Ever since I wrote my article "Designing a Passive Two Way Open Baffle Speaker System" the idea of combining an Eminence Alpha 15A woofer with a smaller full range driver has been something I have wanted to try. Since I am already using an active crossover system with two SS amps, the choice for the full range driver opens up to just about any available 3" or 4" model. In reality the choice of full range driver could also include many 6" and even 8" models. The lure of an inexpensive two way OB design with enough flexibility to not lock me into a single driver combination convinced me to finally build this experimental OB system late last summer.
Over the years, I have accumulated a nice stash of veneered plywood in various sizes and thicknesses. Digging through my collection, I found enough scrap pieces to build this 20" wide by 38" tall OB speaker system. Recognizing that I wanted to try a variety of full range drivers, each requiring a different diameter cut-out and pattern of mounting screws, I devised an easily removable baffle arrangement. In the picture above you can see an 8" x 8" removable black baffle; a close up view is shown below. When building the speaker I cut enough of these removable baffles to try five different pairs of full range drivers, good enough to get me started experimenting.
Brackets to support the OB are attached on either side of the woofer to each of the vertical bracing strips. Two rubber feet are used for each side providing a good stable contact with my carpeted listening room floor. A horizontal stand-off is attached to the brace at the very bottom of the OB for the bi-wire terminal cup. Two speaker wires are run in parallel to the Alpha 15A woofer and the full range driver's barrier strip from the internal tabs of the two sets of binding posts on the terminal cup. Bi amping using my dBx Driverack PA crossover means that two sets of speaker cables, one set from each amp, are attached to the appropriate pair of binding posts on the terminal cup.
Using an active crossover and bi amping the two way OB speaker system opened up a wide range of 3" and 4" full range drivers that can be easily mated with the Alpha 15A woofer. Not having to worry about the relative sensitivity and applying boost or cut to the full range driver to balance the SPL frequency response is a very powerful design tool. I always simulate the drivers being used and derived what I feel would be an applicable crossover configuration, then the final optimization is done by ear changing the active crossovers settings on the fly in my listening room.
So far I have tried the Fostex FF85K, Fountek FR88EX, Mark Audio Alpair 6, and Fostex FE-108E Sigma with great success. Right now the FE-108E Sigma is my favorite of all the drivers I have tried. Each of these drivers eventually produced a nice sounding SPL response after sufficient tweaking of the crossover settings. Several times I was ready to give up on one of the drivers only to find that a slight change in boost level or crossover frequency dialed in the performance to where I was happy. One thing I have observed in my room is that the boost to the woofer usually ends up being 2 or 3 dB more than I applied in the simulation. I am not sure why this is the case but it seems to be consistent for my set-up in my room. It might be different for another system in another room.
This simple OB speaker system has opened up a lot of possibilities for using different small full range drivers and in the future probably a few different 15" woofers. I have on many occasions quickly swapped out a pre mounted full range driver to produce a different sound from my system. Loads of fun, the perfect excuse to expand your collection of reasonably priced full range drivers, easy to build, infinitely adjustable crossover, endless EXPERIMENTATION.