The Beryl Shielded Tower
(Copyright 2002 by Louis Coraggio)

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When my wife actually expressed an interest in having a "simple stereo I can work" for her birthday, I had my mission. I brought out my old Onkyo receiver and asked her where she wanted it. "In the kitchen, that's where I do most of my fun work" Of course, that meant a new pair of speakers. Of all the speakers my wife has watched me build, she thought the Opal Towers were the most attractive. So a set of pecan veneered towers went on to the drawing board. I had more of the Def 6.5" drivers, and a pair of Axon TS1 shielded tweeters. I always wanted to try the Axons. I built Zalytron's German TM for a friend, and thought it was a pretty nice little kit.

After some diddling around I decided on a 22 liter (1380 cu in ) enclosure for the Beryl. (8.25" W X 7.5" D X 36" H External ) out of 3/4 MDF. The box is tuned to about 49 hz with two 1.5" x 4" ports (PE# 260-402) on the back panel. For internal bracing, I cut 1.5" strips of 3/4 MDF. I used these to build a square brace at 15 " from the top. I also used one between the ports glued to the front and back baffle. One more runs horizontally between the drivers on the front baffle. I used a 1/2" roundover along the front edges.

I used PE Black Hole on the top & bottom for damping. I also wrapped acoustic foam behind the woofer, with loose dacron in the lower half. This seemed to suppress any pipe resonance from this tall thin box. The bases are 1" thick, 12 x 12. (Definitely needed when the dogs are playing)


When I measured the drivers in the box, I noted that the Axon had a 5 db dip centered at about 9000 hz. I'm not sure if this is peculiar to my cabinet or inherent in the driver. My first crossover essentially ignored the dip. But, alas it was definitely there. After several iterations in LspCad I came up with this design. Essentially, it's a first order on the tweeter side with a notch filter and a padding resistor.

The woofer circuit looks strange but it works. The undersized Zoebel actually shapes response at the crossover (about 3300 hz) I liked the 2.5 ohm the best, a 3 ohm is a bit brighter. The inclusion of the notch filter rolls off the highs at about 17khz, but gives a pretty flat response from 17k hz.

I haven't measured the final product but LspCad predicts minimum impedance of 7.5 ohm at 250 hz. Peak of 20 ohm at 70 hz. Should be pretty friendly to inexpensive amps. Baffle step is about 4 db. I estimate average efficiency to be about 84db

So how does it sound? Balanced. I think the baffle step is just right. The Def runs out of steam somewhere in the 60's. The bass is a definite improvement over the Topaz and is suitable for full range. Not overwhelming, but you'll hear it. (You true bass heads may not be satisfied) They do an excellent job on my vinyl to CD remasters.

In my 16 x 35 kitchen, (a very bright room) the imaging is excellent and spacious. Vocals are clean, nice horns. Piano seems a wee bit muted. Just a hint of sibilance on female vocals. Head to head with the Topaz, I think the Audax tweeter has a bit more "air" and detail, the Axon seems a bit warmer & richer on vocals and horns.

If I had to pick between the two, I'd give a slight edge to the Audax. At low background volumes, the Topaz seems to hold imaging better than the Beryl. For nearfield, the Topaz topology would be my clear choice.

To finish the Beryls, I wiped them with distilled water to raise the grain. Sanded with 220. Three coats of Fuhr 355 gloss acrylic lacquer. Nice open pore finish. I spray painted the bases. Shot them with a coat of Fuhr 3200 polyurethane.

Before I go I have to plug the pecan/hickory veneer. The color and grain are gorgeous. Cream and tan. It's inexpensive, hard as stone, sands glass smooth. My wife liked these so much, she asked me to leave the grills off! I just bought a boatload of raw hickory veneer on Ebay. New dining room table and entertainment centers are now on the design board.