[ProAudio] Microphones question

Mon Jun 14 12:49:07 PDT 2021

The interesting takeaway from that calculation is that actually given a
reasonable ADC (-97dBu self noise or so), and a reasonable mic (-120dBu
self noise or so) we only need about 30dB of front end gain going into a
modern ADC to make the mic self noise completely dominant (At least for a
condenser mic, a ribbon I am unconvinced about, I would need to do sums).

If that is in fact the case, then all the rest of any required gain can be
done digitally (post capture if need be) without in any way impacting the
noise floor, such a system would be almost immune to clipping anything
except the mic (-90dBu self noise from mic + preamp at ADC, ADC clips at
+20dBu or thereabouts, so you have a theoretical 110dB of dynamic range),
assuming the mic can swing to -10dBu and the preamp can hit +20dBu....

All of a sudden those preamps with 60dB of gain range on the pot start to
seem rather pointless, just do a fixed +30dB and hit the ADC, the mic is
dominant, and a simple multiply in the digital domain gets you any level
you like with little risk of managing to clip anything?

In fact, given that that +20dBu to drive an ADC is purely an artefact of
pro audio expectations (The actual ADC chips usually want about 2V), so you
really only need about +12dB (Which I will grant is hard to make really
quiet). I am wondering about one of those opamp

What have I missed?

On the subject of Ein, does nobody discuss Iin, particularly for a bipolar
input stage it is often a more significant noise source then Johnson once
the source gets above a few k.

Regards, another Dan.

On Mon, Jun 14, 2021 at 7:12 PM Crispin HT via ProAudio <
proaudio at bach.pgm.com> wrote:

