PA - ohms watts - ratings - help / advice needed

[Aeolian]

Basic rule of thumb is to have twice the amplifier power as the speaker's continuous rating.

2000w for a 2-15" Beheringer 2 way with 1.75 horns sounds a little suspicious to me, but it's all about running the thing within it's limits. If you hear distortion, turn it down. Simple as that.

Running an amp into clipping will wreck speakers with much larger power capacity. I've explained the reason for this in a couple of threads.

One specific note here. Running an amp bridged reduces the load on it by half. Meaning that your two 4 ohm speakers will present a 2 ohm load to the amp. You got that part correct. But note that the amp is only rated for bridged operation down to 4 ohms. So your 2 ohm load will cause it to overcurrent and hurt something.

Running amps near their load limits isn't that great of an idea in any event. Running higher end amps (Crest Pro and QSC PLX) I've noticed better sound quality at more conservative loads. Those amps are rated down to 2 ohms and I know of professional sound companies that run them like that, but I noticed that 4 ohms, or not bridging, sounded better. The Crest Pro 9200 is rated at 6500W bridged into 4 ohms (you need a hell of a stiff 30A AC supply to achieve this), but I found that running 4 18Sound LW1400 1000W subs with two a channel at 4 ohms (amp is rated at 2200Wpc at that load) gave a tighter bottom end that was plenty loud for what I was doing. These are the drivers found in the top of the line EAW Sb1000 cabs, not some budget thing.

I've really not a fan of dual 15 PA cabinets. They kind of need to be on the floor, but then the horns are chest high to dancers and ear level to seated folks. They are big and heavy to put up on things, dangerous if they fall. Big time rigs with high quality drivers can get away with 2-15s on top of 2-18 subs. I've never seen a successful budget version.

I would suggest getting higher quality single woofer cabs and then saving for subs. Try to find the largest horn you can. 2" exit at least. 3" or hopefully 4" diaphragm. In order to cross over low enough for a 15" woofer, you need a pretty stout compression driver. And you need a larger throat and horn to handle the 500-800Hz crossover point that a 15 should be used with.

Get hold of the Yamaha Live Sound Handbook. Pretty much everything you need to know to separate the pretenders from the effective is explained in there. You should understand power dissipation in the speakers, pattern control, efficiency, and basic set up. That will make for much more successful shopping and a much happier audience.

[frank ledwith]

Hi there. This is my first time on here. Did ya buy the gear yet ?
Heres a few ideas,
A general rule when driving speakers is to allow a minimum of 50% head room.
i.e. a 1,000w speaker should be driven by a 1,500w amp. Most pro engineers will try and allow 100% headroom though.

When buying gear dont concern yerself with peak,prog or cont. wattage numbers. The r.m.s. (root mean squared) is the most important thing.American sales people use the higher no.s to make the stuff more impressive sounding.

You say the twin 15 speakers you are looking at are 4 ohm, they are probably 2 x 8 ohm 15" speakers wired in paralell giving 4 ohms impedence per cabinet.Now you should run your amp in stereo in my opinion.Therefore you will have a 4 ohm load on each output of the amp.This doesnt result in a total of a 2 ohm load on your amp because in stereo you are dealing with 2 seperate amps effectively.This is a good thing because it keeps you away from the 2 ohm limit of the amp,best not to pressurise cheap gear,right.

That amp will drive those speakers nicely for ya provided you keep to the basic rules of mixing and powering your drivers,

A. Dont send too much signal to your power amp. 0db on your mixer should be your hottest. i.e. no amber lights and definately no reds on your output indicators on your desk (what desk you got?)This will help you avoid clipping the inputs of your amp,known as maintaining unity gain in your system.

b. When turning on your amp during set up keep the master volume pots turned down then bring them up slowly while someone else or yourself counts into the mic. incase there is a problem somewhere. Keep them turned fully up during the gig so that there is enough power available at the master faders of your desk.This will prevent some one pushing too high and creating clipping.

c. If you hear distortion from your speakers-stop. Theres something wrong somewhere. Either you are clipping your desk and sending a noisey signal or clipping your amp inputs or driving your speakers to bits with too much power. Check your settings and turn down and play at a slightly lower level if ya have to.

Seperates are the way to go, 18" bins and 15" 2 way tops are the business. Dedicated speakers for specific jobs. You wont have to spend much more to achieve this.Watch out though, there are 3 options,passive subs,passive subs with a built in crossover and active (powered) subs.I have 2 p.a. rigs and a third one coming tomorrow from music store,its a cheap behringer set up, 2x 18" and 2x 15" all passive for 550euro thats about 450 stg. pounds.

Good luck.


Best of luck

[hdavis]

I have

a pair of Peavey Low Rider 18" subs 800 watt cont. 8 ohm

a pair of JBL MR 925 350 watt cont. 8 ohm

a pair of JBL MRX 512 400 watt cont. 8 ohm


a behringer EP 2500 450 watt 8 ohm / 650 w 4 ohm
bridged mono 1500 w 8 ohm / 2400 w 4 ohm

a behringer E 4000 550 w 8 ohm / 950 w 4 ohm / 1250 w 2 ohm
bridged mono 1750 w 8 ohm / 2400 w 4 ohm

so I want to run an amp parallel to power the subs, or run the amp bridged mono to the speakers connected in parallel.

