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qsc rmx2450 and two 700w 8 ohm cabs

J

jam73

Aug 8, 2009
12
0
#1
Hello all. I've been searching the forum for amp info lately and found loads of stuff so I decided to register as a member.:)
I'm planning on getting a qsc rmx2450. I have two 2-15" cabs. Each cab is loaded with two 350w 4-ohm black widows run in series as an 8-ohm load. I am fine with running the amp bridged using both cabs together as a 4-ohm load. I am also not opposed to rewiring each cab to a parallel configuration to make each cab 2 ohms. I'd also like to frequently power just one cab alone (one at home, one at practice).
My questions are: Would I be happy with the rmx2450 if I swapped my cabs to 2 ohms and ran the amp in stereo? I could then run one cab off each channel of the amp. Or is this really too low of an impedance for the rmx 2450 to handle? Would I be happier and safer just running the amp in bridged mono into both cabs as a 4-ohm load, and likewise bridge mono into one cab at a time as an 8-ohm load?
Another direction I could go would be to rewire from a series 8-ohm load to a two separate 4 ohm speaker configuration in each cabinet, then I would only ever use one cabinet at a time. Would I too easily blow my 350-w speakers out if I did this? I've got lots of ways to go, I'm hoping for some high-power amp owners:bassist: to help me out- thanks
 
#4
So basically you are both saying to just run one cab per side of the amp.

bacan- i studied the manual and I understand what you mean about using the parallel inputs. This would also allow me to utilize one cab at a time, through one side of the amp, since setting for parallel inputs allows for useage of both gain level knobs, am I correct?

and Lindsay- pretty much what bacan said, right?

I was actually hoping it would work this way, but the amp specs say 450w at 8 ohms per side which I thought might underdrive my 700 w cabs.

I was wondering about bridging mainly because I'm worried the amp wont put out enough power for my liking going the one 8-ohm cab per side route.

Thanks guys I appreciate all the input and for now I'll still plan on getting the amp but i'm going to study some more...
 
#5
Yes, each knob would control the level of each cab.

There is no such thing as too little power. Will 900 be enough? I daresay it will be plently. I'm actually planning on getting a QSC poweramp, and am looking for around 500 w at 8 ohms per channel. Remember, on a standard circuit, you will never go over ~1600 watts anyways (I think this is correct) with a class D power amp. This includes everyone else (guitar, PA, etc.) So unless you are going to be playing an outdoor show with a generator and you don't have any PA and need to play for 5 times the number of people in a standard bar, then 900 watts seems more than enough.
 
#6
totallybacan said:
The only issue I can see with your setup is if you keep them at 8 ohms as series. If one blows, then you'll drop below 4 ohms bridged which is a bad thing for your amp. Just run the amp as parallel inputs and keep both volumes the same. IMO
Click to expand...

No you won't. If two speakers are in series if one fries the other is removed from the circuit. The impedance will rise to the 8Ω of the second cabinet.

Paul
 
#7
Paul is right about the load impedance rising if one cab blows (bridged as a 4-ohm load)...
I'm pretty sure the amp will run both my cabs at 8 ohms each at 500 w per side to an audibly acceptable level but I want to make sure I have plenty of headroom, know what I mean?
If I rewire both my cabs as 2-ohm loads in stereo, (1200w per side);) or use them as they are now and bridge the amplifier (2 8-ohm cabs, 1500w into 8ohm or 2400w into 4ohm), I'm thinking that's going to get me where I really want to be.
Bridging seems to be the best way with my current configuration, and i've heard the rmx2450 doesn't like 2 ohm loads sometimes, even though the specs say it can do them.
either way I'm feeling pretty excited about the amp...
bacan- I'll definitely post on how things work out since youre getting one too!
 
#8
The extra "headroom" from bridging will be barely noticable, plus you'll be pushing 1200w to two speakers with a 700w RMS rating. If you're cranking hard on it you run the risk of damaging the speakers from over-powering. If you're careful you may never have a problem, but why risk it for a negligible increase in output?
 
#10
Then what's all this stuff about making sure the amp's got 2 or 3 times the output of the speakers? I've got 2 monster cabs and I want to crank these bastards! What should I do, then, get an amp that puts out 700w at 8 ohms per side? I mean, of course i'm not gonna blindly crank everything and whale on it right out of the box, but i'm not gonna be a wimp either!! I'm gonna slowly turn it up till I'm giggling like a madman!:bassist: Are you telling me this is too much amp for me?

bridged 4 ohm load= 1400w
bridged amp into 4 ohm load: 2400 watts

sounds perfect to me
 
#11
Get a RMX 4050HD and run 8 ohm stereo...thats 800 watts per channel. For more headroom get the 5050. This way the amps would run more efficiently and reliability + performance of the amp would be better than a 2450 in 4 ohm mono.

Also, the 350 watt spec for the speakers...is that rms or program or peak? Most manufacturers publish all three ratings and recommend that the used select an amp that corresponds with the program rating.
 
