[ProAudio] Microphones question

Dan Lavry dan at lavryengineering.com
Fri Jun 11 19:49:07 PDT 2021

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 
> <mailto: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 <mailto: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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