It goes like this: If you're recording with a microphone then you need to use a preamplifier, no questions asked.
It is 100% possible to record without one and you have the same percentage of a chance to get horrible results. The reason I'm so adamant about this is that I recorded my first 30 songs or so without one because I had no clue what I was doing. I was more eager to record my art than to learn the technical aspects of how to do it properly.
The funny part about all of this is that preamps are hidden everywhere. They are built into mixers. They're built into USB mics (for the love of all that is good and holy, don't use one of these). They're built into even a cheap audio interface. Some soundcards even have them. But they're so invisible most of the time that when a newbie starts recording on his or her own, they don't even realize they exist and end up not using one at all!
Obviously they exist for a reason. The question is why? What do they do? Can they do their job poorly? Can they do their job really darn good? What happens if you skip one? You've got the questions and I've got the answers...
The preamp exists due to a characteristic of all microphones... They record a mic-level signal.
We were saying, microphones record their signals in at mic-level. What happens is that acoustic waves come vibrating through the air and jiggle a diaphragm back and forth. This moves a magnet in a wire coil and this generates an electrical signal. But this signal is very weak, so weak that it gets it's own name... mic-level.
Now, all of our recording gear from compressors, equalizers, analog-to-digital converters, the works... it all expects a line-level signal. A line-level signal is at a much higher voltage, which is to say a louder volume. This is the type of signal coming out of electric guitars and keyboards for instance.
So the challenge is to raise a mic-level signal up to a line-level signal. This is the basic purpose of preamplifiers.
That's the basic reason we use preamps.
The first and foremost goal of a preamp is to raise the volume of a mic-level signal and to do this cleanly. You want to boost the signal without raising the noise floor and other non-problems that get picked up along the way like electrical hum.
Pre's that boost your signal cleanly to perfectly reproduce the sound the microphone recorded are called Transparent.
This is the problem with NOT using a preamp. You can run the signal into an instrument input or into a soundcard and just crank up the input gain as high as you can and get a useable signal to record into your digital audio workstation. But you're cranking up the volume of all the noise too. The noise floor turns up with the desired signal. You get a very high signal-to-noise ratio.
Starting with your acoustic treatment, then microphone, and then preamp, you can control your desirable signal while maintaining a minimal amount of noise through gain staging. That's part of the job of a recording engineer.
So back to it. Transparency is created through solid state electronics, but old preamplifiers used tube technology (just like cathode ray tube televisions did). Some newer solid state pre's use transformers.
Pre's that use transformers and vacuum tubes are designed to raise your signals volume while imparting a specific Color.
Coloration, Color, Flavor... these are all terms related to a warmth that is imparted to your signal as it passes through the tubes or transformers. What happens is a very pleasant distortion is applied to the signal. This is a harmonic distortion based on the signal itself at very low volumes and lower frequencies that provide a sense of "warmness" to the signal.
Manufacturers have mastered the art of creating transparent preamps (not that they all are willing to spend the money on the right electronic parts to do it). The big boys that already have their transparent models are also providing colorful models.
The thing is, this is a very subtle feature that 99% of listeners won't detect. If you're out looking to buy the best mic preamp possible and encounter the conversation about coloration, don't worry about it at all. Worry about base quality first. At the top level, flavors are just very similar and very subtle preferences for people to argue about (us studio engineers have learned to hear every peculiarity!).
We know what preamplifiers do in general, but how do we get them to do it?
Most preamps have two types of inputs. The main one is the XLR input that accepts the cable that runs from your microphone and the other is a TRS input that accepts the cable that runs from your electric instrument (keyboard, guitar, bass, etc.). You'll see the second kind labeled as "Instrument" or "Hi-Z," with the second label being an abbreviation for "high impedance." We'll talk about impedance shortly.
You can't just plug in your mic or instrument and magically you're at the right volume. You need to perform gain staging. Each piece of studio equipment expects to receive a signal coming in at a certain level and that includes preamps, compressors, EQ's, and most importantly your analog-to-digital converters. This is what the Gain knob is for.
Gain is the ratio of your output to your input. So the higher you turn the gain knob, the louder your volume becomes. This is you telling the preamp how high the signal needs to be boosted. Some pre's have lights that show you what level you're operating at, otherwise you need to monitor your levels inside of your DAW. You'll also see knobs called Trim which is essentially the same thing as Gain.
There are a few other switches, buttons, and knobs you'll see. On solid state preamps with transformers and on vacuum tube preamps you'll often find an impedance knob. This changes the amount of harmonic distortion your signal will receive, which is the pleasant color saturation we talked about above. Some don't give you this option. It's pretty subtle either way.
Finally, you'll see buttons for things like Invert, which just flips the phase of the signal. This is helpful when you're recording a stereo signal like a double-mic'd acoustic guitar and you're getting constructive or deconstructive phase issues. You'll invert either the left or right channel and that typically solves it. You'll see the ability to deliver 48 volts of phantom power as well. A lot of mics need this and the preamp delivers it. Some microphones have their own built-in or external power source though, so in that case you'd turn off the phantom power.
If you're unfamiliar with different outboard studio gear, you'll wonder why so many different preamplifiers have various shapes. Some are weird shaped boxes that sit on your desktop, while others are long and horizontal that get mounted into a rack of other equipment. Then you have the lunchbox that accepts 500 Series shaped equipment, which can be racked or sit on your desk!
Above is a single channel desktop preamp. There are lots of musicians, such as rappers or singers who only need one high quality channel for their vocals. They don't end up with a lot of other equipment other than a preamp and a mic, so they are happy to have it sit right there on their desk.
For anyone collecting several pieces of studio gear of various types, it makes sense to purchase the rackmount form factor. The width of the chassis and the holes on the side are standardized for 19-inch rack rails. This is what I do, because I have compressors, EQ's, and interfaces as well.
Finally, there is also the 500 Series units. They are racked vertically into a "lunchbox" that often has a handle as pictured above on the left. There are lunchboxes with four slots, six slots, and eight slots. The widest ones can have rack ears attached and be converted to a rackmount style.
500 Series styles are cool because they are less expensive, only after you've paid for a lunchbox though. They are cheaper because they are single channel and only have a faceplate versus a giant chassis. This is good for those who want to collect a lot of different options. You can create your own channel strip if you want by adding in gear of all types into your lunchbox.
Preamplifiers generally come in three configurations:
You've seen the single channel types above in the desktop format and the 500 style. Dual channels are desirable so you can record stereo signals on the same electronics. What I haven't shown you yet are the types that feature four channels or even eight or 16! This is your most cost effective way to collect enough channels to record an entire band live or for a concert. The problem with these is that most of the time you're getting mediocre preamps.
I mean, the really good ones can cost $1,000 per channel. Now multiply that by 8 or 16. Yet you can buy a strip of preamps for $300 that also includes converters. I'm sure you catch my drift. This isn't to say they aren't acceptable quality, but you aren't going to wow anybody either. Something like this might be good for drums if you need a cheap option to get rolling though.
The last thing to mention are channel strips. These are rackmount style and include a preamplifier leading to a compressor that leads to an EQ. It includes all of the main components of a recording chain. You can find some amazing preamps in this configuration, such as with the Avalon VT-737sp.
I was loaned one of these for about six months and it blew me away. It's what got me started on always adding a bit of compression instead of going completely dry to avoid any surprise peaks and distortion. You should be aware that these exist, and the higher-end ones are very viable and great additions to a studio.
You now know everything there is to know about preamplifiers besides how to make them. If you're on the hunt for a preamp, check out our review of the best preamps in each price range. Now you have an in-depth answer for anyone who asks you "What is a preamplifier?"