Question about running brided mono

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digitalphantoms

New Member
Hey guys,
one question.... My mixer has balanced right and left outputs (XLR). If you are running brided mono, what would be the proper way to get both channels to the crossover? I don't have a bridged mono switch on the mixer, or anything that would combine the channels at the mixer. I think using a XLR "Y" splitter backwards would work, but wouldn't that change the impedance? How do you guys do this?
p.s. I obviously can't spell..... sorry for the lack of attention... :oops:
Thanks,
 

#1someguy

New Member
Which amp is it? Many amplifiers will internally mono sum when you are running bridged and present it with two input signals.
 

Jackie

New Member
Using a Y cable to combine signals would not work, it will cause cancellation/phasing

You can get a signal combiner that subs two XLRs into one.. (i think they run about a 100 bucks)

or you can get a new crossover that has a mono-sum output (think rane).
 

Jackie

New Member
digitalphantoms said:
I am using a QSC PLX-2402 and a crown CE-2000.... does the plx have mono sum capability?
nope, only input channel 1 is used during bridged mono
 

DJGK

New Member
Jackie said:
Using a Y cable to combine signals would not work, it will cause cancellation/phasing

You can get a signal combiner that subs two XLRs into one.. (i think they run about a 100 bucks)

or you can get a new crossover that has a mono-sum output (think rane).
I think Jackie nailed it here. Your best best might be to get a crossover that offers a summed mono out. I use a Rane SAC-22 which has this feature. The Mojo series crossovers and the famed dbx Driverack also offer this feature.
 

digitalphantoms

New Member
I have been putting off getting a dbx driverack for sometime... I guess there is finally a "necessary" reason to get one.... These things can do everything except push "play" for you... lol.... so anyways, thanks for the input... p.s. whats the best deal you guys have seen on a driverack and where?
 

Bob Dietrich

Active Member
My 1st question would be why are you considering running the amp in bridge-mono?

Could you give us a complete rundown of the rig, current configuration and your plans for bridge mono operation? What is the ulitmate goal?
 

digitalphantoms

New Member
Bob,
Thanks for taking interests.... As far as running bridged mono goes here is why I had to do it.....First of all, I changed my set up this last year... I went from all crown ce-2000 amplifiers (3) to running active subs and passive tops. Because of the weight issue, current draw, and because everyone here talks up the plx series amps, I decided to sell the ce-2000 I was running my tops with and buy a PLX-2402 to replace it. I figured that the plx was rated higher than the crown and would be a comparable switch...

And well, I don't really want to open this can of worms, but to be honest I will have to... Once I got the 2402 and installed it in my rig, I tested it, and to my surprise I wasn't happy with the output at all.... I checked and re-checked myself and my set up to see if maybe I had something set up wrong and I didn't.... The plx couldn't touch the crown! I was running the crown in stereo mode and it had plenty of power... And if I needed more, I would daisy chain a second set of 15" tops in stereo to bring the load down to 4ohms per side resulting in more output... But I only did that once at a large event.... But I quickly found out that the plx couldn't keep up in stereo mode... So I needed to either add another set of tops for every show, or bridge it... Bringing in another set of tops wasn't an option.... (I decided to go to this whole set up just to eliminate weight and the amount of cabinets) So bridging it was the answer..... I have enough output now that it is bridged but my crossover is not set up to run summed output, hence why I started this topic.... So I know many of you will dissagree with me, but I didn't change anything other than the amps and bottom line, the PLX-2402 can't touch a CE-2000 in my case...

I am not bashing the PLX line at all... they definetely have their place.... if I would have known this before purchasing, I would have stepped up to a 3402 just to power my tops... I love their light weight, reliability, and low power consumption. But I even tried putting the 2402 and ce-2000 both bridged on the same set of 18" subs, and the crown walked away again.... And because of wanting to keep my rack light I will stick it out with the plx, but I have thought a couple of times about selling the 2402 and stepping up to a 3002 or 3402... So go ahead bash away at me, and I know many of you consider the CE line to be entry level, but if I wasn't mobile, I would have a crown powering everything. So Bob, to finish answering your question, my normal rig consists of this:
a PLX-2402 Pushing a pair of AA 15" Molded speakers, and the B-52 LX-18A running itself plus its brother the LX-18.... If I need more output, I run another set of tops (switched back to stereo) and another set of 18" subs...
 

Bob Dietrich

Active Member
Ok, either amp is easily overpowering the top speakers in bridge-mode and could easily lead to premature failure.

2nd, your top speakers are a very poor match for your bottom speakers as you have an extreme mismatch in sensitivity and output. While you have a Howitzer on the bass you have a a 38 snubnose for tops if that makes it any easier.

3rd, If you need more than what a 2402 can produce in a biamped mode for those top cabinets you're already pushing them too far. Further, I suspect you might have a problem with the 2402 as that isn't typical in terms of output when compared with the CE-2000 under normal operating conditions. I'm wondering if the limiters are set unuallsy low clamping the output down far below what it should be.

