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Can you alter the midrange of a PA cabinet - one for the sound guys


Turdus

Question

Have a pair of JBL MRX-512's. These play very loudly, and have a nice midrange that gives a real "in your face" sound.

Have a pair of EV Lixe X 12's. These don't have the output of the JBLs, and have a more mid scooped sound.

Been using both for a Theater production, music reproduction only, and the JBLs seem far superior. Using identical power amps, it's almost as if the EV's are about 2/3's as loud.

Can I modify the EV crossover to get more mid honk, and overall loudness out of the cabinet?

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According to the factory specs, the EV needs twice the power to play as loud as the JBL at any given volume level. Basic rule of amplification is that you double the amp's power output for every 3 dB increase in SPL.

By far the simplest answer is to turn the amp volume to the JBLs down by 3dB to balance out the output at each cabinet. Since the JBL has a slightly lower power handling capacity this is a good thing. If you try to mess with the EV's drivers or crossover at this stage unless you're a mad genius, you're asking for trouble.

The other thing you could do is get an equalizer and boost the midrange to the EVs. The perceived loudness difference may mostly be due to the JBL's stronger midrange. Still, lowering the signal strength to the JBLs is the easy answer.

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Wow JB! Double the power? I'm using a pair of QSC PLX1804's... 600 watts at 8 ohms. I like your suggestion of turning down the amp to the JBL. I will have to experiment with that a bit. The EV is not a bad sounding speaker... but if the opportunity arises.. I'd ditch them in a heartbeat for another set of the JBLs.

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A good EQ for the EVs is the best answer. Turning down your JBLs to meet the SPL of the EVs will make the whole system sound more like the EVs than the JBLs, if that makes sense. Another set of JBLs is optimal, but not the most cost effective solution.

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Wow JB! Double the power? I'm using a pair of QSC PLX1804's... 600 watts at 8 ohms. I like your suggestion of turning down the amp to the JBL. I will have to experiment with that a bit. The EV is not a bad sounding speaker... but if the opportunity arises.. I'd ditch them in a heartbeat for another set of the JBLs.

Yep, that's the basic rule of thumb. If a speaker makes 97 dB at 1 watt input, then it requires 2 watts for 100 dB, 4 watts for 103 dB. To produce its 129 dB max requires about 1800 watts, which is beyond the threshold of pain. There are other factors, however, such as room gain (which can vary). At the other end of things, you lose 6 dB for every doubling of the distance. So if it's 97 dB at 1M, it's 91 dB at 2M, 85 dB at 4M, etc. EXCEPT if the speakers are a line array (which these are not), in which case you lose only 3dB for every doubling of the distance.

Note also that the standard sensitivity rating is a 1Khz test tone measured 1M away. Some speakers put a bump in their speaker around 1K so they measure as more efficient than would otherwise be the case. More useful would be pink noise averaged from 200-5Khz.

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While I believe this is incorrect, I've always looked at the knobs on a power amplifier as a volume control. Looking at the QSC amps today, I see that control fully clockwise says 0, then counter clockwise seems to be negative DB. I have a fairly good handle on setting gain from a board perspective, but never gave much thought to the amps. Usually just turn them up making sure they don't clip.

If I understand what you are saying, just backing off 3 db (amp powering JBLs) would get the speakers to about the same volume, eh?

I'd prefer not to add an EQ. I'm a believer in less components in the path, to reduce failure probability. I don't even use a crossover. just a mono line out into the sub power amp, which has low pass filter.

Call me paranoid, but if the sound goes out in the 1000 seat auditorium, what am I going to do? I can juggle... but not that and fix the sound issue at the same time, lol.

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Yes, you should dial the knobs for the amp for the JBLs to -3dB. That would be the starting point, but you may probably have to tweak it by ear.

This may be why pro sound power amps almost always have volume knobs--to dial in the appropriate gain for the speaker cabinets and the venue.

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This may be why pro sound power amps almost always have volume knobs--to dial in the appropriate gain for the speaker cabinets and the venue.

I believe I once read that to really set a system up right, you get your board gain structure set, then with no speakers hooked up to the power amps, you set those until they clip, then back them down. Now.. that is all fine and dandy, but that is a maxium setting right? So once the speakers are plugged back in, where should the overall volume adjustment be made? At the board, or the power amps? Would you just back down the master faders on the board?

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Normally, I'd think you'd put the amp volumes at full and control overall volume from the board. But in this case where one set of speakers is not as sensitive as the other, you would tweak it at the amp output. That's the simplest answer without getting real complicated.

I'd think this would be analogous to home audio. Home audio power amps generally don't have volume knobs, but some do, including mine. Normally I run it full, but if I were bi-amping into two sets of speakers with different sensitivities, I'd use the amp volume to adjust the speakers to play at the same volume. I could also use it if I were bi-amping a single set of speakers with two amps that have different amounts of gain. A large number of amps provide 29dB of gain, but some are around 26 dB and others as high as 32. Amps with volume knobs make it easy to get this corrected without complicating things.

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