Aux feed subs?

Robert Lofgren

Robert Lofgren

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Oct 5, 2012
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Stockholm, Sweden
Is aux feed subs being used in the the really big concerts/setups or are we still in mono/stereo territory?

If aux feed subs are being used, how do you handle fx inserts that needs to be linked on the main bus and subs bus?
 
Re: Aux feed subs?

What kind of FX are you talking about. All the verbs and delays I use need to stay OUT of the subs...

Yes the big boys are using aux driven subs, however it's a bit different on the bigger rigs. Most big systems have a very full range response. They may play down below 50hz without any problems. Sometimes there will be a line of full range boxes, with a flown line of subs that are tied in to L/R. The aux subs could be stacked on the floor. The aux subs are brought in as extra horsepower for kick drums and bass instruments.
 
Re: Aux feed subs?

Tim Weaver said:
What kind of FX are you talking about. All the verbs and delays I use need to stay OUT of the subs...

Yes the big boys are using aux driven subs, however it's a bit different on the bigger rigs. Most big systems have a very full range response. They may play down below 50hz without any problems. Sometimes there will be a line of full range boxes, with a flown line of subs that are tied in to L/R. The aux subs could be stacked on the floor. The aux subs are brought in as extra horsepower for kick drums and bass instruments.
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I am fairly certain he is referring to stereo linking dynamics processors.

Personally when I run mix big PA's I will run it mono and stereo link the Main and the Sub out..... you may or may not want this depending on the content you are mixing.

This is easy for mono PA + fills and mono subs, for stereo PA + subs you can get creative with your side chain linking if you are on analog or side chain linking / stereo linking is trivial on most digital desks.

I personally don't use a mix buss compressor, but instead use sub-group compression as a preference.

YMMV
 
Re: Aux feed subs?

Greg Cameron said:
40Hz eh? And I thought I was relatively low with my tops at 60Hz. You don't run into LF driver headroom issues running that low?
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Evan said "I usually try to get the main PA to run down to 40hz or so."
The main PA would also include subs, additional aux fed subs (that he may bring in) used for effect .
 
Re: Aux feed subs?

Art Welter said:
Evan said "I usually try to get the main PA to run down to 40hz or so."
The main PA would also include subs, additional aux fed subs (that he may bring in) used for effect .
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Thats what I was trying to say, but I didn't make it too clear.

Often the main PA is a full system with subs included, then we ADD aux subs to that. Nothing like a little overkill!
 
Re: Aux feed subs?

2 main issues to consider with aux fed subs:

1) If its your primary sub (not an effect) Its better to run them as a fixed level send at 0dB than an aux. By changing the level of the sub send, you are moving the crossover point of the PA, which is less than ideal, particularly if your sys tech has worked hard on the freq/phase response and lineup.

2) Depending on your console, you need to make sure the main and sub outputs are in-phase. Some consoles will delay-compensate the outputs. On others, you need to make sure that the 'path length' is the same to achieve this.
 
Re: Aux feed subs?

Chris Johnson said:
2 main issues to consider with aux fed subs:

1) If its your primary sub (not an effect) Its better to run them as a fixed level send at 0dB than an aux. By changing the level of the sub send, you are moving the crossover point of the PA, which is less than ideal, particularly if your sys tech has worked hard on the freq/phase response and lineup.
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In my experience, most big systems with aux subs have the subs on the floor with the tops flown, so they're only going to line up in one spot of the venue anyway. I generally run the sub sends at unity relative to the tops, but there are those times when a sub send for say bass guitar is just too bottom heavy at unity with a particular musician's setup and needs to be dialed back. Yes, you can tweak the channel EQ, but it's not the same. Sure, you're lowering the acoustic crossover point by dialing back that channel's sub send. But in my experience, it isn't a problem.
 
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Re: Aux feed subs?

When I started doing sound I used aux-fed subs so I could get more oomph out of my systems, but as I've been learning and working with better gear I've strayed away from having to mess with gain structure/clipping amps nonstop.
One thing that I still like about aux-fed is that I can pick which channels go into the subs and which are out entirely.
 
Re: Aux feed subs?

Tnx,

belonging in the JV the subs are primary with a cutoff at around 100Hz. I usually call it aux feed subs since this seems to be the common word for it. However, in my case it is run thru a seperate bus fader linked to the main bus.

In my case I wanted to insert a loudness enhancer into the main mix just to realize that the end result doesn't work due to the total signal being split into two paths. Even if I use two fx-processors the end result will become "skewed" due to the units not operating aware of each other.

By the answers so far it seems that the big boys/girls are running mains in full range but aux feed a sub-bass for fx and can therefor use "mastering"-fx on the main bus w/o any real issues.
 
Re: Aux feed subs?

Chris Johnson said:
2 main issues to consider with aux fed subs:

1) If its your primary sub (not an effect) Its better to run them as a fixed level send at 0dB than an aux. By changing the level of the sub send, you are moving the crossover point of the PA, which is less than ideal, particularly if your sys tech has worked hard on the freq/phase response and lineup.
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This is almost correct, what actually happens is that the slope ( which can be quite shallow ) moves position. So lets say the -6db point moves lower and therefore is in a slightly different place on the frequency scale. The actual chosen crossover frequency does not move. Here is to remember that the steeper the slope ( or Filter) the smaller this artefact becomes. On a LR24 this is not a major problem. So depending on the applied Filter this Problem can be fairly big.

Chris Johnson said:
2) Depending on your console, you need to make sure the main and sub outputs are in-phase. Some consoles will delay-compensate the outputs. On others, you need to make sure that the 'path length' is the same to achieve this.
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This should however be a fixed value. So once time and phase aligned this value should be stable. What you probably refer to is that the Filtering in the crossover creates a different phase alignment depending on gain. Again this depends on type of Filter applied and its relationship to the gain used here.

But I have to agree with you in essence. And we could say that : "If we agree on that phase is gain dependant, we must also agree on that sub on aux is by definition wrong"
 
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Re: Aux feed subs?

Gert Sanner said:
This is almost correct, what actually happens is that the slope ( which can be quite shallow ) moves position. So lets say the -6db point moves lower and therefore is in a slightly different place on the frequency scale. The actual chosen crossover frequency does not move. [/B]
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The electrical crossover frequency doesn't move, but the acoustical one does. That's the one I care about.
 
Re: Aux feed subs?

David Sturzenbecher said:
What makes you think phase is gain dependent as well? Because it's not...
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again you could say that if your phasealignment is done lets say for 40 HZ and you reduce the gain of the sub feed your acoustical crossover ( I like that term) moves and therefore your phasealignment is no longer at the right frequency. Thats what I mean by phase is gain dependant. But if you just look at a single value it of course isn't. It is more the relation between the two that has changed.
 
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