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<p>Hi Bill, <br>
</p>
<p>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.</p>
<p>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.</p>
<p>Thanks for all the comments</p>
<p>Dan Lavry<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 6/11/2021 6:47 PM, Bill Whitlock via
ProAudio wrote:<br>
</div>
<blockquote type="cite"
cite="mid:1124007279.4538052.1623462475596@mail.yahoo.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">How
would you propose testing and specifying equivalent input noise
for mic preamps?
<div><br>
</div>
<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>
<div><br>
</div>
<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>
<div><br>
</div>
<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>
<div><br>
</div>
<div>Bill Whitlock</div>
<div>AES Life Fellow</div>
<div>Ventura, CA<br>
<br>
<br>
<div
style="font-family:arial,helvetica;font-size:10pt;color:black"><font
size="2">-----Original Message-----<br>
From: Dan Lavry via ProAudio <a class="moz-txt-link-rfc2396E" href="mailto:proaudio@bach.pgm.com"><proaudio@bach.pgm.com></a><br>
To: <a class="moz-txt-link-abbreviated" href="mailto:crispin@crookwood.com">crispin@crookwood.com</a>; <a class="moz-txt-link-abbreviated" href="mailto:proaudio@bach.pgm.com">proaudio@bach.pgm.com</a><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">
> <a class="moz-txt-link-abbreviated" href="http://www.crookwood.com">www.crookwood.com</a><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"
moz-do-not-send="true">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"
moz-do-not-send="true">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">
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