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<p>Hi again,</p>
<p>Impedance is one thing, but my question is about circuit
generated noise, not impedance. The noise generated by a
theoretical resistor is sqrt(4*K*T*<b>R</b>* BW). It is NOT
sqrt(4*K*T*<b>Z</b>*BW) where R is a pure resistor, Z is the
impedance. <br>
</p>
<p>Here is a simple example is a single 300 Ohm resistor in parallel
with a capacitor 17pF capacitor. The 300 Ohm resistor generates
around 3.13uV at room temp and 20KHz BW. The capacitor generated
noise (BW*K*T/C) is only 0.034uV. Capacitor generated noise is
very tiny unless you use very small cap values (such as in
semiconductor switched cap filters in sigma delta...).</p>
<p>The capacitors may impact the frequency response. In this simple
example In the example (300 Ohm, 17pF), the BW is limited by
single pole at 31MHz. Take a 6mH in series which that translate
into around 753 Ohm at 20KHz. It may alter the impedance
significantly, and impact flatness response some, but the <b>higher</b>
impedance in series, against say 6.8Kohm Phantom load resistors,
actually <b>attenuates</b> the noise and the signal. <br>
</p>
<p>The capacitors and inductors of the mic (equivalent circuit)
together with the load (micpre input) may shape the frequency
response, thus indirectly impact the noise (and the signal) over
the BW of interest, but higher impedance is not a direct cause of
generated noise. Resistors are the thermal noise generators. <br>
</p>
<p>While I simplified things a lot, accounting for noise begins with
figuring the individual components contributions over the BW of
interest, and then accounting for frequency response (integral of
noise over freq). It gets easier for flat response. This is very
different then figuring impedance. Analyzing noise is about
multiple noise sources at each component interacting with the
circuit. Analyzing impedance is about circuit interaction with
signals and not about generating the signals. <br>
</p>
<p>The 300 Ohm Bill suggested for SM57 seems like a good estimate
(ignoring the cap and inductance). Even at 20KHz, with the 800 Ohm
series impedance against the 6.8K input, the impact (attenuation)
is small, and actually reduces the signal and noise slightly.</p>
<p>It seems to me that suggested LCR and impedance measurements are
not appropriate for noise estimation, and the old single resistor
model works better. The question is still the resistance value...</p>
<p>Regards</p>
<p>Dan Lavry<br>
</p>
<p><br>
</p>
<p><br>
</p>
<p><br>
</p>
<p> <br>
</p>
<div class="moz-cite-prefix">On 6/11/2021 11:46 PM, Bill , via
ProAudio wrote:<br>
</div>
<blockquote type="cite"
cite="mid:995149481.4573123.1623480364361@mail.yahoo.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">My
analysis of the SM57 appeared in the chapter I co-wrote with
Michael Pettersen of Shure. It's in the Ballou "Handbook for
Sound Engineers" 3rd edition, Chapter 21 "Preamplifiers and
Mixers," pages 601-602. It may be in a different chapter and
page number in the 4th and 5th editions. Anyway, the parameters
for the equivalent circuit are:
<div><br>
<div>(capsule) =12 Ω and 157 µH in series, feeding the primary
of a</div>
<div>transformer = 1:4.5 ratio, DCRs 1.2 Ω pri and 24 Ω sec,
sec L = 2.75 mH</div>
<div>which, impedance converted through the transformer,
becomes <span style="background-color: transparent;
font-size: 12pt;">equivalent to 6 mH in series with 300 Ω
across pins 2 and 3 (each having 17 pF of capacitance to
the case.</span></div>
<div>The <span style="font-style: italic;">measured</span>
impedance, not surprisingly, never goes below 300 Ω and
broadly peaks at 530 Ω at 150 Hz, again returning to 300 Ω
between 1 and 2 kHz, then rising to 500 Ω at 10 kHz and 800
Ω at 20 kHz.</div>
<div><br>
</div>
<div>If I had to choose, a 300 Ω resistor would be a fair
approximation for calculating noise in the frequency range
(about 1 to 5 kHz) where noise is most audible at low
levels.</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: Bill Whitlock 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:proaudio@bach.pgm.com">proaudio@bach.pgm.com</a> <a class="moz-txt-link-rfc2396E" href="mailto:proaudio@bach.pgm.com"><proaudio@bach.pgm.com></a><br>
Sent: Fri, Jun 11, 2021 10:03 pm<br>
Subject: Re: [ProAudio] Microphones question<br>
<br>
<div id="yiv4358600936">
<div>
<div style="color:black;font:12pt Arial, Helvetica,
sans-serif;">I'll be showing my age here, but the
EIA or Electronic Industries Association set
standards for components used in consumer gear
back in the 50s, when I was young repair tech.
