<div style="color:black;font: 12pt Arial, Helvetica, sans-serif;">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 ...
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<div>Bill Whitlock</div>
<div>AES Life Fellow<br>
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<div style="font-family:arial,helvetica;font-size:10pt;color:black"><font size="2">-----Original Message-----<br>
From: Dan Lavry via ProAudio <proaudio@bach.pgm.com><br>
To: proaudio@bach.pgm.com<br>
Sent: Sun, Jun 13, 2021 2:25 pm<br>
Subject: Re: [ProAudio] Microphones question<br>
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<div dir="ltr">I guess I should be more precise:<br clear="none"><br clear="none">1. Measure noise voltage with a short (with gain).<br clear="none"><br clear="none">2. Measure with 100K (with gain), remove (compute) the resistor noise <br clear="none">and noise voltage, leaves i*R where i is noise current.<br clear="none"><br clear="none">Clearly the sums and differences are not add linearly, you do sqrt of <br clear="none">the "sum or difference" square...<br clear="none"><br clear="none">That would yield noise voltage and noise current of the micpre, and <br clear="none">leaves the mics out of it.<br clear="none"><br clear="none">Regards<br clear="none"><br clear="none">Dan Lavry<br clear="none"><br clear="none">
<div class="yqt4701292098" id="yqtfd89738"><br clear="none">On 6/13/2021 1:00 PM, Dan Lavry via ProAudio wrote:<br clear="none">> Hi again,<br clear="none">><br clear="none">> Let me get away from sales and what people say, and back to the <br clear="none">> technical stuff:<br clear="none">><br clear="none">> Some here suggested to look at the mic output impedance to tell us <br clear="none">> about noise. That is only true for the mic pre noise current <br clear="none">> component. And in most real world cases, that is the smallest <br clear="none">> component. I think there is some confusion regarding the noise <br clear="none">> generated by the mic pre, it does not directly relates to output <br clear="none">> impedance. Say some noise is due to a PNP transistor, how does that <br clear="none">> relates to some output inductor? The combined causes of noise in a <br clear="none">> analog circuit is complex, circuit dependent and component dependent. <br clear="none">> The question is how to model it.<br clear="none">><br clear="none">> I would start by using 100KOhm (not 150 Ohm), with gain it is enough <br clear="none">> noise to be measured well. That noise (divide by gain) is due to <br clear="none">> current noise (the voltage component is negligable). Now I will go <br clear="none">> back to a short and measure the noise voltage of the mic pre (of <br clear="none">> course gain is needed). Now you have in and en (noise current and <br clear="none">> noise voltage).<br clear="none">><br clear="none">> That is what I want to know. I agree, it is difficult to translate to <br clear="none">> the consumer. I am not going to insist on much. Just turned 76, got my <br clear="none">> own problems. fs=24KHz would work fine. Youtube can have a narrow <br clear="none">> bandwidth for the old, you add lossless compression and a 4KHz <br clear="none">> wireless connection to the hearing aid...<br clear="none">><br clear="none">> Dan Lavry<br clear="none">><br clear="none">><br clear="none">> On 6/13/2021 5:06 AM, Scott Dorsey via ProAudio wrote:<br clear="none">>>> Scott Dorsey writes=20<br clear="none">>>>> In a perfect world the lowest noise would be when the input <br clear="none">>>>> impedance =<br clear="none">>>> of the<br clear="none">>>>> preamp matches the output impedance of the microphone, ....<br clear="none">>>> Um, no. Your perfect world would need to redefine a lot of other =<br clear="none">>>> parameters for that to be so. A typical emitter-follower microphone =<br clear="none">>>> output may have an output impedance of 100 ohms but if you load it <br clear="none">>>> with =<br clear="none">>>> that value the distortion will be significant, the output level <br clear="none">>>> severely =<br clear="none">>>> limited. Noise performance depends as Bill mentioned on the operating =<br clear="none">>>> point where current and voltage noise of the input stage is optimal <br clear="none">>>> for =<br clear="none">>>> the source impedance of the microphone. This is why IEC 60268-4 <br clear="none">>>> requires =<br clear="none">>>> specification of both output impedance and minimum load impedance, <br clear="none">>>> which =<br clear="none">>>> is typically 10-20x the output impedance.