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Re: DIY isolation chamber

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  • picnet2
    Hi Eric, The Earthworks ZDT preamp caught most of my attention :) - US Patent 4403199 may be relevant there. Or entering Blackmer; David; E. noise into
    Message 1 of 16 , Sep 1, 2008
      Hi Eric,
      The Earthworks ZDT preamp caught most of my attention :) -
      US Patent 4403199 may be relevant there. Or entering "Blackmer; David; E. noise" into
      google patent search should find the legacy of an audio genius.
      http://www.stereophile.com/news/11303/

      Off topic: US 6,091,829 is also interesting, Ive never come across a mic using that
      structure?

      > The noise of each piece of equipment can be analyzed separately. First the
      > analysis equipment, with its input shorted by a very low impedance the same
      > as that of its source, the microphone preamplifier. Then with the
      > preamplifier in place, input shorted by a representative impedance such as
      > 50 Ohm to 200 Ohm. And then finally the microphone is attached.

      1st step gives reference level, 2nd if using a preamp shows the additional noise, 3rd
      shows the final noise with the mic in place, each step additive, or relative to the last?
      For the edirol recorder, a reference can be taken with the mic input shorted or at least so
      far ive been using a 1k resistor. Followed by the mic attachment. Ive compared the
      internal mics vs an external one, but this is just a relative measurement between the two.
      Baudline has a noise ratio measurement function, Ill check how to use that (RTFM)..

      > In order to do a good job of measuring the sensitivity of a cardioid
      > microphone or other directional microphone one must measure its frequency
      > response, and do that in comparison to a known calibrated microphone. And
      > that's a completely different topic.
      >
      > I hope this helps.

      It helps a lot and thank you for writing. Ill focus later on the mic calibration techniques
      - I have a mic thats been calibrated / came with a plot & data.

      Cheers,
      -MIke.
    • userno232000
      ... itself minus the ... noise? Excuse the pun ... Eric is correct in general but in my case the mic has 20dB of gain right at the capsule so it swamps the
      Message 2 of 16 , Sep 1, 2008
        --- In micbuilders@yahoogroups.com, "picnet2" <picnet@...> wrote:
        >
        > Hi,
        > By noise floor, the noise floor of the recording device, the mic
        itself minus the
        > recorder noise? Or both? Exactly how do you figure out the mic
        noise? Excuse the pun
        > but Im all ears :-)
        >

        Eric is correct in general but in my case the mic has 20dB of gain
        right at the capsule so it swamps the noise of my recorders preamp. In
        this case I was looking for relative answers. The issue of exact
        spectral weighting and calibration as if one was in the business of
        specing mics is beyond my DIY energy :) I did find what I was looking
        for, that the random air motion on the diaphragm's mechanical system
        is a measurable noise effect above the noise of a very quiet FET.
      • picnet2
        Hi, I think my question was more related to the whole setup in general. Ive not got as far as the quiet mic measurements yet - more just characterizing the
        Message 3 of 16 , Sep 1, 2008
          Hi,
          I think my question was more related to the whole setup in general. Ive not got as far
          as the quiet mic measurements yet - more just characterizing the recorder...
          Hence Im still looking for the answer,reading IEEE papers on the same subject, and doing
          some investigations myself. Eric's answer was great and Im still absorbing how to translate
          it into practice :) Some translated in the plots linked below..

          - relatively I can plot the response of a recording devices preamp, at any of its gain
          settings - then plugging in a mic watching for the differences. But since the mic has a
          FET, its powered by a bias supply (2.5vish) in my case. I get the mic FET / amplifier noise
          in addition. Hence I can see the difference when a mic is plugged in and graph it, I can
          compare mics - but does that really show the mic noise level itself or just the relative
          combination of the recorder, the device under (mic, fet + air movement etc) test given
          differences in sensitivity/frequency response?

          Today I 'tried' to characterise the noise as present in the Edirol portable recorder:-
          http://www.urlme.net/blog/?p=119

          Using the select noise ratio in baudline,
          Does +16 dB of noise in an amplifer set to max level (in this case a portable device. bias
          power on)

          "Look about right ?"

          -I have no comparison to benchmark against and this is the first time Ive attempted this
          detailed level of faffing around with mics :)

          -Mike.



