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How can I tell if a microphone with work with a given preamp?

Some preamps don't have the power for dynamic or ribbon mics. How can I tell if a certain mic is extra needy in the gain department? What number on the microphone spec sheet will tell me how much gain I need on my preamp?

I want to look at an information sheet for the mic and preamp and be able to tell what will work together reasonably well - no concern for tonal issues. What about setting jumpers on the preamp which select resistance (Ohms)?

Thanks!

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Do you have an example of a mic/amp combination that doesn't work? I had thought that the voltages were mostly agreed upon so most every mic preamp should work with most any XLR mic (assuming phantom power for condensers, etc is matched) –  Warrior Bob Mar 16 '11 at 20:14
    
From my experience, everything is labeled pretty clearly. With the pres and mics I've used, I've not noticed any significant changes with different pairings. Though, you may have a point about tones. I think the only way to detect such small variances would be with testing equipment. –  d-_-b Mar 17 '11 at 0:21
    
@Bob - The fathead and presonus digimax. Recording quiet sounds and it needs to be cranked - cranking means hissing. SM57 needs gain for doing acoustic guitar. Got a 75db preamp and they both sound great without noise from the pre. –  Sam Greene Mar 17 '11 at 1:57
    
@sims - What labels are you talking about? –  Sam Greene Mar 17 '11 at 2:04
    
Just the stuff like "48v" etc. I've never run into what you are describing. Though, isn't the SM57 dynamic? I've not used the digimax but have the firepod. I suppose the pres a similar. –  d-_-b Mar 18 '11 at 2:30

2 Answers 2

up vote 3 down vote accepted

Well, if you'd like to relate datasheet-listed sensitivity to needed gain, it's pretty straightforward math.

Standard professional line level audio is considered to be +4dBu, so we can reasonably say we want a preamp with sufficient gain to bring up the signal to this level. We just need to compute the output level of the microphone in dBu (almost surely a negative number) and subtract it from +4 to get the gain required in dB.

In case you're bothered by the unit change (from dBu to dB), just know that it's due to the nature of logarithms (and thus, dB). That is, we're comparing two signal levels to a known reference level (the u of dBu), and then comparing them to each other (the u drops out of the equation and we could have used any reference in place of u to get the same result)

We have a sound pressure wave that can be expressed in Pascals ((Pa), standard unit of pressure). The sensitivity can be expressed in (mV)/(Pa). The product of these two quantities is the number of millivolts of electrical potential in the signal sent from the microphone to the preamp in response to that many Pascals of pressure.

The wikipedia page for sound pressure has a table (http://en.wikipedia.org/wiki/Sound_pressure#Examples_of_sound_pressure_and_sound_pressure_levels) that you can check to get a feel for rough Pascal values of various sounds. For the purposes of this explanation, I'm going to pick a value at the top of the approximate range for "normal conversation at 1 m" : [2*10^-3 (Pa:RMS)]. (According to the same table, this is about [60 (dB:SPL)])

Now let's say the microphone has a (somewhat "typical") sensitivity of [10 (mV)/(Pa)]. When transducing the sound, it should send the preamp:

  • [10 (mV)/(Pa)] * [2*10^-2 (Pa:RMS)]
  • = [2*10^-1 mV:RMS]
  • = [2*10^-4 V:RMS]

To convert this into dBu, you have to take the logarithm of the ratio of the voltage you have to the reference voltage for dBu: [.7746 (V:RMS)]

  • 20 Log([2*10^-4 V:RMS]/[.7746 (V:RMS)])
  • = 20 Log(.0002/.7746)
  • = 20 * -3.588
  • = [-71.76 (dBu)]

You can also use this handy dBu calculator and reference page: http://www.sengpielaudio.com/calculator-db-volt.htm (Try typing .0002 V into the rightmost calcultaor, you'll get the same -71.76 dBu result)

The same site also has another reference page about sensitivity: http://www.sengpielaudio.com/calculator-transferfactor.htm

So, anyway, we have a signal of [-71.76 (dBu)], and we want it to be [+4 (dBu)], so:

  • [+4 (dBu)] - [-71.76 (dBu)]
  • = (4 + 71.76) (dB)
  • = [75.76 (dB)]

We need the preamp to give about 76 dB of gain to get the signal all the way up to standard pro audio level, although with standard use, you'll probably want some headroom, and may not want to give it the full 76.

If you run through the math for a few other sensitivity ratings, you can get a feel for what the numbers really mean. Doubling the sensitivity (to [20 (mV)/(Pa)]) doubles the voltage and gives a 6dB boost to the output (that's 6dB your preamp doesn't need to provide). Halving the sensitivity is a 6dB cut. This is perhaps the most useful rule of thumb to know. Consider a mic you are familiar with and find out its sensitivity, then compare other sensitivities to this one with the above rule in mind.

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Brilliant - exactly what I was after! –  Sam Greene Mar 21 '11 at 19:34
    
+1 for linking to the website of Eberhard Sengpiel. (And of course for a very good answer.) –  Pelle ten Cate Mar 27 '11 at 9:49

By their connector. In general if a mic. comes with an XLR connector, it should basically work fine with a normal preamp. (Whether it needs +48V phantom power is a different issue).

Almost all microphones that need a different power source have different connectors. Normally, the power sources that they are shipped with can be used to "convert" that connector to a standard format, so that it can be used at line level or as microphone signal.

Edit: Microphones can have different sensitivities. A SM57 for example is known for needing quite a bit of gain on a regular preamp, so for getting a clean signal on a soft source, either use a different mic. or use a preamp that can give you a lot of gain without getting noisy.

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"What number on the microphone spec sheet will tell me how much gain I need on my preamp? " –  Sam Greene Mar 17 '11 at 23:09
    
The short version: Among others, sensitivity is the most interesting thing to look at. It is displayed in mV/Pa. More mV/Pa means more sensitive mic, means stronger signal. –  Pelle ten Cate Mar 18 '11 at 15:32
    
Again, I'm looking for concrete rules on how the gain relates to sensitivity. More is better when the gain on the preamp is low, but is there a way to avoid - "Oh...shouldn't have bought this mic, my preamps can't power it, now I'm going to have to drop $500 on a preamp", instead of waiting until after you get the mic and then have to sell it or get a new preamp. As much as artists like to think this is all art, there is science happening in the background. –  Sam Greene Mar 19 '11 at 3:29

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