Intro

The drivers for this project were the NSBs and a Meilloon tweeter that PE had on buyout for quite some time. Unfortunately, PE is now out of the Meilloon tweeter, but they may be able to be found somewhere on the used or surplus market. The Tech Talk board would be a great place to try to find some.

Crossover

This was the first project where I tried to use software to help design a crossover. My friend Dave Brown helped me get driver files put together for the NSBs, I spent some time learning how to use Speaker Workshop, and then went to work. At first, I wound up very frustrated -- I just could not seem to get any crossover that looked like it could be decent at all. Finally I got something that looked like it might work and gave it a try. It was a first order on the NSBs and a third order on the tweeter. Unfortunately, the mids were very blarey or shouty, and the highs lacked air.

So off I went to TechTalk and SpeakChat seeking advice. Many many folks responded, including Dave Burke and Curt C. At Dave B's suggestion I lowered the value of the second cap in the high pass filter. Curt C. suggested some different values for the L pad on the tweeter. I wound up actually settling on a parallel resistor value midway between what I had originally and what Curt suggested. Curt also suggested I not reverse polarity on the tweeter. SpeakerWorkshop had been telling me to do so, so I had been doing so. But the minute I went with normal polarity and fiddled with the cap and resistor values, it all fell into place. The shouty mids went away, and the air came into the high end. Then someone else -- who at this point I cannot remember -- suggested I put a bypass cap on the series resistor in the L Pad. This basically creates a shelving network that holds the tweeter response up at higher frequencies. That brightened up the high end a bit more, and I called it good.

I should note that I used the same crossover in all five speakers, including the center channel.

Here is the final crossover schematic:

Note with respect to C1 that you can find nonpolar electrolytic caps in either 31 or 33 values; either will work.

Here is a picture of the assembled crossover, with each part labeled (well almost; see below):

The orange wire is not labeled. It is the Tweeter Positive.

My friend Dave Brown measured the final product for me. Here is a picture on axis:

Reasonably flat, especially given the parts I was working with.

Impedance

These are pretty clearly 4 ohm speakers. I have not had any problems running them on my $200 TEAC HT receiver, but still, I feel constrained to note that you should be sure your equipment can take the lower impedance. The last thing I want or need is someone building the UBHTs, frying a receiver, and claiming its my fault. I'm just a hobbyist and do this for fun. If I can help someone out, I feel very good about that; but I sure don't want to be blamed if things don't go right.

The Enclosure

Front and rear enclosures were built with 1/2 inch MDF except for the front, which is 3/4 inch MDF. The internal dimensions are 6.25 inches W x 19 inches H x 9.25 inches D. External dimensions are 7 1/4 inches W x 20 inches H x 10. 1/2 inches D. All drivers and the port tube are on vertical center. The top woofer center is 3 3/4 inches from the external top; the tweeter center is 8 1/8 inches from the external top; the bottom woofer center is 12 3/4 inches from the external top; and the port center is 17 5/16 inches from the external top. There is a 1/2 inch MDF shelf brace that fits right between the bottom of the top woofer and the top of the tweeter. There is also a cross brace that is a bit above the top of the port tube and halfway down the sides of the cabinet. Here is a picture showing the bracing:

The crossover board is 3 7/8" x 6 1/8", both so it would fit through the woofer holes and because there is not that much space for it on the back baffle. The top of it is located on the back baffle just below the shelf brace; and the top of the hole for the terminal cup is just below the bottom of the crossover board. By the way, I used 5 watt resistors less for budget reasons (using 10 watt resistors only adds about $3 to the total cost) than because of the size of the crossover board.

The port for the fronts and rears is PE part number #260-324, which is 2.88 inches in diameter and 5 inches long. I have left the port at that length.

All interior surfaces except the front baffle and the area over the crossover are lined with egg crate foam.

Center Channel

As noted above, I used the same crossover on the center channel. However, I changed the dimensions of the cabinet a bit, and used a different port and rear ported that speaker. And I must made a confession: I have long since sold these speakers, and I cannot find my records for the dimensions I used. I do know I used a 1.88 inch port trimmed to about 2 inches long, centered above and behind the tweeter, with the terminal cup below, and that I made the front baffle less long and a bit wider, and the cabinet less deep. I put a shelf brace in between one woofer and one tweeter. Sorry to be so imprecise, but knowing that the cabinet volume is .63 cubic feet should allow you to come up with workable dimensions. Against all conventional wisdom, I centered all drivers on the baffle, and did not elevate the tweeter. I built a prototype that way, and did not like the way it sounded as well as with all drivers centered. The picture at the top of the page shows the center from the front; here is a picture of the center from the rear:

Trim Rings for the NSBs

The NSBs are pretty ugly speakers, with little brass tabs sticking out for mounting, and a 1/8 inch tall ring around the surround. After someone else noted they were going to make trim rings for them, I thought that was a good idea. So here is how I did it.

I used what Home Depot calls “standard hardboard” – 1/8 inch thick. I did several test runs with the Jasper jig trying to find the right hole to use for the outer and inner diameters. I should note I am using a 1/4" spiral upcut bit from MLCS. I finally arrived at using the 5 9/16" hole on the jig for the outer diameter and the 3 15/16" hole for the inner diameter. But not so fast, it is not that easy. It turns out that at least with my bit, 3 15/16" is a bit too small, while 4" is more than a bit too big. So, I had to cut the 3 15/16" hole, then carefully freehand the inner diameter with the router to get to the right size. I wound up with a size that snaps down over the ring with a little pressure.

More dirty details. Everyone has their methods, and mine are no doubt stupid. I duct tape the board to a working board. Then I cut the outer diameter. Once I knew the right hole to use, I cut the other three trim ring outer diameters one after the other by taping a piece of the hardboard to my working board and drilling 3 center holes the right distance apart. Then I taped the resulting circle to the board again and did the inner diameter pieces. This time I had to switch each time in the same place, that is, I did not try to do all three in a production line like I did with the outer diameters. Then I carefully lined those up and used masking tape along the edges to tape them together, then lined that bundle up and taped them to the model ring I had made (that is, the first one I was able to make that came out right). Then I carefully lined up the bearing on my flush trim bit with the model ring, and trimmed the inner diameters of all three of the other rings at once.

Then I put them on an NSB, marked screw holes and drilled them. You can paint them black or use truck bed liner if you want.

After the paint dried, I used a plastic knife and spread Goop on the brass tabs and pressed the rings into place. Then I put on latex gloves and freehanded Goop all around the ring on the back side to seal them.

Thereafter, flush mounting with a trim ring was a breeze!

Parts

I am not going to provide a parts list, partially because you cannot find the Meiloons at PE anymore, and partly because PE has begun carrying some different inductors now. Suffice it to say that you can get away with nonpolarized electrolytic caps -- using more expensive caps would be a waste. You can get by with cheaper, smaller gauge inductors too.

Final Word

These are very efficient speakers. They can be run off a boom box, so they are a fun project to do with a child or teenager, and will provide much better sound than the speakers that came with the boom box. A number of folks who have heard them say they sound much better than they have a right to. I choose to consider that a compliment.