Essential tips for mix bus processing

Spend any time whatsoever investigating the art of mixing and you’ll likely hear the words “mix bus processing.” First, you’ll be confused: what is a mix bus, and why should you process it? 

Then, when you learn the language, you’ll be even more confused: “hang on, isn’t this what the mastering engineer is supposed to do?”

Fear not: you’ll learn all about the mix bus in this article, as well as some tips for how to process your mix bus and how to holistically think about the mix bus processing chain. 

Follow along with iZotope Ozone and plugins from Plugin Alliance.

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What is a mix bus? 

Even when you’re mixing a 200 track arrangement, you’re only listening to one stereo track – the funnel through which all the others are heard. This single solitary track is called the mix bus, though it does have other names:

Some call it the stereo bus. Others call it the 2bus or the master bus. Some spell the word like the vehicle, while others prefer the double ess.

However you call it, the mix bus is the end of the line – the final frontier of your mix. Beyond the mix bus, there is nothing: all your instruments, effects, and submixes arrive at this main left/right stereo output. 

In most DAWs, you can apply processing right onto this output, adding color, saturation, and compression, tailoring the song to your needs in the process. 

Many engineers love to do this. They use their favorite compressors to glue the mix together, or their favorite equalizers to give everything a little openness in the top end. 

In this article, we’ll provide concrete tips for processing your mix bus. 

What should go on my mix bus?

Anything can go on your mix bus. But in practice, engineers tend to favor the following: 

EQs, compressors, and saturators – all used subtly. 

Some engineers may use a limiter, but it’s not good practice to rely on it when you’re starting out, because you want to manage your peaks within the mix itself.

You almost never see modulation tools like flangers and phasers on the mix bus, unless the engineer is automating one in for a brief moment. The flanged end of “Life in the Fast Lane” would be one such example. 

Many novice engineers throw stereo-width enhancement on their own mix bus willy nilly, but this is quite dangerous. You run the risk of creating a blurry, ill-defined mix. Try widening out things like pads, background vocals, and non-percussive synths in the mix itself, so that you don’t mess up the impact of your drums and lead instruments. 

Every once in a while, an engineer may use a touch of reverb on the mix bus. However, this is so rare that almost no engineer would ever admit to it (cowards).

Wait a minute, how is this different from mastering?

Mastering is the final step before a song is released into the world. If you’re at all familiar with this dark, secretive art, you might wonder how mix bus processing differs from the definitions of mastering. 

Don’t mastering engineers apply EQ, compression, and limiting to your songs to make them competitive in the marketplace of content?

Not quite. Remember this: a square is a kind of rectangle, but a rectangle is not a kind of square.

Mastering may include similar processes, but that’s hardly the totality of it. Mastering involves sequencing all the songs, correcting any inharmonic errors in the recording with tools like RX, embedding metadata, exercising extensive quality control, and exporting the record into various file formats. 

To the extent that “mix bus processing” is employed in both, the spirit of the processing is totally different: 

Mastering engineers make their decisions to ensure the song works across every conceivable playback system, in a variety of formats, at various playback levels, and in many different contexts. To that end, mastering engineers tend to keep their processing as transparent as they can. Vibe is only implemented when needed, and as subtly as possible.

Mixing engineers, on the other hand, are not worried about maintaining a transparent hand if they see an opportunity to get the vibe exactly right. They’ll pump their whole mix through a flanger if they have to (again, “Life In The Fast Lane.”)

A mixing engineer wants their EDM song to have more of a snap to it; that’s how they choose their compressor. 

The mastering engineer knows the drop needs to playback at -6 LUFS short term or higher to compete in the genre, and uses the compressor to make sure they get there transparently. 

5 tips for mix bus processing 

1. Know the plugins you are using intimately

As a mix engineer, it's essential to understand the tools at your disposal. Before applying any processing to your mix bus, take the time to familiarize yourself with the plugins you plan to use. Experiment with different settings, understand the controls, and make your own associations as to what each tool accomplishes. 

I’ll give you an idea of my own thought patterns. Maybe they’ll help you, maybe they won’t, but it’ll illustrate what I mean.

We’ll use this mix from the artist PANTLER as an example, and after each entry, I’ll play the example again, running it through the processor I’m talking about.

We’ll start with compressors, as they arguably have the most pronounced effect on your mix bus.

Compressors

Vintage style mu compressors

Vintage vari-mus can bring a fun, bouncy feel to the mix, one that feels round and musically timed to the song. They have a way of anchoring the kicks and snares in the center of the image while letting everything else bounce around a bit – especially when used (judiciously!) in m/s mode. An example of a vintage-style mu compressor is the NEOLD WUNDERLICH. 

Modern mu compressors

I like to think of modern mus as offering a smooth and glassy effect. The ones I like tend to curtail the low midrange a little while preserving the subs. When used properly, you can keep a mix anchored in place with a fair amount of gain-reduction, yet it’ll still sound fairly natural. Try them in dual-mono for enhancing the stereo spread. Again, be judicious with this! 

Here’s an example of a modern mu compressor sound with SPL Iron. 

Opto compressors 

Depending on the circuit, I like to think of an opto as giving me the bouncy feel of a vintage mu with the smooth glassy color of a modern one. Let’s listen to an example with the NEOLD U2A. 

Please note, engineers do not generally use things LA-2A emulations on the stereo bus, but the U2A’s recovery control and R37 parameter allow you to tune the attack and release times, while drive parameters and a mix knob let you dial in a suitable – and subtle – amount of saturation.

API style VCAs 

API VCA compressors have a way of bringing the center image to the front while spreading out the stereo field (at least, I tend to think so). In terms of compression, they have a recognizable style: grabby, yet warm, with an “on-top-of-the-beat” pump when used to enhance transients. Here’s the Lindell SBC demonstrating an API VCA compressor sound. 

SSL style VCAs 

These are also great for adding snap to transients. But they can also add a density engineers like to call “glue.” As a rule, SSL style comps don’t give you the same depth/stereo presentation as the API style VCAs. Here’s an example with Solid Bus Comp. 

Diode bridge compressors

In addition to taming transients, diode-bridge comps have their own grainy color. I think of them as shooting on grainy 35mm film stock. Diode bridge comps can be very aggressive, so they require the right material, and usually backing off the mix knob is a good idea. This example includes the Lindell 254E.