The difference in bridge mono and dual channel operation is significant to be sure at maximum output, 1200 wpc compared to 425wpc. While definitely significant numerically speaking, in terms of actual speaker output we're talking less than 6db and likely far less than that due to speaker compression. If we're realistically talking about a 3db to 5db difference it clearly shows you're simply expecting far too much from your top cabinets...especially if you run them full range.

I would recommend running the mixer output into the B-52 active subwoofer input, then use the hi-pass outputs back into the PLX-2402 and you have now effectively cut the frequencies below 120Hz from the signal for the top cabinets...something you definitely don't need anyway with the groundpounders you have. This should easily yield more than enough output to achieve a decent balance.

Keep in mind, there's absolutely no reason to use the top cabinets full range with the subs as they have so much more output in the 120Hz range and down that they easily cover up any bass made by the top cabinets. The top cabinets are also robbing precious power from the amplifier reproducing these frequencies that cannot even be detected. Hence, the whole idea behind bi-amping to begin with. This is to make a system more efficient overall.

While I still feel the speakers you have is quite a missmatch, I feel that if you run the system as I mention above it will yield far better results without the need to run bridge-mono...and one final thought. I would NOT consider running bridged when running this configuration as it's simply overpowering the top cabinet and you can easily toast a driver with that much power for only mid/hi's.

I hope this helps! Let us know.
 

digitalphantoms

New Member
Bob,
Sorry for the confusion... I forgot to mention that I do have the plx crossed over.... That is one reason why I started this topic... I am using a beringer crossover that only has 1 input for bridged mono.... I am currently crossing over the plx somewhere between 175-200hz.... sometimes higher... So that was one reason why I couldn't understand why the amp wouldn't power these tops... As pointed out, normally you don't need as much wattage for your highs as you would the lows, thats why I thought I bought an amp with plenty of headroom running stereo... When I was testing the plx out in stereo mode it wasn't clipping(mainly because of the limiters) but it was frequently at the -10db led, which made me believe that it was running with not much headroom.... So when I bridged it, I got the headroom I needed... normally running at the signal and -20db leds.... no where near the -10db.... So I don't think I am overpowering these speakers, especially since the woofer doesn't recieve any signal less than 200hz on most occasions... My main concern was overpowering the tweeters, so I really spent time with my EQ taming some frequencies down... But anyways, I don't think I mentioned which American Audio speakers I have.... here is the link...I of course have the unpowered version...http://www.adjaudio.com/product.asp?ProductIDNumber=1227&cat=Speakers Bob, to be honest I think the American Audio's handle the B-52's with ease! Matter of fact, I have run 4 subs with only 1 set of the American Audio's on frequent occasions, without pushing the AA's that hard at all! I bought 2 sets of them thinking I would need the other set for big events, but I have found myself leaving them in the trailer more often than not... Maybe its just me, but I think you would agree if you had the chance to work with this system, The AA's with the PLX kinda leave the B-52's longing for more..... I agree sending them 1200 watts a piece isn't a good idea, and when I am realling leaning on the system the PLX is probably only sending half that to the AA's....

But I agree with you on the limiters on the plx... I noticed that when I turned them off, I had a ton more headroom with out clipping the amp.... Hope this helps a bit.... But I might just go totally active.... what do you think? I might sell the plx and the AA's and go with active tops out of the B-52 crossover like you mentioned....
 

NickyB

Gear and Equipment Moderator
DigitalPhantoms -

Just for grins try this and your financial investment is minimal.
Get yourself a pair of low level, two RCA female to one RCA male Y adapters from Radio Shack. Y jack the inputs from your CD player(s) to your mixer into the left input for their respective channels. Now turn the stereo pan control to play only the left channel. Use the mixer's left master output as your signal out. This will set you up for Mono mode out of your stereo mixer.

Now I agree with Bob in that there is seriously something wrong when a PLX-2402 cannot out perform a CE-2000 in every way except current draw from the wall. How well does the 2402 perform driving your subs? The efficiency of your tops could very well play a part here. Also double check all your cabling and the settings on your Behringer crossover. If you're using the Speakon outputs from the amp, did you re-wire the connector for bridged mono use? If using banana connectors, were you using both 'red' speaker outs or one red/one black?

NickyB
 

DJGK

New Member
digital, where do you run the gains on your PLX? One thing I noticed when I first switched from my Mackie 1400i's to PLX 3002's earlier this year is that the PLX's seem to like a "hotter" input signal to reach their potential vs. the Mackies. I typically run a pair of EV SX300's along with a pair of CV SUB 15's, crossed over by a Rane SAC22. Although the PLX's have more than enough power for the cabinets I use, I also felt that they didn't have the "oomph" I was expecting but once I adjusted the outputs of my EQ and crossover, the PLX's kicked butt and never looked back. Upon checking the specs for both amps, sure enough, the input voltage requirement of the Mackie is lower than the PLX for full power output (1.23 V vs. 1.7 V). You may want to experiment a little more before giving up on the PLX 2402, I suspect you may find the same thing I did. Just make sure you aren't clipping anything in your signal chain but you make want to see if pushing the amp a little more fixes your problem.
 

