Topaz Shielded Monitors
The Topaz began life as an economical sealed monitor with a small facade that could be used as L-R-C in a home theater or as part of a multimedia setup. The original customer for this system wanted a vertically rounded baffle with recessed drivers. The PE 299-712 clearance driver looked like a good candidate. I've never been a fan of horizontal MTM center channels, so I needed a small footprint tweeter that would allow me to build a vertical, TM center channel. The Audax TMF0251 looked like a good choice for performance, small footprint, and ease of flush mounting. Since this tweeter also comes with a truncated flange, I planned on using that version for the center.
The sealed version of the Topaz is now my office monitor. I've spent quite a bit of time listening to them. My wife was very happy with a set of towers I built for her using the same woofer, but thought the Topaz was a bit better high end performer. She asked me to build a pair of Topaz towers for her choir director. (See her review) The Topaz Tower is 22 liters tuned to about 49 Hz with two 1.5" x 4" ports.
#299-712 High-End Buyout Woofer
#276-152 Audax TM025F1 Tweeter
The Topaz is my first venture into a measured design using JustMLS and LSPCad. It took me awhile
to iron out the kinks with JustMLS. Once I did my homework, cursed a bit, read some more, tried stuff etc. I finally got the measurements working. I setup at about 39 inches with my mike directly on axis with the tweeter. I took measurements for each driver, and for the two drivers wired in parallel. I also used JustMLS to measure impedance curves. After loading the measurements into LspCad, I verified that the combined curve looked very similar to my measured, two-driver curves by adjusting the position of the drivers. The design process took a few hours, a prototype Xover, some more measurements, and some more diddling. The crossover shown belows is my 4th pass.
The final crossover is at 2600. The woofer did not need a zobel, nor did a second order crossover improve it's response. The tweeter has a definite rise with increasing frequency. After some experimentation, I found that a 0.1 mH coil flattened the Audax response nicely. The pseudo "zobel" allows shaping the tweeter response a bit. It also flattens the overall impedance response. I used a .67 mH coil in the final version but a .7 would be fine. Values of 3.7 to 4.3 on the tweeter "zobel" do not change the response drastically. I used a 16 ga coil on the woofer, but simulations suggest an 18 wouldn't change things greatly. In this trim, the Topaz is a nominal 8 ohm design. Minimum impedance is 7.5 ohm with a max of 20 ohm at 1500 Hz. It should present no problems to any amp out there.
I ran a quick measurement on the tower version. This is a merged near + far measurement taken out on the driveway on a windy day. I did not add in port effects (which would add a bit to the bottom end). I worked on the dip at 8000 (it's a remnant of tweeter diffraction at the cabinet top). Flattening the response completely made the measurements better, but it added too much sizzle to the high end. The 1/2" tweeter offset that I tried on the towers, broadened the dip a bit, but did not completely eliminate it. (My microphone runs out of gas at about 15,000 so the final dip isn't really there)
The sealed construction is fairly straight forward. 3/4 MDF with one small piece of bracing glued behind the baffle between the drivers. I use biscuits and clamps exclusively. After sanding the boxes square, I used my 3/4" radius bit to roundover the two front edges.
For the sealed version, I used leftover 10 mil paperbacked cherry veneer with solvent based contact cement. I first veneered the back, then the wraparound front (always start from the center and wrap each side individually) I place the box on a 1/4" high scrap to set the veneer overhang. I allow 1/4" extra top and bottom with 2" extra along the wrap. For paperback, I trim the edges by hand with a utility knife.
The cherry finish was an experiment. I mixed a few drops TransTint Red-Brown dye stain with a
1 lb cut of dewaxed shellac and used that for a sealer. Followed by four light coats of Fuhr 355 waterbased Acrylic with my Walcom STM spray gun. I added a couple of drops of Trans Tint Amber to the Fuhr. It didn't completely eliminate the Cherry "splotching" but it did warm up the wood nicely. I've pretty much retired my Deft now. I left all the veneer intact when finishing. I then used my flush trim bit to trim the holes. Followed by my rabbeting bit to finish the recesses. I use closed cell self-adhesive window insulation (3/8 x 1/4) to seal the drivers with black #6 wood screws. The cabinets got dinged while I assembled. I'll probably spray them again. The square cross section is far from optimal for internal resonance. To compensate, I lined the inside with acoustic foam and filled the rest lightly with Dacron.
For the tower version, I added a brace (same size as top) about 15" from the top. (3x5 hole in the center). I lined the upper portion with acoustic foam and lightly stuffed the lower section with dacron. The bases are a sandwich of 3/8 & 3/4 MDF slabs. I cut them about 12 x 13. After running my 3/4 roundover around all edges, I sanded the edges and sealed them with a 2:1 mix of white glue and water. Then I shot them with two coats of spray paint and let them dry. A quick scuff sand with 220, and two coats of Fuhr and they were done. For the veneer, I used a 1 lb cut of pale dewaxed shellac to seal. After three coats of Fuhr 355, I hand sanded with a rubber block. First with 600, then 1200, finally 2000 grit.
H, 8-1/2 W, 8-3/4 D
H, 8-1/2 W, 7-1/4 D
Volume (cu in)
The sealed version of the Topaz is now my office monitor. I've spent quite a bit of time listening to them. My wife was very happy with a set of towers I built for her using the same woofer, but thought the Topaz was a bit better high end performer. She asked me to build a pair of Topaz towers for her choir director. (See her review) The Topaz Tower is 22 liters tuned to about 49 Hz with two 1.5" x 4" ports. The Topaz is a smooth, refined sounding speaker that does a great job with vocals and upper bass. The Audax tweeter is very detailed with exceptional off axis response. This combo images nicely, with good depth and placement. F3 on the sealed version is about 89 Hz. The vented tower realistically rolls off in the high 50's. (I never got a chance to add in port boost for my measurements). Lowest bass isn't quite as authoritative as some of the Peerless or Vifa drivers I've heard. The Topaz is tight and true, but it won't rattle the walls. I was deliberately conservative with box tuning and baffle step (about 4 dB). Trying to get that last 5 Hz. of bass too often destroys the upper bass and lower mid clarity (IMHO). Since the electrical parts cost under $70 each, I don't think too many folks would find them lacking.
"What can I say? I'm speechless! These speakers are awesome in every sense of the word! ... The sound is pure and clean. With a choral director and a drummer in the household, we listen to a wide variety of music styles and each one is heard with precise nuance and clarity"
--Review of the Topaz Towers
mH 18 Ga coil
mH 18 Ga coil
mH 18 Ga coil
ohm Resistor (2 in parallel)
(2 per speaker)
I'm a "mature" college professor/systems analyst/application developer living in southern Arizona with my wife and four dogs. Since I don't golf, my time in the shop (building speakers and furniture) gets me out of my chair and away from my square headed girlfriend.
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