Aux fed sub question

[rschultz]

Hi,

Occasionally we experience trouble in controlling the low end at my church. One of the worship leaders suggested an aux fed sub arrangement. I've read about this and it seems to me there are pros and cons to it. I'm wondering if it would or would not be a good idea for my situation:

Two Main L/R speakers: Renkus Heinze TRX82
Yorkville 700P Sub - this sub is overkill for those mains IMO.
DBX Driverack 260, splits L/R from board to L,R,Sub
Room is 50' x 50' with low ceiling.

Currently the crossover in the DBX and the back of the 700P is set at around 120Hz I think. I was wondering if it would do basically the same thing as an aux fed sub if I shifted the crossover to like 80 Hz. This would keep the guitars and vocals out of the sub, and is the low shelf point for the channel EQ on the board - A&H GL2400. So instead of feeding the sub by aux, you could essentially adjust the sub in each channel with the low shelf.

Thoughts?

Thanks.
Ryan

[cmchamp]

Yes, I would try moving the x-over to 80Hz first. 80Hz is where I like to try to keep mine crossed over when possible, and your mains can handle it.
Yes, the 700P is overkill in your room, but you've got thump if you need to go out.

[prochlea]

I've never been a big fan of the aux fed sub. I guess it can be useful in situations where you don't have good speaker control, but you've got decent processing, so I would stay away from it. First - I'd turn off the cross over in the sub itself, you don't need it, use the DR 260. Then I'd start by moving your crossover down to 90Hz or so. I don't know the TRX82 that well - and it says it's rated down to 60Hz, but I'd guess it starts to fall off in the 70s or 80s. If you move you crossover down, you're sending less to the sub, and it should give you more control over the LF in the room. 120 is way too high.

[Arlin]

In addition to the channel low shelf filter, you can use the channel high pass filter button. That looks to have frequency cut-off of 100Hz.

I asked a professional about aux fed subs once, and his opinion was that they were more trouble than they were worth. Especially with digital consoles with adjustable High-Pass roll-off filters.

[cmchamp]

I have been in the business in my area for almost 20 years, and as yet, have not experienced a situation which I was responsible for that had high enough SPL at the stage to require aux fed subs to keep LF from open mics out of the subs. That said, the majority of work I do is for southern gospel quartets with and without live bands. I have done live contemporary christian festivals locally, but even then, haven't found it necessary to use an aux fed sub.

[rschultz]

Thanks for all the suggestions. I too am not in favor of the AFS.

I need to dig into the DR260 again to make sure, but my memory suggests I set the crossover at like 105hz... and I think i did what prochlea suggests in turning off the low pass on the sub, it's set way above the 105 (I think).

Side bar: some where complaining that the bass/toms/kick sounded weak. I checked the gain and it was turned down to -10 or so, but there was a piece of tape that said +20 on it. I can't believe anyone would have put that tape there besides ME. So I turned it up, then we had LF feedback issues. I think +20 is too high, but -10 was too low.

So, I have to balance the gain knob on the back of the sub with the gain on the mains amp, right?

[cmchamp]

Yes, if you've got an FFT, or even an RTA with a pink noise source, your room should be fairly balanced between sub and the full range RH's.
One with experience can generally balance that by ear.
C.

[Brad Weber]

There are a lot of misconceptions regarding aux fed subs and many poor implementations of them. They definitely have pros and cons and are not the best solution for everyone. That being said:

Changing the speaker system crossover frequency will will not keep a vocal mic from picking up low frequency stage sound and thus prevent that from being reproduced, it will simply vary which speakers reproduce any low frequency sound that the mic does pick up.

A variable high pass on each input channel is much closer to doing the same thing as aux fed subs, but you may have to consider the slope of the channel filter. Many variable low pass filters are variable frequency but have a fixed, and fairly sometimes, shallow slope so you may still get signal well below the frequency set for the filter.

I agree that you probably want to avoid the dbx as a crossover in conjunction with the crossover in the sub. Unfortunately, the LS700P does not let you bypass or defeat the integrated low pass filter, only vary it between 100Hz and 150Hz. Since you can't defeat the low pass in the subwoofer then instead of a crossover in the dbx you might want to high pass the main left/right and send the subwoofer a full range signal, then use the crossover in the sub. Either that or set the LS700P for the highest crossover point setting and be sure to crossover well below that frequency using the dbx.


Also keep in mind that what matters is not the settings of the electronics but rather the acoustic crossover between the outputs of the speakers. This is one aspect where aux fed subs can be more difficult but also useful. For example, you could have the sub with a 100Hz low pass and the mains with a 100Hz high pass but as you vary the relative level of the two the resulting acoustic crossover changes. Increase the sub level, or decrease the mains level, and the acoustic crossover moves up in frequency. Decrease the sub level, or increase the mains level, and the acoustic crossover point moves down in frequency. By being able to easily vary the subwoofer level aux fed subs provides the ability to turn down the subs and let the mains carry more for speech and then turn up the subs and let the subs take more of the low end for music. A nice trick but one that you have to realize what is happening in order to take advantage of it.

Another aspect of this is don't lock yourself into thinking that the high and low pass filters that make up a crossover always need to match in frequency and/or slope. If you run the subs high you may want to drop the low pass frequency or roll of the high frequency response more quickly. Don't be afraid to play with minor changes to some settings just to hear the results.

[rschultz]

Originally Posted by cmchamp

Yes, if you've got an FFT, or even an RTA with a pink noise source, your room should be fairly balanced between sub and the full range RH's.
One with experience can generally balance that by ear.
C.

DR260 has an RTA that I've used a few times to tweak the frequencies. But more grossly, adjusting the gain I've done by ear as you say.

I just wanted to confirm that raising the gain would in fact make it louder. I've read/heard some people say to open the amps up all the way, that it won't get louder unless the signal increases. I don't believe that though, I think the gain is basically a volume knob.

Correct my thinking if I'm wrong

[rschultz]

Thanks Brad, Cory,

What about using the Mono fader for the sub (bypassing the DBX and using the hardware crossover), and L/R fader for the mains going through the DR260 (with HP filter)? That way you literally could mix the low end to taste.

I did this once when the DR260 was in the shop, used a 2 channel 31-band EQ on the mains, no EQ on the sub.

I like this option better than AFS. But I'm not sure it is even necessary. Taking the sub out of the DBX and doing it manually is probably like a manual transmission instead of an automatic: some people like it, but it's a pain in traffic

[cmchamp]

Regarding crossover slopes:
Situation dependent, my crossover slopes between subs and mains vary from 6dB to 24dB.
As Brad mentions, when you increase the Sub level, slope change or not, you effectively change the crossover frequency. For instance, if the low pass (high frequency filter) is set at 100Hz with a 6dB slope, if you increase the output of the sub 3dB, you've also changed the effective crossover frequency to around 150Hz causing your sub and your mains to duplicate the frequencies between 100Hz and 150Hz creating boomyness.
With that same filter, increase your sub level 6dB and change the resultant crossover frequency to 200Hz.

[cmchamp]

You could run off the mono output, but your DR260 only has 2 inputs, so if you're running your mains stereo, you won't be able to utilize any DR260 processing for the sub.