digitalphantoms

New Member
NickyB,
I was thinking of trying what you mentioned regarding the y-cables...but wasn't sure if that would change the signal the amplifier was getting... so i didn't do it yet.... I am positive everything is configured correctly...I used the bananna outputs on both the plx and the CE. All I did was switch out the amps, and changed nothing... and the CE just had more output in stereo mode... like I mentioned, the plx was hitting the -10 db led right before the clip light really soon... Now I am sure the efficiency of the speakers has something to do with it, but using the same speakers, the CE just had more output, bottom line... it wasn't a ton more, but you could definetely tell it powered the AA's better than the plx... Regarding the subs.. With boths amp in bridged mono pushing a set of 18" eminence subs Dave Nye built the crown again won... It again had more output... The subs had more of a nice punch, and it really seemed that the crown had better control over the woofer at lower frequencies... And I never ran the CE to clipping... Never once saw the clip light flash in either tests...
DJGK,
I think I remember reading some posts regarding the sensivity issue of the Plx's and having to run the gain a bit more on these units compared to others....... I normally have the plx starting to get close to clipping at 28 or 30 of the total 32 on the gain when my rack gear is close to 0 db.... Which compared to the gain level on the CE for the same volume is much higher... For the same output I would normally run the CE between 1/2 and 3/4......
 

Bob Dietrich

Active Member
If the -10db lights is just flickering on the 2402, you're leaving a LOT of current on the table...or in the amp. -10db amounts to about 50wpc of the 425 available. There's a monumental difference between when these -10db lights flicker and when actual clipping begins.
 

digitalphantoms

New Member
Bob, thats what I thought..... But why does the amp limit itself down this far then? When I run the gains up on the amp it will keep itself to just flashing itself into the -10db area, but if I turn the limiters off it sounds like there is a ton more headroom, and the -10db light is on 95% of the time...why wouldn't qsc put another light like a 0db in between the -10db and clip light if there is lots more headroom....I never drove the amp into clipping so to tell you the truth I assumed that the -10 db light was very close to maximum output.... Like you mentioned, I allready knew these speakers were getting too much power, so I wasn't even about to actually push the amp into clipping just to see "for sure" when it would clip... then i surely would have lost them.... I just assumed that the amp was at 100% volume when the limiters kicked it.... So I am thinking I almost want to hook the plx back up to the subs in bridged mono mode and turn the limiters off and push the amp until I hear an audible clip... what do you think?
 

Jackie

New Member
it's extremely hard to tell when the amps are putting out equal amounts of power. QSC has always required a hotter signal than other amps to acheive it's full potential, that's why other people think that the other ampis more powerful.

compare the amps when they are both just barely touching the clip lights - and leave the clip limiters on! Don't be afraid to give the PLX a more powerful input!
 

Elloit

New Member
digitalphantoms said:
Bob, thats what I thought..... But why does the amp limit itself down this far then? When I run the gains up on the amp it will keep itself to just flashing itself into the -10db area, but if I turn the limiters off it sounds like there is a ton more headroom, and the -10db light is on 95% of the time...why wouldn't qsc put another light like a 0db in between the -10db and clip light if there is lots more headroom....I never drove the amp into clipping so to tell you the truth I assumed that the -10 db light was very close to maximum output.... Like you mentioned, I allready knew these speakers were getting too much power, so I wasn't even about to actually push the amp into clipping just to see "for sure" when it would clip... then i surely would have lost them.... I just assumed that the amp was at 100% volume when the limiters kicked it.... So I am thinking I almost want to hook the plx back up to the subs in bridged mono mode and turn the limiters off and push the amp until I hear an audible clip... what do you think?
I think you should direct your question to QSC. From what you
are stating leads me to believe that the amplifier is compressing
the signal @ - 10dB.

Seeing, that I don't own a PLX, I can't guarantee anything, but,
as far as the EX, and, PL is concerned, the limiter activates at
clipping or 1% THD.

DJGK, is spot on with the input sensitivity between the CE, and, the
PLX. This will play a bigger factor when using two different brand
amplifiers, than using the LED indicators as reference among the
two.

The CE 2000 offers two Sensitvity Settings 1.4 & 2.83 @ 8 ohms

The PLX 2402 offers a 1.5 volt setting @ 8 ohms

So, you'll need to check to see what Crown settings are.

The fastest, way to calibrate both amps, is to feed the amplifiers
with Pink Noise (With no speakers attached) and raise both gains
until they clip.

Its not 100% accurate (97 - 98%) but, it will give you a better
outlook on where you stand than what you are currently doing.

Another thing you need to take in account is the low frequency
filters on these amplifiers. Both will have their own shelving,
and, will alter the sound. So, you should try both without any
filters engaged.

Perform your test having both amplifiers running with no
limiters, and, no low frequencies filters. They should sound
the same. If they still don't, well use the amp that works
bests for the given frequency.

I own various brands of amps, and when it comes down to
the bass department, different amplifiers respond differently
to those very low frequencies.

Best Regards,
 
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