Quick Question On JBL MRX 515 and QSC amp

[EvinBrady]

I just bought a pair of JBL MRX515 and a QSC PLX 1804 amp. Now I'm finding out that the amp is too small and there is a possibility it could blow my speaker due to lack of power! The 1804 is not bridgeable so I can't get the full effect (Is was I hear from guitarcenter) they said i needed the 1802 and it would be good... do i have to sell the 1804 and get a new amp now? crown xti2000? or something... what would be the best amp for these speakers and should i be worried about the plx1804 pushing them currently? I also has the allen and heath pa12-cp that has built in amps does anyone have a brillant plan that i can use these speakers for my acoustic show this week.... I also have a pair of jrx monitors. and would love to have an acoustic monitor mix during this show.... help please!

[JeffBarnett]

Hold on there...

Amplifiers don't kill speakers. People kill speakers.

Although people kill their speakers more frequently through under-powering than over-powering, it's still possible (completely safe, and in many cases recommended) to run your speakers with an amplifier that is rated at less than 2x their continuous power rating. You cannot blow up a speaker by giving it too little power. If that was true then anybody turning down the volume to less than 100% of the system's capacity would kill their speakers.

The reason that people say it's easy to blow up your speakers by under-powering is that some operators try to push the SPL of their system beyond the capacity of the amps and speakers by pushing up the volume at the mixer. When they do that, they clip the amp, and that's a sure-fire way to kill the speakers

When you clip an amp, the amp sends a square wave to the speaker, in effect asking the driver to defy Newton's laws of motion by going the the full limit of its excursion, then stop on a dime. The speaker can't do this, and the result is damage to the speaker. (Yeah, I know that's an oversimplified explanation, but we don't really need to dig into all of the mechanics of how a driver dies.)

So, if you want absolute maximum performance out of your system, you can run your speakers off of an amplifier that will supply the program wattage (2X the continuous) or even a little more. That way, you can push your speakers to the limit and get the SPL you are after without clipping the amp. However, it's important to make sure that you aren't clipping anything in the signal path prior to the amplifiers, and it's important that you manage low frequencies appropriately through high pass filtering to ensure you don't exceed the excursion limit of the driver.

If maximum performance isn't what you're after, just safe reliable operation, you can go with an amplifier that supplies less than the program wattage, but in this case you have to know what your system is capable of and make sure you never clip the amp.

It's also helpful to keep in mind that doubling the power supplied to the speakers will result in a 3 dB increase in volume. That's a noticeable difference, but not a huge one.

Let's look at the numbers specific to your situation...

- The MRX515s are rated at 400W continuous / 800W program / 1600 W peak at 8 ohms.
- The PLX1804 is rated at 550W at 8 ohms stereo. It cannot be bridged.

So the PLX1804 isn't a terrible match to the MRX515. You could go with something that has a little more horsepower, but it's not necessary. Just know your gear and its limitations, and take care not to clip anything.

[Krynos]

The XTI2000 would be a good choice. I was running my two MRX515's with an XTI4000. Bridged and linked parallel, feeding the speakers 1600 watts each. Sounded great, just be sure to get some sort of limiting

[EvinBrady]

What do you recommend? I know these speakers are awesome and I'd like to get the sound out of them they were made to produce without having the volume pushed way up!

[Krynos]

I'd recommend trading your amp in for another. I started with a QSC RMX1450 bridged on the MRX 515's, 700 watts into each. . wasn’t enough. I then went to the XTI2000, sounded great, but I wanted more. So I went to the XTI4000. Sounded really good. I'd say get an XTI 2000 or 4000.

[EvinBrady]

I ended up getting the xti2000 bridged mono. Its loud and considerign al I have got to play is acoustic shows with it the sound quality is amazing! however I want more haha ( Don't we always) I want to be running stereo so I can pan different things to different areas. If I ran an additional xti2000 would it be too much. I just figured it would be a cheaper alternative than trading in the xti 2000 I have now and getting a xti4000

[Krynos]

I think you should just get an xti4000 to bridge and run both speakers with. Set the limiter on the amp to -6db.

You can find B-stock xti4000's on ebay for $650.

[Bugzie]

There are 2 ways to destroy a speaker. Overheat it or push it beyond it's physical capabilities. JBL is at being realistic with its RMS/program/peak ratings. Any decent amp built today can double its rms rating for brief durations. If the 1808 can push 550 wpc in stereo, then you are at 137.5% of the rms rating before taking this into account. If you are using subs and a crossover, thats a pretty decent amount. Any more than that and you are going to be pushing the physical capabilities of the speaker. A dropped mic or runaway feedback can take out the drivers in a heartbeat. The limiters built into amps today are pretty good at protection. To push a speaker with its peak rated power is playing russian roulette. Sooner or later you are going to fry a voice coil or push the speaker beyond its designed capabilituies. The person at GC actually gave you some decent advice IMHO. If you need more volume than what that amp can deliver to those speakers thaen you simply need more rig. For longevity run the amp full out and keep an eye on the clip lights. Check the owners manual on the amp and see if they indicate clipping or if the limiters are being hit when they are engaged. Different amp manufacturers do it differently. By the way Jeff it's refreshing to see someone give an opinion based on good info rather than conjecture. As usual Sweetwater isn't just trying to sell what makes them the most profit. Can't say that about the advice that I've heard in person at Ukelele Center.

[amuskop]

I would run your moniters on one side of the amp and your mains on the other. Just don't drop the amp load below 2 ohms (no more than three cabs). While the amp is not ideal I ran JBL's simliar to that on a QSC 1602 for years with no problems.

[jomama]

[TimmyP1955]

I ended up getting the xti2000 bridged mono. Its loud and considerign al I have got to play is acoustic shows with it the sound quality is amazing! however I want more haha ( Don't we always) I want to be running stereo so I can pan different things to different areas. If I ran an additional xti2000 would it be too much. I just figured it would be a cheaper alternative than trading in the xti 2000 I have now and getting a xti4000

In most rooms, panning is a disservice to 2/3 of the audience.