvements. I like being able to switch between the stock pzm's
              > and the mkh-110s. Right now I have it set with separate outputs so I
              > can record with two separate recorders and take my pick of the
              > better. I will switch this to shared balanced outputs and be
              > switchable. Once I am happy with the end result I will finish my
              > second sass to match and then I will have four channel back to back
              > or Front room / Back room for a compact 5.1 surround recording mic
              > package.

              I hope you got the SASS parts before Crown upped the price. Or, if not
              that you have a crown dealer who will cooperate in getting them for you.
              I'm wishing I had of bought a few more. Crown must have noticed all the
              mods being made. I've built several for folks for them to use with MKH-20's.

              If your setup works well, the next magic trick would be to organize it
              as a all in one surround mic of a more compact design.

              I'm headed a different way in experiments with the SASS. Staying with
              stereo, but want to experiment with how much the field shape can be
              varied. Meld it with what I've picked up about curved plate PZM's. Which
              is not a lot, unfortunately. The Crown PZM dish I have is the 12" one,
              not the much rarer 24" one. Reports on that indicate some interesting
              properties. It was not considered all that useful as it tended to pick
              up distant things too well. They were annoyed when they not only picked
              up the quarterback in the huddle, but people talking on the opposite
              side of the field...

              That is if I ever have enough time to get into it.

              Walt
              wwknapp@...
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