A Bipolar Transmission Line Project -- the TLB

Section 6 Construction Details

The TLB construction consists of two modules: the TL column and the tweeter module. They are separate since the tweeter is time aligned to the cone of the woofer. The main case is a double panel construction. Interior panels are 0.25" thick free floating on a 'liquid nails adhesive' layer. This method is used to dampen the low frequency side-wall vibration. The woofer mounting technique is a Lars Mittigen innovation; the woofers are supported by the magnet structure, ie the greatest mass of the woofer which is coupled to the case wall. This is used to dampen the basket vibrations and the subsequent cone coloration. This construction requires a certain amount of precision and while described is a trial and error procedure. If you are uncomfortable in attempting this you may use the conventional rim mounting procedure. Make sure that you have attached the driver's electrical leads before attempting the next step. click image for larger plans with dimensions Fig. 6.0 TLB Dimensions

It is assumed that you can work out the general panel cutting dimensions from the given data and that only the unusual steps have to be detailed. I would highly recommend using biscuits for panel construction since this would allow a dry fit before the final glue-up. However dry wall screws can be substituted. After the MDF has been cut and the dry fit verified, glue up the column leaving one side and the top not glued. Next secure all the damping panels using a liberal amount of 'liquid nails' the sub-floor variety which remains elastic. Next secure the open side with screws and proceed to the step of fitting the drivers and the magnet brace detailed on the next page. Once the dimensions of the magnet brace have been set, it is time to commit yourself. I would recommend that you remove the loose side and install the wool stuffing up to the brace line leaving a hand full for the very top. This will allow you an easy way to distribute the fiber so that it is homogenous. Once the fiber is in place glue the lose side of the column, but make sure that the top plate is loose. Now you need to epoxy the brace to the sidewalls and proceed with the driver mounting as detailed on the next page. After you have finished the driver installation fish the driver wires through the top plate and seal the wires to the top plate. Using some of the latex gasket material secure the top plate using screws. Make sure that there are no air leaks around the plate and especially where the wires exit the top plate, since this will destroy the TL's terminus gain. Next install and wire up the tweeter module. Next make the base plate and using it as a template drill the holes for the 3/8" dowels that form the legs and the TL Terminus opening. click image for larger plans with dimensions Fig. 6.1 Damping Panels Before installing the woofers, a damping compound–Daft Silly Putty or equivalent is applied to ribs of the woofer's basket to deaden any resonance.

The Push-Push woofer mounting detail, for the column top

Fig. 6.2 Magnet Brace Details click image for larger plans with dimensions It is not possible to give precise dimensions for the brace to which the magnet base is secured, since the case as well as the flush mounting depth will vary. The dimension given in the drawing of 5.75x1.8x1.5 assumes that the column interior is 5.75" and that the space between the magnets is about 1.77", thus both dimensions will have to be adjusted to reflect the actual construction dimensions. The procedure should be as follows: Build the brace oversize from 2 pieces of 0.75" plywood glued together. Don't use solid wood since the expansion of the multicore plywood is much more stable. Determine the interior dimension of the column and cut to tight fit. Don't secure at this stage. Test mount the two woofers flush with the column surface, but allow approximately 0.1" gasket thickness to float the speaker's mounting rim. I would use metal washers for temporary spacers. With both drivers in place measure the clearance between the magnets and cut the 1.8" width of the brace to fit. Install the brace and screw and epoxy it to the column side walls. Remove the drivers and remove the washers used as spacers. Using the latex caulking compound, run a bead to secure the one driver, in the cut out. Use epoxy on the base of the magnet structure to secure it to the mounting bracket. Press the driver into place until a firm contact is made to the mounting bracket. Let the epoxy cure and repeat for the second driver. Note that the woofer's mounting flange is floating in the latex compound and screws are not used. After the epoxy has set apply an additional bead on the top to secure the structure. The concept is to have the woofers coupled to each other and mounted by the mass of the magnet structure to minimize any induced vibration in the baskets. Note that the caulking bead has to provide an air tight seal around the driver. Leakage will destroy the TL's terminus gain.

Detail of TL column Top Plate Two holes are drilled in the top plate for the connecting wires to the woofer drivers. Make sure that the wires are long enough and securely epoxied to the bottom of the plate and form an airtight seal The two wires form a parallel connection to the terminals of the Tweeter Case assembly. The top plate is screwed to the TL column allowing future access to the drivers. Again the top plate must have an air tight seal. click image for larger plans with dimensions Fig. 6.3 Column Top Plate Details

Fig. 6.4 Tweeter Module The Tweeter module is build as a separate assembly containing the two Vifa D27TG-45-06 1" soft dome drivers and the blocking capacitor Two sets of speaker terminals are mounted in a recessed side panel, allowing the option of biamping the speaker. The tweeter coupling capacitor used: General Electric (41L2100) polypropylene, 10uF 250vdc, obtainable from Madisound (good and cheap), is connected to the input terminal and the body is hot glued to the tweeter case. click image for larger plans with dimensions

Fig. 6.5 Base Plate click image for larger plans with dimensions Base Plate Detail A critical construction detail is the alignment of the dowels that support the TL column with the base-plate. An easy way to accomplish this is : Cut the base to the specified dimensions. Draw the diagonal lines to the corners, this will provide the centering reference for the column. Center the TL column and trace out the outer case outline. Trace the case wall thickness Mark the location of the dowel diameter on the base-plate and drill the 3/8 dia. holes in the base plate. Make sure that the drill is orthogonal to the base plate. Use a drill press or a reference square for a hand drill. Align the base with the column base and using the base-plate as a template, drill the dowel holes into the column sidewalls. Glue in the dowels, making sure that you have the 3" hight from the base to the column terminus. This defines the TL throat opening. Making it too small will attenuate the Fr amplitude. The base can be made from 0.75" plywood and a 0.25" plate glued to the bottom forming a base for the dowels supporting the TL case, with the edges hardwood glued and lightly beveled. The whole painted a high gloss black. A luxurious and functional option is a marble base about 1" thick.

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