This design has its roots in a speaker kit called the Opal, a 2-way tower design that Audio Concepts introduced several years ago. This was before I had any measuring equipment or crossover skills, and I ordered a pair, along with their cabinets. The Opal used the Vifa M21 8" treated paper woofer and a pair of Vifa D25 (later D27) tweeters, and received a rave review in the Audio Critic. Although the sound was basically uncolored in the midrange, the overall presentation was leaden and closed in. I have since learned to mistrust Audio Critic reviews, and have also been able to develop a series of crossover designs that I think substantially improved on the original. I present here my latest, and definitely my last, take on the subject.
This kit is intended for people who want a basically full-range system with a minimum of complexity and cost. The Vifa M21W039-08 woofer is good down to 32 Hz in the ported tower cabinet, and provides surprisingly satisfying midrange performance for such a large driver. The Vifa D25AG35-06 aluminum tweeter delivers very smooth response with excellent dispersion. Although the resulting sound is not quite as focused as, say, my MBOW1 3-way, the Vifa tower costs much less and the crossover is far less complex. The total cost for the drivers and crossover parts is only about $220 a pair. You do, however, have to be able to build a large cabinet. To the best of my knowledge, there are no appropriate prefabricated cabinets available from the usual hobbyist sources.
There are some misguided souls who think the Vifa D25 tweeter is harsh and bright. I think thatís because it looks bright and shiny, and people hear what they see. Rather than arguing the issue, I have provided an alternative crossover using the ScanSpeak 9500 fabric dome--a unit that I donít think anyone thinks is inherently bright. To my ears, the basic difference is $112 in extra cost. But I certainly donít think the 9500 version is inferior.
The original Opal cabinet is 38 1/4" high, 10" wide, and 12.5" deep, with 3/4" mdf all around except for 1.5" on the bottom. I would use at least two shelf braces, and target about 1.8 cubic feet volume net of the bracing. The tweeter center is down 3 1/4" from the top, and the woofer center is down 6.5" from the tweeter center. The tweeter is offset 1" horizontally from the woofer center in mirror image pairs. The cabinet is vented, with the port mounted on the bottom rear. I used a 5 1/4" long, 3" wide (ID) port, which produced a tuning frequency of about 34 Hz. I would start with a 6" length and experiment for your room. Here is a link to complete cabinet details: http://subwoofers.cc/HomeAud/Kits/opal/opal.asp
Here are the schematics using the D25 tweeter. Donít be confused by those little resistors in line with the woofer inductors. If the value is less than 1.0 ohm, the resistor doesnít exist. Thatís just the dcr of the inductor itself. But all the other resistors are very real, including the 50 Ohm bypass resistor for the big woofer coil, and any resistor in line with a shunt inductor in the tweeter circuit. I used a Madisound steel laminate for the 3.0 woofer inductor, and their 16 Ga Sidewinder air cores for all the other coils. My crossover price estimate of $220 assumes Eagle resistors (Madisound) and Axon poly caps (http://www.gr-research.com).
Here is the revised Net 2 for the ScanSpeak 9500 tweeter:
Here is the on-axis response with the Vifa D25 tweeter:
And here is the response with the tweeter polarity reversed. It shows the crossover point of 2200 Hz:
Here are the response curves at 30 degrees and 45 degrees off axis horizontally. There are no serious problems, and the dispersion at the top of the woofer response is quite good
Finally, here is the on-axis response with the SS 9500 tweeter: