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Old 20th January 2002, 01:14 AM   #11
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The Atma-Sphere amps are quite happy with an 8 ohm load (4 ohm too, for that matter). But, yes, they are also good for higher loads. I haven't paid as much attention to the other models. The old Futterman layout is always a possibility, too.
Diy OTLs are not difficult, and once you're free of the output transformer, amazing things are possible (Bandwidth to a quarter megaHertz? Is that good enough for you? Man, you can talk to bats with these things, and they're stable beyond anything solid state people can even dream of...). I've got two different designs I've been tinkering with...but you know the line...time, I need more hours in a day.

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Old 20th January 2002, 01:31 AM   #12
JohnR is offline JohnR  Australia
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Grey, know what you mean... I would be interested to hear more about your designs when you do have the time :-). Perhaps we should start another thread. Here's the most viable design I came up with so far:

http://www.tubeaholic.com/projects/S...2/totl-two.gif

JohnR
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Old 21st January 2002, 08:23 PM   #13
Bill F. is offline Bill F.  United States
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Man, this thread has all my favorite hot-button words: line array, high impedence, low impedence, OTL amps...

I'm just drawn to that wacky stuff. Maybe it's because the rest of you wise ones already have the mainstream honed so fine, you make it look easy.

Yes, my interest ranges from 1/2 ohm to multi-Kohm speaker loads--with not much in between. (I'm the guy who wants to wire a line array of 32 8-ohm drivers in parallel, or wind one 2Kohm full-range driver.)

Anyhow, I currently listen to a pair of line arrays I built--12 4-ohm 4-inch wide-range drivers per side in 80L bass reflex boxes tuned to about 48Hz. They are very efficient and capable of *BIG* sound at very low excursion. The bass energy was a surprise to me--Not all that deep, but extremely solid and coherent. Many hands make light work. Even during a 50-watt blast (and at 96+dB/W 50 watts is a blast), each driver sees only a little more than 4 watts. Normal listening pushes them with only a fraction of a watt each, so they operate in a very linear range of motion.

My original idea was to try a bessel array (mathematically approximates point-source dispersion), but I read that the wavefront didn't unify until it had propogated at least 3x the array hight. So I decided to try a tapered array, ala D'Appolito.

In my case, I wired the center 4 drivers in series--these are the acoustic center. I wired four more drivers and two parallel pairs in series. Finally, I wired these two series lines in parallel. The second four go two above and two below the first four, and the parallel pairs are on the ends, two on top and two on the bottom. The net effect is power delivery that is concentrated in the center four drivers of each line and tapers off to the ends. In my case, the nominal impedence for each line is 8.88 ohms. Crossover is 1st order at 6KHz to a cheap dome tweet.

By wiring a tapered array, I sacrificed some overall efficiency for (I hope) better imaging. Wired normally, each line would have given me about 99dB/W.

(An aside: purchasing the drivers on an internet auction and roughing the cabinets together myself, I spent a little less than $100 on this project)

Bill
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Old 21st January 2002, 09:37 PM   #14
Bill F. is offline Bill F.  United States
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One thing I do wonder about is comb filtering in line arrays. (Anybody feel free to correct me if I'm mistaken):

My understanding is that pressure waves from each driver in a line array interact with pressure waves from every other driver, independant of what is between them. (Waves pass through each other, summing to anything from zero to 2 depending on angles of phase and intersection at all points in space.)

Therefore, the top and bottom drivers of a line array, being far apart, will interact to create comb filtering beginning at quite low frequency. The fact that other drivers are firing between the extremes (and setting up their own different comb filter interactions) would tend to mask the response ripples set up by any two drivers, but those ripples are still there. In a typical line array of 10-20 drivers, the interactive combinations are very numerous and I imagine that the comb filter interactions are extremely complex. Do all these ripples of slightly different phase combine to form flat response? I don't know.

I get a kick out of my line arrays, but I still wonder if I'm truly ascending to higher levels of Fi.


I've got these two cases of 32 full-range 4" drivers each, just burning a hole in my basement floor, so I bull-headedly continue to dream of ways to use them--all at once.

What's currently on my brain is a "matrix" array, for lack of a better word. I'm picturing a 32"x16" open baffle with a 4x4 (=16) driver matrix in the middle. This will be the midrange. Since all the driver centers will be close to their fellows, comb filtering shouldn't begin till higher in the band. Above and below the 16-driver matrix, I'll put 16 more drivers (8 above and 8 below) and roll them off around 150 to fill in the lower end.

