Aux fed sub - one or two lines in?

[websterama]

Hi,

First time poster.
Did search for answer to this question, but no luck (perhaps because it's such an obvious/stupid one).
I'm going to be using my 1608 for my daughter's band next week.
I've done that a couple times already, with no issues, but i'm adding a new piece to the puzzle and don't want to screw things up.
We use 2 JBLPRX712s and we've just added a single sub, the JBL PRX715 xlf.
I know the default/path of least resistance is to just to run the main L/R outs  into the sub, and then feed the mains (with the subs built in cross-over handling things).
The sub has two inputs and passthroughs for this purpose.

But from what I've read I might get better (if not simpler) results by feeding the sub from the aux outs, enabling me to only send the instruments I want (kick, bass, floor tom?).

My question is this:
The band has a keyboard player that uses true stereo out with panning effects.
I'll be feeding that into the 1608 by linking two of the 1/4" inputs. Simple enough.
My question is whether I should be adding the keyboard to the sub-aux mix?
And if I so, would that mean I should be feeding a stereo signal into the sub by using two of the aux outs, linked?
Or just feed the sub a single aux-out and assume the board is mixing down both channels of the keyboard into a single mono out for the sub-aux?

Sorry if this is a super naive question, I just would like this to go smoothly and sound great and i've got a new variable in the mix.
And if there's a general primer on best practices in doing aux-send to sub, could you point me to it?

thanks!

[Greg C.]

Feed each channel of the stereo keys to the mono sub send. No need to burn up a 2nd aux. Since you'll be sending 2 channels of sub send from the same source, you might need to drop the sub send level to -3dB on each channel since they'll be summing at the aux send.

[websterama]

Thanks!
That's very helpful. Makes sense.
With regard to setting levels for the sub-mix, is there a best practice for  that?
I mean what to set the gain at on the sub itself vs. what to send out via master fader.
And i'm assuming, during the performance, that the one "drawback" of this kind of operation is that when I adjust the volume of the mains that has no impact on what I'm sending to the sub right? So i'll need to adjust those two things manually in tandem?

[Greg C.]

On most mix desks these days with good noise floors, I always run the main outs at unity anyway as well as the aux masters. On bigger desks, I'll assign a VCA/DCA to the mains and sub aux so that in the event I need to drop the levels to the mains, I can track both subs and tops with one control. As far as setting the sub level, you'll just have to "ear-ball" it with a known music source and set the sub so it's balanced with the tops relative to what you're playing. You can use measurement gear if you're so inclined. Make sure you high pass your tops properly since you're not using the crossover in the sub to do it.

[websterama]

Thanks, that's helpful guidance. Looking forward to putting this sub into service on Friday

[websterama]

I have one last question, prompted by your last comment.
I know I should put a high pass filter on the tops (since I'm not feeding them via the sub, but rather directly) so that I'm not duplicating output at the same frequencies.
I'm just not entirely certain where to cut. The manual for my sub http://jblpro.com/www/products/portable-market/prx700-series/prx715xlf#.UyDirlFdWZ8 says that the loop-out crossover has a "120 Hz analog 24 dB filter slope".
Since it was designed to be used as a companion to my tops, should I take that to mean I should replicate that in Master Fader?
And conversely, I'm assuming I can feed the full signal (without a filter) to the sub itself. Since it has an internal crossover that only attempts to reinforce low frequencies.
Again, sorry if this is super-noobville but I haven't use the 1608 this way.

[Greg C.]

I took a quick look at the sub's specs. They indicate that the sub is internally low-passed at 100Hz with a 48dB per octave filter which is very steep. You should be able to cross your tops as low as 100Hz too, but they probably chose 120Hz to reduce the overlap a bit. Don't be afraid to shift that high pass point down to 100Hz if it sounds better. Experiment. And you're correct that you don't need to low pass the sub. It's taken care of.

[websterama]

Super. Thanks so much!
I love this forum. 

[stevegarris]

I have a similar system, and can offer a different approach:

Run your Left Main Output cable to your sub.
Run your Right Main Output cable to your 712's (jump from one to the other).
Be sure and run your 712's on the full-range setting.

Now you can control how much of the channel goes to the sub, top or both, via the balance control on each channel strip.
The kick drum, bass guitar and keyboards would be set at 50/50.
The guitar, other drums and vocals panned hard right.

