[ProAudio] Microphones question

Dan Lavry dan at lavryengineering.com
Mon Jun 14 09:01:19 PDT 2021

Hi Bob, I am not designing a mic pre right now, and my interest is not about specific micpre. I am interested in noise and distortion of the gear that people connect to my converters. My older Lavry Gold was mostly for mastering, not tracking. The new Gold (Lavry Savitr) offers very low latency and can be used with micpres. It would be good to  to get a sense from ear people, such as yourself, about what is needed in terms of noise, for the most extreme cases (highest gain), because that is the bottle neck of the noise floor.I am told here that any fool can come up with 128dBu ein. Some engineers I respect would take offense to such comment. I agree that at low gain it matters less. But it would be good to know if the 128-130dBu (or whatever the state of the art) is limited because we can't do better, or because it is good enough for all practical  cases. If it is limited by technology, a mic designer can look forward to pushing the state of the art. If it is good enough, there is no point in improving...If I get convinced that there is a market for -135dBu, I would consider doing so, right after all the other stuff I want to do. My last project took me 3 years. Doing  a -135dBu will take a year or more. I have other things on my plate. I see the new developments regarding digital microphones, based on one bit modulator. The cable carries one bit digital, and some very new IC's provide decimation via 4 or 5 pole filter. I saw the data and specs. So far it is far from state of the art, and aimed mostly at mass production lower cost stuff. I can't comment about the mic quality, but i see much data related to the IC's involved. one bit (at the present day upsampling rate) and 5th order is 90's technology. It may be fine for many uses, but the results are not state of the art...  I do expect the digital mics IC's to improve in the future. The concepts are there, such as multibit and faster clock. And it may be that sigma delta will be surpassed. I can't predict...Did you try  to use digital mics? If so, what do you think?RegardsDan LavrySent from Samsung Galaxy smartphone.
-------- Original message --------From: Bob Katz via ProAudio <proaudio at bach.pgm.com> Date: 6/14/21  7:42 AM  (GMT-08:00) To: proaudio at bach.pgm.com Subject: Re: [ProAudio] Microphones question 
    Dan: As you can see, there are so many variables, that all we can
      do is an approximation using 150 ohms, in order to come up with an
      EIN value for the preamp, which will be an approximation. Feel
      free to specify alternate EIN values in the preamp spec sheet in
      addition to the widely-accepted 150 ohms, for example, with 50 ohm
      source at 40 dB gain. Maybe it will help characterize your preamp
      better to the reader. Maybe someone will discover that all
      microphones should have a lower output impedance to get better
    But as Scott pointed out, why not eliminate the issues and go
      with a digital output microphone. My concerns there are jitter.
      But there are ways around that with a superior PLL at the
      receiving end. 
    Best wishes,
    On 6/11/21 10:49 PM, Dan Lavry via
      ProAudio wrote:
      Hi Bill, 
      I am not trying to change the standard. Certainly not before I
        have the understanding of what is going on. Of course in an
        ideal world, each mic would come with an impedance information.
        In a somewhat less ideal world, each mic would come with, at
        least, the best resistor value. But here we are stuck to the
        "typical" old dynamic mic.
      I know that most people don't understand technical details, and
        it will confuse customers. But I am not talking to customers
        here, or level playing field. I asked the opinion of mic
        experts. I want to know what real impedance levels are. That is
        really all I want to know. So far, I learned about the 150 Ohm
        relation to the old dynamic. There must be more information, I
        thought I would start here.
      Thanks for all the comments
      Dan Lavry
      On 6/11/2021 6:47 PM, Bill Whitlock
        via ProAudio wrote:
        How would you propose testing and specifying
          equivalent input noise for mic preamps?
          Using the 150 Ω dummy source at least levels the playing
            field, even though it's not accurately predictive for all
            mics.  Specifying e and i noise separately (and their own
            spectrum, if you're going to be rigorous) will further
            confuse buyers - most of whom can barely understand why a
            shorted input is unrealistic.
          As I recall from my tests of the SM57, its impedance
            varied from under 150 Ω at very low frequencies to over 300
            Ω at resonance - and continued to rise at higher
            frequencies.  I'll try to find the data - I did the tests as
            research before writing Jensen AN-005 about mic splitters.
