I'm thinking of building a pair of JBL CBT 50LA clones for surround duty in my home theater. These are passive speakers that use a line array of eight 2" full-range drivers.
The drivers are arranged into four groups in the following configuration: 1-8, 2-7, 3-6, and 4-5. The drivers in the middle (4-5) operate at full level, while the surrounding drivers' frequency responses are shaded (tapered) to minimize interference caused by the center-to-center spacing between the drivers. Also, JBL uses a clever trick to make these straight columns of speakers behave as if they are curved by applying delays to the drivers near the edges, further minimizing interference. These are supposed to be great as surround speakers because their level doesn't vary much even if you sit very close to them.
Fortunately, JBL published the crossover schematic in a technical manual, and I've whipped up a quick model in VituixCAD using the exact crossover filter values. To my surprise, the CBT (constant beamwidth technology) effect seems to be working fine, even though I plan to use a different 2" driver: the Lavoce FSN020.71F. I believe driver selection isn't overly critical, as long as all drivers are identical and correctly attenuated and delayed.
Here's the vertical directivity (about +/- 15 degrees nominal):
The crossover and its effect on frequency response and phase can be seen below. The notch filter at the top left wasn't part of the original design, I added it to tame a peak.
Please let me know if I'm missing something before I go ahead and build these. Thanks!
The drivers are arranged into four groups in the following configuration: 1-8, 2-7, 3-6, and 4-5. The drivers in the middle (4-5) operate at full level, while the surrounding drivers' frequency responses are shaded (tapered) to minimize interference caused by the center-to-center spacing between the drivers. Also, JBL uses a clever trick to make these straight columns of speakers behave as if they are curved by applying delays to the drivers near the edges, further minimizing interference. These are supposed to be great as surround speakers because their level doesn't vary much even if you sit very close to them.
Fortunately, JBL published the crossover schematic in a technical manual, and I've whipped up a quick model in VituixCAD using the exact crossover filter values. To my surprise, the CBT (constant beamwidth technology) effect seems to be working fine, even though I plan to use a different 2" driver: the Lavoce FSN020.71F. I believe driver selection isn't overly critical, as long as all drivers are identical and correctly attenuated and delayed.
Here's the vertical directivity (about +/- 15 degrees nominal):
The crossover and its effect on frequency response and phase can be seen below. The notch filter at the top left wasn't part of the original design, I added it to tame a peak.
Please let me know if I'm missing something before I go ahead and build these. Thanks!
Last edited: