Transmission Line -- Fostex FE 107

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Fostex FE 107s formed the start point for the next project. Given the flange width and magnet cross-section of the 107 I chose a 4" ABS elbow to be the basis of the enclosure. Care should be taken with driver selection in that magnet diameter can occlude a good portion of the limited space in the throat of the line. The smaller shielded magnets of Fostex's A/V drivers seem OK but I would be concerned about the larger diameters of the non shielded drivers. AlNiCo
FE 103As (or the Monarch 4" AlNiCo FR) look particularly well suited.

I made a baffle from a plywood circle and a small MDF ring glued to its back surface. The MDF ring is sized to slip onto the outside of the flange of the ABS elbow and is siliconed in place. The other flange accommodates the 4" ABS pipe which forms the line. Wool blanket fabric works well in the flange area directly behind the speaker baffle. The ABS elbow flange has quite a step where the pipe coupling diameter abruptly steps down to the elbow internal diameter. Visualizing an early reflection from this step I tried tapering the step smoothly by applying body-filler in the flange area. I achieved a good taper but the results didn't seem any better than the method I use now. I cut chunks and strips of blanket and using silicone seal as a glue build up a fabric taper to the depth of the step in the ABS flange.

Pondering how to taper the line I came up with a solution which kills two birds with one stone. The 2x4. I cut a 2x4 to taper the chosen length of the pipe. The 2x4 extends from the bottom of the line and is a handy place to bolt on a couple of pieces of bent strap iron to form a crowfoot arrangement. The 2x4 occludes the terminal cross-section of the pipe by approximately 40%. I also, now, attach the stuffing to the 2x4 with staples before insertion into the pipe. This seems to prevent wadding and clumping in the narrow pipe; it also keeps very light stuffings from settling down the line. Wool blanket fabric and/or bonded polyester quilt batting stapled to the 2x4 seem to give good resonance control if one wishes to not stuff heavily.

The sound is just lovely. While the low end goes only into the mid-bass my friends and I are now starting to hear the excellence of these little Fostex drivers. There are calls for More! More!

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