[ProAudio] Microphones question

Dan Lavry dan at lavryengineering.com
Sat Jun 12 11:17:05 PDT 2021

Hi Scott,

Thanks for the information. So lets put some numbers together (room 

50 Ohm resistor only 0.127uV, -135.7dBu

150 Ohm resistor only 0.221uV, 130.9dBu

3000 Ohm resistor only 0.988uV, 117dBu

So a micpre for 50Ohm can benefit from a mic pre with pretty high noise 
current, but low noise voltage. The 3Kohm  will benefit less from the 
extra high dynamic range, and it would be best to avoid high noise current.

I know it is not easy to explain to customer, and I am not asking the 
manufacturers to specify it. It seems to me that a good micpre for 
condenser can be poor for ribbon. That 150 Ohm in the middle leaves some 
to be desired. I am not going to fight it. And thanks, you were informative.

Dan Lavry

On 6/12/2021 9:34 AM, Scott Dorsey wrote:
> Oh, we can measure the microphones and plenty of folks do.  But if you
> do that, you'll find a huge range of values.
> If I were wanting to give a number on the datasheet, I'd want to do it
> with a 50 ohm source impedance or maybe even a 22 ohm source impedance
> because that is a reasonable reflection of a typical modern condenser
> microphone.  Since most of the cases where people are dealing with low
> volume sound sources are using condenser microphones, that would seem
> appropriate.
> On the other hand, you could make the argument that your reference
> source impedance should be about 3k ohms, because that's about the
> impedance of an RCA ribbon microphone.  Since ribbon microphones have
> very low output, most of the cases where the primary noise source is
> the mike preamp are cases involving ribbon microphones.  So that would
> also seem appropriate.
> Seeing that these are both perfectly appropriate ways to measure noise
> because they both apply well in the cases where noise is a problem, I
> think it might be best to split the difference and pick something around
> 150 ohms or so to model both cases somewhat.
> --scott

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