Rob's OBLA-33 (Open Baffle Line Array) project.

Rob41

Don't stop believin'
I think I need to first disclose that this is my very first speaker build. I have modified prebuilt speakers before, but have never done a complete build from the ground up.

I have recently gone the way of using an active crossover/dsp using a Minidsp. This has opened up a whole world of possibilities to me allowing me to have abilities as a novice that would have been impossible 30 years ago.
I use vintage amps like those in my signature, I listen primarily to various styles of rock but desire a system capable of critical listening. While I tend to appreciate healthy, yet accurate bass, I generally listen at moderate levels.


Choosing a type of speaker

I wanted to try building a line array for some time but got a bit side tracked by the concept of an open baffle. Thanks a lot Poultrygeist! That's when I thought about combining the two types of speaker designs.
From the offset I hadn't really seen this before but after doing a bit more research I discovered although not particularly common, others have been down this road so O.B.L.A. it is.

One of the advantages that drew me to line array's was the fact that distortion is greatly reduced due to the sheer number of drivers and how little each of them is pushed.

My listening space is very live and untreated and line arrays lend themselves well to this kind of environment due to interacting less with the floor and ceiling.

The O.B. type of speaker interested me because it's free of the resonances associated with a typical enclosure and I've been told they interact well with ribbon or planar type drivers. I'll find out if I like the dipole effect of the mid-bass drivers, but if I don't I can always add treatments to the rear wall.


Which type of Line Array to choose


This is where a lot of research cam into play as well as reading many white papers on the topic. Let me just say, at this point, I feel very comfortable having an in depth conversation on the science behind Line Array's as long as the other person in the conversation is doing all of the talking.

For my room of 8' tall and approximately 20'x30', I decided a straight and essentially floor to ceiling array would suit my application best. There are other designs such as CBT where the array has a convex shape like the kit Parts Express sells. I've read some very excellent reviews of this design but all of the research I've done points me in a different direction. IMO, the CBT is akin to pro type line arrays where a vast area in an open environment needs to be covered, i.e. farfield listening environment. Perhaps it is also a good choice where the cost of the additional drivers to achieve a floor to ceiling array is too great. Lot's of enthusiasts and designers are very pleased with a CBT style array, it's just not the direction I want to take.

In the end, I decided to go with a straight, floor to ceiling array where the floor and ceiling reinforce the cylindrical wavefront.


Driver selection, layout and frequency coverage

Midranges:
I knew I wanted to build some line arrays so when Parts Express had some 6 1/2" mid/mid woofer buyouts with the specs I was looking for I got them, 48 of them.

http://www.parts-express.com/6-1-2-poly-cone-midbass-woofer-4-ohm--299-609
Midrange.jpg

With a Qts of 0.95 they should work well in an open baffle arrangement and go as low as I need them to without putting up too much of a fight. I plan to cross them from 75Hz to 3300Hz if the combing of a 6 1/2" driver at these high of frequencies isn't too noticable. I'll have to see what happens on the bottom end as well since they have an Fs of 52Hz and should actually be crossed two octaves above the resonant frequency which would be 208Hz. I guess I'll find out about combing givin the size and high frequencies I want to use.


Tweeters:
I thought of keeping it budget minded but it seemed the choices I had that would work in a line array would either sound like the cheap tweeters they are or would be cost prohibitive. I ultimately decided to go with Dayton PT2C Planar tweeters. I've used these as substitutes in a couple pairs of speakers once I ditched the passive crossovers and used them with a MiniDSP active crossover.

http://www.parts-express.com/dayton-audio-pt2c-8-planar-tweeter--275-085
PT2C.jpg

I like the way these planar tweeters have an effortless, airy quality to them and have been told when used in multiples i.e. an array, their qualities improve appreciably.


Augmented bass:
Eventually I would like to get dedicated subwoofers in a distributed bass set-up but for the time being I'm going to use a pair of Dayton PA drivers that I've come to appreciate.

http://www.parts-express.com/dayton-audio-pa380-8-15-pro-woofer--295-034
PA380.jpg

Although they have an Fs of 28.3Hz and modeling indicates significantly higher Hz, I have used them successfully below 20Hz with dsp while still allowing the necessary spl's. I've found the output to be very tight and clean for my taste. Unfortunately, their low Qts doesn't allow me to go O.B. for my woofers which will have to come later.

Final (preliminary) design:
I started out wanting to build a single array of mid/mid bass drivers with an outboard array of the planar tweeters. Fortunately the flaws of doing this were quickly pointed out to me so I settled on mounting all of the drivers in a single baffle.