Im not sure: if I want to push 1600 watts to each low rider , do I bridge the Ep 4000 mono and push 2400 watts into the Low Riders connected from one into the next, parallel which will make the low riders 4 ohm?

Which amp will give me the correct power to speakers

and the same for the the JBL Mr 925 pair.

right now I can not afford crown power , so I realize I might need a nother amp to get the MRX 512,

If someone could help me out here I would appreciate it.

[dboomer]

If you have to ask about bridging then you likely would be a good candidate to NOT bridge anything. It's not gonna make much real difference but it will be a lot more likely to have problems operating bridged.

You didn't mention crossover. You do plan to use one ... correct?

[museAV]

Originally Posted by frank ledwith

A general rule when driving speakers is to allow a minimum of 50% head room.
i.e. a 1,000w speaker should be driven by a 1,500w amp. Most pro engineers will try and allow 100% headroom though.

Many pro engineers will try to figure out how loud they want it to be out in the audience, the headroom desired, etc. and from that and the speaker sensitivity calculate how much power they actually need, then use the speaker ratings just to verify the speakers can handle that. That is the 'engineering' approach.

Originally Posted by frank ledwith

When buying gear dont concern yerself with peak,prog or cont. wattage numbers. The r.m.s. (root mean squared) is the most important thing.American sales people use the higher no.s to make the stuff more impressive sounding.

The problems with this start with the fact that there is no such things as RMS Watts, it is just Watts even if based on RMS voltage. It is the Continuous or long term rating that is typically actually measured for speakers using a defined standard procedure and test signal with the common 6dB crest factor of the test signal defining the generally four times greater assumed Peak value.


Fergies Watch, perhaps you can clarify how many speakers you plan to run.

I'm also not sure where you got your numbers but you might want to look at http://www.behringer.com/assets/NU10...ebBrochure.pdf and the "RMS" values for the NU3000 as those lower numbers are probably more realistic, although they still don't identify any of the test methods or limits and I hate their erroneously supporting the use of "RMS" power.

[ProSonicLive]

Originally Posted by museAV

Many pro engineers will try to figure out how loud they want it to be out in the audience, the headroom desired, etc. and from that and the speaker sensitivity calculate how much power they actually need, then use the speaker ratings just to verify the speakers can handle that. That is the 'engineering' approach.


The problems with this start with the fact that there is no such things as RMS Watts, it is just Watts even if based on RMS voltage. It is the Continuous or long term rating that is typically actually measured for speakers using a defined standard procedure and test signal with the common 6dB crest factor of the test signal defining the generally four times greater assumed Peak value.


Fergies Watch, perhaps you can clarify how many speakers you plan to run.

I'm also not sure where you got your numbers but you might want to look at http://www.behringer.com/assets/NU10...ebBrochure.pdf and the "RMS" values for the NU3000 as those lower numbers are probably more realistic, although they still don't identify any of the test methods or limits and I hate their erroneously supporting the use of "RMS" power.

I agree RMS is not truly indicative of how loud you will be SPL numbers are what determine that, but do not ignore them as RMS ratings give an IDEA of how much juice your speakers will handle regardless of volume level.
All the different power ratings simply tell you what is presumed safe given a certain sine wave and peak only tells you how much juice the things can handle given a large surge in power in a split second.

Your speakers should be fine, But i would avoid bridging them for a few reasons. just run them in parallel and try not to turn your volume knob to %100 while doing songs with heavy bass.

[museAV]

Originally Posted by ProSonicLive

All the different power ratings simply tell you what is presumed safe given a certain sine wave and peak only tells you how much juice the things can handle given a large surge in power in a split second.

The power ratings typically tell you at what 'power' (actually voltage and current) a noise source with a specified frequency response that has been intentionally clipped to provide a specific crest factor can be applied for a specified time before some type of failure occurs. That defines the 'continuous' or long term rating.

The 'peak' rating is not a measured value, it is simply assumed to be the continuous rating plus the crest factor of the test signal, which is most commonly 6dB. Basically, it handled a test signal with peaks that high so it must be capable of at least handling those peaks, but it might actually be capable of handling much greater momentary peaks. The 'program' rating is usually just assumed to be midway between the continuous and peak ratings.

Note that what is not identified is how the speaker fails or if its performance changes. For example, it is fairly easy to put a protection circuit on the high frequency driver that allows it to absorb much more energy but with an increasing amount of that energy going into the protection circuit rather than the driver thus resulting in increasing relative LF output at higher power levels. So that allows the speaker to have a higher power rating and helps protect the HF driver but not necessarily in a manner that you would want to actually implement during use. Some people have pushed for power ratings that relate to any audible (3dB) changes in the response but that would result in much lower ratings for many speakers so it has not been well received by the manufacturers.