Connecting an External Microphone Preamplifier to Your Interface

Posted by Hairball Admin on Sep 27 filed in DIY Resources


Connecting your Elements or Lola microphone preamp into your workflow is not as complicated as it may seem. It is, however, different depending on the type of analog to digital conversion you are using in your workflow.

Why Use an External Microphone Preamp?

The purpose of a preamp in any setup is to boost the low-level signal captured by your microphone to a level that can be: a) recorded by your tape machine, or b) converted to digital signal within your A->D converter then recorded into your DAW software. In general, signals leaving your microphone will be less than -20dBu, and signals being recorded should be around line level, which in the case of pro audio is +4dBu. That is not to say your signal should be +4dBu; it really depends on what you are recording and where you feel it will end up in your mix. For example, if you’re recording a vocal that is captured by a microphone at around -40dBu, it will need to be boosted significantly to be useful for your tape machine or DAW. This level boost is handled by your preamp.

Boosting a signal by 40-60dBu is no easy task when the final signal is part of a professional audio mix. The preamp must able to increase the signal significantly while rejecting power supply and environmental noise.  In addition, all of the choices made in the design process can shape frequency response, phase shift, slew rate, and harmonic distortion. The degree to which these elements are shaped is what imparts “color” into a recording.  The design of the gain stage, an op-amp for the Lola and Elements, as well as transformer choice has the greatest effect on these elements. If you remove the transformers and replace the discrete gain stage with a generic monolithic IC, you’ll get a very transparent preamp, which may be what you want.

These transformer-less and generic monolithic IC preamps are the ones you’ll find in A->D converters that include built-in preamps. They are cheap, small, and very easy to design. While they work well and may be what you need in certain circumstances, they can be pretty boring and a far cry from the classic console preamps made by companies like Neve and API.  Preamps like the Lola and Elements series bring classic console design and color to your 500 series lunch box.

About Connectors

To get the most out of your awesome outboard preamps, they really need to be connected and gain staged properly. Let’s start with the connection. Your 500 series rack will generally have two balanced connections for each channel. Typically, these are XLR or TRS connectors, but some may use DB25 connectors.

Regardless of whether or not you make your connections DB25, TRS, or XLR (and whether you choose to use a patchbay), all connections need to be made using balanced and differential connections. In your studio, you can safely assume any connector that is XLR, TRS, or DB25 is both balanced and differential.

XLR                 TRS                 Connection

Pin 1               Sleeve             Cable Shield

Pin 2               Tip                  Hot (+)

Pin 3               Ring                Cold (-)

Pin 2 and Pin 3 will have the same impedance to ground and that is what makes the signal balanced. Pin 2 and Pin 3 also carry the same signal in opposite polarity (180°), which makes the signal differential. In pro audio people often say “balanced” when they mean “differential”.   A differential will always be balanced, but a balanced signal may not be differential. If you want to know more about balanced and differential connections, you can read our article on the subject here.

Gain Staging

Using your pre is pretty simple. However, one function that can confuse users is the addition of an output control. The output control has two functions: 1) it can trim the output gain; and, 2) it allows you to saturate the preamp gain stage and transformers without clipping your converters.

If you would like the cleanest possible signal, turn your output control fully clockwise and leave it. Fully clockwise should be your default position. If you have stepped gain and you find one gain step leaves the signal too low and the next step is too high, you can use the higher gain step then roll back on the output a little to the exact level.  That is how you would trim your output.

A classic technique used in recording for years is to saturate or “push” the preamp to impart some musical distortion into the signal. Not necessarily the type of distortion you associate with guitar amps, but a subtle clipping that can almost function as a compression stage.  To achieve this, you’ll want to turn up your gain while you roll back the output.  This allows the signal to clip through the pre but allows you to lower the overall level at the output to avoid clipping your converters.

Connection to your Interface

When connecting the preamp to digital set-ups, what you are trying to accomplish is simply this signal chain:

Mic -> Preamp  -> A/D Converter -> Digital Audio Workstation (DAW)

The first part, Mic -> Preamp, is easy. Simply connect the XLR from your mic into the input connector of the 500 series rack channel where your preamp sits. If your mic requires phantom power (+48VDC), activate that selection on the preamp front panel.

The last part, A/D Converter -> DAW, is even easier. That is simply the firewire, USB, or Thunderbolt connection between your converter and DAW. There are also drivers and software but all that takes care of itself (joke!).

The tricky part can be connecting your preamp to your converter, especially if your converter includes onboard preamps. You need to locate a balanced and line level signal input. Let’s go through the common types you’ll encounter and how you should make the connection.

Type 1 – Pure A->D Converter (Lynx Aurora, Apogee AD/DA, Digi 192)

In this group, you’ll find most of the professional converters. They do one thing and they do it exceptionally well; they convert A->D and D->A.  They have no onboard preamps. They will have a balanced input and output for each channel. In this case, you would connect the preamp output to the input of a channel; let’s say channel 1, and that is it. In your DAW, you would open a mono channel and select channel 1 of your converter as the input for that channel. 

The outputs of the converter are not typically used in the preamp setup unless you are re-amping.  They are used for monitoring and for dynamic processing outboard gear like an EQ or compressor.

Type 2 – A->D Interface with Optional Preamps (DIGI 003 RACK, UA Apollo, RME Fireface)

These interfaces include onboard preamps, but not on every channel. On the 8 channel versions, you will typically encounter onboard preamps on channel 1-4 and line-level inputs on channels 5-8.  The line level ins/outs do not have onboard preamps, so they will not have gain knobs associated with them.   They are also typically TRS connectors. In short, if your interface has balanced line level inputs, use those.

Type 3 – A->D Interface with Switchable Preamps (Apogee Duet, Motu Ultralight, Focusrite Clarett 8 Pre)

These interfaces do not have dedicated line inputs, but you can (mostly) bypass the onboard preamp by switching to line.

In the case of the Duet, the ¼” jack is for unbalanced connections only (TS like a guitar). So, for the Duet, you’ll want to use the XLR input and use the software to set the input to +4, which is a balanced line level.

The Motu Ultralight and 8 Pre have unity gain/line options in the software.

These interfaces are often the trickiest and they are all a little different. Again, you really need to read the manual and determine how to make a balanced connection and switch it to line level (+4).

Type 4 – A->D Interface with Fixed Preamps (Focusrite Scarlett, Focusrite Saffire Pro 40, Digidesign M-BOX)

These interfaces have a preamp on every channel that cannot be bypassed. In this case, you’ll have three knobs; two on your Lola or Elements pre, and one on the channel of your interface.  You want to gain stage this type of set-up so that a) most of your gain comes from the sexy Lola/Elements pre rather than the onboard pre and b) you’re not clipping your signal.

The most basic approach is to turn the gain on the interface preamp to its lowest setting. That should work in many cases. However (depending on the interface), this may attenuate or effectively turn off the input. If you notice you’re not getting enough gain from your Lola/Elements pre, then you’ll need to increase the gain on the interface pre until you have the appropriate level.

Additional Focusrite Information

Focusrite has an extensive line of interfaces. Using your Lola or Elements pre can be a little different than described above depending on the model you choose. Please refer to the Focusrite link below for a general overview of using external preamps.

*Please note we do not own these interfaces (other than the Lynx Aurora). We’ve written this based on past experience and user manuals. If you find a unit’s functionality has changed or is incorrect, please let us know and we’ll update the article.

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