These guys take the relatively low mic-level output signal and boost it to a usable level.
When choosing this piece of your rig, a critical consideration is how many individual microphones you plan to use.
You will need a mic preamp for every mic you want to record at the same time!
You will also need one of these for every mic level signal (such as the one coming out of a direct box) that you plan on recording.
For example, if you want to record an electric guitar & bass via direct boxes with 2 mics for vocals - you will need a total of 4 mic preamp inputs.
This will allow you to record each instrument and vocal on a dedicated track on your DAW Recording Software at the same time. This is a big deal.
Why? Because it gives you the ability to alter the level, assign processing FX, and generally edit each input without affecting the others in your mix.
But are these built-in mic preamps any good? Or should I buy individual mic preamps instead??
The onboard Mic Preamps found on the commercially available audio interfaces I discuss on this site are quiet and powerful enough to make beautiful recordings with.
You do not need to spend a king's ransom buying these individually, even though that is an option you may consider at a later time.
I'd recommend against this until you have some more experience because it does get a bit more complicated, and it's more than you need to worry about starting off.
Mic preamps tend to get swallowed up quickly so err on the side of getting an interface with a few more than you think you'll use.
After considering the number of mic preamp inputs you need, the next question is how many other inputs and outputs will you require.
Some instruments like keyboard synthesizers and drum machines can be added via a line level input.
These are different than the mic inputs listed above because these instruments' output levels are much stronger than the tiny output signals that come from microphones.
Line level inputs can come in 2 different flavors:
One accepts the input for a 1/4" instrument cable only, and the other is known as a combination (or just combo) jack that can take either a 1/4" instrument or XLR cable input.
The combination jack is becoming very popular because of its versatility, but it's also slightly more expensive for manufactures to produce so don't look for them on the entry level budget units.
Just like the mic preamps above these get eaten up fast, especially if you have a left / right output such as that from a digital piano or drum machine.
On the output side, all units will have at minimum a stereo main out.
Some units may have multiple additional outputs that can expand the possibilities of your home recording studio setup.
Additional outputs usually come in stereo pairs and are useful for sending specific tracks to outboard (hardware based) effects or to a custom headphone mix via a Headphone Amp.
Even in the higher priced units, the number of outputs can vary drastically, so make sure you consider this carefully when choosing one.
