<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">How would you propose testing and specifying equivalent input noise for mic preamps?
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<div>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.</div>
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<div>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.</div>
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<div>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.</div>
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<div>Bill Whitlock</div>
<div>AES Life Fellow</div>
<div>Ventura, CA<br>
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<div style="font-family:arial,helvetica;font-size:10pt;color:black"><font size="2">-----Original Message-----<br>
From: Dan Lavry via ProAudio <proaudio@bach.pgm.com><br>
To: crispin@crookwood.com; proaudio@bach.pgm.com<br>
Sent: Fri, Jun 11, 2021 6:24 pm<br>
Subject: Re: [ProAudio] Microphones question<br>
<br>
<div dir="ltr">Yes of course the input noise has to be taken into account AFTER <br clear="none">amplification. So say a micpre has 120dBu noise (referenced to the <br clear="none">input), with say 60dB gain the noise is at 60dBu. That is easy to <br clear="none">measure and hear...<br clear="none"><br clear="none">My point of interest was not about measuring microphones. But we have <br clear="none">dynamic, ribbon and condenser with phantom, and a wide range of <br clear="none">implementations in each category. So the use of one value resistor seems <br clear="none">to be arbitrary. I mentioned earlier that the input noise is made of <br clear="none">both noise voltage and noise current components. The noise current <br clear="none">(today's technology) will have low impact for 150 Ohms resistor. So why <br clear="none">is the resistor there? The answer is to give us a better idea of how the <br clear="none">micpre works with a mic instead of a short. And so we lump all mics into <br clear="none">a simple model. A 150 Ohm resistor.<br clear="none"><br clear="none">I think we should remove that 150 Ohm resistor. The resistor noise is <br clear="none">-130.9dBu (room T). If future technology will enable a shorted input <br clear="none">micpre to reach 130dBu noise (referenced to input with acceptable gain), <br clear="none">the outcome with 150 Ohm is -127.4dBu. Further down the line,135dBu <br clear="none">noise (referenced to input) only improves the outcome to -129.5dBu.<br clear="none"><br clear="none">I just wonder if there is some information about the real impedance of <br clear="none">real mics including different types relative to that 150 Ohm. It would <br clear="none">be good to have some better detail...<br clear="none"><br clear="none">Regards<br clear="none"><br clear="none">Dan Lavry<br clear="none"><br clear="none">On 6/11/2021 1:32 PM, Crispin HT wrote:<br clear="none">> 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.<br clear="none">><br clear="none">> 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.<br clear="none">><br clear="none">> Hope this helps.<br clear="none">><br clear="none">> Kind Regards<br clear="none">> Crispin Herrod-Taylor<br clear="none">> Managing Director, Crookwood<br clear="none">> www.crookwood.com<br clear="none">> Tel: +44 (0)1672 811 649<br clear="none">> Mobile:+44(0)7910 637 634<br clear="none">> <br clear="none">> Sign up for our great newsletter here! and keep up to date with the audio world<br clear="none">><br clear="none">> -----Original Message-----<br clear="none">> From: ProAudio <<a shape="rect" ymailto="mailto:proaudio-bounces@bach.pgm.com" href="mailto:proaudio-bounces@bach.pgm.com">proaudio-bounces@bach.pgm.com</a>> On Behalf Of Dan Lavry via ProAudio<br clear="none">> Sent: 11 June 2021 20:44<br clear="none">> To: <a shape="rect" ymailto="mailto:proaudio@bach.pgm.com" href="mailto:proaudio@bach.pgm.com">proaudio@bach.pgm.com</a><br clear="none">> Subject: Re: [ProAudio] Microphones question<br clear="none">><br clear="none">> My question is about mic output impedance, in relation to noise:<br clear="none">><br clear="none">> 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...<br clear="none">><br clear="none">> 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.<br clear="none">><br clear="none">> 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.<br clear="none">><br clear="none">> I assume that the resistor modeling is a simplification. I would be interested in comments from the mic experts here.<br clear="none">><br clear="none">> Thank You<br clear="none">><br clear="none">> Dan Lavry<br clear="none">><br clear="none">><br clear="none">> --<br clear="none">> This email has been checked for viruses by Avast antivirus software.<br clear="none">> <a shape="rect" href="https://www.avast.com/antivirus" target="_blank">https://www.avast.com/antivirus</a><br clear="none">><br clear="none">> _______________________________________________<br clear="none">> ProAudio mailing list<br clear="none">> <a shape="rect" ymailto="mailto:ProAudio@bach.pgm.com" href="mailto:ProAudio@bach.pgm.com">ProAudio@bach.pgm.com</a><br clear="none">> <a shape="rect" href="http://bach.pgm.com/mailman/listinfo/proaudio" target="_blank">http://bach.pgm.com/mailman/listinfo/proaudio</a>
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