I've been thinking about building an OB array using the $3 Madisound Foster extended range buyout drivers ever since I read Zobsky's thread about his Foster OB array several months ago. I finally got around to it last week.
My version is an extreme budget array (about $100 per speaker) using the Foster drivers with a centrally located point source tweeter, in this case the Eminence ASD1001 compression driver with a Dayton waveguide. They're up and playing, with the Foster arrays running full range and a 2nd order crossover on the tweeter. Nobody will confuse these with Magnepans or with my Selah Audio XT-8 arrays, but they do offer amazing bang for the buck considering the cost to build. They were also very easy to build - the baffles are 16 wide by 88" high sheets of 3/4" birch plywood with 2 1/2" deep side rails to increase their rigidity. (A 7' 4" tall sheet of unreinforced plywood sways quite a bit.)
Now a description of the sound: I was surprised to find the Foster drivers sound very smooth running full range by themselves with no crossover. According to my Radio Shack sound meter the sound is within 3dB from 80hz to about 4khz, above which it tapers to -6dB at 5khz and falls off quickly above that frequency. The $25 Eminence driver is also treble limited above about 12khz, so these are not true full range speakers, but distorition is acceptable for both the Fosters and the Eminence, and their sin is one of omission rather than comission:) With the speakers set about 3' from the back wall the soundstage reminds me very much of the Maggie 3.6's I used to own - very open and natural sounding with no box coloration. The bass doesn't extend much below 80hz, so I'm augmenting them with a diy sealed box sub using a Dayton 12" budget DVC sub and an MCM buyout plate amp (another below-$100 project).
I used 14 midwoofers per side in a 4-3-3-4 power taper, but that makes them too tall, placing the tweeter too high for my taste. I may cut them down to 12 midwoofers and use a 4-4-4 wiring arrangement instead. The stands are 2.5" by 24" pine boards with Lowe's 12" by 14" shelf brackets ($1.77ea) securing them to the speakers. They do sway some, but no more than Maggies do with their factory supplied legs.
If you're looking to build the last OB speakers you will ever want to own choose another project like the Orions, but if you're not concerned about WAF and want a fun budget project that's easily built with hand tools, this would be a good one. Picture here:
http://fredt300b.smugmug.com/gallery/132721#187653047
