I had three Polk PSW650 subwoofers. The amplifier on one of them unfortunately died (which seems common with subwoofers) and I was left with a deadweight with a couple of decent drivers. After contacting Polk, I was dismayed that the amplifier was not repairable and that it was not available anymore. I tried to replace the amplifier with a third party alternative but could not find one that fit because the Power Port vents were too large on the rear panel, limiting the area for the plate amplifier.
During the Christmas holidays, I decided to do something about it. I tore apart the old cabinet, keeping the vent tubes, the Power Ports, the grills and the rubber grommets for the grills. I picked up the Dayton Audio Speaker Analyzer (DATS) and the speaker cabinet software from Parts Express. Unfortunately, the software could not come up with a viable solution for a new design (I was really hoping for an isobaric design). Since I could not come up with a suitable alternative, I decided to rebuild the PSW650. I bought their Dayton SA250 250-watt (into 4-ohms) subwoofer plate amplifier. The original Polk amplifier was 165 watts and I did not want to go overboard and drive the speakers beyond the excursion limits. The speaker themselves were measured at 2-ohms each and were wired in series giving a nominal value of 4-ohms.
To keep the speakers within design specs, I kept the same dimensions...I measured the original cabinet at 16"x17x18" with 1" driver baffles and 3/4" baffles for the other. I bought some 3/4" and 1/2" MDF boards for the build. It was fairly obvious that the layout had to be rearranged. The amplifier was 10"x10"x4", so it occupied the rear panel. That meant the vents had to go somewhere else. Looking at some vintage Polk subwoofers, I decided to mount the vents on the bottom. I designed the cabinet to be 18" deep, 17" wide and 16" deep. I also had to decide how to finish the cabinet. I was thinking of piano gloss black but thought that my skills and tools would not make an even finish so decided on a walnut veneer.
Let me be up front, I am not a carpenter and I did not have the proper tools for the build. I used a circular saw with a old RotoZip and a cheap sander. The side panels were made of two 1/2" panels glued together to make a 1" thick panel. The outer panel was cut with a 10" circle to fit the driver. The inner panel was cut to 9" for the driver to rest on, making it somewhat flush with the outer panel. The RotoZip can cut decent circles but it was very hard to get them the exact size as the tool kept on shifting in diameter. It would have been better to get a decent router and a Jasco circle cutter guide but i wanted to keep expenses to a minimum. The rear 3/4" panel was cut with 8x8" opening and countersunked with a 10"x10" 1/4" area to flush mount the amplifier.
The bottom panel posed a problem. The sub cabinet was 18" long. The two Power Ports were 7"x8". Mounting them next to each other would take 14". The 18" minus the 1.5" for the front and rear panel left me with 16.5" inside. The amplifier had an internal depth of 4". The vent tubes were 3" in diameter but had large flared ends on the inside. I could not get the entire design to fit within the 18" without some compromise. My decision was to trim 1" off the edge of the Power Port. So the side panels were 18" long and 16" wide. The top and bottom panels were 18" long and 15" wide. The front and back panels were 15" by 14.5". The Power Ports were screwed in a bit and I drew a line across the screw holes to find the center point to cut out the vent holes. I then cut the power ports vents in the bottom panel and rounded the edges on the bottom like the original.. I also added two vertical 18" braces to stiffen the cabinet.
I then started assembly. Bit of advice. You really need a good table saw. Even with a good table saw, you cannot cut the wood perfectly. Doing a test fit showed that there was an awful lot of raised joints. I glued and screwed the whole cabinet together. I got out a belt sander and ground down the raised edges but got a bit overly aggressive on the top (there is a slight depression) and under aggressive on the front, but it came out decently.
I had some unused walnut veneer and applied it to the cabinet. I ran the grain from the front, over the top and on the back with one long piece so that the grain matched at the edges. The sides had the grain running vertically. I then sanded the veneer with 180-grit paper, then 220 and finally 320-grit, being careful not to cut through the thin veneer. I applied three coats of oil based Oak stain (I did not want the cabinets to be too dark). After that, I applied three coats of Danish Oil. After that dried, I then wet sanded (with thinned Danish Oil) it with 0000 grade steel wool to even out the luster and applying satin wax over the finish. I then had to drill in the holes for the grill grommets. I was kind of stumped with this for a bit but got an idea. I drew a line horizontally along the top edge of the cutout and repeated it with a vertical line that intersected the horizontal line. I repeated this process at all four corners. I used a long ruler and measured diagonally a 13" (the grill pins were 13" in diameter across the grill) point equidistant (i.e. 1.5" from the circle). I made sure that the marks were all the same distance diagonally as well as adjacently. I used an ice pick for a starter hole and drilled a 3/8" bore to fit the grommets. It came out perfectly.
I stuffed the cabinet with the original filling from the PSW650 and added a bit more of dacron fill. I then screwed the Power Ports into the bottom of the cabinet. The ports had a depth of 1-3/8" so I got a dowel and cut four pieces in that length. I then attached a 17"x18" MDF board on the bottom mounted to the dowels. I then screwed the cabinet to the dowels. I screwed in the drivers, wired the amp in series and screwed the amp in.
I took the sub (which by this time was awfully heavy) into my house and connected it to my system. i was at first sorely disappointed by the weak bass until I realized I had the system playing stereo with no sub signal. I changed it back to 7.1 and got the sub working.
The sub was a bit more powerful than the original and had a bit more dynamic punch. My guess is that it bottoms out at 30-35Hz (like the original). In retrospect, I should have made the cabinet an inch or two larger, because the interior volume is a bit smaller than the original, because of the added vertical braces and that the amp was a bit larger inside the cabinet. I am pretty happy with the end result and must admit the final finish looks really nice.
The attached pictures are of the cabinet before staining but after wood conditioner was applied. The top picture is how the subwoofer originally looked.... Note that the indoor shots are inaccurate in color due to the fluorescent lighting. The outdoor shots represent the actual colors...