          If you want to change this, I'd encourage you to join an
            AES standards committee and make your case.  Membership in
            working groups is open to all.  Working group SC-05-05 is
            currently trying to change the ways equipment manufacturers
            describe inputs and outputs - in the interests of avoiding
            interoperability issues and unexpected results.
          Bill Whitlock
          AES Life Fellow
          Ventura, CA
            -----Original Message-----
                From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
                To: crispin at crookwood.com; proaudio at bach.pgm.com
                Sent: Fri, Jun 11, 2021 6:24 pm
                Subject: Re: [ProAudio] Microphones question
                Yes of course the input noise has to be
                  taken into account AFTER 
                  amplification. So say a micpre has 120dBu noise
                  (referenced to the 
                  input), with say 60dB gain the noise is at 60dBu. That
                  is easy to 
                  measure and hear...
                  My point of interest was not about measuring
                  microphones. But we have 
                  dynamic, ribbon and condenser with phantom, and a wide
                  range of 
                  implementations in each category. So the use of one
                  value resistor seems 
                  to be arbitrary. I mentioned earlier that the input
                  noise is made of 
                  both noise voltage and noise current components. The
                  noise current 
                  (today's technology) will have low impact for 150 Ohms
                  resistor. So why 
                  is the resistor there? The answer is to give us a
                  better idea of how the 
                  micpre works with a mic instead of a short. And so we
                  lump all mics into 
                  a simple model. A 150 Ohm resistor.
                  I think we should remove that 150 Ohm resistor. The
                  resistor noise is 
                  -130.9dBu (room T). If future technology will enable a
                  shorted input 
                  micpre to reach 130dBu noise (referenced to input with
                  acceptable gain), 
                  the outcome with 150 Ohm is -127.4dBu. Further down
                  the line,135dBu 
                  noise (referenced to input) only improves the outcome
                  to -129.5dBu.
                  I just wonder if there is some information about the
                  real impedance of 
                  real mics including different types relative to that
                  150 Ohm. It would 
                  be good to have some better detail...
                  Dan Lavry
                  On 6/11/2021 1:32 PM, Crispin HT wrote:
                  > I'm not a mic expert, but in designing preamps
                  over the years, we’ve found that the EIN of most mics
                  sits around the -118 to -122dB mark.
                  > The relevance of this, is that you need to
                  amplify a mic, and it's noise to use it.  Often quiet
                  mics have low outputs, so need to be amplified more,
                  and the real test of a mic pre's EIN is not at 60dB
                  gain, getting an EIN of better than -124dB at gains
                  around the 20-40dB.
                  > Hope this helps.
                  > Kind Regards
                  > Crispin Herrod-Taylor
                  > Managing Director, Crookwood
                  > www.crookwood.com
                  > Tel: +44 (0)1672 811 649
                  > Mobile:+44(0)7910 637 634
                  > Sign up for our great newsletter here! and keep
                  up to date with the audio world
                  > -----Original Message-----
                  > From: ProAudio <proaudio-bounces at bach.pgm.com>
                  On Behalf Of Dan Lavry via ProAudio
                  > Sent: 11 June 2021 20:44
                  > To: proaudio at bach.pgm.com
                  > Subject: Re: [ProAudio] Microphones question
                  > My question is about mic output impedance, in
                  relation to noise:
                  > Both the mic and the micpre contribute to noise.
                  The micpre generates some noise voltage which can be
                  measured by replacing the mic with a short (0 Ohm).
                  But there is also mipre generated noise current, which
                  is no problem for 0 Ohm, but real mics have some
                  > At some point, it was decided to model a mic
                  noise with replacing the mic with 150 Ohm resistor.  I
                  am not proposing to change it, just trying to
                  understand why 150 Ohm.
                  > The value 150 Ohm makes 1.568nV/sqrtHz (at room
                  temp), so for 20H-20KHz noise voltage of .225uV. Given
                  that we are interested in noise power, we can use the
                  dBu scale to realize that the resistor itself sets a
                  limit on the noise floor at -130.9dBu. But say the
                  impedance is 1K, then we have -122.8dBu.
                  > I assume that the resistor modeling is a
                  simplification. I would be interested in comments from
                  the mic experts here.
                  > Thank You
                  > Dan Lavry
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