How Microphone Works

At some point you have probably heard someone talk about a microphone preamplifier, or sometimes called a mic pre or simply preamp.  


Well what is a mic preamp and why do you need it, and DO you need it? And why are some preamps under 50 bucks while others are over 2000, man?


Well, in this video we’ll discuss mic preamps, and we’ll hear a few examples from a couple of different models. An amplifier is a device that allows us to increase the amount of electronic signal that is present in an electronic device.Now usually when we talk about amplifiers we’re talking about speaker amplifiers – the kind of amps that allow us to produce sound through a loudspeaker. 


But in this case we’re talking about amplifying a signal from mic level to line level.  Huh?  Okay, a basic sound system looks like this – microphone,amplifier, speaker.  


The microphone captures the sound and turns it into an electronic signal. The amplifier then boosts that signal to a high enough amplitude that it can compel the speaker to move.  


But there’s problem with this.  Ifwe literally plug the microphone directly into the amplifier it wouldn’t produce much sound at all.  [muffled scream] this is because the amplifier’s input is expecting a line level signal, which means, in simplest terms, on average,around 1 volt.  


And for you pros out there I realize that the average operating level of a line level device is +4 dB 1.228 volts across a 600 ohm load, butI’m trying NOT to confuse people right now, so bear with me! 


Microphones, however, by the very nature of their design, produce a tiny, tiny signal.  Much less than 1 volt.  So in order for the amplifier to have a usable signal, the microphone signal needs to be boosted up to line level This is where the mic pre comes in.  


The microphone preamp takes the signal from the microphone and boosts it up to line level.  Now the amplifier has a usable signal that it can use to amplify through the speaker. [loud scream] Now if you’re wondering why we’re talking about sound systems and not recording, it’s because the need for mic preamps is universal.


Nearly every audio device operates at line level.  Amplifiers, mixing consoles,recorders, and outboard devices like reverb and compressors, all operate at line level.  


And the interface of your DAW operates at line level if you take a microphone and plug it directly into an input that’s expecting a line level signal, you won’t have a very strong input.  Now having said all that, you may be wondering why you don’t always need a mic preamp.For example, maybe you have a recording interface that allows you to plug a mic right in.  And what about a USB mic? 


You don’t have to plug that into a preamp.Well, in these cases, the mic pre is built in.  This Avid Mbox has two built-in mic pre’s, so it’s not necessary to add one.  But this Avid HD-IO is line level only, so if we’re going to record microphones, we have to supply the microphone preamps.This mixing console doesn’t need separate microphone preamps because they’re built into each channel.  This section up here that’s labeled gain is amic pre. 


This active speaker allows a mic input with a flip of a switch right here.And a USB mic has the mic preamp built-in as well.  In a USB mic themicrophone’s capsule, the preamp, and the analog to digital converter are all integrated together.  So mic pre’s are used anytime we are using a microphone.  


Sometimes they’re integrated within a device, sometimes not. The important thing to know is that they’re necessary, and don’t assume that this is a built-in microphone preamp just because you see a 3 pin XLR connector.  Alright, we know what they are and why we need them, so now let’s answer the nagging question about the devices themselves. 


Why are there so many different models, and why the big difference in price range? Well, we could talk about that for quite a while, but it basically comes down to two things – operational options and sound quality.


Operational options cover a wide variety of things from number of channels,whether it provides digital outputs, various controls it has like phantom power, polarity reverse, high-pass filter, and so on. But sound quality is the real kicker. 


The choice of your microphone preamp is very much like your choice of microphone. You choose them because they have a certain sound – a certain character to them that works very well with one instrument and may be not so much for another. Many engineers like to experiment with different microphone and preamp combinations to achieve different results. Assuming we have all the other puzzle pieces in place, a well-chosen microphone preamp can take the quality of your recordings to that next level. 


On the other hand, a crappy microphone and crappy acoustics are not going to be saved by a $2000 microphone preamp.  So before you go spending a lot of money, make sure you have all your other puzzle pieces in place.  


Okay, so inclosing, let’s listen to a few different mic preamplifiers on a couple of different instruments.  For our instruments we’ll listen to piano and snare drum, and for our mic preamplifiers we’ll listen to four different models.  


The Rupert Neve 5024, the Avalon2022, the built-in preamps on the Digidesign C|24 console and the Rollsmini mic preamp MP-13 which retails at about 50 bucks.  


Now, before we play these examples, are you listening on a good set of speakers or a decent set of headphones? 


If you’re listening on the little speaker on your phone [groan] you won’t be able to hear much of a difference between these examples. 


Also keep in mind that the media compression that occurs over the Internet is going to be a factor as well. Anyway, here we go! So, maybe you heard a difference, may be you didn’t.


But I encourage you to try these experiments for yourself.  If you have more than one mic preamp at your disposal,do a comparison and see which one you like better.  Well, that’s it for this Article.


If you have any questions please leave them in the comments section below.

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