HOW TO: Master FabFilter Pro-MB's Pan and Gain

How to use FabFilter-MB's in-band and gain controls
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How to use FabFilter-MB's in-band and gain controls
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FabFilter Pro-MB is arguably the most powerful multiband dynamics plug-in available today. But with that power comes the potential to screw things up if you don’t use it properly. This is especially true when using the gain and pan controls for each band in Mid-Side mode, as they work together in ways that are not obvious. Improper use of these controls can lead to unintended EQ changes in the mid or side channel.

In this article, I’ll explain how Pro-MB’s gain and pan controls work and show you how to use them to get great results. I’ll illustrate my points using real-world mastering applications.


An advanced technique used in mastering is to boost the gain in a single band to accentuate a key element, while compressing the same band to automatically prevent another element in the same frequency range from also being enhanced. For example, Pro-MB can be used to boost the bass guitar in a previously recorded mix without also goosing the kick drum.

Fig. 1. When adjusting the output-gain control for a filter assigned to the mid or side channel in FabFilter Pro-MB, use the filter’s pan control to reverse the gain change in the opposing channel. To pull this off, create a filter in the bass band and assign it to the mix’s mid channel (see Fig. 1). Key the filter’s sidechain with the mid channel and restrict the sidechain’s bandwidth to 30-80 Hz to weed out everything but the kick drum and bass guitar from its signal. Set the bass filter’s range control to -6 dB and its ratio to 100:1, and use the fastest attack and release times possible along with at least 1 ms of look-ahead detection. Now the magic begins. By boosting the bass filter’s output gain 6 dB and compressing the same band an equal amount—using the proper threshold setting—you end up boosting bass frequencies in the mid channel (where the bass guitar lives) at all times except when kick-drum hits occur.

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But here’s the rub: Raising the bass filter’s output gain in Mid-Side mode will apply bass boost to both the mid and side channel—even if the filter is assigned to only one of those channels—making any hard-panned guitars and reverb also sound more bassy. To prevent this from happening, you must use the bass filter’s pan control to cut an equal amount of gain (in this example, 6 dB) in the opposing channel (the side channel). Specifically, drag the filter’s pan control counter-clockwise to the “Mid 0 dB/Side -6 dB” position.

The takeaway is that a band’s pan control affects how much filter gain will be individually applied to mid and side channels—even if the band is applying dynamics processing to only one channel. Note that a band’s pan control has no effect on the balance between left and right channels, even when used in stereo mode. (That’s what the plug-in’s master or global pan control is used for.)

There’s a keyboard shortcut you can use to keep yourself out of trouble and consistently dial in the perfect amount of compensatory panning: Whenever altering (boosting or lowering) output gain in a mid or side channel’s band, alt-drag (using a Mac, option-drag) the band’s output-gain control. Doing so will automatically apply the correct pan amount to preclude making a gain change in the opposing channel’s band.

When a band’s output gain is set to 0 dB in Mid-Side mode, you should generally keep its pan control set to the noon position. Any other pan position will cut gain from unity in one of the two channels. Of course, there are times when that’s exactly what you want to happen. One example involves again our old friends, the bass frequencies.


Fig. 2. A low-shelving filter’s pan control (see mouse pointer) is used in Pro-MB to collapse a mix’s bass frequencies to mono. The filter’s Range control has no effect on the bass band’s panning to center and is set to -6 dB here solely to make the filter’s bandwidth more apparent for illustrative purposes. In many if not most cases, it helps to focus a mix’s bass frequencies by collapsing them to mono in the stereo field. To do this using Pro-MB, create a low-shelving filter at roughly 100 Hz and crank the filter’s pan control fully counter-clockwise to the “Mid 0 dB/Side -INF dB” position (see Fig. 2). Doing so will completely strip the band’s bass frequencies from the side channel, seating them dead-center in your mix. You don’t need to boost or cut the bass band’s gain or activate dynamics processing for this to work—a 1:1 ratio will still get ’er done. In fact, you don’t even need to assign the filter to the mid or side channel. Simply panning it fully counter-clockwise will collapse its frequencies to mono.

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Armed with the knowledge imparted by this article, you’ll hopefully be able to handle any audio-related situation—even NAMM conventions—with aplomb. The next time your fellow engineers bemoan the devaluing of music and its attendant revenue destruction for studios, tell ’em it’s okay—you’ve become a panhandling expert!