[ProAudio] Microphones question

[Crispin HT]

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
 
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-----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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