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New passive PA system, very confused
New passive PA system, very confused
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Old 30th September 2016, 01:26 PM   #1
gullis is offline gullis
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Default New passive PA system, very confused

Hi! I just registered here, after countless hours searching the web and reading forums and articles. I arrange techno clubs and have recently bought a new soundsystem (we used to rent). We got a really good deal, but there is a lot about the operation that i dont understand. I hope that you can help me straighten out a few questionmarks!

The system consist of:

Amps:
2 x alto mac 2.4 Link
1 x alto mac 2.3

Speakers ( homebuilt, seemingly good carpentry):
4x passive 12" monitors
4x 15" bases - JBL 2225H Link
4x 12" + horn - P. Audio C12 Link

Misc:
Mixer
3way splitting filter
Compressor
EQ


Currently the smaller amp drives the monitors. So far, so good. Here the confusion starts. Reading some articles about power, i come to the conclusion that the amps are way oversized. Reading others, i get the impression that i im way underpowered. I cant make heads or tails of the numbers here. Ohms, RMS, EIAJ, continous program power, program powre and sensitivity.

If someone can tell me how these specs of the speakers relate to the rateings of the amplifiers, i would be very grateful!

The other question i have is regarding expanding the system. Our Idea is to sell the monitors, and get one active monitor. Than use the smaller amp to power the tops, use the 4x 15" speakers to run lower mid, and build a subbase (double 18" perhaps?) to be run by the second big amplifier. What do you think of this idea?

I realise an active system would be better matched to our skill, but its a question of money, and also of wanting to learn.

Thanks so much for taking your time to read this, and i hope i made myself atleast somewhat understood
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Old 1st October 2016, 02:26 PM   #2
conanski is offline conanski  Canada
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Matching amplifiers to speakers is a complicated issue but the bottom line is if you can get the SPL you want without overdriving the amplifiers or damaging the speakers then you're good. Your plan is also fine but I'd suggest not using the compressor as it won't do anything useful and could actually increase the potential to blow drivers. If you want to add some protection then sell off the mixer, compressor, and EQ and buy a Behringer DCX, this unit has all those functions plus adjustable limiters.
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Old 1st October 2016, 06:41 PM   #3
bear is offline bear
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Assuming you have one channel of amplifier for one channel of speaker - be it a two or three way with a passive crossover, you have no big problems except one. That problem is the potential of running too much power to your speakers.

Paralleling IDENTICAL speakers per channel of amp is ok, IF the lowest impedance does not drop below 4 ohms IN REALITY - not just based on some label.
Different speakers ought not be paralleled on any channel of any amp, especially at high power levels.

There is a practical SPL limit for the speakers, and that limit is WRT to some power level that they can handle. You have to set the power limit to LESS than the max for the "weakest link", that being the speaker that can handle the least power (after taking into account things like pads and attenuators in a xover).

Then the way to keep from blowing up speakers is with a soft knee limiter where the output level from that unit is NEVER CHANGED, set up properly and drives the amps directly. Pro audio companies frequently put covers over level pots on amps, to keep idle hands off them. To be clear that level out needs to be such that it drives the speakers to just under their max long-term power limit... speakers get hot over time and blow up. Some technical ability is required to do this, but it's not that difficult.

There's another trick that uses a light bulb for protection of mostly tweeters and high mids, you can read up on that here and elsewhere on line.

Also you can not get the full power out of those amps by using an extension cord off a 15amp 120vac plug in some random room/club. Btw.

The amps on paper have more power than the speakers can handle.

Maybe a wiring diagram, some pix of the speakers, the xovers, and the like would help to give you better information...

Btw, beware of EQ, in particular BOOSTs eat up a lot of power and headroom.
Keep in mind that just a 3dB boost = 2x the power!! 6dB= 4x and 12dB = 16x!!

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Last edited by bear; 1st October 2016 at 06:46 PM.
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Old 2nd October 2016, 02:33 PM   #4
gullis is offline gullis
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Thank you for your replies. I will ignore the smaller amp and the monitors, since we are planning on selling those.

My wiring is as follows:

SL2442FXPRO mixer to
Samson s-3way splitter to
-> Low signal R+L (<900hz) -> amp -> 2 paralell bases for each chanel.
-> Hihgh signal R+L -> amp -> 2 paralell tops for each chanel.

The speakers are identical. They are connected paralell with two to each channel, thus giving a total resistance for each chanell of 4 ohm i think. The amplifiers are rated on the website as such:

Stereo Mode:
2 x 1800 Watts @ 2 Ω (EIAJ)
2 x 1100 Watts @ 4 Ω (EIAJ)
2 x 910 Watts @ 4 Ω (RMS)
2 x 670 Watts @ 8 Ω (EIAJ)
2 x 560 Watts @ 8 Ω (RMS)

My interpetation of this is that the amplifier can deliver 910w to a pair of paralell speakers. The tops are rated 600w program, and the bases at 400w. This is confusiong to me, because i had the impression that the jbl bases were the better speakers here. But atleast the tops then should be fine running with the amp almost at clippping evels, and the bases somewhat lower?

I also suspect that there is something fishy going on with the specs here, orr rather, my understanding of them. It would seem that the amp could deliver 1800w per chanell at 2ohm, which i believe would be four paralell speakers. That cant be right?

As for the power issue, im in sweden with 16a/230v in the socket. This would ofcourse no be enough in theory, but hopefully in practice.

I dont have the speakers here, but the next time im where they are, i will get some pictures. They are however just black, homebuilt boxes. What should the wiringdiagram contain? And what is an xover?

