It's often claimed that expensive mic preamps will take your recordings to the next level. But is that really true, or are you just wasting money that would be better spent elsewhere? And how can you know whether an external mic preamp will be any better than the ones built into your audio interface?
Hello, my name is Sam Inglis, and I’m the Editor In Chief of Sound On Sound magazine. I get asked a lot of questions, and a lot of those questions begin “Do I need a...?” The most asked ‘Do I need...?’ question of all is: do I need a mic preamp? Well, do you?
If you’re asking this question, what you mean is: "at the moment I’m plugging my mic straight into the preamps on my audio interface or on my mixer. But people on the Internet keep telling me that that’s not 'pro'. If I want to get that pro sound, I need to have a fancy outboard preamp. Is that really true?"
Well… it might be. There are three reasons why you might want to add another preamp to an audio interface that already has preamps in it:
One is that you just need more preamps — perhaps your interface only has two mic preamps, but you want to record a drum kit. The second reason for adding another preamp is if the preamps in your interface are of less than optimal quality and not up to the job. You’re trying to record the sound of a field mouse scratching its leg, you have the gain all the way up, and you still aren’t getting enough level into your computer. Or you are trying to record a drum kit, your gain’s turned all the way down, and it’s still clipping. Finally, the third reason for adding another preamp is to get that magical classic retro vintage warm, liquid up-front, punchy solid, vibey 3D sound that certain preamps are supposed to have.
So, three good reasons for buying a preamp. Now out of these, only the first one is really cut and dried. If you want to record more mics simultaneously than you have preamps, you’re definitely going to need more preamps. End of. The real question in this case isn’t ‘Do I need more mic preamps?’ it’s ‘How can I add more mic preamps?’ And the answer to that question depends on what expansion options your audio interface offers.
Usually, expansion means digital inputs, and if you have an S/PDIF input, you can add a mono or stereo mic preamp with an S/PDIF output. If you have an ADAT optical input, you can add an eight-channel mic preamp if it has the right connector. These are sometimes called ADAT expanders. If you don’t have any extra inputs at all on your interface, then you need to buy a different audio interface.
So that’s the first reason to buy a mic preamp. What about the second one? You’re recording speech or recording acoustic guitar or the sound of moss trembling, you have the gain turned right up and you still aren’t getting much level into the computer. Can you fix that by buying a mic preamp?
Yes you can, but do you actually need to? The ratio of signal to background noise in speech is maybe 40 decibels (dB). The input on your audio interface has a dynamic range of at least 100 decibels. So from a technical point of view, it actually doesn’t matter whether your signal peaks at -10dB, -20dB or even -40dB. Record it, turn it up in your DAW and it will still contain exactly the same amount of information, and the noise floor will be exactly the same.
So low levels don’t matter from a technical point of view. But from a practical point of view, low levels are annoying to work with, so adding a mic preamp might make your life easier. If you just need more level, the easiest option is one of those fixed-gain booster preamps, (such as the Cloudlifter or Royer dBooster) that goes between the microphone and your existing preamp. That way you are, in effect, running two preamps in series, so you get the gain of your interface preamp plus the fixed gain of the booster.
Alternatively, you could add a separate preamp using the digital inputs on your audio interface, but how do you know it’s going to be any better than the built-in preamps? The best way to do this is to check the specifications. These should list a gain range in dB and a maximum input level in dBu, and if you subtract the one from the other, you’ll know the quietest signal that can deliver a full-scale input.
So for instance, if your audio interface has a 50dB gain range and a maximum input level of +10dBu, then a preamp with a gain range of 60dB might actually be less suitable, if the maximum input level on that preamp was +24dBu.
So the first two reasons to buy a mic preamp are kind of boring. What about the third one?
Well, you’re probably not going to get much lower noise, or a flatter frequency response, or less distortion, by buying a third-party preamp. But, if you do it right, you might get more distortion — and that’s what people are getting at when they say that your choice of preamp makes a difference to the sound.
...you might get more distortion — and that’s what people are getting at when they say that your choice of preamp makes a difference to the sound.
If you operate any mic preamp within its comfort zone, it’ll sound pretty much identical to any other mic preamp — and that includes the ones on your audio interface. When people talk about preamps having a sound, they’re talking about operating them outside their comfort zone. You can’t do that with the preamps on your audio interface, and if you could, it would sound horrible.
It means to use more gain than you need, so that transformers or valves or op-amps begin to saturate and add distortion. That can be a very pleasing effect, and it is indeed one that a lot of professional sound engineers use all the time. To do it, you need a preamp that actually sounds good when you overdrive it, but that’s only useful if you can exploit that feature. And that means you need the ability to turn the signal back down again after it’s preamplified and before you feed it into your audio interface. In other words, you don’t only need a gain control, you also need an output level control, so that you can intentionally apply too much gain and then compensate for that after the fact. Not all outboard hardware preamps have this, so if you’re buying one for its sound, choose wisely.
How much difference does it actually make? Well, if you push it too far, you turn your preamp into a fuzz box, and that’s definitely not 'pro'! Keep it clean and you won’t notice any difference at all. Somewhere in between there should be a sweet spot.
If everything else in your recording chain is right — you’re using the right mic in the right place in a well treated, good sounding room — then yes, an outboard preamp can make a difference.
If you decide you do need a mic preamp, of course, the next question is “Which one?” And to find the answer to that question, you need to read Sound On Sound magazine regularly (or better still, subscribe!) or visit our website, where we review everything you might possibly need in your studio.