Backhorn for RS 40-1197 By John Lapaire This project is an adaptation of the Buschhorn MK2 and it transforms the tiny 1197 into a speaker that you can actually use as your mains. I started with the excellent information available at Pit Hinder’s website. He says, and I agree with completely: PLEASE don´t use this plan for commercial purposes, or make sure to contact the copyright holder Dr. Andreas Buschhorn Golmckesgraben 13 37120 Harste Germany (He would surely like to receive some feedback, anyway - and so would I, for that matter.) Even though this plan is not exactly the same and doesn’t use the same driver, we understand that the design work was done by Dr. Buschhorn and we were helped to find it by Pit. The order of assembly as I did it is as follows: Set rip fence to 15” and cut out 4 sides from the 4’ x 8’ sheet of ¾” plywood. Set rip fence to 5 ¼” and rip up the rest of the sheet. Cut strips to lengths given in the plans. Dry-assemble the pieces on two of the sides to make sure you’ve got them right. We’re building both speakers at the same time. It doesn’t take much longer than building one. Mark the pieces that will need bevels, corner rounding, or driver cutouts and do those operations. Reassemble dry to check, and if OK, glue them in place. Lots of different assembly options could be used here, I just machined to close tolerances and glued them with yellow glue (Titebond). A clamp or two here and there if necessary holds things in place Add stuffing and internal damping materials. I used very light polyfill in upper chamber, felt carpet backing in the rest. In the next set I’ll use the felt only in the first foot or so of horn. Install the wires, bring them out through the connector holes wherever you decide to put them. If you put the connectors up high on the back of the cabinet, you will be able to reach them any time and can put them in later. Add fine, DRY sand to the sealed cavities and vibrate them for a long time to settle the sand. I did this by clamping an old woofer to the unfinished enclosure and playing loud music while we left for the day. If you choose to use spray foam in addition to or in place of the sand, make sure you’re ready for the next step because you’ll need to do it immediately. Glue and clamp the other sides on. I glued one, set the other speaker on top, glued that side on, and clamped the whole thing together. Make sure you can put pressure on the middle areas as well as the edges. Sand bags might work well for this. I used a couple of sticks that reached to the ceiling of the shop, with wedges to apply pressure against scrap pieces of wood that protected the nice plywood sides. Mount the drivers and fire them up. You can do the finish work any time or never. These make the most bass if you can put them in corners, about 6” or 8” from the walls, but also sound good at up to about a foot away from a straight wall. Best imaging for me is facing exactly into each speaker so you can’t see any of the sides. They’re about 6’ apart, 8’ from listening position. As with most of the single-driver designs, the actual sweetest spot is small, and you’ll hear the soundstage open up as you get the positioning right. They image better than most speakers, regardless of cost. Bass response is larger and tighter than with the same driver in any of the pipes and boxes I’ve tried this driver in. The pair of 1197s currently mounted have been tweaked with 2 coats of thinned Dammar on the cones, phase plugs, Duct Seal on the baskets and magnets and felt inside the basket legs. All of these things improved something a little bit, but the stock drivers sounded pretty good right out of the box, and great after a week or so of more or less constant playing. If you get around to it, you might try one of the 4” Fostex drivers in these horns. I expect I will sometime. Top