This way you can stay on your main mix page and just utilize the pan control on each channel to get the needed lows. I use this process with my PRX615's and xlf18's. I run the 615's at about 11 o'clock, and the sub's at around 3 o'clock (volume on back of speakers).

The above configuration gives me a noticeable improvement over running everything through the speakers and relying on the built-in filters.

[websterama]

That's a really interesting idea. I'll test it out during rehearsal and see if it makes sense.
I thought the conventional wisdom was not to send the same content to both sub and top?
Or is that one of those hotly debated religious issues I've just accidentally stumbled into?
Thanks for the suggestion.

[Greg C.]

That's a really interesting idea. I'll test it out during rehearsal and see if it makes sense.
I thought the conventional wisdom was not to send the same content to both sub and top?

That would be correct. Sending full range signal to the tops does 2 things: 1) is that it causes excessive overlap of frequencies being produced by both sub & top which leads to uneven frequency response as well as possible comb filtering of those overlap frequencies; 2) it causes the headroom of the woofers in the tops to get eaten up and increases intermodulation distortion of those woofers.

When the subs are already reproducing the lower frequencies and the tops are properly high passed, the tops usually gain clarity and have more headroom in their optimal band pass instead of wasting amp and woofer resources at frequencies where they're not as efficient. And the more low frequencies you throw at a driver reproducing vocals, the more IM distortion you get. Sometimes it's ok to have some overlap and it can increase the "punch" of certain instruments depending on the rig. This is where experimentation on the crossover point vs sub/top gain comes in. But to let the tops operate as low as they can go is probably not the way to go IMHO.

[stevegarris]

That's a really interesting idea. I'll test it out during rehearsal and see if it makes sense.
I thought the conventional wisdom was not to send the same content to both sub and top?
Or is that one of those hotly debated religious issues I've just accidentally stumbled into?
Thanks for the suggestion.

No - not debated at all. You must send a signal to both sub and top for instruments that require the full range, i.e. kick drum, bass guitar, keys.
For things like vocals and guitar, you can simply use your tops full range or blend in you sub's to taste. I prefer to keep vocals, guitar & snare out of the sub's entirely.

My method is essentially the same as running aux fed subs, except you only have 1 fader to control each channel instead of 2 (good).
Running from an aux mix, if you have your kick drum going to the tops on LR out, and going to the subs on let's say Aux 6, then you need to use (2) faders to adjust the volume. With my method, only (1) fader on the LR main page needs to be moved.

[sam.spoons]

That method seems fine on the face of it but, as GregC says, it involves sending a full range signal to the tops, this would be fine for certain speakers (QSC K12 for one) which have a crossover built in, it is not suitable for the OP's PRX712s which don't (and you mention setting your tops to full range which would be wrong for all the reasons Greg stated). Used with an external active crossover it would work but that defeats the biggest advantage of the DL1608.

The object of aux fed subs is not so much to send certain things (kick, bass and keys usually) full range but to keep the mics on things which don't have low frequencies out of the subs, the theory is that it works better to not send them to the sub at all rather than the normal method of using a HPF on the channel. The advantage is reduced 'stage rumble' and the consequential improvement in clarity.

[websterama]

Ok, I think I get the general philosophy on this now. All of your feedback has been very helpful.
On a more tactical level, I've been playing around with the Master Fader app in order to better understand where to implement this.
If I'm understanding it right, seems like the procedure is to select the LR output channel, tap the EQ button, and then swipe to the panel with the parametric EQ.
On that panel, I selected the HPF slider at the bottom, and moved it up to the desired level. In this case, for my tops, seems like I'll be experimenting between 100-120hz.
The default HPF setting appears to be an 18db/octave curve. Other options include 6/12/24 db. Any reason not to leave at default?

And I don't need to bother putting a LPF on the aux that is feeding the sub. Just only raise the levels on the few instruments I'm sending there.

As for my 3 stage monitors, I'm assuming it might be worth setting a HPF on those as well?
I typically only send vocals, guitar and some keys to those monitors, so not sure it matters.
Thanks again for helping a fellow 1608 brother out!

[Greg C.]

I'd use the 24dB per octave filters for all high pass needs. Not a fan of odd order filters. By all means use HPFs on the monitors. I do it all the time - especially since I have old Mackie SRM450s for wedges that tend to overheat easily with a lot of low frequency content.