KEF 104 woofer

The KEF 104 uses an active 8" woofer (on the left), and an 8" by 10" oval passive radiator (on the left). Note the unusual shape of both frames, so a special jig was needed to route the recess for each of the drivers.

The active (driven by the amplifier) 8" woofer used in the KEF 104 is a KEF model B200 SP1039. This was a special variant to the KEF 8" B200 series with a larger magnet, designed for vented or passive radiator systems. As you can see from the right hand sticker in the photo, these drivers were made in 1977.

This gives you an idea of the relative size of the B110 driver used in the LS3/5a (on the left) and the B200 driver used in the KEF 104. That extra size will be important because the woofer will be required to move a lot more air at low frequencies (not that an 8" woofer is all that large).

In addition to the 8" woofer, the KEF 104 used a passive radiator, the KEF model BD139 SP1042 (a passive, no magnet version of the famous B139 oval woofer, with a styrofoam cone). Passive radiators are sometimes used in lieu of ports/vents to reduce port noises, but the function very much like a vented speaker design. The date code on this BD139 indicates 1980.

The first step in construction was to route the irregularly shaped recesses for the two drivers. This requires making templates for each driver. In fact, it requires making a (supersized) template to make the actual template for the driver. The several step technique I used was as follows, and can be used for any irregularly shaped driver:

1. Wrap the driver to keep dust out. I used a couple layers of plastic wrap.

2. Screw the driver to a 1/4" oversized piece of masonite (2' by 2' in this case).

3. Using a 1/4" bit, route around the perimeter of the driver, making sure to keep the routers base against the driver at all times (thats the hard part). That will give you a supersized cutout in the shape of the driver. That template will next be used to make the actual driver template.

Now the driver can be removed and put away, and the cutout center portion can be tossed or reused elsewhere.

4. Attach a second piece of 1/4" masonite under the template created above, and use the first (large cutout) template to route a 1/4" cutout in the lower masonite that will be closer to the driver size.

5. Use that second template with a 1" router guide and a 1/2" router bit to make the actual recess for the driver at the specified depth.

Here you can see the 10mm deep cutout I've made in the speakers baltic birch face for the frame of the BD139.

6. I used a Jasper Circle cutting jig to cut the thru holes, which required a bit of creativity for the oval BD139 cutout, seen underway here.

The two baltic birch baffles with driver recesses.

My cabinet construction technique follows the recommendations of George Short, formerly of North Creek Music Systems, in his Cabinet Handbook. That technique requires that the front and rear baffles be 3/4" baltic birch and 3/4" MDF, laminated together with Aleene's Tacky Glue (which makes a soft glue joint) for both strength and to dampen vibrations.

Short also recommends using MDF for the cabinet top, bottom and sides, and baltic birch for extensive internal bracing, as baltic birch doesn't compress under pressure as MDF does (making MDF a poor choice for internal bracing).

Laminating the front and rear panels of Baltic birch and MDF.

The front panels, front and rear sides, after final machining showing routing recesses on the Baltic Birch front (on the bottom in the photo) and round-over for airflow on the MDF rear side of the panels (top in the photo).

All panels and internal braces machined and ready for assembly. Note that almost all of the panels have dado cuts to position braces and each panel to the others, making for an easy (guaranteed-square) assembly.

Partially assembled cabinet showing some of the side braces in place. I also installed a channel in the rear of the speaker to run a cable from the base up to the top for the LS3/5a atop this cabinet.

Fully assembled cabinets (no veneer or trim yet) with the baltic birch internal (window-frame) braces in place. Note that although the cabinet sides and top/bottom are 3/4" MDF, the front edges are trimmed down to 1/2" wide to better match with the look of the LS3/5a cabinets. The cabinets are also as thin as possible given the driver dimensions (much thinner than the original KEF 104s), also to look better with the LS3/5a's atop them.

Once the cabinets were assembled, I selected the veneer - I happened to have some Santos Rosewood veneer in stock that would fit the bill nicely. Here you see one top panel veneered and I'm about to start the side panels.

Once the top and sides were veneered, I cut thin (1/8" thick) strips of solid rosewood for the 1/2" wide face trim. Solid wood on the front edge makes it less susceptible to damage from the occasional impact than the thin veneer used on the sides.

This shows the cabinets veneered with the solid front trim in place and trimmed, ready for staining.

I did a good bit of experimenting with stain color to try to match the Stirling LS3/5a cabinets. In the end, I got the color pretty close, but the Stirling cabinets are a bit darker than my woofer cabinets.

The cabinets have been stained here, but have not yet had a coat of clear polyurethane to protect the finish. That process took weeks as rosewood is an oily wood, and oil based polyurethane takes a very long time to dry between coats.

Next up: the bases. Here are the parts, machined and ready for assembly. I use solid oak for the perimeter of the base as its intended to take the brunt of impacts from vacuum cleaners, etc.

The smaller blocks with the off-center holes are to mount Tee-nuts that for the feet.

The Tee-nuts being assembled. The column of blocks on the left shows how the pieces will be assembled into the base. The Tee-nut itself is glued between two layers of plywood block so it can never move or come loose.

Bases being assembled. Lastly, I cut a bevel on the outside so they look a bit more attractive, and the base is wider than the veneered cabinet to protect the veneer.

Finished bases, upside down, getting a coat of black paint. Once completed, they'll screw into the bottom of the speaker cabinets so they'll be removable to repair damage and/or repaint if necessary.

This shows the completed bases, with the Soundcare Superspike feet and binding posts - Superior BP-30. Those footers use an internal spike but with an integral base to protect the floor.

Next: the easy step of mounting the drivers, using closed cell foam for gaskets to seal the cabinet.

Once the drivers were installed, I was able to do some crossover prototyping. As you can see here, I use an OmniMic test setup and lots of clip leads!

I originally tried variations of the crossover used for the Rogers AB1 bass module, but that couldn't get that to work well for this setup. I eventually settled on a crossover designed by Jeff Bagby, extensively documented here, for his LS3/5A-based Continuum speaker, and that seemed to work quite well.