The Pros and Cons of Aux Fed Subs, Pt. 2

Image courtesy of Martin Audio (CSX218)

Image courtesy of Martin Audio (CSX218)

This week, we’re discussing aux fed subs. Last time, I explained how aux fed subs differ from a full range feed. Today, we’ll get into some of the advantages of feeding your subs from an aux. Before I go too far, it’s occurred to me that there are variations on this theme including a matrix fed sub and group fed subs. I’ll circle back to those later. For now, we’re going to talk about a true aux fed sub situation, where the only way a signal gets into the subs is when you turn up the aux for that channel and send it there. 

Without further adieu, here are what I perceive to be the pros of aux fed subs. 

Granular Control Over Sub Content

With aux fed subs, the engineer has complete, discreet control over what ends up in the subs. The only way something gets there is to be turned up in the aux. In a typical worship band situation, that is probably going to mean the kick, floor tom, bass, and synth. And tracks if you have them. That’s pretty much it. And you can control how much of each channel goes there. So if you want a lot of kick but just a touch of synth in your subs, you can do that. It makes for a very clean sub feed.

Separate Sub Processing

Because the subs are on an aux, you can do some additional “group” processing on it. For example, you could put a compressor on the sub aux and add a little dynamic control. This may or may not be a good idea, but it could be done. You could also add a plug in like Lo Air to synthesize some additional low end content. Again, this may or may not be a good idea. But you could do it. 

Variable Sub Level Control

This is the big one everyone seems to go after; the ability to push the subs up or down with a single fader. As we all know modern music is “all about dat bass” and everyone loves their bass. Except for those who don’t. And when it becomes so overwhelming that the audience can’t hear the vocals. But hey, with aux fed subs, you can turn the subs down just as easily! The fader goes both ways. Aux fed subs make it easy to tweak the level of the subs on a per song, or even per chorus basis.

Those are some of the pros. However, as I’m writing this out, I’m thinking of rebuttals to each. So in the spirit of the most excellent and helpful presidential candidate debates [that was sarcasm], here are the rebuttal answers to each.

Granular Control

You can do this in a full range fed system by using a high pass. If your high pass filter is set above the level of the sub cross over, very little if anything will end up in the subs. And pretty much everything besides the kick, floor tom, bass and synth should have a high pass on it. And tracks if you use them. So there’s that.

Separate Sub Processing

You could just as easily do this on a group that feeds the main output. Though I’m still working on justification…

Variable Sub Level Control

This is really a mix issue. If you want more kick in the mix, turn up the kick. If you want more bass, turn up the bass. If you want more floor tom, well, you get the idea. 

So, while it may be alluring at first to have this amazing, discreet control over the subs, it’s not the only way to do it. Moreover, it creates some problems that are hard to overcome down the road. And we’ll tackle those next time. And I’ve probably tipped my hand as to which way I currently lean, huh?

DPA Microphones

The Pros and Cons of Aux Fed Subs, Pt. 1

One of the great things about the pro audio is that one has the ability to change one’s position on a particular topic. This is a fun one for me as I have argued on both sides of the issue at various points in my career. I suspect that the position I currently take might change again someday. Or not. One never knows. 

Full Range Fed

Before we get into the pros and cons of aux fed subs, I figure I should define what we’re talking about. There are primarily two ways to route audio to subs. The first is to send a full-range mono or stereo signal to some kind of a crossover which splits the signal into appropriate bands. This can happen in a number of different ways. In most modern PA systems, a DSP will split the signal to the subs, full-range mains and possibly fills and delays. Usually, the crossover from low to high frequencies in the full-range mains happens inside the speaker box itself.

There are other ways to do that, but for now, we’ll concern ourselves with two bands—sub range, which is typically 40-100 Hz, give or take a little bit, and everything above 100 Hz. And yes I know, sometimes subs go lower than 40 and above 100. We’re taking concept here. 

In this concept, the engineer simply mixes a full-range signal and sends it all to the DSP for splitting up amongst the appropriate drivers. The person who tunes the PA is the one who decides how loud the subs will be relative to the mains, and what the crossover frequency is. Once those parameters are set, the system acts as a cohesive whole and aside from making mixing and board-level EQ adjustments, the system is what it is. 

In a non aux-fed system, every channel is set up to send signal to all the speakers—subs and mains. The only things that determines how much of the channel goes to the sub are the high-pass filter and the amount of low frequency content. 

The top example is a full-range feed, the lower is aux fed.

The top example is a full-range feed, the lower is aux fed.

Aux Fed

The other way to handle the frequency division is to send two (or three in the case of stereo) signals to the DSP. The main would be a mono or stereo signal that will feed the main speakers as in our previous example. The additional signal is typically derived from an aux send and drives the subs. 

In this case, for a channel to show up in the subs, the engineer has to dial up the level in the appropriate aux on that channel. The engineer has to make a conscious decision to add something (or subtract it) from the subs. I’m not sure when the idea for this came about, but it’s an interesting concept. By controlling very tightly what goes to the subs, the net result should be cleaner low end. And of course, if the show is really bumping and you want some more, just push that aux master up and get you some more bass! Rock.


The famed economist Thomas Sowell once said, “There are no idea solutions, only trade offs.” That is equally true in live audio. While it might seem that aux fed subs are the bee's knees because you can very discretely control exactly what goes into them, there are some hidden gotchas. 

Today we’ve set the stage for how these two systems work. Over the next two posts, I’ll describe the pros and cons of feeding your subs on an aux. And I will issue this disclaimer; before you go out and start re-feeding your PA, make sure you think through the issue carefully. There are reasons to feed the PA both ways, though I do think one is better most of the time. But I’ll save that conclusion until the end.

