Mono vs. Stereo with DRPA and JBL Subs

robberns

First time poster here....
Getting ready to launch my new PA (for small/medium sized clubs, bands mostly).
Have a pair of JBL PRX 415M's for the mains and PRX 418S (pair) for subs.
Powering them with QSC GX5 (mains) and GX7 (subs). Feeding from an Allen & Heath GL2400.
First question would be, any advantages to running the system in mono vs. stereo?
I run everything in the center anyway.
Second question would be crossover points and settings for the PRX418S subs. They say "80Hz" is recommended in the literature.
I've fiddled with the DRPA using the 2-Way Stereo preset. Just not sure where to go from there.
Thanks for any advice....

Dra

I have not done "band" work in years, but Gadget is fond of saying that 80hz is generally a very bad place to point a x-over point because it sets right on top of the kick's fundamental.

If everything is pan to center it doesn't matter. If the audience is between the stack then some "stage" steering could be used (guitar left panned to mostly stack left, for example). The other benefit is stereo delays. Keys also benefit from stereo. Even toms can be steered for neat effects.

That cleared things up for you didn't it?

DRA

robberns

Thanks for the thoughts, I was wondering about JBL recommending 80Hz for the subs.
Not sold on the idea of stereo though.

Gadget

My main problem with 80 hz is the kick drum, but it's not the only one... generally that would be considered "optimal system config" meaning you had the exact amps that they proscribe and the processor and everything.. However, if you don't have the "right power" ( that means a speaker rated at 1200, 2400, 4800 watts had a MINIMUM of 2400 watts per speaker and 4800 to give headroom...) How many of us have THAT kind of power? I know I don't... so we move the crossover point up to a more reasonable number like 100hz-120hz...

OklaPony

Try setting them up as mono subs and stereo tops.

The only PRX418S I've set up in the real world thus far I found to be very impressive, especially for the price point. I'm hoping that JBL will produce a 2 X 18 version utilizing the same Selenium driver.

robberns

Thanks for reply.
I spoke to a nice fellow at JBL who gave me some starting numbers for crossover points/slopes.
Loaded them into the DRPA and am giving them a listen.
So if you think I should run the subs in mono, would it make sense to run the amp I'm feeding them from in bridged mono or keep it in stereo?

Dra

I wanted to make sure (now confirmed) , but the GX series don't have a bridge option, nor go 2 ohms.
What are these startling numbers?

robberns

Son of a gun, you are right. For some reason I thought the GX7 had a bridge option.
So I guess just a mono feed from the DRPA and into each channel of the GX7. Not sure if there is any advantage to that or just running stereo panned in the middle?
I don't have the settings from the JBL guy with me, but I seem to remember 30Hz - 75Hz on the subs and 80Hz on the tops (PRX415M).

robberns

Checked the settings given by JBL:
Subs (PRX418S):
Output Lo Shape BUT 18
Output Lo Frequency 30Hz
Output Hi Shape LR 24
Output Hi Frequency 75Hz

Tops (PRX415M):
Output Lo Shape LR 24
Output Lo Frequency 85Hz
Output Hi Frequency Out

Dra

10hz separation? Interesting. That is not a little bit in that region. About 1/3 octive. One speaker or the other must have a bump in that region.

DRA

robberns

Yeah, seemed a bit weird to me. With that much of a delta between the two systems I'm wondering what happens to anything in between.

Dra

If you have access to a tone sweep, whether recorded or an actual generator, you could listen for a lull in the area.

DRA

Gadget

I take issue with the setups because they don't take into consideration the amount of power (or lack there of) of the system... The GX7 has only 800 watts @ 8 ohms and 1000 @ 4.. for sub duty that's not a lot, and I see no point in 30 hz power robbing HPF...although would be about right for the tops if set like that.

OklaPony

Gadget wrote:

I take issue with the setups because they don't take into consideration the amount of power (or lack there of) of the system... The GX7 has only 800 watts @ 8 ohms and 1000 @ 4.. for sub duty that's not a lot, and I see no point in 30 hz power robbing HPF...although would be about right for the tops if set like that.

