I built these super-tweeters to extend the high frequency output of my Yamaha NS-1000M speakers (http://www.homebuilthifi.com/project/6043) which roll off steeply over 15K.

They use quite an inexpensive AMT (air motion transformer) drive unit by Dayton Audio. The drivers are a precise push fit in to a laser cut piece of 12mm acrylic. The base is 12mm Corian and the series capacitor is a ClarityCap ESA (polypropylene). The tweeters sit on three small rubber feet and can be adjusted for toe-in and angle. I chose these drive units specifically because they are reasonably flat to 40K and have wide dispersion characteristics.

Just after the top picture was taken I made a pair of anti-diffraction rings that fit over the face plate of the AMT units. These rings are shown in the second picture.

I was surprised how much difference adding these super-tweeters made and how positive their contribution was.
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EDIT: I have recently added the last picture and this shows how the tweeters are at present. The anti-diffraction ring is fitted and the capacitor has been replaced with a Mundorf Supreme (1.5uF). The wiring has also been replaced by a custom made umbilical cable constructed from, high purity, solid core, Teflon insulated copper.

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