It's always touchy trying to claim one piece of gear the most important piece in the studio. After all, there are many critical pieces of gear -- monitors (your gotta be able to hear what you're recording and mixing!), microphones (first link in the chain), converters, recording platform, acoustic treatment, etc... all important to getting great results. Of course there are the artistes of the engineering world who will always interrupt such conversations with the obligatory, "your ears are the most important piece of gear in the studio," or some other such truthful, yet completely off-topic comment to show the richness of the depth of their experience and wisdom. Psshh.
I'm talking about gear here -- stuff you purchase. Everyone with a recording program wants to know what gear they need to get great results, and with that ever-present question in mind I will venture an opinion -- one developed over many years of observation.
What is the most important piece of gear in a studio (i.e. where should you commit your dollars)? The microphone preamp.
Not monitors? Nope. Every single day folks all over the planet do great work on modest monitors (NS10s anyone?). They don't have to be full-bandwidth, killer, high-dollar, self-powered client impressors. They just have to be familiar to you. You need to be so familiar with their response that you can trust what you're hearing from them, so you can make mixing decisions with confidence.
What about the mic? It's the first part of the chain. Yes, your choice of mic is critical, but now, more than ever, there are tons of solid-performing, reasonably-priced mics on the market, from tube and solid-state condensers to dynamics and ribbons. Most folks can assemble and impressive mic cabinet without too much difficulty or expense. And I'd much rather record a modest microphone with a killer mic preamp than a killer mic through a modest preamp. "Why," you ask?
Professional microphones produce a balanced, low level output signal. For this signal to be useful it must first be amplified a LOT to feed the input of a balanced, line-level signal processor or recorder input. Most any microphone signal will see a minimum of 20dB of gain to be useful. That's a 10x increase over the mic's output. If you require 30dB of gain, that is a 31.6x increase. 40dB is 100x. And any signal that requires 60dB of gain has to be increased 1000x! That's a lot of amplification, and the manner in which it is accomplished has everything to do with determining the quality of your signal when it moves to the next device in line.
Comparatively, an inexpensive line-level device (such as a compressor) has both line level inputs and outputs (essentially a unity gain device). If your gain reduction is around 10dB (which is a lot, actually), then the compressor's output stage has to produce only just over a 3x increase in signal to produce the same level of output on louder passages... a much simpler task that can be done well with even modest circuits. But the mic preamp.... your microphone's low-level output, no matter how pristine it may be, will require such gain before it becomes useable that it is literally at the mercy of your preamp. Skimp here and your $3000 tube mic could lose much of its pricey luster. And once the damage is done there is no whiz-bang device that will restore what has been lost. Nothing.
A quality mic preamp will produce a solid, focused, fully intact signal ready for recording or further processing. Some designs produce solid, but accurate signals (sometimes referred to as a 'straight wire with gain'). Being a rock dog I'm personally a fan of preamps that bring a little sex to the party, and give the signal not only a solid focus, but some rich color as well to add some visceral spice. For these reasons I'm a fan of older style designs with transformers that saturate, round transients, and generally thicken the whole affair in a glorious way.
Mmmmm...... transformers.... (Okay, I'm getting off topic...)
Like I say, I'd rather record a modest mic through a killer preamp than a killer mic through a modest preamp. Ideally both would be killer, but if I gotta choose one I'm going with the preamp. It'll improve my results with every mic I plug into it.
Thanks for reading!