As for compression, we are at the moment not using any, since we almost only play already mastered music which shouldnt have a lot of peaks. Or is this faulty reasoning?
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Old 2nd October 2016, 10:24 PM   #5
conanski is offline conanski  Canada
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A 900hz crossover between the bass and mids seems unnecessarily high, I'd suggest you lower that to about 500hz. Don't get too hung up on the rated power handling of the drivers, suffice it to say they will be fine with the amplifiers you have especially after you balance(blend) the output from each section to sound natural, the mids and highs will be turned down relative to the bass and subs. So for example if you had the same amplifiers powering all drivers the sub amp would be the only one to reach full output, all the others would run at incrementally lower levels.. probably 3dB less for the bass and 6-9dB less for the mid-highs, and that is because human hearing is more sensitive to mid and high frequencies, because it takes more power to reproduce lower frequencies, and because modern music is recorded with a hugh sub/bass boost.

Quote:
Originally Posted by gullis View Post
As for the power issue, im in sweden with 16a/230v in the socket. This would of course no be enough in theory, but hopefully in practice.
That is another area with direct math does not apply, music is not a sine wave so you can't compare AC wall power directly with music power. In practice the ratio will vary but even for heavily compressed music it could be 2 or 3:1

Crossover = Frequency Dividing Network

Last edited by conanski; 2nd October 2016 at 10:32 PM.
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Old 3rd October 2016, 01:59 AM   #6
bear is offline bear
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One way to balance the speakers by ear is to run some pink noise through them, and adjust them so that the sound from the different speakers is more or less indistinguishable, that is nothing sounds louder than the other... it should sound more or less like a powerful waterfall - but watch ur levels.

xover = crossover, btw.

unless you need to recover some money, keeping the other amp around is a good plan.
worst case you have a backup if the main one smokes!

The alleged power handling ratings are likely not accurate, since even pro compression drivers are limited to <100watts max. With a crossover and pad, higher of course.

So, your "splitter" is the crossover.

You're better off with a more sophisticated unit, even a Behringer, that does EQ in the passband, delay and probably limiting... yes, limiting is a VERY good idea... if for no other reason than it will protect ur speakers against a stupid move and/or a fault in the signal chain.

Oh and yes, minus losses and IF the amp is capable of delivering current then power doubles as the impedance is halved (with a low output Z amp, aka a solid state amp - not a tube amp)
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Old 3rd October 2016, 03:46 AM   #7
conanski is offline conanski  Canada
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Quote:
Originally Posted by gullis View Post

Stereo Mode:
2 x 1800 Watts @ 2 Ω (EIAJ)
2 x 1100 Watts @ 4 Ω (EIAJ)
2 x 910 Watts @ 4 Ω (RMS)
2 x 670 Watts @ 8 Ω (EIAJ)
2 x 560 Watts @ 8 Ω (RMS)


I also suspect that there is something fishy going on with the specs here, or rather, my understanding of them. It would seem that the amp could deliver 1800w per channel at 2ohm, which i believe would be four parallel speakers. That cant be right?
Yes that would be 4 speakers in parallel but there is something telling in those specs.. notice there is no RMS spec for 2ohms. That is not surprising to those in the know because building an amplifier that will happily deliver that amount of power into 2ohms continuously is expensive, and these aren't expensive amps so the only conclusion is that the amp won't really drive 4 speakers per channel for any useful period of time before going into protection.
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Old 3rd October 2016, 07:54 AM   #8
chris661 is offline chris661  United Kingdom
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Okay, so you have 4x 1x15" subs, and 4x 1x12"+horn tops, and a pair of fairly powerful amplifiers.

Here's how I'd go:
Set the crossover for, say, 150Hz.
Use one amp for your left side, one for your right. 2x15"s on one channel, 2x12"s on the other. This keeps your speaker cable runs short. Should be 4ohm on each channel of the amp, which is fine. I wouldn't trust these amps at 2ohm, but there's very few that I would.

The 15" JBLs may be the weak link here, depending on what sort of music you'll be playing. Most bass is fairly peaky, so you'd have to be pushing really hard to burn the drivers. If it'll be EDM and dubstep, you're gonna need some good limiters, since they can approach sine-wave-like content, which kills drivers very quickly unless you can cut back the power. Either way, make sure the subs do not receive signals below the tuning frequency of the cabinet. Its very very easy to do mechanical damage when there's no highpass filter in place.

Chris
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Old 3rd October 2016, 01:07 PM   #9
bear is offline bear
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Hang on... there are "horns" that load some 12" drivers made by "P Audio" for the highs?
No HF horns or other HF drivers??

How low the crossover can be set will be determined by the lowest usable frequency of the 12" horn loaded drivers, in the horn. Without knowing what those are, it is impossible to set the LF xover point for them.

Probably that accounts for the "900Hz" crossover frequency.

And, yes a HP filter on the bass set just below or at the -3dB point of the bass enclosures is a very good idea.

Again, something like one of the Behringer boxes (cheap, especially used) has that sort of feature.

Agree, do not attempt "2 ohm" operation, ever...

Think you may be in need of some HF speakers - although excessive HF in PA/SR can sound very annoying, imo. Maybe you want to hold on to the smaller amp for running the HF?

It's not that difficult to put together a good HF array, DIY... could be done fairly inexpensively for a system of this size and power...
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Old 3rd October 2016, 02:43 PM   #10
conanski is offline conanski  Canada
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Quote:
Originally Posted by bear View Post
Hang on... there are "horns" that load some 12" drivers made by "P Audio" for the highs?.
I assumed "12+horn" meant a traditional 2 way PA box with a high frequency compression driver, you could be right but since this was apparently a complete working system it would be strange if it didn't include any high frequency components.
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