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Training Audio Volunteers, Pt. 2

image courtesy of sergio_leenen

image courtesy of sergio_leenen

Last time, we started talking about how to train audio volunteers. We start with the system—getting them comfortable with how things are wired and connected. Then we move on with the basics of console operation; gain structure, faders and VCAs. Today, we start to get into the fun stuff. 

Ear Training—Listening, Deconstructing and Reconstructing

After set up and before the band arrives, we’ll pull up recordings for each of the songs we’ll be doing that weekend. By listening to the songs, we can discuss the leading instruments, the placement of the vocals, effects styles and the overall feel of the song. Then we talk about how we are going to recreate that with our band that week. By having the example recording fresh in their minds, it’s easier to build a good representation of it.

Again, virtual soundcheck is a great tool to use at this stage of training. On our training nights, we’ll do the same thing; playing back the original then recreating it with the tracks. I will start them off using my snapshots as a starting point, then as they get comfortable, change the playback levels of various tracks to help them learn to adapt to players playing louder and softer. Once they have a handle on building a basic mix, we move onto the next phase.

Advanced Skills—EQ, Dynamics, Effects, Plug-Ins

With more and more churches are going digital with their mixing consoles, it may be tempting to start off teaching new engineers all about EQ, dynamics and effects. I’ve found this to be problematic. Learning to mix is a bit like drinking from a fire hose, so I’ve tried to throttle it back a little bit and help them learn techniques in smaller, more manageable pieces. Only after they have a handle on building a mix do we start talking about the advanced stuff. Most of our starting EQ points are saved in our show file, so when starting off, the new engineers don’t have to do much anyway. 

As they gain proficiency, we start talking about EQ and how that relates to the mix. Again, virtual soundcheck allows us plenty of time to play with various settings using actual sounds without worrying about the effect on a service. If you have access to an RTA (Real-Time Analyzer), explaining the concepts of frequency distribution can be illustrated very visually. Dynamics controls and effects are taught the same way; lots of time on the console with tracks. 

While it is possible to train without virtual soundcheck, it is much more difficult. However, if you have a mid-week rehearsal, you can experiment quite a lot without affecting the monitor mixes. If you can do it, even recording one or two channels at at time will prove valuable as you teach how fast or slow compression attack times affect various instruments. 

Don’t Forget Relationships

Woven throughout the training process is an emphasis on getting to know the band. I tell my A2s, “Never pass up an opportunity to go talk with the band.” Some of the most successful engineers are not necessarily the best technically, but they have excellent people skills. By helping our trainees learn to relate to and communicate with the band, we do ourselves, the engineer and the entire church a huge favor. 

It’s a Process

I tell new recruits that it may take 3-6 months before they get much hands-on time with the console. After that it might be another 3-6 months before they’re mixing a service. Training an audio engineer is not like training an usher; it’s a big task, but one that comes with big rewards. Both the trainer and trainee have to be committed to the process and willing to spend the time it takes to make everyone comfortable. In smaller churches, this process can go much faster; larger churches may require more time. The important thing is to develop a process, then work the process. When done well, everyone wins.

DPA Microphones

Training Audio Volunteers, Pt. 1

Image courtesy of Nacho Torres

Image courtesy of Nacho Torres

Training new audio engineers is possibly one of the most difficult tasks the church tech faces. Mixing audio is half art, half science and half politics. Even in a mid-sized church, the audio systems can be quite complicated with plenty of opportunities for things to go wrong. Having trained dozens of audio engineers over the years, my training process has evolved steadily into a fairly structured process that starts with learning the system, progresses to basic equipment operation, mixing and learning how to interact with the band. 

Start With The System

While I used to start people off right at the board, I’ve found a better long-term strategy is to first teach aspiring engineers how the system gets put together each week. In my program, new audio team members start off as A2s and learn how to set the stage each week. By getting the mic’s, DI’s, monitors and other equipment out of the audio locker and onto the stage, they are in a much better position to troubleshoot when things go wrong. Because they are the ones actually plugging the mic lines into the snakes, making sure powered DI’s have power and putting batteries in the wireless mic’s, if something amiss is discovered during line check, they know how to fix it.

Here is a perfect example of this: The click track from our tracks computer was delivering a very low and distorted level, something we discovered during soundcheck. As we began troubleshooting, my A2—a high school student who had been on the team for about 8 months—suggested that perhaps the 1/4” wasn’t plugged in all the way into the audio interface. Sure enough, that was it. By actually doing the hard work of learning how the system is wired together, troubleshooting becomes second nature. As part of the training process, I would also set the stage up with a few things wrong and have the team troubleshoot and figure out what the issues are. We purposely build time into our weekend set up schedule to practice things like that. Once the A2 has demonstrated proficiency with the system set up process, we move them onto the next phase.

Learning Basic Mixing Controls

Let’s face it, once a mixing console gets beyond about 32 channels, they can be pretty daunting. For a new person, stepping up to a modern, digital mixer can feel a little like walking into mission control. So rather than try to explain every single function on the board at this stage, I work on a few basic functions—those functions will form the basis of a solid mix regardless of what console they would ever find themselves on. 

In the early stages of mixing training, we focus primarily on input gain, digital trim (if we have it available), faders and groups/VCA’s/DCA’s (varies by mixer). Getting the gain structure of the console right is the foundation for a good mix; and if the band is good and the system tuned well, everything else is decoration. 

We spend a lot of time getting gain structure correct. Following the time tested model, I’ll have them observe me run a sound check, then talk them through doing it themselves, and finally turn them loose on their own. Virtual soundcheck can be an invaluable tool here. By playing back tracks into the system, we can spend a lot of time with the A2s as they get comfortable with how loud things should sound (and where each input should end up on the meters). 

Building a basic mix is the next step, and for that we move on to the next phase; ear training. And for that, we’ll be back next time. Stay tuned…

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