April 09, 2012, by Dennis A. Bohn
Selecting the right preamplifier for a given microphone, or conversely, selecting the right mic for a given preamp, involves two major factors along with several minor ones.
First, the two big ones:
Input headroom. Do you have enough?
Noise. What will the preamp add to your mic?
You need to determine whether the mic, under worst-case conditions, is going to overload the preamp input stage, and also whether the preamp is going to materially degrade the noise performance of the mic.
Actually, mics have relatively few specifications. Most are sold on sound, reputation and price. Specifications rarely enter into it. Even so, enough exist to make the right decision.
Another issue is proper input impedance. The trend has been moving toward higher input impedances, with many now rated at two kilohms and higher. Because the connected impedance (i.e., mic plugged into the preamp) determines the noise performance, and the mics are low impedance (150 - 200 ohms), then there is no noise penalty for providing higher input impedances.
Yet another aspect to examine is phantom power. Is it provided? Do you need it? Is it the correct voltage, and does it source enough current for your mic? This is an area requiring informed decisions. There is a huge myth that mics sound better running from 48 volts, as opposed to, say, 12 volts, or that higher phantom power increases the dynamic range of a mic. For the overwhelming majority of microphones both of these beliefs are false.
Most condenser microphones require phantom power in the range of 12-48 VDC, with many extending the range to 9-52 VDC, leaving only a very few that actually require just 48 VDC. The reason? Internally, most designs use some form of current source to drive a low voltage zener (usually five volts; sometimes higher) that determines the polarization voltage and powers the electronics.
The significance is that neither runs off the raw phantom power; both are powered from a fixed and regulated low voltage source inside the mic. Increasing the phantom power voltage is never seen by the microphone element or electronics; it only increases the voltage across the current source. But there are exceptions, so check with the manufacturer, and don’t make assumptions based on hearsay.
Final selection details involve checking that the preamp’s gain range is enough for your use, that there are overload indicators or metering to help in set up, that the plumbing is compatible with your wiring needs, and that the color doesn’t clash with your tour jacket.
FINDING THE DATA
Determining input headroom compatibility requires knowing the mic sensitivity rating and the maximum sound pressure level (SPL) allowed. The sensitivity rating is usually the easiest and least ambiguous number to find on the data sheet, rated at 1 kHz and expressed in millivolts per pascal (mV/Pa). One pascal is the amount of pressure resulting from a loudness level of 94 dB (written as 94 dB SPL). For example, a sensitivity rating of 20 mV/Pa tells you that when a sound equal to 94 dB SPL strikes the mic element, resulting output voltage is 20 millivolts.
The sensitivity rating provides a voltage level at one reference point; now all that’s needed is the mic’s maximum SPL to calculate the maximum output voltage. Then this is compared against the maximum input voltage rating of the microphone preamp. The maximum allowed SPL is stated in several ways: maximum SPL often with a stated total harmonic distortion (THD) level; maximum acoustic input; and sound pressure level for “X” percent THD. All are variations for the same rating.
With these two specifications, it is a simple matter to calculate the maximum output level in volts and convert that into the familiar dBu units found on microphone preamp data sheets.
To make this even easier, see Table 1. To obtain the microphone maximum output level in dBu, find the mic’s sensitivity rating on the left side and then move right until directly below the mic’s maximum SPL rating.
Table 1: Mic maximum output level (dBu). (click to enlarge)
As an example, for a microphone with a sensitivity rating of 20 mV/Pa and a max SPL equal to 130 dB, Table 1 tells us that the maximum output voltage is +4 dBu. You now have what you need to compare preamps regarding maximum input level.
Another example using Table 1: block out all possibilities that could overload a specific preamp. For example, the red triangle area represents all combinations that could overload Rane’s MS 1b Mic Stage. Its maximum input level is rated at +10 dBu, therefore all microphone sensitivity and maximum SPL combinations resulting in greater than +10 dBu are excluded from consideration. Used this way, any new mics can be quickly checked for overload threat.
Caveats. Remember that this output level only occurs under the worst-case condition of SPL equaling the maximum allowed by the mic, meaning that if an application has sources that cannot achieve the maximum SPL, then the input overload requirement can be relaxed accordingly. For instance, if you know your source is never going to exceed 110 dB SPL, and your mic is rated for maximum levels of 130 dB, then you can take 20 dB off the levels shown in Table 1, widening your preamp choices considerably.
Note also that input overloading is a strong function of the preamp’s gain control setting. Most preamp manufacturers measure the maximum input level with the gain control set at minimum. This means there is a real danger that even after carefully matching the output and input levels of a mic and preamp, the mic can still overload the preamp.
This happens when the system needs the preamp gain turned up (correspondingly reducing input headroom) and the microphone is used for a wide dynamic range source. Not only do you have to worry about matching your mic and preamp, but also about real-world sources and gain settings.
INDIVIDUAL NOISE FLOORS
Microphones and preamps each have their own noise floors.
When selecting a preamp, it’s important to know to what degree the preamp’s noise degrades the noise of the mic.
Different mic technologies use different terminology to describe noise.
Dynamic microphone data sheets rarely list noise as a specification because there is no active circuitry to generate noise; there is only a magnet and a coil. This category of mic is properly called electromagnetic or electrodynamic.
Output noise is very low, so low it’s not listed. However, some noise is generated, and this can be calculated by knowing the mic’s impedance.
