Another post from my voiceover blog here. This one is an introduction to microphone pre amps. It's a subject that is as important in music too, so please excuse any references to the world of voiceover.
Last time I spent some time discussing microphones, their pickup patterns and how you can use them to get the best possible sound for your project. In this post I am going to spend a little time talking about Pre Amps.
What is a Pre Amp?
We use a pre amp to boost the signal of a source to line level, line level being a healthy signal into your recording device. Microphones have a very low output, whichever microphone you use (condensers/tube mic's have a slightly higher output than dynamics) that needs to be boosted to a point that is usable within your DAW. Line level is the goal (http://en.wikipedia.org/wiki/Line_level).
The output directly out of your microphone is called "Mic Level", and out of an instrument such as a guitar jack is called "Instrument Level". Although instrument level is a much higher output than mic level, it still needs boosting to line level.
Why do I care about Pre Amps?
Well they are the next step (after the mic) along the signal path for your voice before it hits your AD converter and then into your DAW of choice. This stage influences the sound of the signal as well as the level, and so it cannot be ignored. Mic pre's come in many forms and you can disappear down all manner of rabbit holes trying to find the "best" one. The truth is, there is no "best" mic pre, only the one that sounds best to you, compliments the source/mic, is affordable to you and reliable.
If you have some kind of outboard interface such as an M-Box that has a pre-amp built into it, you can plug your mic into it, it boosts that signal to line level, converts it to digital information and sends it down a firewire/USB/Thunderbolt cable to your computer. These days the mic pre's on these consumer devices are very good, and are certainly usable in most applications. However, if you have started to lust after an expensive microphone you may also want to think about improving your whole signal chain, which includes your mic pre. Your sound is only as good as the weakest part of your signal chain, so don't waste your money on a $3000 mic and plugging it into an M-Box. That's like buying a Ferrari and fitting it with shopping cart wheels.
You could choose a tube or solid state pre, a channel strip, a 500 series, or a vintage re-issue: just make sure it gives you the sound you are looking for, to enhance whatever it is you are recording. If your main mic is a dynamic mic, it will more significantly effect the sound of your pre amp and how it performs, so take that into consideration when choosing.
So you have an M-Box (for example) and you want a new mic pre, do you have to replace the M-Box and get a new AD (analog to digital converter) box too? Nope! If you are happy with the audio to digital conversion of your M-box, you can simply bypass the pre on that and use your new pre while using the M-Box solely as an AD converter.
Let's say you buy an API 512 pre (nice!), you plug your mic into that, make sure it is set to mic input, then turn up the gain as you see fit. The API will be outputting a line level signal, simply plug that line level signal into your M-Box, making sure that the input is set to Line Level. It then knows that it doesn't have to apply it's pre and as long as the input knob is down to zero it will not provide any of it's onboard gain boosting from its pre. You are then only using its AD conversion technology.
After you have saved up your pennies you may consider buying a dedicated AD conversion box and replace your M-Box altogether, which may well improve your sound a little more.
It should be said that replacing a pre or converter may or may not improve your sound, but it will certainly change it. If you have a somewhat muddy, warm sounding voice, adding a Neve 1073 (for example) will only serve to enhance that, and exaggerate any problems associated with your voice, so it is best to research and then test out any pre's you are thinking of purchasing.
I am a strong proponent of as good a signal chain as you can afford, but as stated earlier you should be aware that mixing pro quality and consumer quality components will result in a consumer quality sound. If that is the only way you can afford to complete your pro chain (let's face it, how many of us can drop $5-10k on 3 components), by all means play the long game, knowing that you will ultimately end up with a great sound.
As a final point I would like to say that there are a lot of sources on the internet "debunking" the idea that you can't get a pro sound from consumer products. Well you can get close, but that final 10% of quality costs, and in the professional environment the difference is noticeable. People who can't hear the difference tend not to have real world experience of either using pro equipment, or working in a professional environment.
I am a British Producer, Mixer & Musician who is privileged to live in New York City.