[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Mon Jun 14 10:43:42 PDT 2021 

I don't want to derail this thread, but since you mentioned jitter ... I wonder how many designers have considered how microphonic a crystal oscillator is, which translates into jitter of a reference clock when the gear is in the listening environment.  And then there are the ubiquitous ceramic capacitors, some of which are highly microphonic. And, as long as digital signals have finite rise and fall times, jitter will always translate into amplitude aberrations (spectral impurity) downstream.  Ditto for common-mode noise issues in digital signal interconnections.  Just a little food for thought ...
Bill Whitlock


-----Original Message-----
From: Bob Katz via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com
Sent: Mon, Jun 14, 2021 7:42 am
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 performance. 
  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, 
  
  Bob 
  
  
  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...
 
 Regards
 
 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 impedance...
 >
 > 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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