[ProAudio] Microphones question

Mon Jun 14 19:48:21 PDT 2021

We are talking about audio fiction, so here we go:

A new (year old) TI AD IC, the PCM6240-Q1, on figure 37 of the data 
sheet states:

Figure37 and Figure38 show how to connect a DC-coupled microphone for a 
differentialand single-endedinput,respectively.Thevalue of the 
externalbias resistor,R1, must be appropriately chosen basedupon 
themicrophone impedance. For a differential input,the value of the 
externalbias resistor is recommendedto beused for half of the microphone 
impedance,whereas for a single-ended input,the externalbiasresistor is 
recommendedto be the same as the microphone impedance.

They are advocating matching the  load to the mic output impedance 
(obviously for maximum power transfer). That optimal setup is of course 
only true if the mic could support such load while "behaving well" 
(linearly, no clipping or other issues).

I do respect TI. It is a good company with good reputation. So can we 
sort out facts from fiction? I think the answer is simple, but I will 
let you experts debate it if you wish. I am logging off for now, I am 
starting to feel like an old guy talking too much, and this "mic 
loading" is a whole other issue...

Regards

Dan Lavry

On 6/14/2021 3:48 PM, Bill Whitlock via ProAudio wrote:
> The following article, "11 Myths about Analog Noise Analysis," is a 
> great read for most of the topics raised in this thread:
>
> https://www.analog.com/en/technical-articles/11-myths-about-analog-noise-analysis.html#
>
> Bill Whitlock
>
>
> -----Original Message-----
> From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
> To: David Josephson <dlj at josephson.com>; proaudio at bach.pgm.com
> Sent: Mon, Jun 14, 2021 1:14 pm
> Subject: Re: [ProAudio] Microphones question
>
> Yes, we have to pick our battles. The 150 Ohm that is talking about 
> has near 0 DC.
> But the 6.8K resistor to 48V phantom your mic is connected to has 
> DC... that is where material may matter.
> Yes, there is more to learn every day...
>  Regards
> Dan Lavry
>
>
>
>
> Sent from Samsung Galaxy smartphone.
>
>
> -------- Original message --------
> From: David Josephson via ProAudio <proaudio at bach.pgm.com>
> Date: 6/14/21 12:11 PM (GMT-08:00)
> To: proaudio at bach.pgm.com
> Subject: Re: [ProAudio] Microphones question
>
> The microphone measurement standard recommends the use of A-weighted rms
> noise specifications in addition to ITU-R BS.468-4, which specifies not
> only a frequency weighting curve but a quasi-peak detector which
> penalizes for low frequency pops and clicks, which can be a factor for
> some poorly executed microphone designs.
>
> Yes, even some of the experts in the standards world talk about "low
> noise" resistors. We have to pick our battles.
>
> On 6/14/2021 12:01 PM, Dan Lavry via ProAudio wrote:
> > Excess noise is important in some cases, and it is material 
> dependent. I
> > agree that the 150 for the noise measurement can be "any: 150 Ohm.
> >
> > Regarding "A weighting", I just publish both figures. In most cases it
> > is 2dB difference.
> >
> > Regards
> >
> > Dan Lavry
> >
> > On 6/14/2021 11:03 AM, Bill Whitlock via ProAudio wrote:
> >> Dan, I'm glad you caught that one!  Noise is a "stand alone" that
> >> needs no reference except a unit of measure - and, as frequently
> >> omitted by marketing types, a stated bandwidth.  And I have no problem
> >> in using weighted figures as long as it's clearly stated.
> >>
> >> In my mind, the biggest barrier to understanding noise and its
> >> implications is the lies, distortions, and half-truths perpetrated by
> >> marketing folks!  A measurement, with test conditions and references
> >> fully disclosed, is not subject to interpretation.  Sadly, many folks
> >> are "educated" by such misleading information and will believe, for
> >> example, that the 150 Ω resistor used for testing preamp noise must be
> >> a special "low-noise" resistor.  In fact, thermal noise has no
> >> dependence on the resistive material at all.  So-called "low-noise"
> >> resistors are low in "excess noise" that occurs when DC current flows
> >> in it - and there's a huge difference among those!  Anyone whose ever
> >> heard me lecture is well aware that no love is lost between me and
> >> marketing people.
> >>
> >> Bill Whitlock
> >>
> >>
> >> -----Original Message-----
> >> From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
> >> To: Mike Rivers <mm1100 at yahoo.com>; proaudio at bach.pgm.com
> >> Sent: Mon, Jun 14, 2021 9:27 am
> >> Subject: Re: [ProAudio] Microphones question
> >>
> >> I think you are confusing things. EIN does not need to be measured at
> >> full scale, and it is a real representative of the noise contributions.
> >>
> >> Micpre noise is low and not easy to measure directly. Say you test
> >> system can measure down to 1uV, but your signal is 0.1uV. It would be
> >> difficult to measure the noise directly. Say you set the gain at
> >> 60dB, the noise will become 100uV, and a 1uV system can measure that.
> >> At 40dB gain you get 10uV, still good enough to measure. In the first
> >> case Ein is 100uV/1000, in the second case 10UV/100, both yield 0.1uV
> >> which is the real input noise.
> >>
> >> Of course that will not hold if the design is poor. Any reasonable
> >> design will yield very similar ein at 60dB and 40db.
> >>
> >> Regards
> >> Dan Lavry
> >>
> >>
> >>
> >>
> >> Sent from Samsung Galaxy smartphone.
> >>
> >>
> >> -------- Original message --------
> >> From: Mike Rivers via ProAudio <proaudio at bach.pgm.com>
> >> Date: 6/14/21 8:07 AM (GMT-08:00)
> >> To: proaudio at bach.pgm.com
> >> Subject: Re: [ProAudio] Microphones question
> >>
> >> How about a little discussion on the value of knowing the EIN of a
> >> preamp and how the figure is useful to the designer?
> >>
> >> Here's why I ask:
> >>
> >> When I was writing reviews regularly, EIN was (and still is) often
> >> quoted in the product's specs. The numbers ranged from -125 dBu (just
> >> a couple) to a majority being either -127 or -128 dBu. Manufacturers
> >> liked to tout it because it was a nice low number with "noise" in its
> >> name. And it was always measured at a level within a dB or so of
> >> clipping since that's it looked the best.
> >>
> >> But unless I missed the appropriate chapter, EIN is a calculated value
> >> - the measured noise level with the gain subtracted out. So a preamp
> >> with 60 dB of gain that advertised EIN=-128 dBu could be expected to
> >> put out -68 dBu of noise, measured, of course to the advantage of the
> >> marketing department with the appropriate input termination (0, 100,
> >> or 150 ohms usually) and output load.
> >>
> >> So, among design engineers, what's the big deal about EIN? Is there a
> >> better way of measuring it that's more meaningful? And if you can
> >> squeeze another dB of EIN out of a design, how significant will this
> >> be to the user?  I
> >>
> >> , know "it depends."
> >>
> >> --
> >> For a good time callhttp://mikeriversaudio.wordpress.com 
> <http://mikeriversaudio.wordpress.com/>
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