|Appendix | Modifying
|While it isn't absolutely necessary to add the Ready Mixed Bondex Concrete & Mortar Patch (31084) to the magnets of the tweeters and to the magnets and the outside areas of the baskets of the main speakers, it is an easy and inexpensive way to improve their performance. This material is sold at Wal-Mart. I don't know if it is their house brand or if it is sold by others as well. It is the only material that is the right consistency for use directly out of the container that I have found. It's water soluble until it hardens, so clean up is easy. There are no toxic fumes to harm speaker builders or speaker adhesives. Drying time is pretty quick. You could probably install the speakers in a transmission line in a couple of days. I'd wait awhile longer if I was using a sealed box that doesn't have a TL's free access to open air.||
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|When potting the tweeter
magnet remove the paper label that covers the hole in the magnet (or at least puncture
the paper with a well aimed finger so the patch material can enter and fill the hole.
Keep the material inside the circular lip that surrounds the lower edge of the magnet
and keep it well away from the connecting terminals. As you go up the outside surface
of the magnets keep the diameter of the potting material about the same. The top
of the magnet is covered with a hemisphere (more or less) of the same diameter. Then
everything is blended together into a smooth monolithic structure. I want to thank
SB reader Gordon Burkhart-Schultz for creating this rather impressive looking architectural
Current versions of this tweeter have a metal shielding can over the magnet. It would be advisable if you scratched this up with coarse sandpaper or a file to allow a better gripping surface for the patching material.
If I had my wits about me I would have told you, before you got yourself into that messy situation, that a tongue depressor makes a nice tool for this job, as do your fingers. In the latter case it might be well to wear a pair of disposable vinyl gloves.
When potting the main speakers I started by forcing the compound into the deep spaces between the basket and the magnet. Then I covered the magnet with about 1/4" of the stuff (Maybe about 1/2" on what would be the top with the speaker facing down.) Finally I ended up putting about 1/8" along the basket legs, on the outside only, tapering it off towards the flange end so that it wouldn't interfere with mounting the driver. Make sure you don't allow any of the compound protrude into the spaces between the legs of the basket. This could interfere with the breathing of the speaker. When I use this material to damp larger speakers I usually also place material carefully on the inner sides of the basket legs. It is much easier to reach in and the basket legs are usually concave on the inside so the compound can be placed without blocking the air passageway behind the cone.
I find it easier to modify the speaker cones after the speakers are mounted into the enclosures. The speakers are firmly held in the correct position for working on them and both hands are free to handle the operation. This also gives you a chance to compare the speakers, before and afterward. Modify one of them and listen to your heart's content. Actually it won't be a completely fair comparison because the unmodified speaker will be considerably more sensitive than the modified one, and will sound out of balance with the tweeter. To solve this, merely disconnect the tweeters. The main speakers are essentially "full range'' types and you can get a fair idea of what the modification does.
To paint the full strength Elmer's polyvinyl acetate (PVA) white glue onto the speaker cones and dustcaps I used a 1/2" flat nylon bristled artist's brush. Nylon bristles are what artists use with acrylic paint. They are cheaper than sable brushes and more durable for our purposes. Before each use wet the bristles with water and squeeze the water out with your fingers. This will make it easier to clean the brush after use.
Apply the glue full strength using a flowing motion. Start up at the joint between the cone and the cloth surround. Don't get any glue on the cloth surround. If you accidently get some on the flexible part of the surround, stop everything and remove it with a damp tissue. Don't allow it to dry on the surround.
Use horizontal strokes of about 1 1/2" work from the top down to the dustcap. I use light vertical strokes from the bottom up to smooth out the glue. You will never get it perfectly smooth, so don't try. If you keep going back over the glue as it dries, you may lift the glue right off the cone. If this happens, put some fresh glue on the brush and stipple some glue onto that area. After the cone is covered, proceed to the dust cap.
Don't panic when coating the cone. The glue is opaque when wet but dries completely transparent (if the glue is applied at a temperature over 60 degrees F). When you're done, clean your brush with soap and cool water.
To lower the cone's resonant frequency and increase the mechanical Q, you must add about 5 grams of weight to the junction of the cone and the dust cap. I usually use 1/8" diameter solid solder wire for adding mass to my speakers, but you may not be able to find it.
As an alternative, use BB size split lead fish sinkers, obtainable at sports shops (and probably Wal-Mart). 9 of these weigh 5 grams. They are split almost all of the way through, so all you have to do is slip a single-edge razor blade into the split and slice down vertically (carefully) through the remaining lead. Rocking the blade slightly while pressing down makes the going a bit easier. Don't use the kitchen cutting board for this process as lead is highly toxic.
Glue the split sinker halves around the cone and dustcap junction (Fig. 4) like petals of a flower. Just brush a ring of glue around the cone where it meets the dust cap, then place the weights, flat side down, with their little pollywog tails pointing up toward the surround, on the glue. Move them around until they look their best. Go out and listen to your old speakers for awhile and let them know you still love them.
When the weights are firmly attached, paint a second (final) coat of glue on the cone and dome, covering all of the weights so they are cast in place and won't buzz or rattle. When this is dry to the touch you are ready to hook up the Wallflowers and listen. I hope you are pleased with your efforts.
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