In the most basic sense, the role of a microphone preamp is to provide voltage gain to a signal. A microphone’s signal on its own is usually too weak for further processing or recording. A mic preamp will increase the voltage to line level, the appropriate level for a console, tape machine or AD/DA (Converter) on the way to the DAW.
For the past several decades, outboard mic preamps have also taken on the role of not just increasing voltage but also imparting character, making it possible for several flavors for different uses. However, should you decide that transparency is what you desire, there are clean preamps also. This all depends on circuit design.
If you’re looking for a clean preamp, something that imparts no character to your signal, you’d usually be looking at solid state designs (i.e. no transformer). This design uses transistors, which can impart minimal distortion as gain increases. But be careful because not all solid state preamps actually lack character.
Tube preamps use vacuum tubes, also known as valves, to create gain. Most of these preamps impart a great deal of character. People typically consider these to be “warm” or “fat” sounding because as the gain increases the tube produces distortion (even-order harmonics). When used in moderation, it’s usually pleasing to the ear. Some tube preamps may produce deeper bass content or open top end, “air” as it’s usually called.
A preamp also gets its color from the transformers. A transformer bridges the input with the output by magnetic coupling and induction. It’s providing isolation and circuit protection because there is no direct wiring. In most cases it also imparts a sonic characteristic, especially if there are more than just one transformer in your signal path. I’ll go into more detail about transformers, tubes, transistors, op-amps and other goodies in a later article.
Another class of preamps are the hybrid preamps. These designs may be solid state on the preamp side and tube on the output side, offering coloration as you drive the output stage. Tube saturation equals harmonic content, and you can flavor to your personal taste.
There are digital preamps, too. Some digital preamps include preamp modeling, after it converts the analog signal to digital. This digital signal can be sent to your DAW via a card that installs into your computer or it may be built into the preamp itself. These are more often considered interfaces, but they still perform the same duty, generally speaking. In my personal experience, I’ve not come across preamp/converter designs that do both processes better than any stand alone units with just a few exceptions. It’s more a matter of convenience when quality is sacrificed for either budget and/or portability with small form factor.
There is yet another category for instrument preamps but I’m saving that discussion for a later article where we will discuss and review DI (Direct Injection) as well as channel strips, which do a number of processes in one box. Both of those topics transcend the scope of this discussion and deserve more attention individually.
Which Microphone Preamp to Use?
First and foremost, as always, use your ears. They won’t lie. Get familiar with the way preamps sound on different sources with different microphones in different spaces. I’ve personally found where I’ve not liked a preamp on a vocalist on one session but with a different mic or different vocalist, it was the chosen winner. A microphone preamp’s character, or lack of character, is a factor in why you may choose one over the other, but how it responds to transients (slew rate), and how it pairs with your microphone will also be something to consider. Only your ears can decide.
Budgetary constraints, channel count, character, whether or not you need a lot of gain for ribbon microphones, transient response, are all considerations for choosing a preamp. It’s my opinion that variety is most helpful in your studio. Being able to cover all the bases will provide you with not only the best selection for what your recording but it’s the tonal variety in the final mix that helps quite a bit too. Even when studios had an SSL or Neve console, engineers were looking to boutique preamps for recording certain elements because at the end of the mix, using all of the same channels on everything they recorded just didn’t sound as good to them. (Still lusting for a classic console?)
Do the best you can with what you have, use your ears to make your decisions, and the end result will surely please. Also, keep in mind that any time you think you need a piece of gear you just don’t have in the arsenal, you can likely rent it for your session. And that sure beats breaking the bank sometimes.