Your audio interface already contains a preamp, so why would you want another? How will it help you make better recordings?
Yes, your audio interface already has a microphone preamplifier, perhaps two and perhaps even more.
So you can plug your microphone into your interface, switch on the phantom power (for a capacitor mic) and start recording. All is well with the world.
But sooner or later you'll start wondering whether a separate preamp will improve your work. Perhaps you saw an ad in a magazine or on the web, or perhaps you watched a YouTube video of a producer in a $1,000,000 studio who wouldn't dream of recording without his or her $2000 preamp.
A separate preamp just has to be better, right?
Reality check - There are many things you can do that don't cost a penny that will improve your recording much more than any preamp (assuming your audio interface is of at least basic professional quality, like a Focusrite or PreSonus in the lower price ranges for instance).
Concentrating on learning and improving your skills is an absolute must. A new preamp won't let you off the hook for that.
But if you've considered every relevant factor and you're almost sure a preamp is the thing you need. Will it make a difference?
If you aspire to a quality preamp because you want a more accurate sound, then you would be right. A better preamp can give you a more accurate sound, in terms of translating what comes out of the microphone into an exact but more powerful replica to send to the line input of the audio interface.
But the thing is...
...that the differences are small. Modern preamp design is way better than it was in the 1970s, 1980s, and even the 1990s for the most part.
Even budget preamps these days approach the theoretical limits of perfection extremely closely.
So you could buy an expensively accurate preamp and raise your quality level from 99.3% to 99.9.
Unless the preamp in your audio interface is noticeably noisy, distorted, or has a poor frequency response, you will have to listen extremely closely to hear a difference.
I'm all for progress and technical perfection, but there is the law of diminishing returns in play and you can pay a lot to achieve only a tiny improvement.
But if that's the way you want to go, then go for it. To achieve accuracy, normally you would expect to get that from a transistor preamp. A nice unit I like to use as an example is the Focusrite ISA One. With your mic plugged into this, you will not be complaining of any lack of accuracy from the preamp.
Now this is where things do get interesting. The preamp in your audio interface may be technically good enough, but it will - unless there's a fault - be lacking in any interesting kind of texture.
There's a word we use - warmth.
Improving your preamp is a great way to achieve warmth. And to my ears, achieving warmth in the microphone or in the preamp is far superior to anything you can do to the signal later on with processors or plug-ins.
For this you'll need a tube preamp. The example I like to use for this is the Universal Audio Solo/610. You can set the controls for a high degree of accuracy, maybe not quite so much as the Focusrite but close enough. Or you can set it for as much warmth and texture as you want, and a little more just to be sure.
When you research tube preamps, be sure to look for controls both for gain and level (sometimes called 'output').
You can balance these controls for the kind of sound you like. If you set the gain low and the level high, then the sound will be highly accurate.
If you boost the gain and control the level downwards with the level control, then you'll get a shedload of warmth and texture. Trust me - you'll love it.
I'm going to give you my personal opinion on this, so be aware that other opinions will differ.
Objectively, I don't need a more accurate preamp. I can get all the accuracy I need from the preamp in a basic audio interface.
But on the other hand, I would gain a lot of confidence and certainty from using a pedigree preamp with performance as close to perfection as it is possible to get. I know that because I've been there, done it, and still wear the t-shirt (getting a bit ragged and holey now). Subjective and emotion-driven it may be to a large extent, but I know that I like it.
What I am more certain of however is that it is an incontrovertible objective fact that a good tube preamp will offer a range of sounds textures that will make your vocals and instruments sparkle and shine. And while the Solo/610 I mentioned earlier is pricy, you'll be surprised at the quality you can get from a modest tube preamp such as the Behringer Ultragain MIC100. I know because I took one to Abbey Road Studios and tested it against much costlier units.
So there you go - You probably don't need a preamp. But if you want certainty then go for a top-of-the-range transistor preamp. If you want warmth and texture, a well-designed and well-built tube preamp is the way to go.
Great home recording starts with a great home recording studio. It doesn't need to be expensive if you know how to select the right equipment for your needs.