<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">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 ...
<div><br>
</div>
<div>Bill Whitlock<br>
<br>
<br>
<div style="font-family:arial,helvetica;font-size:10pt;color:black"><font size="2">-----Original Message-----<br>
From: Bob Katz via ProAudio <proaudio@bach.pgm.com><br>
To: proaudio@bach.pgm.com<br>
Sent: Mon, Jun 14, 2021 7:42 am<br>
Subject: Re: [ProAudio] Microphones question<br>
<br>
<div id="yiv6022184408">
<div>
<div>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. <br clear="none">
</div>
<div>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. <br clear="none">
</div>
<div><br clear="none">
</div>
<div><br clear="none">
</div>
<div>Best wishes,</div>
<div><br clear="none">
</div>
<div><br clear="none">
</div>
<div>Bob</div>
<div><br clear="none">
</div>
<div><br clear="none">
</div>
<div><br clear="none">
</div>
<div class="yiv6022184408yqt7745746079" id="yiv6022184408yqtfd87901">
<div class="yiv6022184408moz-cite-prefix">On 6/11/21 10:49 PM, Dan Lavry via
ProAudio wrote:<br clear="none">
</div>
<blockquote type="cite">
</blockquote></div>
</div>
<div class="yiv6022184408yqt7745746079" id="yiv6022184408yqtfd46295">
<div>
<div>Hi Bill, <br clear="none">
</div>
<div>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.</div>
<div>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.</div>
<div>Thanks for all the comments</div>
<div>Dan Lavry<br clear="none">
</div>
<div><br clear="none">
</div>
<div class="yiv6022184408moz-cite-prefix">On 6/11/2021 6:47 PM, Bill Whitlock
via ProAudio wrote:<br clear="none">
</div>
<blockquote type="cite">
</blockquote></div>
</div>
<div>
<div class="yiv6022184408yqt7745746079" id="yiv6022184408yqtfd14793">
<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 clear="none">
</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 clear="none">
</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 clear="none">
</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 clear="none">
</div>
<div>Bill Whitlock</div>
<div>AES Life Fellow</div>
<div>Ventura, CA<br clear="none">
<br clear="none">
<br clear="none">
<div style="font-family:arial, helvetica;font-size:10pt;color:black;"><font size="2">-----Original Message-----<br clear="none">
From: Dan Lavry via ProAudio <a shape="rect" class="yiv6022184408moz-txt-link-rfc2396E" ymailto="mailto:proaudio@bach.pgm.com" href="mailto:proaudio@bach.pgm.com" target="_blank" rel="noreferrer noopener"><proaudio@bach.pgm.com></a><br clear="none">
To: <a shape="rect" class="yiv6022184408moz-txt-link-abbreviated" ymailto="mailto:crispin@crookwood.com" href="mailto:crispin@crookwood.com" target="_blank" rel="noreferrer noopener">crispin@crookwood.com</a>; <a shape="rect" class="yiv6022184408moz-txt-link-abbreviated" ymailto="mailto:proaudio@bach.pgm.com" href="mailto:proaudio@bach.pgm.com" target="_blank" rel="noreferrer noopener">proaudio@bach.pgm.com</a><br clear="none">
Sent: Fri, Jun 11, 2021 6:24 pm<br clear="none">
Subject: Re: [ProAudio] Microphones question<br clear="none">
<br clear="none">
</font>
<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 shape="rect" class="yiv6022184408moz-txt-link-abbreviated" href="http://www.crookwood.com/" target="_blank" rel="noreferrer noopener">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" target="_blank" rel="noreferrer noopener">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" target="_blank" rel="noreferrer noopener">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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