> Hi Dan,
>
>
>
> Please excuse me if I caused offense, I was being flippant, quoting
> Molesworth (UK reference) - most analogue audio engineers at some point
> have played with mic amp circuitry and getting -128dB EIN at 60dB gain is
> very easy and affordable.
>
>
>
> This is my thought experiment for you:
>
>
>
> The maths, as you know, is straightforward: the EIN figure for preamps is
> quoted at 60dB gain, 150 ohm source and 20K bandwidth. It’s also assumed to
> be at room temp, though not often mentioned.
>
> This EIN represents an amplifier input noise of about 2.3nV root Hz.
> Because shunt feedback is used, the effective feedback resistance can
> easily be only an ohm or so (1k feedback res, 1 ohm to ground), so its
> noise is negligible.
>
> The 150R input resistor has a noise of 1.6nV root Hz, so this allows the
> amplifier voltage noise to be around 1.6 nV root Hz, which means very
> affordable op-amps/ input transistors can be used for affordable preamps.
> Some cheaper pres use a 50 ohms test source, which make it even easier to
> quote this figure.
>
>
>
> Most mics have a noise floor of about -118 to -120dBu (6nv root Hz), so to
> make an invisible mic pre ( noise wise) you just need an EIN of better than
> about 124-125dB,(3.1nV root Hz) but as Bill says, across the gain range.
>
>
>
> At 20dB gain, our simple amplifier above could have an effective feedback
> resistance of about 150R, so the input noise from the resistors is now
> 2.3nV root Hz, meaning your amplifier has to have a very quiet 0.8nV root
> Hz.  This makes it a very different beast.  So as Bill says, you need to
> build amplifiers that can drive low feedback resistances, in the analogue
> world to at least 15V.  For interfacing to converters, you can cheat a bit
> because the input levels are typically a couple of volts, but amps also
> exhibit other issues when driving lots of current out of their outputs, so
> all of this needs to be factored in.
>
>
>
> So that’s the circuitry around a mic re noise – we can design mic pres
> that do not increase the native noise of a mic. However we then need to
> think about what we are doing with these mics & pres.  Typically, we
> connect them to ADCs., with a 0dBFs of about 20dBu say.  A normal pro ADC
> has a non weighted noise of about -117dBFs, or about -97dBu.  A mic with
> 20dB of gain on it will equal this noise, and with a typical 40dB, exceed
> it.  So, the mics again become dominant.
>
>
>
> The final recording will ideally want to aim for 70-90dB of dynamic range.
> This means we want our noise floor to be about -60dBFs to -70dBFs say.
> With good modern condensers, running at 40dB gain, without gating, with our
> perfect preamps and decent converters, we can run 10-60 open mics before we
> hit these limits.
>
>
>
> I’ve approximated some of these figures, but hopefully you can get the
> drift – I don’t think there is a need to improve on the analogue noise
> floor or either mics, pres or converters, as long as we use the best
> available parts on the market today, and not the pro-sumer parts.
>
>
>
> Apologies if my sums don’t add up 😊
>
>
>
>
>
> Kind Regards
>
>
> *Crispin Herrod-Taylor**Managing Director, Crookwood*
> www.crookwood.com
>
>
> *Tel: +44 (0)1672 811 649Mobile:+44(0)7910 637 634*
>
>
>
> *Sign up for our great newsletter **here*
> <http://crookwood.com/newsletter/>*!** and keep up to date with the audio
> world*
>
>
>
> *From:* ProAudio <proaudio-bounces at bach.pgm.com> *On Behalf Of *Dan Lavry
> via ProAudio
> *Sent:* 14 June 2021 17:01
> *To:* bobkatz at digido.com; proaudio at bach.pgm.com
> *Subject:* Re: [ProAudio] Microphones question
>
>
>
> Hi Bob,
>
> I am not designing a mic pre right now, and my interest is not about
> specific micpre. I am interested in noise and distortion of the gear that
> people connect to my converters. My older Lavry Gold was mostly for
> mastering, not tracking. The new Gold (Lavry Savitr) offers very low
> latency and can be used with micpres.
>
>
>
> It would be good to  to get a sense from ear people, such as yourself,
> about what is needed in terms of noise, for the most extreme cases (highest
> gain), because that is the bottle neck of the noise floor.
>
>
>
> I am told here that any fool can come up with 128dBu ein. Some engineers I
> respect would take offense to such comment. I agree that at low gain it
> matters less. But it would be good to know if the 128-130dBu (or whatever
> the state of the art) is limited because we can't do better, or because it
> is good enough for all practical  cases. If it is limited by technology, a
> mic designer can look forward to pushing the state of the art. If it is
> good enough, there is no point in improving...
>
>
>
> If I get convinced that there is a market for -135dBu, I would consider
> doing so, right after all the other stuff I want to do. My last project
> took me 3 years. Doing  a -135dBu will take a year or more. I have other
> things on my plate.
>
>
>
> I see the new developments regarding digital microphones, based on one bit
> modulator. The cable carries one bit digital, and some very new IC's
> provide decimation via 4 or 5 pole filter. I saw the data and specs. So far
> it is far from state of the art, and aimed mostly at mass production lower
> cost stuff.
>
>
>
> I can't comment about the mic quality, but i see much data related to the
> IC's involved. one bit (at the present day upsampling rate) and 5th order
> is 90's technology. It may be fine for many uses, but the results are not
> state of the art...
>
>
>
> I do expect the digital mics IC's to improve in the future. The concepts
> are there, such as multibit and faster clock. And it may be that sigma
> delta will be surpassed. I can't predict...
>
>
>
> Did you try  to use digital mics? If so, what do you think?
>
>
>
> Regards
>
> Dan Lavry
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> Sent from Samsung Galaxy smartphone.
>
>
>
>
>
> -------- Original message --------
>
> From: Bob Katz via ProAudio <proaudio at bach.pgm.com>
>
> Date: 6/14/21 7:42 AM (GMT-08:00)
>
> To: proaudio at bach.pgm.com
>
> Subject: Re: [ProAudio] Microphones question
>
>
>
> Dan: As you can see, there are so many variables, that all we can do is an