Obtain the dynamic microphone impedance rating from the data sheet and use Table 2 to convert that into units of dBu, A-weighted. This noise is the white noise generated by the resistance of the wire used to create the coil, plus a correction factor of 5 dB for A weighting. (This is somewhat arbitrary, as true A weighting may decrease the level anywhere from 3-6 dB depending upon the nature of the noise, but agrees with Holman’s article and measured results).
Table 2: Output noise for dynamic mics (20 Hz - 20 kHz, 20 degrees C/68 degrees F). (click to enlarge)
The noise of the measuring standard 150 ohms (200 ohms for Europe) source resistor makes a good noise reference point. In Table 2, it equates to -136 dBu (A-weighted; -131 dBu when not). This means that you cannot have an operating mic stage, with a 150 ohm source, quieter than -136 dBu (A-weighted, 20°C/68°F, 20 kHz BW). Looking at Table 2 confirms that dynamic microphones, indeed, are quiet.
Use Table 3 to compare microphone output noise with preamplifier equivalent input noise (EIN). As an example, if your dynamic microphone’s output noise equals -136 dBu, and you are considering a preamplifier with a rated EIN of -136 dBu, then the difference between them is 0 dB.
Table 3: Output noise for condenser mics (dBu). (click to enlarge)
Table 3 illustrates that this preamp with this microphone will degrade the total noise by 3 dB. That is, the combination of mic and preamp adds 3 dB noise to the total. More on how this table works shortly.
Condenser, capacitor, or more properly, electrostatic microphone technology involves a polarizing voltage network and at least a buffer transistor built into the microphone housing, if not an entire preamp/biasing/transformer network - all of which contribute noise to the output. Electrostatic microphones are quite noisy compared to dynamic designs, but are very popular for other reasons.
Different manufacturers use different terminology on their electrostatic microphone specification sheets for noise: Self-Noise, Equivalent Noise SPL, Equivalent Noise Level, Noise Floor, and just plain Noise all describe the same specification. Microphone noise is referenced to the equivalent sound pressure level that would cause the same amount of output noise voltage and is normally A-weighted.
This means the noise is given in units of dB SPL.
A noise spec might read 14 dB SPL equivalent, A-weighted, or shortened to just 14 dB-A (bad terminology, but common).
This is interpreted to mean that the inherent noise floor is equivalent to a sound source with a sound pressure level of 14 dB.
Problems arise trying to compare the mic’s noise rating of 14 dB SPL with a preamp’s equivalent input noise (EIN) rating of, say, -128 dBu. Talk about apples and oranges!
Luckily (again), tables come to the rescue. Table 4 provides an easy look-up conversion between a microphone’s output noise, expressed in equivalent dB SPL, and its sensitivity rating, in mV/Pa, into output noise expressed in dBu, A-weighted.
Using Table 4, a direct noise comparison between any microphone and any preamp is possible. The example shown by the blue column and row is for a mic with a noise floor of 14 dB SPL and a sensitivity rating of 20 mV/Pa, which translates into an output noise of -112 dBu, A-weighted.
Table 4: RMS noise summation for connected mic and preamp. (click to enlarge)
Now, time to return to Table 3. Unfiltered electronic noise, whether from a resistor, a coil, an IC, or a transistor is white noise consisting of all audible frequencies occurring randomly. Due to this randomness you don’t just add noise sources together, you must add them in an RMS (root mean square) fashion. Mathematically this means you must take the square root of the sum of the squares - which is why Table 3 is so handy - it does the RMS conversion for you.
Use Table 3 to convert a mic’s rated noise output into units of dBu. Find the difference in dB between the mic’s output noise and the preamp’s input noise. Find that difference in the left column of Table 3 and read what the preamp added noise will do to the mic’s noise in the right column.
For example, if the mic’s output noise translates into -120 dBu, and the preamp has an EIN of -127 dBu, then the difference between the mic and the preamp is -7 dB. That is, the preamp is 7 dB quieter than the microphone. Table 3, at the row marked -7 dB, tells you that this preamp will degrade the mic’s noise by only 0.8 dB. Looking at Table 4 tells us that after about a 10 dB difference, the noise added by the preamp becomes insignificant.
Similar to Table 1, you can use Table 4 to map out a preamp’s A-weighted noise to show the combinations that add insignificant noise. If you use a -10 dB difference figure as a guide, then the preamp’s noise amounts to less than 0.4 dB increase.
The red-shaded triangle area in Table 4 shows an example of this. The areas not shaded represent all possible combinations of mic sensitivity and noise specifications that can be used with Rane’s MS 1b Mic Stage, for instance, and add less than 0.4 dB of noise.
If you allow 1 dB net added noise, then even more combinations are possible. (The shaded area is figured by taking the EIN of the MS 1b at -128 dBu, reducing it to -133 dBu with the 5 dB factor for A weighting, and using the -10 dB difference found in Table 3 for 0.4 dB added noise, resulting in all combinations less than -123 dB being blocked out.)
The author would like to point out that this note was inspired by an article authored by Tomlinson Holman, published in September 2000 Surround Sound Professional magazine, titled “Capturing the Sound, Part 1: Dynamic Range.”
Dennis Bohn is a principal partner and vice president of research & development at Rane Corporation. He holds BSEE and MSEE degrees from the University of California at Berkeley. Prior to Rane, he worked as engineering manager for Phase Linear Corporation and as audio application engineer at National Semiconductor Corporation. Bohn is a Fellow of the AES, holds two U.S. patents, is listed in Who’s Who In America and authored the entry on “Equalizers” for the McGraw-Hill Encyclopedia of Science & Technology, 7th edition.