I am less than excited to see that none of the images work in my post. Anyway, you can head over to my Picasa album to see the pictures that are mentioned in the post.
Thanks to everyone on the forum for inspiring me to get serious about modifying my pair of RTA15TL speakers. I am the original owner of these speakers. I bought them in 1994 at Great Sounds in Muncie Indiana while I was attending Ball State.
There are approximately one billion screws in a pair of RTA15s, so getting everything out of the cabinet is a non-trivial affair. I cannot begin to stress how important it is to label everything before you take it apart. I also highly recommend only disassembling one speaker at a time, so you have a fully assembled, functioning reference. Here is what the cabinet looks like when empty. You can see the tape I used to label every connection, and the bowl I used to collect the screws.
The first modification to the cabinet was to paint it the interior with Cascade VB-1X damping compound.
This attenuates panel resonances nicely. One gallon is more than enough to do both cabinets. Shave your arms or put on a long sleeved shirt before applying this substance! Be patient, and let each coat dry before applying the next. Trust me, you'll want the break!
There is an intermediate step not pictured in which you pour over the Polk forums and Tony Gee's cap shootout on the internet at work for weeks trying to decide what brands of components to use. It's vastly superior to doing actual work while you are at work. ;)
Here are my components on scrap piece of MDF. You need to decide on a layout. Obviously your new components are multiple orders of magnitude larger than the old ones, so they aren't going on that tiny little PCB screwed to the input cup. In a later step, we'll rip that off and throw it away!
Remember that the cutout for the front passive radiator is only 9 inches, and your crossover has to fit through that opening. The input cup is in the back, and the drivers are in the front, so try to arrange the input and output sides of your crossover with that in mind.
I mounted my components to a piece of maple 1x6. I probably obsessed over the stain on my board a little too much considering that it goes inside the speaker never to be seen again! There are more high-tech ways to fasten the components to the board, but I got-back-to-basics with my hot glue gun.
Though I didn't take a picture of it, remember to connect all of the pigtails that go to up to the drivers and out to the terminal cup when you are wiring the crossovers. Now would be a good time to assemble your new wiring harnesses, also. Use the old one as a template to create the new one. The original Polk factory instructions for assembling the harness can be downloaded from forum if you'd like an additional technical reference.
I used Neotech solid-core teflon insulated wire to make new harnesses. I used exactly the specified gauge for the tweeter. I slightly increased the gauge of the woofer harness, since they see far more power on average than does the tweeter. I used exactly the specified lengths.
This requires more attention than you might think. First you need to remove the old crossover. There are four screws holding it in place. Remove those, and then snip all of the wires to get it free. Then comes the fun part; removing the binding posts from the cup. The nuts on the back are covered by a giant dollop of hot glue to prevent them from turning when the binding posts are tightened. Scrape away the glue to expose the nuts. I opened the binding post all the way, inserted a small screwdriver through the speaker wire opening, then used my socket to remove nut inside the cup.
Depending on how insane your new binding posts are, you may need to drill out the holes in the terminal cup to accept them. My Cardas rhodium posts fell squarely into that category.
After tremendous profanity and a half gallon of Diet Dew, the binding posts were installed as pictured below.
Do not tighten the nuts down with your nuclear impact wrench! Later, you'll need to remove the nuts to mount the ring terminals of the crossover pigtails.
Though I lacked the vision to do it; I hope that someone out there is man/woman enough to use the extra space on the terminal cup to mount a second set of binding posts. Since you are wiring the crossover point-to-point, a second set of posts is the only thing standing between you and all of your wildest bi-wiring/bi-amping fantasies!
Cabinet Part II
I lined the inside of the cabinets with a 1.125 inch foam/vinyl barrier product similar to BlackHole. This provides even more panel damping, and the works to attenuate flutter echo inside the cabinet. Most importantly, it absorbs some of the woofer's backwave. Remember; there's just as much energy radiating off the back of the speaker cone as the front. The backwave bounces around inside the cabinet and eventually passes out through the cone. This delayed copy of the signal wrecks accuracy in the time domain. Attenuate the backwave with the foam/vinyl barrier panels as pictured below.
The vinyl layer has the consistency of floor tile, which makes the panels fairly rigid, and your knife fairly dull Panels are self-adhesive, and the adhesive is serious stuff. Test fit each piece before removing the backing to expose the adhesive. If you had any arm hair left after the VB-1 application, its about to come off during this step :( Here's one cabinet very nearly complete.
Slide your crossover into the bottom of the enclosure first. Connect the pigtails to the terminal cup, then begin installing and connecting the drivers. When complete, use a 9v battery to confirm that all of the drivers are wired in phase. Do NOT connect the speaker to your amplifier until you verify the impedance of the speaker. Use your multimeter to confirm that it is approximately 7.5 ohms. Once you are satisfied with the phase and impedance, then connect the speaker to your amplifier. Start at a very very very low volume and increase it gradually until you are satisfied that nothing is going to explode.
Your speakers will sound like smashed dog turds until everything burns-in. Do not do any critical listening at this point. Now would be an excellent time for you to travel to your neighborhood mini-mart for a cola product of your choice. :D
Mods not pictured
I also installed the new silk dome tweeters from Polk If you have the SL3000 tweeter like my RTA15s, it'll be a tasty little improvement. If you have one of the older tweeters, it'll be like God reached down and healed your speakers.
My RTAs now ride on Vibrapods mounted to 2 inch Cusinart end-grain cutting boards. This serves to uncouple the speakers from the floor. When the speakers are spiked to the floor, bass travels through the cabinet walls, through the spikes, and into the floor. The floor is a very efficient bass radiator, because it has massive surface area coupling it to the air in the room. A very small amount of cabinet resonance can produce significant output through the floor. This is a way, way, way bigger deal than it might seem. If you are looking for something you can do without opening up your speakers, the Vibrapods are cheap and the improvement is massive.
Thanks for all of the support, and good luck to everyone who decides to take the plunge and modify their vintage Polks.
Let me know if I can help in any way