          --- In micbuilders@yahoogroups.com, "userno232000" <scott.wurcer@...> wrote:
          >
          > --- In micbuilders@yahoogroups.com, "picnet2" <picnet@> wrote:
          > >
          > > Hi,
          > > By noise floor, the noise floor of the recording device, the mic
          > itself minus the
          > > recorder noise? Or both? Exactly how do you figure out the mic
          > noise? Excuse the pun
          > > but Im all ears :-)
          > >
          >
          > Eric is correct in general but in my case the mic has 20dB of gain
          > right at the capsule so it swamps the noise of my recorders preamp. In
          > this case I was looking for relative answers. The issue of exact
          > spectral weighting and calibration as if one was in the business of
          > specing mics is beyond my DIY energy :) I did find what I was looking
          > for, that the random air motion on the diaphragm's mechanical system
          > is a measurable noise effect above the noise of a very quiet FET.
          >
        • userno232000
          ... attention :) - ... David; E. noise into ... That is actually one of several DBX system patents. I talked to Dave about his preamp. If you look at the
          Message 4 of 16 , Sep 1, 2008
            --- In micbuilders@yahoogroups.com, "picnet2" <picnet@...> wrote:
            >
            > Hi Eric,
            > The Earthworks ZDT preamp caught most of my
            attention :) -
            > US Patent 4403199 may be relevant there. Or entering "Blackmer;
            David; E. noise" into
            > google patent search should find the legacy of an audio genius.
            > http://www.stereophile.com/news/11303/
            >

            That is actually one of several DBX system patents. I talked to Dave
            about his preamp. If you look at the noise vs. gain you can figure out
            the values that would fit in the standard THAT1512 type circuit (but
            with discretes) to achieve the noise performance. Doubleling up on the
            input devices and running them hot enough is enough to achieve .6nV.
            If one was ambitious you could build a little daughter card (and
            change the gain resistor values) for one of those Seventhcircle kits.
          • userno232000
            ... Dave ... out ... (but ... the ... achieve .6nV. ... kits. Just noticed their C84 preamp is just that, should just about do (at least with respect to noise)
            Message 5 of 16 , Sep 1, 2008
              --- In micbuilders@yahoogroups.com, "userno232000"
              <scott.wurcer@...> wrote:
              >
              > --- In micbuilders@yahoogroups.com, "picnet2" <picnet@> wrote:
              > >
              > > Hi Eric,
              > > The Earthworks ZDT preamp caught most of my
              > attention :) -
              > > US Patent 4403199 may be relevant there. Or entering "Blackmer;
              > David; E. noise" into
              > > google patent search should find the legacy of an audio genius.
              > > http://www.stereophile.com/news/11303/
              > >
              >
              > That is actually one of several DBX system patents. I talked to
              Dave
              > about his preamp. If you look at the noise vs. gain you can figure
              out
              > the values that would fit in the standard THAT1512 type circuit
              (but
              > with discretes) to achieve the noise performance. Doubleling up on
              the
              > input devices and running them hot enough is enough to
              achieve .6nV.
              > If one was ambitious you could build a little daughter card (and
              > change the gain resistor values) for one of those Seventhcircle
              kits.

              Just noticed their C84 preamp is just that, should just about do (at
              least with respect to noise) what the Earthworks does.
            • Richard Lee
              ... Mr. Wurcer, which preamp patent are you looking at? I tried Google patent but only got a load of stuff about gain controlled stages which I remember fondly
              Message 6 of 16 , Sep 2, 2008
                > > The Earthworks ZDT preamp caught most of my
                > attention :) -
                > > US Patent 4403199 may be relevant there. Or entering "Blackmer;
                > David; E. noise" into
                > > google patent search should find the legacy of an audio genius.
                > > http://www.stereophile.com/news/11303/

                > That is actually one of several DBX system patents. I talked to Dave
                > about his preamp. If you look at the noise vs. gain you can figure out

                Mr. Wurcer, which preamp patent are you looking at?

                I tried Google patent but only got a load of stuff about gain
                controlled stages which I remember fondly from the early 80's.
              • userno232000
                ... entering Blackmer; ... genius. ... Dave ... figure out ... That s what I said, they are the DBX compander patents. The Cohen circuit dates from at least
                Message 7 of 16 , Sep 2, 2008
                  --- In micbuilders@yahoogroups.com, "Richard Lee" <ricardo@...>
                  wrote:
                  >
                  > > > The Earthworks ZDT preamp caught most of my
                  > > attention :) -
                  > > > US Patent 4403199 may be relevant there. Or
                  entering "Blackmer;
                  > > David; E. noise" into
                  > > > google patent search should find the legacy of an audio
                  genius.
                  > > > http://www.stereophile.com/news/11303/
                  >
                  > > That is actually one of several DBX system patents. I talked to
                  Dave
                  > > about his preamp. If you look at the noise vs. gain you can
                  figure out
                  >
                  > Mr. Wurcer, which preamp patent are you looking at?
                  >
                  > I tried Google patent but only got a load of stuff about gain
                  > controlled stages which I remember fondly from the early 80's.
                  >

