[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Sat Jun 12 13:54:51 PDT 2021


Your probably aware of it, but there's a great Original National Semiconductor application note AN104, now re-issued under the TI umbrella, at:
https://www.ti.com/lit/an/snva515c/snva515c.pdf?ts=1623508150433&ref_url=https%253A%252F%252Fwww.google.com%252F
that thoroughly explores this subject.  The impedance of the mic is important only because it converts the preamp's current noise into additional uncorrelated voltage noise.
Bill Whitlock


-----Original Message-----
From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com
Sent: Sat, Jun 12, 2021 7:42 am
Subject: Re: [ProAudio] Microphones question

 Hi again, 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*R* BW). It is NOT sqrt(4*K*T*Z*BW)  where R is a pure resistor, Z is the impedance. 
  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...). 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 higher impedance in series, against say 6.8Kohm Phantom load resistors, actually attenuates the noise and the signal.    
  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. 
  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. 
  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. 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... Regards Dan Lavry
  
  
  
   
  On 6/11/2021 11:46 PM, Bill , via ProAudio wrote:
  
 
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: 
 (capsule) =12 Ω and 157 µH in series, feeding the primary of a transformer = 1:4.5 ratio, DCRs 1.2 Ω pri and 24 Ω sec, sec L = 2.75 mH which, impedance converted through the transformer, becomes equivalent to 6 mH in series with 300 Ω across pins 2 and 3 (each having 17 pF of capacitance to the case. The measured 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. 
  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. 
  Bill Whitlock AES Life Fellow Ventura, CA
 
 
 -----Original Message-----
 From: Bill Whitlock via ProAudio <proaudio at bach.pgm.com>
 To: proaudio at bach.pgm.com <proaudio at bach.pgm.com>
 Sent: Fri, Jun 11, 2021 10:03 pm
 Subject: Re: [ProAudio] Microphones question
 
   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. 
  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. 
  Bill
 
 
  -----Original Message-----
 From: Jim Brown via ProAudio <proaudio at bach.pgm.com>
 To: proaudio at bach.pgm.com
 Sent: Fri, Jun 11, 2021 8:25 pm
 Subject: Re: [ProAudio] Microphones question
 
  On 6/11/2021 6:47 PM, Bill Whitlock via ProAudio wrote:
 > 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.
 
 I remember seeing that data, perhaps in your chapter in the Ballou 
 Handbook for Sound Engineers.
 
 To others -- it's important to realize that the impedance of a dynamic 
 mic is complex, because it's equivalent circuit is complex. Remember 
 that a dynamic mic is the analog of a single driver loudspeaker. If I 
 remember correctly, the nominal impedance of a mic is defined by the 
 manufacturer as 1/5 of the minimum load impedance. Someone will correct 
 me if my memory has failed me. For loudspeakers, it's the minimum value 
 of its impedance when plotted vs frequency, and it's impedance typically 
 varies by at least two orders of magnitude over its operating range.
 
 Jim Brown 
 
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