> approximation using 150 ohms, in order to come up with an EIN value for the
> preamp, which will be an approximation. Feel free to specify alternate EIN
> values in the preamp spec sheet in addition to the widely-accepted 150
> ohms, for example, with 50 ohm source at 40 dB gain. Maybe it will help
> characterize your preamp better to the reader. Maybe someone will discover
> that all microphones should have a lower output impedance to get better
> performance.
>
> But as Scott pointed out, why not eliminate the issues and go with a
> digital output microphone. My concerns there are jitter. But there are ways
> around that with a superior PLL at the receiving end.
>
>
>
>
>
> Best wishes,
>
>
>
>
>
> Bob
>
>
>
>
>
>
>
> On 6/11/21 10:49 PM, Dan Lavry via ProAudio wrote:
>
> Hi Bill,
>
> I am not trying to change the standard. Certainly not before I have the
> understanding of what is going on. Of course in an ideal world, each mic
> would come with an impedance information. In a somewhat less ideal world,
> each mic would come with, at least, the best resistor value. But here we
> are stuck to the "typical" old dynamic mic.
>
> I know that most people don't understand technical details, and it will
> confuse customers. But I am not talking to customers here, or level playing
> field. I asked the opinion of mic experts. I want to know what real
> impedance levels are. That is really all I want to know. So far, I learned
> about the 150 Ohm relation to the old dynamic. There must be more
> information, I thought I would start here.
>
> Thanks for all the comments
>
> Dan Lavry
>
>
>
> On 6/11/2021 6:47 PM, Bill Whitlock via ProAudio wrote:
>
> How would you propose testing and specifying equivalent input noise for
> mic preamps?
>
>
>
> Using the 150 Ω dummy source at least levels the playing field, even
> though it's not accurately predictive for all mics.  Specifying e and i
> noise separately (and their own spectrum, if you're going to be rigorous)
> will further confuse buyers - most of whom can barely understand why a
> shorted input is unrealistic.
>
>
>
> 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.
>
>
>
> If you want to change this, I'd encourage you to join an AES standards
> committee and make your case.  Membership in working groups is open to
> all.  Working group SC-05-05 is currently trying to change the ways
> equipment manufacturers describe inputs and outputs - in the interests of
> avoiding interoperability issues and unexpected results.
>
>
>
> Bill Whitlock
>
> AES Life Fellow
>
> Ventura, CA
>
> -----Original Message-----
> From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
> <proaudio at bach.pgm.com>
> To: crispin at crookwood.com; proaudio at bach.pgm.com
> Sent: Fri, Jun 11, 2021 6:24 pm
> Subject: Re: [ProAudio] Microphones question
>
> Yes of course the input noise has to be taken into account AFTER
> amplification. So say a micpre has 120dBu noise (referenced to the
> input), with say 60dB gain the noise is at 60dBu. That is easy to
> measure and hear...
>
> My point of interest was not about measuring microphones. But we have
> dynamic, ribbon and condenser with phantom, and a wide range of
> implementations in each category. So the use of one value resistor seems
> to be arbitrary. I mentioned earlier that the input noise is made of
> both noise voltage and noise current components. The noise current
> (today's technology) will have low impact for 150 Ohms resistor. So why
> is the resistor there? The answer is to give us a better idea of how the
> micpre works with a mic instead of a short. And so we lump all mics into
> a simple model. A 150 Ohm resistor.
>
> I think we should remove that 150 Ohm resistor. The resistor noise is
> -130.9dBu (room T). If future technology will enable a shorted input
> micpre to reach 130dBu noise (referenced to input with acceptable gain),
> the outcome with 150 Ohm is -127.4dBu. Further down the line,135dBu
> noise (referenced to input) only improves the outcome to -129.5dBu.
>
> I just wonder if there is some information about the real impedance of
> real mics including different types relative to that 150 Ohm. It would
> be good to have some better detail...
>
> Regards
>
> Dan Lavry
>
> On 6/11/2021 1:32 PM, Crispin HT wrote:
> > I'm not a mic expert, but in designing preamps over the years, we’ve
> found that the EIN of most mics sits around the -118 to -122dB mark.
> >
> > The relevance of this, is that you need to amplify a mic, and it's noise
> to use it.  Often quiet mics have low outputs, so need to be amplified
> more, and the real test of a mic pre's EIN is not at 60dB gain, getting an
> EIN of better than -124dB at gains around the 20-40dB.
> >
> > Hope this helps.
> >
> > Kind Regards
> > Crispin Herrod-Taylor
> > Managing Director, Crookwood
> > www.crookwood.com
> > Tel: +44 (0)1672 811 649
> > Mobile:+44(0)7910 637 634
> >
> > Sign up for our great newsletter here! and keep up to date with the
> audio world
> >
> > -----Original Message-----
> > From: ProAudio <proaudio-bounces at bach.pgm.com> On Behalf Of Dan Lavry
> via ProAudio
> > Sent: 11 June 2021 20:44
> > To: proaudio at bach.pgm.com
> > Subject: Re: [ProAudio] Microphones question
> >
> > My question is about mic output impedance, in relation to noise:
> >
> > Both the mic and the micpre contribute to noise. The micpre generates
> some noise voltage which can be measured by replacing the mic with a short
> (0 Ohm). But there is also mipre generated noise current, which is no
> problem for 0 Ohm, but real mics have some impedance...
> >
> > At some point, it was decided to model a mic noise with replacing the
> mic with 150 Ohm resistor.  I am not proposing to change it, just trying to
> understand why 150 Ohm.
> >
> > The value 150 Ohm makes 1.568nV/sqrtHz (at room temp), so for 20H-20KHz
> noise voltage of .225uV. Given that we are interested in noise power, we
> can use the dBu scale to realize that the resistor itself sets a limit on
> the noise floor at -130.9dBu. But say the impedance is 1K, then we have
> -122.8dBu.
> >
> > I assume that the resistor modeling is a simplification. I would be
> interested in comments from the mic experts here.
> >
> > Thank You
> >
> > Dan Lavry
> >
> >
> > --
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