The Afterburner Project - My version by Andrew Brandon This project came about for two reasons, firstly I enjoy this audio hobby and am always looking for something new to try and secondly because I was rapidly growing unhappy with my existing speaker setup. With two valid reasons (in my book) here we go… The existing speakers were Fostex 208 Sigma's in Voigt horn cabinets, which are in reality, folded TQWT's. The Fostex is a great speaker I think but the bass was just not there in terms of quality and quantity, plus there was unevenness to the bass. I built a powered subwoofer to cure this shortcoming but instead of a cure I just exchanged one problem for another. I had the quality now but the quantity was just not right, on some material it integrated well but on other material it was too much or too little bass and after awhile I found that I just left the subwoofer turned off rather than fiddling with it for every song. I saw a post by Thorsten on the Joenet about his DDDLQD idea, and decided to give it a whirl except during my emails with Thorsten one of the 208's developed a scratchy sound and killed that project before it really got started. I was a bit pissed at this and in my pissy mood bought one of the Eminence Beta 12 LTA's that I saw in the A.E.S. catalogue figuring I would just give it and try and see if I could get something half decent going using this driver, after all this is the base driver for the Hammer Dynamics Super 12 albeit the Super 12 is heavily modified, to the stage of being a different animal really, but I figured there must be some merits to this driver otherwise why it would it be used for that highly regarded speaker in the first place? I emailed Thorsten about my bad luck and purchase of the Beta 12's and he then told me of his Afterburner project and the story goes on… I posted a message on the Full Range Driver Forum re the Afterburner project and got several responses, one in particular had a listing of all the posts regarding the project. So further into the land of the internet I went and got a good lot of info to wade through . After about a week I had a good idea of what I wanted and using my WinISD software came up with a 127.4 litre cabinet with a 4-inch diameter port and tuned to 40 Hz. I used 3/4 inch "Brazilian Plywood" which is a good quality plywood with a nice veneer on it although I later found out that it was not void free but not to worry, the voids can be filled. The interior I first lined with a double layer of tar paper or roofing felt, the heavy variety, I think they call it 90 pound tar paper on about 70% of all interior walls except the front where the speaker and port are. Then it was bubble wrap, the "small bubble" variety on all interior walls except again the front baffle. Finally is the1 inch polyester batting on the same surfaces again. My thinking here is the tarpaper will damp cabinet vibrations / resonances, the bubble wrap will break up standing waves and the padding will absorb what's left. Figure 1: Internal view. Four sides assembled. Shelf brace installed Figure 2: Front view with holes cut Figure 3: Here you can see the support for the rear of the speaker Figure 4: Internal view with interior damping etc installed Taking a page from the Goodmans cabinet damping methods, I also used a piece of felt behind the driver placed vertically from the top of the cabinet to about midway down the cabinet. Goodmans recommends the felt be the full height of the cabinet but with all my interior linings I felt behind the driver only to be sufficient. I used a second piece of plywood to double the front of the cabinet from the top of the cabinet to a little below the middle where I installed a "figure eight" shelf brace to tie all four sides together. The front was routed so as to facilitate flush mounting of the Beta 12, which was mounted near the top of the cabinet. On the rear of the cabinet I mounted two plywood "blocks" which act as braces for the Beta 12. When the Beta 12 is mounted, the magnet rests against the "blocks" and in this way the driver is supported from the magnet structure as well as the mounting flange from the front. For mounting the driver I used "T" nuts. The port as I said before is a 4 inch unit that is flared, I initially used PVC but decided to get a flared unit at the same time I got some other stuff from Parts Express. They are cheap enough at $1.09 each and definitely give the speaker a more finished look, much better than the PVC. I also got some L-pads that I use to adjust the tweeter output level. I found that I needed to attenuate the tweeter some, maybe its just more sensitive than the Beta 12 or maybe because of the Damar coats on the Beta 12. The tweeter is the Radio Shack "super tweeter" part no. 40-1310B. The Beta 12 has three coats of Damar varnish applied. I have also covered the speaker frame with the 1-inch batting. I am considering also applying some modeling clay to the frame to damp it but with the the way in which I have mounted the driver, I am not convinced it will make a difference, I guess I will at some stage do one and see if I can hear a difference. Wire is a biwire job using two runs of cat 5 for the Beta 12 and a single run for the tweeter, a popular approach. The speaker is hard wired. I have mounted the speaker on 3 brass spikes which are approximately 1-1/2 inches tall each, one each at the front corners and the other centered at the rear. I have finished them with a clear coat over a red mahogany stain and they are very dark and look quite nice I think, matching my favorite amp which I used Walnut as a base for with the same colour stain. The crossover cap is the crossover, I do not like crossovers, but am sure they have their place in certain designs. Maybe this speaker would sound even better with one. Figure 5: One completed with PVC port and a quick listen Rather than having to remove the speaker and then get inside the box to make changes, I made an external "half box" on which I mounted the L-pad. It's attached to the rear panel of the speaker with Velcro. I am going to mount the 1.5 uF crossover cap here as well; presently it's soldered to the tweeter. In this way I can quickly swap caps to optimize. This may change later of course when I think I have it worked out okay and in the interest of neatness I may put it all inside the cabinet, who knows what the future holds. So, how does it sound ???? Well, much better than it has any right to, well maybe that statement is unfair. Yes it's cheap to buy the components for it but as I learned before, money has little to do with it. I built a pair of modified J-horns last year using the Radio Shack 40-1197 and was simply amazed at the performance of them. These were a lot more work though. The Afterburner is of a much higher standard and is even more amazing. I do not want to get into the usual audio adjectives but will say this. Its all here, from the lows to the highs. They component cost is very low, I have not totaled it up but I have less than US$400.00 in them and that includes shipping ($3.00 / LB) and duty (20% cost + shipping). The Beta 12 was $37.xx each! I did some quick frequency tests with my old HFN&RR test CD (I have to update it soon) and had output at 20HZ, not shaking the walls but there was no doubt of its presence. At 31.5 Hz it seemed to be all there. I will admit to cheating a bit here, the speakers were about a foot away from the side and rear walls. When I get my SPL meter I will do things properly. My hearing only goes to a little above 16kHz but the tweeter supposedly goes way, way past there, to the region of around 40kHz. Figure 6: My Afterburner in position In comparison the Fostex's, I find the Afterburner to be much more dynamic and open. It also has more highs, I never thought I was missing anything with the fostex's but know realize that I definitely was. The Fostex I find to have a slightly "faster" sound and a more laid back sound with a different tone. When I get an SPL meter I will make some measurements. Did I say that I was hearing things I never heard before? … Highly recommended. Questions / comments email me at ajbrandon@yahoo.com or post a message on the forum. Thanks to Thorsten on the idea, the forum members for their input and to James for a great site to share with others our knowledge and experiences in this totally consuming audio hobby. Top