[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Sun Jun 13 18:41:57 PDT 2021

At the risk of belaboring the point, extremely low-noise mic preamps were built using quality PNP bipolar transistors such as the 2N4403 or 2N4124, both having e-noise of 1 nV per root Hz across the audio band and corresponding i-noise making their optimum source impedance around 200 Ω to 800Ω.  Today's transistors, even including the LM394 (50 NPN transistors connected as one) won't do any better.  Granted, if your design world is ICs or FETs, they've gotten way better.  But mic preamps, built with today's "state-of-the-art" transistors and "gee-whiz" circuit topologies are insignificantly quieter than a good one built 50 years ago.  I submit that a single low-noise PNP transistor is the "state-of-the-art" input device (PNP devices are inherently quieter).  While differential pairs and op-amps make circuit design simpler (you don't have to deal with those pesky h-parameters for one), simply using a differential pair increases noise over that of a single device.  I'm often dismayed by today's convenience-driven "cut and paste" circuit designers.
But I do agree that, if one is to accurately predict noise performance of any given mic and preamp, both the reactance and resistance of the mic are necessary.
Bill Whitlock

-----Original Message-----
From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com
Sent: Sun, Jun 13, 2021 5:33 pm
Subject: Re: [ProAudio] Microphones question

 I agree that "basic principles" stay the same. But I think that a lot has changed. In 1971, I met sales reps from semiconductor, resistor, name it companies, and they left behind tons of data books and specs. Today, I can get a data sheet (internet) in seconds or a minute... The materials are better, the process is tighter, the test equipment is better and more affordable. I keep reading everyday, articles and about product introductions. One can wait for technology to advance, and it has since 1971. We can get better results as technology moves forward. But yes, I would agree that the basic principles of networks, components, theorems and circuits are the same. I know that, I am a circuit designer... 
  Regards Dan Lavry
This is all well-known and much written about - and well explored territory - and was when I got into pro audio in 1971 at Quad-Eight, and at Jensen dug even deeper to understand how input transformers can greatly benefit noise performance of vacuum tubes, for example. Nothing has really changed ... 
  Bill Whitlock AES Life Fellow
 -----Original Message-----
 From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
 To: proaudio at bach.pgm.com
 Sent: Sun, Jun 13, 2021 2:25 pm
 Subject: Re: [ProAudio] Microphones question
 I guess I should be more precise:
 1. Measure noise voltage with a short (with gain).
 2. Measure with 100K (with gain), remove (compute) the resistor noise 
 and noise voltage, leaves i*R where i is noise current.
 Clearly the sums and differences are not add linearly, you do sqrt of 
 the "sum or difference" square...
 That would yield noise voltage and noise current of the micpre, and 
 leaves the mics out of it.
 Dan Lavry
 On 6/13/2021 1:00 PM, Dan Lavry via ProAudio wrote:
 > Hi again,
 > Let me get away from sales and what people say, and back to the 
 > technical stuff:
 > Some here suggested to look at the mic output impedance to tell us 
 > about noise. That is only true for the mic pre noise current 
 > component. And in most real world cases, that is the smallest 
 > component. I think there is some confusion regarding the noise 
 > generated by the mic pre, it does not directly relates to output 
 > impedance. Say some noise is due to a PNP transistor, how does that 
 > relates to some output inductor? The combined causes of noise in a 
 > analog circuit is complex, circuit dependent and component dependent. 
 > The question is how to model it.
 > I would start by using 100KOhm (not 150 Ohm), with gain it is enough 
 > noise to be measured well. That noise (divide by gain) is due to 
 > current noise (the voltage component is negligable). Now I will go 
 > back to a short and measure the noise voltage of the mic pre (of 
 > course gain is needed). Now you have in and en (noise current and 
 > noise voltage).
 > That is what I want to know. I agree, it is difficult to translate to 
 > the consumer. I am not going to insist on much. Just turned 76, got my 
 > own problems. fs=24KHz would work fine. Youtube can have a narrow 
 > bandwidth for the old, you add lossless compression and a 4KHz 
 > wireless connection to the hearing aid...
 > Dan Lavry
 > On 6/13/2021 5:06 AM, Scott Dorsey via ProAudio wrote:
 >>> Scott Dorsey writes=20
 >>>> In a perfect world the lowest noise would be when the input 
 >>>> impedance =
 >>> of the
 >>>> preamp matches the output impedance of the microphone, ....
 >>> Um, no. Your perfect world would need to redefine a lot of other =
 >>> parameters for that to be so. A typical emitter-follower microphone =
 >>> output may have an output impedance of 100 ohms but if you load it 
 >>> with =
 >>> that value the distortion will be significant, the output level 
 >>> severely =
 >>> limited. Noise performance depends as Bill mentioned on the operating =
 >>> point where current and voltage noise of the input stage is optimal 
 >>> for =
 >>> the source impedance of the microphone. This is why IEC 60268-4 
 >>> requires =
 >>> specification of both output impedance and minimum load impedance, 
 >>> which =
 >>> is typically 10-20x the output impedance.=20
 >> This is all true, unfortunately.  It is far from a perfect world.
 >> I'm waiting for a noiseless transformer that gives me free voltage 
 >> gain and
 >> I'm not expecting to see one any time soon.  But once I get one I'll be
 >> able to get the largest possible signal into an input in order to 
 >> swamp the
 >> noise.
 >> I'll point out that specifying minimum load impedance is sufficient for
 >> a condenser microphone but that a dyanmic microphone should also be 
 >> specified
 >> for maximum load impedance.  In some cases with weak coupling where the
 >> electrical load does not make a large proportion of the total damping 
 >> that
 >> maximum might be infinite, but not all microphones are like that.
 >>> Dan is pointing out that one needs to know the output impedance of 
 >>> the =
 >>> microphone to design an optimal preamp input stage, and this 
 >>> information =
 >>> is seldom supplied. There are preamp designs that don=E2=80=99t play =
 >>> well with very low output impedance mics, for instance, because of 
 >>> their =
 >>> negative feedback structure (applying NFB to the input stage in 
 >>> parallel =
 >>> with the input signal.) There are many mics that don=E2=80=99t do 
 >>> well =
 >>> when loaded with anything less than about 1500 ohms, and if you 
 >>> parallel =
 >>> a few consoles at a venue without considering this, performance may 
 >>> be =
 >>> impacted. Performance may also be different depending on impedance 
 >>> from =
 >>> each side of the input to ground, which may be related to the =
 >>> differential output impedance, or not.=20
 >> This is all true, and it's why some preamps perform better with some
 >> microphones while other preamps perform better with other microphones.
 >> But nobody sells an "optimized for condenser microphones" or "optimized
 >> for moving coil dynamic microphones" preamp although there are a 
 >> couple of
 >> "optimized for ribbon microphones" preamps out there.
 >> --scott
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