They defined speaker impedance as the value on the
impedance vs frequency curve as the first minimum
after the first maximum as frequency is increased
... and the most common standard was 3.2 ohms. In
most cases, it represented the lowest impedance
over the audio frequency range.
<div><br clear="none">
</div>
<div>And I think you're right that I published
both the equivalent circuit and the impedance
curve for the SM57 in the Ballou book. Today, I
only checked the version posted at the Jensen
website, which I think is the 3rd edition. I'll
check my manuscripts for the piece in the 4th or
5th edition. If I find it, I'll post it here.
I'm guessing that there are a lot of impedance
plots that are never published by mic
manufacturers.</div>
<div><br clear="none">
</div>
<div>Bill<br clear="none">
<br clear="none">
<br clear="none">
<div class="yiv4358600936yqt7638123744"
id="yiv4358600936yqt89899">
<div style="font-family:arial,
helvetica;font-size:10pt;color:black;"><font
size="2">-----Original Message-----<br
clear="none">
From: Jim Brown via ProAudio
<a class="moz-txt-link-rfc2396E" href="mailto:proaudio@bach.pgm.com"><proaudio@bach.pgm.com></a><br
clear="none">
To: <a class="moz-txt-link-abbreviated" href="mailto:proaudio@bach.pgm.com">proaudio@bach.pgm.com</a><br clear="none">
Sent: Fri, Jun 11, 2021 8:25 pm<br
clear="none">
Subject: Re: [ProAudio] Microphones
question<br clear="none">
<br clear="none">
</font>
<div dir="ltr">On 6/11/2021 6:47 PM, Bill
Whitlock via ProAudio wrote:<br
clear="none">
> As I recall from my tests of the
SM57, its impedance varied from under <br
clear="none">
> 150 Ω at very low frequencies to over
300 Ω at resonance - and continued <br
clear="none">
> to rise at higher frequencies. I'll
try to find the data - I did the <br
clear="none">
> tests as research before writing
Jensen AN-005 about mic splitters.<br
clear="none">
<br clear="none">
I remember seeing that data, perhaps in
your chapter in the Ballou <br
clear="none">
Handbook for Sound Engineers.<br
clear="none">
<br clear="none">
To others -- it's important to realize
that the impedance of a dynamic <br
clear="none">
mic is complex, because it's equivalent
circuit is complex. Remember <br
clear="none">
that a dynamic mic is the analog of a
single driver loudspeaker. If I <br
clear="none">
remember correctly, the nominal impedance
of a mic is defined by the <br
clear="none">
manufacturer as 1/5 of the minimum load
impedance. Someone will correct <br
clear="none">
me if my memory has failed me. For
loudspeakers, it's the minimum value <br
clear="none">
of its impedance when plotted vs
frequency, and it's impedance typically <br
clear="none">
varies by at least two orders of magnitude
over its operating range.<br clear="none">
<br clear="none">
Jim Brown
<div class="yiv4358600936yqt9809924749"
id="yiv4358600936yqtfd11571"><br
clear="none">
<br clear="none">
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