The BassZilla Evolution
By Dick Olsher
This update reflects the status of the BassZilla loudspeaker as of February 2, 2004. The good news is that a replacement woofer has been selected for the original Audax PR380M2: the Eminence Magnum 15HO. The Magnum 15HO is a gorgeous, made-in-the-USA woofer, that is READILY available. No more Audax drivers for me! This is the third time Audax has pulled the rug out from under my feet with driver model cancellations.
My first impression was that the Magnum 15HO sacrificed some low-end extension, but sounded more powerful and precise with more midbass output relative to the Audax PR380M2. And best of all, there are presently no changes to the bass cabinet or crossover, other than of course the front baffle cutout which is now 14.0 inches in diameter. The measured impedance magnitude of the Magnum 15HO in the BassZilla bass enclosure (with the same vent as before) is shown below. The box tuning is now 42 Hz, which is a tad lower than the 43.5 Hz prediction of LEAP 5.0.
Since this woofer’s Qt is in the overdamped category (0.27 is stated by Eminence - or 0.264 as measured by Vance Dickason in Voice Coil Magazine), the obvious idea would be to investigate an Extended Bass Shelf (EBS) alignment to obtain maximum bass extension. An EBS alignment is characterized by a box tuning frequency equal to the resonance frequency of the woofer (33 Hz). In this case, the vent would have to be downsized to 9.25” L x 1.5” H x 5” D. The LEAP 5.0 simulations for both the standard vent and the EBS sized tuning are shown below.
The EBS prediction indicates another 10 Hz or so of bass extension relative to the higher box tuning. Unfortunately, I was unable to confirm the predicted EBS response in practice (though this could be a function of the in-room measurement technique used). LEAP also predicts an F3 of 45 Hz for the standard vent. The best quasi near-field response (30” distance from woofer cabinet - without any low-pass filter) I have measured was with the standard vent (see below).
The measured in-room F3 was about 52 Hz, which may improve slightly as the woofer is broken in further. Downsizing the vent, resulted in a reduction in bass extension. Hence, I am recommending no change to the vent dimensions at this time.
Excellent progress is being made in finalizing the new Fostex BassZilla version (to be dubbed the Platinum Edition), which uses the new FE208EΣ wide-range driver. As far as a matching tweeter, I have given up on the sound of the Fostex FT17H horn tweeter. Its frequency response is the flattest I’ve ever measured. But sonically, it spits, instead of clearly enunciating transients, something that drove me crazy in the long run. In the end, I’ve decided to deploy the familiar and sonically exceptional Aurum Cantum G2Si as a super tweeter. That means that except for the choice of wide-range driver, the Fostex Platinum Edition will share the same driver complement with the Lowther Diamond Edition.
Finally, David Pettoruto provided the following pictures of his (somewhat modified) Fostex BassZilla. He says: “I am thrilled at how great they sound!!! Thank you very, very much!!!” Well David, you’re most welcome.
These fabulous cabinets were built for David by Lee Taylor. Be sure to check out his site (www.taylorspeakers.com) under “kit Cabinets” and then “Custom Work.” I love the zebra wood finish.