Dave Smith, (former designer who has worked for JBL, KEF, Meridian, McIntosh, Snell and PSB etc.) suggested to me to go in a slightly different direction than I had originally planned. He pointed out to me that I would experience some serious lobing, but if I went with an MTM approach, the lobing would then be symmetrical which would be much better. Thank you Dave!

So my cast in stone.....or yet to evolve design will consist of a line of 9 Dayton Planar drivers flanked on each side by an array of 12 6 1/2" drivers on each of two baffles. O.B. line array MTM. I'm going to call tham OBLA-33 due to the number of drivers on each baffle.
Here is crude drawing of the basic layout:

[URL=http://s1223.photobucket.com/user/Rob41/media/O.B.L.A.%2033_1.png.html][/URL]

The material I'm using for the baffles are maple work surfaces measuring 1 1/2" thick x 2' x 8' long.

http://www.sears.com/craftsman-8%27-butcher-block-work-surface/p-00914965000P
Maple top.jpg

I'll be giving my router and circle jig a workout for sure. Maybe the 132+ solder connections will be the most fun.

Once completed they will be approximately 20" wide by 7 1/2' tall. I will be tri-amping and using my MiniDSP to control most aspects of the driver behavior....I hope.

The fun will begin in ernest tomorrow when I put together some mock ups and I'll do my best to provide some measurements during testing. I only have REW but I am using a calibrated mic.

I'd just like to say, if not for the wealth of information fellow AKer's share here my project wouln't even be possible. Thank you! :thmbsp:

The pictures will begin flowing tomorrow. This is my first build thread so I apolagize as I'm sure there is a lot of information missing. I'll be happy to answer any questions if anyone is interested.
 

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It ended up getting a bit bigger than I had originally planned. I'm really looking forward to building the OBLA-33's. :banana:

If my work ends up being a fraction as good as yours Zonk, I'll be a happy camper for sure.
 
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Man, I read his whole thread and can tell you my talents are nowhere even close to his. His skills are other worldly.

I would love to go with many small full range drivers and may explore that on my next build.

My reasons are two fold to go with the 2-way design. All of the full range drivers I looked at were physically too large to prevent serious combing, especially at the higher frequencies. Also, I wanted the array to reach down to 70-75Hz to the point where the bass becomes non directional, at least as far as nearfield listening is concerned. That's where the augmented bass comes in below 70-75Hz.

Even the BG RD75 is only capable of 150Hz with a recommendation of 300Hz. Plus, they are no longer available.

Mcintosh used 1/2" tweeters in some of their line arrays due to the 1/2 wavelength issues and this goes to show the difficulty using full range drivers. I have not seen any 1/2" drivers that will run 70Hz to 20kHz and do it well.

Like pretty much all speakers there are compromises. My design has two that stand out. One, is that with 6 1/2" drivers there should be significant combing because I'm going to run them up to about 3200Hz. I have done some preliminary testing by ear and I'm not hearing it. I'm sure I can take some measurements to indicate it is present though.

The other issue that was pointed out to me is one of lobing because of the proximity of the mids and tweeters, and the fact that they cross eachother at around 3200Hz. The solution to the lobing was pointed out to me and consists of doing a MTM configuration so the lobing is symmetrical whic is supposed to be more desirable.

They are going to be physically larger than I had originally intended but function is going to trump form on this build. No WAF here.

I have actually done a very basic mock up with a single tweeter and one line of 12 mid/mid woofers and it looks very promising to say the least. Those cheap PE buyout drivers have so far exceeded my expectations. That's also the reason I decided to pony up for more of the Planars and more of the mids. It looks like the quality of sound is going to achieve more than I had expected.
 
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Lookin' good, Rob! :thmbsp: And this revised driver configuration is very interesting. :yes:

Self proclaimed dipole/line source/scale junkie here, so :lurk:

I was seriously considering a DIY build with the RD75's, but like you said they're no longer available.....

....so my upcoming DIY project found an alternate, equally radical path. :scratch2: :naughty:
 
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Sounds like a great project. My only suggestion is you include the fact that it's a dipole in the name: Open Baffle Line Array Dipole, OBLADI :p
 
Lookin' good, Rob! :thmbsp: And this revised driver configuration is very interesting. :yes:

Self proclaimed dipole/line source/scale junkie here, so :lurk:

I was seriously considering a DIY build with the RD75's, but like you said they're no longer available.....

....so my upcoming DIY project found an alternate, equally radical path. :scratch2: :naughty:

I initially wanted to use the RD75 drivers but the more I read about how many people are adding a tweeter the less I wanted them.

I look forward to hearing more about your project. Radical is a good thing!

Sounds like a great project. My only suggestion is you include the fact that it's a dipole in the name: Open Baffle Line Array Dipole, OBLADI :p

Lol, I was wondering if someone would make that connection. I suppose it would be cool to have a Beatles song connection. :D
 
Even using 8" drivers (almost touching edges) in a OB short line, I fail to hear problems from the listening position from comb filtering. You can raise and lower from normal seated, move left and right, and just not hear radical shifts. Sweet spot has focus, yes, but not tonal shifts. I suspect, climb back in the box, and it'd be different.

I've now got a few weeks on a new line of 16 3"-er's. Also hear no issue from comb filtering on them either.

Both efforts, I was targeting keeping crossovers to just augmenting rolls. I find great value in that. Otherwise, pretty similar thinking to what you're doing. I'll follow along with great interest!

EV3
 
That's good to hear. I've read many accounts from people who should be experiencing combing, that in actual listening it often is not detectible. what I've experienced in my mock up supports that as well.
 
Got wood?

I picked up the wood from the cabinet shop today where they had ripped each piece down to 20" wide and took 6" off the top for a total length of 7 1/2 feet.

It's the first time I got to see the maple work surfaces since they arrived from Sears. They look much better than I had expected:

[URL=http://s1223.photobucket.com/user/Rob41/media/DSCN1129.jpg.html][/URL]

They are 6" shorter than my mock up that is a 1x12x8'. They will end up being a few inches taller once I add the base and maybe some spikes.

You can just see where I mounted a single PT2C planar tweeter on the inboard side of the mock up.

This is just a picture to show the scale of the mock up's. The actual build will have twice as many mid woofers so rather than being 12" wide, they will end up being 20" wide:

[URL=http://s1223.photobucket.com/user/Rob41/media/DSCN1125.jpg.html][/URL]

I couldn't resist putting a few of the drivers on the maple baffle. :D

[URL=http://s1223.photobucket.com/user/Rob41/media/DSCN1131.jpg.html][/URL]

The cabinet maker who is a super nice guy, tried using his industrial sander on a piece that was cut off but the finish on the maple gummed up his belt. So guess what I'll be doing for the next couple of days?

Once the sanding is done I'll be able to start the routing and countersinking. I may be new at it now but by the time I finish doing that for 66 holes I should have the hang of it.

I'll post pics and try to give a decent description of the routing process.
 
That's a good idea but I already got a bunch of sandpaper. I didn't even think of that. :scratch2:

I tried a small section already and it seems to come off pretty easily. Besides, aren't speakers supposed to sound better based on how much blood, sweat and tears you put into them? :D

My wallet's already hemorrhaged a bit so that might count for the blood part....I hope.
 
Thanks. I'll have to remember the old saying "measure twice and cut once". Can't afford another maple surface.
 
Wonderful project- I have built small line arrays, 4~6 per side, loved the sound!- bet you will too. You have selected very good parts!
DC
 
What is your reasoning behind deciding to go with a 2-way line array? I would think this would be an ideal situation for going full range, as in this thread at DIYaudio:
Note in your link much smaller drivers were used like you find in Roger Russell's IDS array. I think his choice of the 6.5 inchers lends itself to two way operation like a Scaena. I guess I would begin with one bank of woofers to minimize the lobing.

Regardless, this should be a fun build to follow, and I look forward to it.
Yep. I've been a line array fan for decades.
 
All I can say is "Holy $h!t !"

We could all come over to listen, but I think we'll be able to hear them from here, if you crank it up. :D

Watching with interest as a mostly armchair speaker builder. :thmbsp:
 
With regards to you mock up, with the drivers just set on the board...remember to allow a little tiny bit of lack of perfection. At least a good idea for me!

If you try to touch the drivers you are asking for yourself to come up short somewhere. Another 1/16th of inch on each side won't totally blow up center to center spacing.

The more you have in a line, the easier it'd probably be to get off a touch. With careful marking, drilling a center pivot hole trying to keep the drill square while drilling and then using the Jasper circle jig, I was able to get just a tiny sliver between recessed drivers and still be uniform on each. A little thought in advance can help lower the risk on the wood of a cutting error.

Also, might want to draw out your markings for the driver cuts before you get all the sanding done.

EV3
 
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