[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Mon Jun 14 11:03:08 PDT 2021

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. 
RegardsDan 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 call http://mikeriversaudio.wordpress.com _______________________________________________
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