This design would probably give up some horizontal dispersion and add some vertical dispersion compared to a line array (though open baffle cancellation would lessen room reflections).

Anyway, whatta y'all think?

Bill
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Old 22nd January 2002, 12:48 AM   #15
Super is offline Super  United States
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I believe Wayne J over at SpeakerBuilder tried something like this using the "wonder" drivers that were a Parts Express special, with a hefty matrix-style array. He angled the rows of the array slightly outwards each time, giving a slight curvature. However, he gave up after realizing that the crossover needed to tame the problems would be far too complex.
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Old 22nd January 2002, 04:12 AM   #16
JohnR is offline JohnR  Australia
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Bill, sounds like fun! Did you consider 32 per side... ? :-)
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Old 22nd January 2002, 11:10 AM   #17
Bill F. is offline Bill F.  United States
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That's what I meant: 16 full-range in the middle with 8 more above and 8 more below rolled off at 150Hz = 32 drivers per side.

That's the surface area of a 20" driver!

Bill
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Old 22nd January 2002, 05:25 PM   #18
Won is offline Won
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So what are different ways to wire a line array? It sounds like you made it such that the center drivers recieve the most power (I didn't get that much from your description Bill...I should try rereading it). What's the differences between distributing power equally or putting more power towards the ends?

Also, I'm also considering building a "short" line array of 6x6.5" midbass woofers and 2 dome tweeters (MMMTTMMM) that would stand around 4' tall. Is this a sufficient number of drivers/height to be considered a line source?

Thanks, Won
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Old 22nd January 2002, 07:11 PM   #19
Bill F. is offline Bill F.  United States
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Won,

(This is kinda long)

The cool deal with a perfect line source (infinite length, single transducer) is that it launchs a hemicylindrical wavefront. This pressure wave is different from a point-source hemispherical wavefront in that its width spreads as it travels outward, but its height remains the same (theoretically infinite). A point-source wavefront spreads in width and hight and its pressure decays at 6dB per doubling of distance, but a perfect line source wavefront, spreading in only one dimension (width), decays at only 3dB per doubling of distance.

This effect is a definite bonus if you're trying to illuminate long distances as evenly as possible, such as a crowd at a large concert, ergo the popularity of large line array PA systems.

There is no infinitely tall loudspeaker, so there is no such thing as a perfect self-contained line source. However, if you physically limit the height dimension with a floor and ceiling and illuminate that space with a floor-to-ceiling line source, you still end up with a fine hemicylindrical wavefront and all the joy that it brings.

Speakers that are not infinitely tall or loaded by a ceiling also launch a hemicylindrical wavefront, but that wave will begin spreading vertically. Over distance, it will become more hemispherical and tend toward the corresponding 6dB decay rate.

I don't remember exactly, but I think the wavefront from a finite line souce in free space loses much of its line source characteristic after traveling 2x the line height. So if you want to hear line source effect from 4' tall array in free space, you'll probably have to keep your head no more than 4' from the middle of it.

Incidentally, line arrays (multiple drivers) in free space only approximate a line source at wavelengths longer than total array length. At higher frequencies, the beams begin to decouple and develop interference effects--lobing & comb filtering. (See my previous post about line array comb filtering.) Also, in your MMMTTMMM situation, the Ms will behave more like a line source than the Ts will, so they will have different decay rates (highs will drop off faster as you move away).

All this is to say that, strictly speaking, a line source isn't a line source unless A) it is loaded by a floor and ceiling or B) you sit within its shadow.

And none of this is to say that a 4' tall MMMTTMMM quasi-not-really line source couldn't sound glorious

Bill
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Old 22nd January 2002, 07:48 PM   #20
Bill F. is offline Bill F.  United States
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As I've read it, the benefit of tapering the power delivery is rooted in psychoacoustics: Listening to a vertical array of speakers, your brain will tell you that all sound is coming from A) the loudest speaker or B) the speaker nearest your ear. So, in a set of non-tapered arrays, the center of the stereo image will move vertically as the level of your ears move.

I chose to fix the vertical reference point by making the center four drivers louder than the rest. It makes my array behave less like a line source and throws away some efficiency, but I wonder if it might improve the comb filtering situation since the drivers farthest apart are also the quietest.

I know, I know: you're asking what the point of a diluted line array is. I dunno. Just playing around and having fun. I ran the wiring externally so I could remove the taper if I got the notion.

Bill
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