I was having a conversation with a friend the other day about the sound of a digital console. While that does sound a bit crazy, and aside from the processing algorithms involved with dynamics and summing that can characterize and color the sound, the real sound of a digital all consoles comes from the input gain stage; the preamp. Coincidently enough, I found this article from Rane Audio talking about the importance of matching microphones and preamplifiers.
This is actually something we take for granted. When gathering specifications on a new system, we just think that the most math-heavy end will be the end of the signal chain: power amplifiers to speakers. But that’s the easy part – amplifier and speaker manufacturers speak the same language when publishing specifications. Microphone and preamp manufacturers do not.
Two things often not though about in the selection process is what are the impedances involved? Many preamps allow high input impedances, but the one that matters is the actual impedance of the microphone, which is relatively low. This scenario does not add any extra noise, but going in a reverse manner could.
A second thought concerns phantom power. Do you need it and does a certain preamp provide it, and at what voltages?
And finally most importantly: headroom. Is your mic’s dynamic range higher than what a preamp can handle? This is where the math comes in since there is no one unit of measure that will clearly give a concise answer to this big question. It starts with knowing the sensitivity of a microphone and its maximum source SPL. The sensitivity is expressed as millivolts per pascal, which equates to a certain amount of voltage output when a microphone receives 1 kHz at 94 dB SPL (which is one pascal). So through mathematical comparisons using logrithms, you can find what the maximum voltage output is for a microphone. With that known voltage, you can convert backwards into dBu and compare apples to apples with the input specification for a preamp.
This method is the most straightforward answer to ensure that your microphone won’t clip the input of the preamp. However, this figure comes by using a microphones maximum source SPL. You may be content with accepting this number as the ultimate purchasing decision towards a preamp, but in the case of a vocal microphone, you can actually stretch out the truth. For instance, say a certain microphone has a sensitivity of 20 mV/Pa and a maximum SPL of 150 dBSPL. In the case of some (extremely cheap) preamp, 20 mV/Pa at 150 dBSPL might actually be too high. But thinking realistically, 150 dBSPL is an outrageous sound-pressure level that the human voice is incapable of reproducing. So you can actually get away with skewing the numbers in situations like this. But just be sure you remember that for the next you go for the same setup when micing a kick drum.
One last thing that can be more annoying than problematic is noise. Back to what I was saying earlier: different impedances can cause noise. In the case of dynamic microphones, the higher the impedance, the louder the noise-floor. However, condensers have active circuitry, which will induce noise into the signal. As part of a condenser’s spec sheet, noise is listed as being equivalent to a certain dB SPL, A-weighted, which is usually shortened dB-A. By using the same method as above, you can compare the millivolt output of a microphone to its noise-floor equivalent dB SPL to find the actual noise floor expressed in dBu.
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