U3 Compact Shielded Three Way March 26,2006 Lou's Homepage

I was emailed by Jeff in San Francisco who asked if I would be interested in doing a small three way that could eventually migrate into a 3 way center channel. Jeff wanted something reasonably sized, useable as a standalone in his apartment, but supplemented with a sub when neighbors were cool. He also thought a 3 way would provide the "ruthless midrange" he prefers. We exchanged many emails and phone calls before agreeing on the U3 drivers. Since budget was a major consideration, we chose the Dayton RS150-8 for a woofer; the Tang Band W3-871S for a mid; and the Audax TM020J5 for a tweeter.

While it seemed like a waste to do a 3 way with this small a woofer, the center channel idea made it more sensible. We had considered using the HiVi B3S for a mid, but it's just too inefficient to use as a single driver. Jeff liked the Tangband in another application, so we went with it. The Dayton RS 6" provided reasonable efficiency, decent bass extension and a very reasonable price. While I pushed for the ND20 on the tweeter, Jeff thought the Audax's would be more attractive. At $5 they met the budget, and I had 3 in stock.

The box is 1/2" baltic birch with a net volume of 12 liters (750 cu in net), tuned to 46 Hz with 1.5"D X 4" flared port.
Outside box dimensions are: 16" High X 7.5" Wide X 10.25" Deep

For a mid chamber, I cut some 4" thin wall PVC pipe on the table saw. During assembly, I used hot melt to glue it to the front and back baffle. I drilled a couple of holes in the pipe to pass wires. I stuffed the mid chamber with dacron/polyester stuffing before final assembly. Estimated volume of the mid chamber is about 125 cu in.

The crossover took about 5 passes to get right, but the final version isn't too bad for components. A coil & "pseudo zoebel" for the woofer. The mid is essentially a 1st order electrical on both ends with resonance filter. The tweeter is also a cap/coil. Unless otherwise specified, coils are 19-20 gauge. I used NP electrolytics for the 50 & 68 caps. All components were surplus from stock or straight from Parts Express. The slopes are pretty flat with a lot of overlap. The Tangband has only 0.5 mm of Xmax, so I wanted to push the MW slope as high as possible.

The modeled response and impedance are shown below. Impedance is pretty benign dropping to 4 ohms, but pretty flat otherwise.

For final finishing, I scrounged enough Pecan from scrap to do the boxes. Jeff wanted a maroon baffle and asked for the silver screws on the woofer to add "visual interest". I didn't do my best job on sealing and priming the baffle. I primered with dewaxed shellac, painted the box baffle, then sprayed the whole package.

Despite my misgivings, these turned out to be really primo. They are very neutral and sweet, with surprisingly punchy bass into the high 40s. I asked the AZ 2006 attendees to give them a critical listen. Nobody could find anything to really improve. These aren't big on wow factor, but they don't do anything bad. They handled every piece of music we threw at them. Nice clear midrange (no BBC dip here), good sparkle and air. No fatigue factor, good off axis response, nice soundstage. They do everything well -- except play at ridiculous volumes or fill bass in a big room.

I will probably build another set with Dayton ND20's and a matching center. Cost wise, I'm not sure the U3 is any more costly than a decent MTM and the adaption to center channel is sure an attractor.