Rudy Bozak tried his best to push the state of the art forward but his designs suffered from infancy in technology. Many things were done very, very right and others - well not so much. Thiele-Small did not come along until the 1980s so he was just basically winging a lot of things. The biggest issue was high frequency response, that above 8KHz. He also did not flush mount drivers so his understanding of first reflections was missing. But he did understand low-mass cones and to some degree dispersion.
I thought about trying to keep the original design and after looking at it for a few days decided only to keep the original box for the woofer and move everything else out to a satellite. If I decided later to changer the woofer box, the satellite would already be finished. So the mid and tweets were removed from the original box as was the crossover network (moved to the back panel allowing easy access for tweaking) giving the most volume to the woofers as possible. I also bought another pair of woofers and four more tweeters so I had a total of eight tweeters, two midranges, and four woofers to work with.
The 2" tweeters, although an attempt at high frequency extension by using aluminum center caps, just had too much mass to be considered state-of-the-art today. They did work well up to about 7-8KHz and started to die pretty sharply after that (see Before SPL graph). I used an asymmetrical truncated pyramid box and stuffed the tweeters in a vertical array yielding the best horizontal dispersion pattern. To improve the vertical dispersion, I also arced the array and added a large chunk of Bondo body filler to the baffle board thus providing a low-reflection surface and smooth transition.
Although quite good sounding in their limited range, the lack of HF response was disappointing. I temporarily added a super tweeter (ST) I had lying around by resting it on top of the cabinet. From its limited contribution, I knew that this was the correct direction to go. Searching for a ST that would be efficient ans still sound like music proved to be a challenge. The 1-watt sensitivity on this system was about 96dB/W/m so finding something with a sensitivity above that was a long process. Finally, a friend recommended considering the Audax TW025A28, a gold dome that if nothing else would be aesthetically pleasing.
So what does it sound like? My wife is the best test to such changes since she is an artist, not an engineer. Her reaction was an immediate thumbs up tossing phrases like "amazing" and "it's like listening to a whole new speaker" since she was truly unaware of what was measured to be missing. She listens to them louder than before, another sign that she is listening to the details rather than merely tolerating the sound. Speaking of measurements, here are the before and after RTA results.
So what does this mean to the high end? Adding super tweeters adds enjoyment (duh!) but the physical location of these drivers determines how well the system will sound. Adding the ST and resting it on top to "test" its effects did not permit the driver to perform to its full potential. Putting it in the proper place made a world of difference in the tonal balance.
Copyright © 2015 by Philip Rastocny. All rights reserved.