#12
Here's my speakers:

1502DT-4 Black Widow (4 ohm speaker)
SPECIFICATIONS
Power Handling
Continuous 350 W,
Program 700 W*
AES 600 W<
, Continuous power per EIA Standard RS-426A
* Program rating with a minimum of 3 dB amplifier headroom
< Power capacity for 2 hours per AES Standard 2-1984

2 per cabinet, 2 cabinets. Each cabinet has two 4-ohm speakers wired in series. There is an amp selector program on the QSC website,


here's the link, its pretty cool:

[Invalid or Expired Link Removed]


I used the box on the right, for a sub with no horn, and checked the box for "enter the sub.'s specifications" and played around with it every way I could. No matter which way I plug in the numbers, the rmx 2450 gets listed as an option even when I tried to get it to reproduce for 8 ohms per side (4 ohm, 350w cont., 2 subs per channel) only it spit out a 2-ohm answer. By the way if anyone is actually comprehending what i'm saying i really appreciate it.

The way I see it, the rmx2450 is the most amp (used)I can get for my money (budget) that fits the cabinets I have. I'm gonna be running a Sansamp RPM thru it with a Fender Am. Deluxe P-bass.

I'd have to get a freaking humongoid power amp to run the cabs on separate channels at 8 ohms according to QSC's recommendations of "powering them with an amplifier that is rated for at least two times the loudspeaker's continuous power rating" Each of my cabs would come out to: 2 speakers x 350w continuous each=700w continuous. 700w x two times = 1400w PER CHANNEL.

Even the 4050HD is only 800w per channel at 8 ohms. The power, performance, and efficiency of the amp would be better but I still don't think it would be optimizing the total potential of the whole system unless it was bridged.

Besides i'm already committed to the 2450. I'm going to bridge it, too.

Baris-- I fully plan on "reducing... the amount of input required for the amp to reach full power";) in order so that i don't blow my speakers. And by the way, wouldn't 1200 potential watts into a 700w RMS cab qualify as "headroom"? Seems like a lot more headroom than you'd get pushing 500 w to a 700w RMS cab.
 
#13
I am running the 2450 with my rig, one channel per cab only, using one Goliath III and one JBL JR118S, though I have a pair of each. I can say, without reservation, that this is plenty of power for almost any gig. I run against two guitar players in one of my current bands, and he is half deaf from using his Marshall Major. (I keep my earplugs in for this one). It's a good match.

If things work out well enough, I'll be buying another 2450 for the other side.

Anyway, point being, your concerns about headroom should be satisfied with this amp. It is posdef a real workhorse. I cannot recommend anything else for someone on a budget (like me).

disclaimer, of course: I do not work for QSC, or, for that matter, any music equipment manufacturer.
 
#14
THANK YOU!- very much for the vote of confidence. That's good to hear- I cant wait to hook it up!!

I just want to be ready for any situation: inside, outside, no PA, two PA's, generators, pissing off the neighbers, whatever. Mostly I just want to be able to keep up with anyone I play with.
 
#15
jam73 said:
Here's my speakers:

1502DT-4 Black Widow (4 ohm speaker)
SPECIFICATIONS
Power Handling
Continuous 350 W,
Program 700 W*
AES 600 W<
, Continuous power per EIA Standard RS-426A
* Program rating with a minimum of 3 dB amplifier headroom
< Power capacity for 2 hours per AES Standard 2-1984

2 per cabinet, 2 cabinets. Each cabinet has two 4-ohm speakers wired in series. There is an amp selector program on the QSC website,

I'd have to get a freaking humongoid power amp to run the cabs on separate channels at 8 ohms according to QSC's recommendations of "powering them with an amplifier that is rated for at least two times the loudspeaker's continuous power rating" Each of my cabs would come out to: 2 speakers x 350w continuous each=700w continuous. 700w x two times = 1400w PER CHANNEL.

Even the 4050HD is only 800w per channel at 8 ohms. The power, performance, and efficiency of the amp would be better but I still don't think it would be optimizing the total potential of the whole system unless it was bridged.

Besides i'm already committed to the 2450. I'm going to bridge it, too.
Click to expand...

Wire the speakers in parallel.

2 ohms per cab.

Use the 2450 in 2 ohm stereo (same as 4 ohms bridged). This would deliver ~1200 watts to each cab, 600 watts to each speaker which should work great.

4050HD in the same configuration would be much better especially if you want to "keep up with anyone you play with", but if you can't/wont do it, then thats your prerogitive. The HD models are very well suited for 2 ohms per channel, hence the HD for "heavy duty".
 
#16
Holllowbody- I'll try the 2-ohm rewire eventually. Don't get me wrong- I'd love a 4050HD or something even more but I gotta settle for the 2450 at least for now 'cause of cash. It should be coming to me soon!!
 
#17
Well, I told you guys I'd let you know how the amp worked out. I hooked it all up tonight for a test run. I ran my old sr400 Ibanez through my new sansamp RPM through my new RMX2450 bridged into one of my old 8 ohm cabs. Cranked the ch.1 QSC input attenuator and used the level control on the sansamp.

Holy sh-t there is no question about it this setup is totally bad. I can't wait to hook up my other cabinet too! I felt like Marty McFly when he strummed that power chord through Doc's huge speaker system in Back To the Future.

And I was worried that the QSC's input sensitivity (1.23V, +4.0dBu) would hold something back from the combo, something I had been studying on these forums. WHATEVER!!

If anyone has the same concerns I did about enough power, input sensitivity, with a similar setup as me, I can tell you that as far as I'm concerned, the Sansamp is driving the QSC just fine. Any self doubts I had about this setup not pumping enough juice have been laid to rest.

For anyone searching these forums for the same kinda stuff I have been I hope you come across this thread cuz I know it takes a lot of searching to find all the good info that is scattered across TB. But it's a great site and helped me out a lot- :bassist:
 

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