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Old 2nd July 2009, 08:47 PM   #1
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Default Problem with distortion with OB line array

Hi, I quickly prototyped some open baffle line array speakers using cheap 8ohm 120Hz - 16Khz "full range" 88dB/W 7.5cm dia drivers. They are wired in parallel series strings to present an 8ohm load. The problem is that at anything above moderate spl they distort quite nastily. I would describe the distortion as asif hearing a very loud sound (so loud that the ear produces a distirtion). I would guess standing waves of some sort as that is what causes the distortion in the ear at very high sound levels. Just to clarify the speakers are not outputting anything near that level before the distortion kicks in.
Anyway as a solution I was thinking of increasing the width of the baffle and the distance between the top and the bottom of the speakers and the top and the bottom of the panel (increasing overall height). I think also reducing the legnth of the longer stiffening strip may also help. Anyway any advice is apriciated as I am very novice at all this.
I am planning to use a pair of the improved version of this speaker with a couple of subs so thought the 120Hz point would be low enough for the crossover (poor student not everything can be hi-fi), however they apear to be cutting off way before that with the narrow baffle. Putting a sheet of MDF against the baffle side apeared to fix that though therfore increasing baffle width should help that aswell.
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Old 2nd July 2009, 08:55 PM   #2
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here is a pic of the original plan (missing one driver ^ ^).
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Old 2nd July 2009, 10:40 PM   #3
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A 7.5cm dia driver is rather small and even if you have 8 of them it does not total up a large radiating area. Plus if the drivers are very cheap then I am not surprised that they distort at higher sound levels.

I would find out the driver details, such as frequency response, impedance, Xmax, break-up regions, etc, before rebuilding the baffles.

Regards,
Bill
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Old 2nd July 2009, 11:18 PM   #4
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OB, those will be inefficient and you'll be driving them quite hard for even low sound levels;
What sort of filtering do you have on them? try a series cap to limit bass levels below 150-200Hz and see if that helps?
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Old 2nd July 2009, 11:24 PM   #5
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I am surprised they are distorting at moderate levels. Do a quick check of the DCR with your multimeter to make sure you're not overdriving the amp. Those should be able to go quite loud before you hear the speaker distortion. And perhaps do as Peter says with the high pass cap. Also triple check your wiring to make sure there's no phase cancellation.
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Old 2nd July 2009, 11:54 PM   #6
tinitus is offline tinitus  Europe
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I dont know
The idea is that cheap drivers are ok, if theres enough of them
Only, that rather "simple" way of looking at it mostly relates to woofers

Maybe you could try crossing them at about 300hz

Anyway, if you have a rising response curve, and no BSC, it could easily sound like its distorted
Phase issues could cause distortion too

And all those voice coils may take more power than you imagine
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Old 2nd July 2009, 11:58 PM   #7
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its more off a high frequancy distortion problem, I have tried cutting the bass out and there was very little improvment the dc resistance is fine. the drivers are connected like this:

top

1---<+
2
3--->-

1---<+
2
3--->-

1---<+
2
3--->-

bottom

so 3 parallel strings of series drivers. Unfortunatly don't have any more detialed specs. Pretty sure the amp isn't overdriven as the same output level is fine when pluged into my 8ohm monitors.
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Old 3rd July 2009, 12:04 AM   #8
tinitus is offline tinitus  Europe
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Have you tried one single driver on its own

If it distorts, you will probably just get nine times that
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Old 3rd July 2009, 12:10 AM   #9
Andy G is offline Andy G  Australia
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poly cone, tin dome.. there's gunna be one heck of a breakup mode somewhere.
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Old 3rd July 2009, 12:28 AM   #10
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hmm I shall build a box for a driver on its own and see what that does, it would be a shame to have wasted money on some totaly awfull speakers.
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