The Pros and Cons of Aux Fed Subs, Pt. 4

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Today we’ll wrap up my three part series on aux fed subs. Yes, I know this is part four; consider this my “one more thing.” As you have been thinking through the pros and cons of feeding the subs on an aux, you may be wondering how else we could do this. Is there a way to gain the benefits of an aux fed sub without the drawbacks? I’m glad you asked, because yes Virginia, there is. Or are. We have a few ways to do it. Which one you choose will depend on your console’s capabilities. 

Group Fed Subs

If I were the only person mixing on a system, this is how I would probably set it up. To use this method, you would use two groups, one for the main speakers and one for the subs. Assign all your channels to the mains group and only the channels you want to appear in the subs to the subs group also. The nice thing about this is that the gain structure is maintained. On most consoles, sending a channel to a group is a unity gain send; that is, there is no change to the gain when you assign it to the group. This keeps the crossover and thus the sub timing in tact. But you still get to choose what goes to the subs. 

On the output side of the console, you would just route the groups to outputs and run those to the DSP. Not all consoles will let you send a group directly out, however. This can be a challenge with analog consoles as well. But it doesn’t mean we’re done yet. You can combine this method with the next one to get the two groups out of the console.

Matrix Fed Subs

If you can’t route a group directly out, you can probably route a group to a matrix. In this case, you’d need two matrix mixes, a sub and mains. Doing this adds another level of gain staging, so you do have to be careful how you set everything up. It’s important that the levels are the same at the group level and the matrix level. 

On many digital consoles, you can route individual channels to the matrix. Some, Yamaha’s M-7, CL and QL for example, let you route any or all of the channels to a matrix. The matrix can either act like an aux with variable level for each send or like a group with fixed level. You want to use the fixed level mode for this to work properly. Remember, we don’t want to have level control between the sub feed and the mains feed; we simply want routing control. 

Bonus Round

What I do on Digico consoles (and perhaps others can do this as well) is build a subs group in addition to my master group. I then gang the faders together so they track at the same level. I often won’t even bother assigning the sub fader to the surface because it’s simply going to track the master. Then it’s a simple matter of assigning the channels I want to go to the subs to the subs group and I have the best of both worlds. 

Like I said, how exactly you do it may depend on the capabilities of your console. It may take some experimentation to find the easiest way to do it that doesn’t put an unnecessary burden on the volunteers, but gives them the control. Of course, standardizing your inputs, building a baseline show file and working from that file every weekend goes a long way to making sure everything works as expected. But that’s another post. Or three.

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The Pros and Cons of Aux Fed Subs, Pt. 3

Image courtesy of L'Acoustics (SB28)

Image courtesy of L'Acoustics (SB28)

Continuing our series on aux fed subs, today we’ll discuss the cons. Last time, we talked about the pros—and there are some—and before that, I told you how this concept works. 

All good pro/con lists have both. And this concept is no exception. I’ve given you the pros, and now it’s time too look at the drawbacks. 

Constantly Changing Crossover

This is one of the big issues for me. The crossover point is defined as the frequency at which the two drivers are the same level. In our hypothetical example, let’s say that at 100 Hz, the subs and the mains are the same level. Below that, the mains drop off, as do the subs above that. When it comes to setting the timing between the subs and mains, we want them to be reproducing the crossover frequency at the same time. That is, if we send a 100 Hz burst out of both the subs and mains, it will happen at the exact same time. This makes for very clean and tight low end.

The best way to set that timing is by aligning the phase for both boxes. Phase is time, and it’s easier to see time in the phase trace than it is in say an impulse response. So we line up our phase with delay and the system is aligned. 

However, if we’re constantly messing with the level of the subs using that aux master fader, we’re sliding the crossover frequency up and down. As we do that, the timing is going to start to drift. Not a lot, but it can enough that the low end starts to smear and become less clear. Flabby, loose and muddy are all terms we hear when the bass isn’t aligned to the mains. Aux fed subs make it really hard to lock this down. 

Opportunities for Errors

Whenever we add complexity to a system, we increase the opportunity for failure or error. In this case, it’s all too easy to dial way too much of something into the sub aux, thus skewing the mix. Most instruments produce sound over a wider range than just the sub coverage. 

The kick for example has plenty of sub-100Hz content. But it also has a lot of information above that, and to make the kick sound nice and clear and punchy, we need it. If someone dials the sub aux on the kick all the way up to 11, pushing the fader up all the way will make the kick too loud. So it will get turned down. The low end will still be there, but the top end will trail off. A novice engineer might try to compensate with EQ which only further exacerbates our timing issue. Eventually, the low end is mush. Multiply this by 4-5 more channels and you have a recipe for ugly bass.

Added Complexity

This is similar to the above, but I point it out especially for churches that use volunteer, non-professional help behind the console. Aux fed subs are not necessarily hard, but it is harder to get it sounding right, and there is a bigger opportunity for things to go wrong. When setting up systems for volunteers, I like to give them as many opportunities to succeed as possible. Honestly, I don’t believe there are enough pros to outweigh the cons in an aux fed sub situation for most churches, so I go for simple when possible. 

Conclusion

By now, you probably know where I stand. Though what you may not know is that for years I was a staunch aux fed subs guy. I wanted the control and I felt I could do it better that way. However, as my understanding of PA tuning has expanded, I now believe I can get better sound from a full range fed PA. Having mixed on both, I know I can make either sound good. Overall though, I think a system where the subs are part of the main feed will generally result in better mixes for more engineers. Hear me on this; I’m not saying aux fed is categorically wrong. I simply think in most cases, for most churches doing a full range feed is going to produce better results. 

I can make a case that from a very technical standpoint a timed in sub system is going to sound better than an aux fed system (see the previous post). But again, I know some very good engineers who can make an aux fed system sound really good. However, they really know what they’re doing and they effectively treat it like a full range system.

As I write that, it occurs to me that I have one more thing to say on this issue. But that will wait until next time.

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?

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