How To Hook Up An External Preamp [Reader Question]

2011 Apr 04, 2011

If you ever venture into the world of external mic preamps to compliment the ones built into your audio interface then you may be wondering what is the best way to hook it up. It is a common issue, but an important one to address. One of my readers on Twitter recently purchased an ART Tube MP on my blog’s recommendation for bass recording and had this question:

“I picked [the ART] up last night. Subtle but much more rich. For guitar I’ve been cranking the input into the red for a cool driven sound. One question…If you need more level into ProTools, do you increase the ART preamp output or the audio interface input?”

This is a great question. Let’s start at the beginning…


Mic Level To Line Level

When you plug a microphone into your audio interface input, you never have to really think much about whether it is a mic level or line level source. But in reality, microphone audio levels are very quiet, and the built in preamp in your interface is turning that signal up to what’s called a line level source, which is much more useable. If using an audio interface, that’s all you need to know. It turns your signal into a digital one and you’re golden.

However, if you start to use an external preamp, like the ART Tube MP mentioned above, then you need one extra step. You plug your microphone (using an XLR cable) into the external preamp’s mic input. Then using a balanced 1/4 inch cable (called a TRS cable) you run the output of that preamp into an available line input on your interface.

This is important. Your interface may have some microphone XLR inputs as well as some 1/4 inch TRS inputs for line level sources. Plug your preamp’s output into a line level input. Some interfaces have a combo connection that allows you to plug in XLR cables and TRS cables in the same input. If this is the case, you’ll likely have a switch that tells the interface that you want to use Mic or Line level on that input. Set this to Line. Keep in mind that Line inputs are different than Instrument inputs although both use a 1/4 connection.

Proper Gain Staging

Now that you have your preamp hooked up to your interface, the last thing we need to look at is your gain staging and signal flow. The gain knob on your interface controls the preamp on that interface. Since we are using an external preamp to bring the microphone signal up to line level we don’t need the gain on the interface.

If you have a line input with no gain knob associated with it, use that one. If not, follow this advice. Start by turning the interface gain knob all the way down. Then use the input (and sometimes output) knob(s) on your external mic pre to get the desired audio level into your DAW. If you need more gain into your software, adjust it on the preamp, not the interface as this would mean your signal is being boosted twice by two different preamps.

Remember, you don’t need really hot levels going into your DAW and you definitely don’t want unneeded noise. Avoid all of this by using conservative gain staging in your external preamp, and avoiding the extra gain from your interface’s pre when running a line in from an external.

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