My interface has built-in mic pres. Why do I need an external one as well?
Microphone preamps are analogous to colors of paint. Fundamentally, their primary function is to boost your mic signal to an audible level. Every mic pre sounds different, and in many cases, the differences between them are not subtle. Mic pres color the sound, and each one does it a bit differently. Typically, we can break down all the available mic pres into two simple categories: transparent and colored.
Transparent pres aim to replicate the signal accurately when amplified, nothing more. They essentially provide a mirror image of your sound. A colorful mic pre, on the other hand, adds a recognizably unique “character” or tone to the sound. This coloration is accomplished with components in the circuitry and some clever electrical engineering. As for the mic pres that come built into the recording interface, many are considered average. In other words, they will get the job done.
You may want to consider the cost of the available options on the market. An audio interface could cost anywhere from $99 to thousands of dollars. The $99 interface comes with built-in mic pres and is designed to deliver a convenient and cost-effective all-in-one solution. However, the mic pre built into the $99 interface will not compare in quality to an external preamp that costs $500 or more. Side by side, even a beginner will notice the difference between the two. “You get what you pay for” is a good rule of thumb in audio. The quality and emotional impact of your recordings are largely (but not completely) determined by the microphone preamps used to capture your signals.
Recording tip: When implementing an external mic preamp into your signal chain, be sure to use the line input (as opposed to the mic input) connection and setting on your interface. Switching the signal to line level (on the front/rear panel of the interface or an internal setting in the software) effectively bypasses the mic pre built into your interface so your external mic pre can do all the heavy lifting. The last thing you want to do is amplify the signal twice. Doing so can significantly reduce the quality and impact of your signal. You likely put a considerable investment in your external mic pre. Typically, that piece of equipment should bear 100% of the responsibility. In some cases, an artistic decision can inform the use of two different mic pres (two different colors) to amplify and re-amplify the same signal. If you have a couple of great pres, give it a shot. The key is understanding the role of each and having clear intent in creating the desired result.