                  That's what I said, they are the DBX compander patents. The "Cohen"
                  circuit dates from at least 1969 and is in the public domain. The
                  Earthworks preamp is another spin, as far as I can tell, on the
                  instrumentation amplifier as differential audio preamp. Double pairs
                  of npn's running at 5mA a side and a 1 Ohm gain resistor should
                  do .6nV easily. BTW someone sent me a link to a 65pV/rt-Hz amplifier
                  for SQUID's just to put this in perspective. Not relevant here since
                  it requires a .5 Ohm source resistance, perfect for ribbon mics
                  though. It uses two custom transformers with some "magic" grinding
                  down of gaps, it seems probably not a good DIY candidate.
                • Eric Benjamin
                  ... Scott makes a very important point here. For a DIY microphone builder, the relative answer is the important one. If I m designing a new microphone, or
                  Message 8 of 16 , Sep 3, 2008
                    --- In micbuilders@yahoogroups.com, "userno232000 " wrote:
                    > In this case I was looking for relative answers.
                    Scott makes a very important point here. For a DIY microphone builder, the
                    relative answer is the important one. If I'm designing a new microphone, or
                    modifying and old one, the important thing to know is whether things got
                    better or worse, and by how much. And for that, one only needs to calibrate
                    the relative sensitivity of the microphone capsules under test.


                    >Eric is correct in general but in my case the mic has 20dB of gain
                    >right at the capsule so it swamps the noise of my recorders preamp. In
                    >this case I was looking for relative answers. The issue of exact
                    >spectral weighting and calibration as if one was in the business of
                    >specing mics is beyond my DIY energy :) I did find what I was looking
                    >for, that the random air motion on the diaphragm's mechanical system
                    >is a measurable noise effect above the noise of a very quiet FET.
                  • userno232000
                    ... general. Ive not got as far ... recorder... ... Mike, You re getting way ahead of yourself. The stuff about replacing capsules with capacitors to try and
                    Message 9 of 16 , Sep 4, 2008
                      --- In micbuilders@yahoogroups.com, "picnet2" <picnet@...> wrote:
                      >
                      > Hi,
                      > I think my question was more related to the whole setup in
                      general. Ive not got as far
                      > as the quiet mic measurements yet - more just characterizing the
                      recorder...
                      > Hence Im still looking for the answer,reading IEEE papers on the \

                      Mike,


                      You're getting way ahead of yourself. The stuff about replacing
                      capsules with capacitors to try and verify theory is fairly esoteric
                      stuff mainly for those with an EE bent. Also try

                      There's some good info at avisoft about portable recorder noise.
                      http://www.avisoft.com/recordertests.htm

                      or Rob's site for mics http://www.uwm.edu/~type/audio-
                      reports/MicSpecCharts/MicNoiseSensv01.htm
                    • picnet2
                      Hi, This is the general idea, get way out of depth and fill in the missing knowledge by learning new things as I go (=motto of the lab blog). The sites linked
                      Message 10 of 16 , Sep 5, 2008
                        Hi,
                        This is the general idea, get way out of depth and fill in the missing knowledge
                        by learning new things as I go (=motto of the lab blog).

                        The sites linked below are good examples, in particular EIN is used with A weighting, and
                        the unweighted 20hz -20khz. Now I have some reference when making similar
                        measurements.

                        > You're getting way ahead of yourself. The stuff about replacing
                        > capsules with capacitors to try and verify theory is fairly esoteric
                        > stuff mainly for those with an EE bent.

                        I try to keep the blog free of any spice simulations.

                        > Also try
                        >
                        > There's some good info at avisoft about portable recorder noise.
                        > http://www.avisoft.com/recordertests.htm

                        Good site, the R09HR Measurements look odd, the dBu for low should
                        be high and visa-versa.

                        > or Rob's site for mics http://www.uwm.edu/~type/audio-
                        > reports/MicSpecCharts/MicNoiseSensv01.htm
                        >

                        Is what prompted me to buy the NT1A :-)

                        Cheers,
                        -Mike.
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