I agree that those settings are kind of suspect, maybe the 75 in the lpf was a typo? If it were me I'd start here:

Subs (PRX418S):
Output Lo Shape BUT 18
Output Lo Frequency 45Hz
Output Hi Shape LR 24
Output Hi Frequency 100Hz
Tops (PRX415M):
Output Lo Shape LR 24
Output Lo Frequency 100Hz
Output Hi Frequency Out

then move the x-over point down 5 hz at a time after listening sessions to see where it operates best with respect to your amps. It appears that you're massively underpowered on your subs. I powered one of them with a bridged XTI4002 (2400w @ 8 ohms), you're not even in the same zip code as that.

Gadget

Agreed... but moving the xover point lower will only rob power from the tops...

robberns

Okay, I get the message about under-powering the sub.
So the JBL spec says:
800 W / 1600 W / 3200 W
(Continuous/Program/Peak)

What would I need from a power amp (I'd like to run one amp if possible to save having to lug more gear around).
Thanks

Gadget

Well, I think I would be trying out the new Crest Pro-LITE 3.0...

http://www.crestaudio.com/news/article. ... iew/id/561

The really good news is it only weighs 13 lbs :shock:

And the REALLY good thing is it's CHEAP

http://www.daleproaudio.com/p-10897-cre ... MgodUHEACw

Bridged Mono > 3000 watts

robberns

Wow, that is cheap!
So would you run one amp per sub in bridged-mono mode at 8-ohms?
1,770W bridged at 8 ohms @ 1%

Gadget

Hmm.. you would theoretically be only adding about 200 watts.. which isn't much.... But it would be more efficient that way...1575 watts per speaker in Bridged mode... If you have the power available to run them (could conceivably draw more that 25 amps with that one amp if you push it) I'd go with just one and see what happens...

These also SOUND amazing and I think have a 5 year warranty! :shock:

robberns

Okay, let me just get a reality check.
I could run the subs in mono (via the DRPA), and run one of those Crest amps in bridged mono to both subs.
Since the subs are 8-ohms, would I be giving the amp a 4-ohm load if the subs are in parallel?

Gadget

Reality check complete! Yes...

Otherwise there are some cheap 4000 watt and 6000 watt amps but nothing in that price or weight ratio.

800 watts = RMS, 24 / 7 full power would not damage the speakers but pretty lack luster performance...

1600 watts = Program running, 1/5 and even 1/3 power you should be fine if there is NO clipping (typical rock runs 1/5th power average if your not pushing it

3200 watts = Peak, only momentary peaks only, mostly headroom to help prevent power compression and power supply fatigue, and voice coil overheating... you would need to be careful here...

robberns

Have been doing some testing with the QSC amps I have and wondered if you could answer a (not so) obvious question on power.
If I run the JBL subs off each side of the QSC GX7, they'd be getting 700W each (based on them being 8-ohms).
If I run them off one side, connected in parallel to give the amp a 4-ohm load, the amp will give the subs 1200W.
But would they really only be getting 1200/2 = 600W each, since they are running in parallel?
Not sure if I'd gain anything from this configuration. Kind of a bummer that you can't run the GX7 bridged.

Dra

2 reasons to run 1 sub per side 700w is greater than 600w. (Not a lot, but still is.) and... though not as important as once thought, damping factor is the ability of the amp to control the "push / pull" of the speaker. The higher the number the more control. Amps have increased damping at higher ohm ratings.

If considering a larger amp to get more output, you are almost always better off adding subs to the amp to get a 4 ohm load. 4 subs @ 600w each gives more output than 2 subs would on a bridged amp (regardless of power).

robberns

Thanks for that info.
So was I correct in assuming that the 2 subs run in parallel would be halving the power?
I'm thinking of getting 2 of the Crest Pro Lite 3.0 amps and running each sub off one amp bridged (1770 watts)

Dra

Nowhere near halving the power. 100 watts would probably not yield any audible loudness change, though would be easier to run into clipping.

FYI. The difference in output from 700w to 1700w I only 3-4db. Adding a 2nd sub (600w + 600w) on the same amp yields 3db and setting the subs together yields another (up to) 3db.

Weigh your options. Cost of adding a amps vs cabs, factoring transport and setup of more gear, and don't forget... a sub running with 600w will last a tremendous time longer (as long as you stay out of clipping).

DRA