=20<br clear="none">>> This is all true, unfortunately. It is far from a perfect world.<br clear="none">>><br clear="none">>> I'm waiting for a noiseless transformer that gives me free voltage <br clear="none">>> gain and<br clear="none">>> I'm not expecting to see one any time soon. But once I get one I'll be<br clear="none">>> able to get the largest possible signal into an input in order to <br clear="none">>> swamp the<br clear="none">>> noise.<br clear="none">>><br clear="none">>> I'll point out that specifying minimum load impedance is sufficient for<br clear="none">>> a condenser microphone but that a dyanmic microphone should also be <br clear="none">>> specified<br clear="none">>> for maximum load impedance. In some cases with weak coupling where the<br clear="none">>> electrical load does not make a large proportion of the total damping <br clear="none">>> that<br clear="none">>> maximum might be infinite, but not all microphones are like that.<br clear="none">>><br clear="none">>>> Dan is pointing out that one needs to know the output impedance of <br clear="none">>>> the =<br clear="none">>>> microphone to design an optimal preamp input stage, and this <br clear="none">>>> information =<br clear="none">>>> is seldom supplied. There are preamp designs that don=E2=80=99t play =<br clear="none">>>> well with very low output impedance mics, for instance, because of <br clear="none">>>> their =<br clear="none">>>> negative feedback structure (applying NFB to the input stage in <br clear="none">>>> parallel =<br clear="none">>>> with the input signal.) There are many mics that don=E2=80=99t do <br clear="none">>>> well =<br clear="none">>>> when loaded with anything less than about 1500 ohms, and if you <br clear="none">>>> parallel =<br clear="none">>>> a few consoles at a venue without considering this, performance may <br clear="none">>>> be =<br clear="none">>>> impacted. Performance may also be different depending on impedance <br clear="none">>>> from =<br clear="none">>>> each side of the input to ground, which may be related to the =<br clear="none">>>> differential output impedance, or not.=20<br clear="none">>> This is all true, and it's why some preamps perform better with some<br clear="none">>> microphones while other preamps perform better with other microphones.<br clear="none">>><br clear="none">>> But nobody sells an "optimized for condenser microphones" or "optimized<br clear="none">>> for moving coil dynamic microphones" preamp although there are a <br clear="none">>> couple of<br clear="none">>> "optimized for ribbon microphones" preamps out there.<br clear="none">>> --scott<br clear="none">>> _______________________________________________<br clear="none">>> ProAudio mailing list<br clear="none">>> <a shape="rect" ymailto="mailto:ProAudio@bach.pgm.com" href="mailto:ProAudio@bach.pgm.com">ProAudio@bach.pgm.com</a><br clear="none">>> <a shape="rect" href="http://bach.pgm.com/mailman/listinfo/proaudio" target="_blank">http://bach.pgm.com/mailman/listinfo/proaudio</a><br clear="none">>><br clear="none">><br clear="none"><br clear="none">-- <br clear="none">This email has been checked for viruses by Avast antivirus software.<br clear="none"><a shape="rect" href="https://www.avast.com/antivirus" target="_blank">https://www.avast.com/antivirus</a><br clear="none"><br clear="none">_______________________________________________<br clear="none">ProAudio mailing list<br clear="none"><a shape="rect" ymailto="mailto:ProAudio@bach.pgm.com" href="mailto:ProAudio@bach.pgm.com">ProAudio@bach.pgm.com</a><br clear="none"><a shape="rect" href="http://bach.pgm.com/mailman/listinfo/proaudio" target="_blank">http://bach.pgm.com/mailman/listinfo/proaudio</a><br clear="none"></div>
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