Sonosub - An Inexpensive Powered Subwoofer (page 4)

Building the Speaker (continued)

• At this point, I evaluated the appearance of the endcaps. The top was furniture grade plywood, but I wanted a cherry look to better contrast with the granite green finish of the tube. The veneer was birch, and when I applied a cherry stain, it looked horrible. So I decided to add another layer of cherry veneer plywood I already had.
  • Glue 3/8 inch thick cherry veneered plywood to the top endcap. Use a router with a pattern following bit to cut the plywood flush with the existing endcap.
  • For the exposed edges, apply cherry edge banding to both endcaps. This is an iron-on strip of real wood veneer with pre-applied, heat activated glue.
  • Applied Watco Danish oil to cherry surfaces. Topcoat with a thin layer of wax.
• Final assembly.
  • Glue the plastic port tube to the bottom endcap/baffle using RooClear plastic glue.
  • Attach two wires from the inside of the terminal cup. Solder connectors to the driver ends of the wires to allow easy connect/disconnect to/from the driver.
  • Mount the rectangular terminal cup to bottom endcap/baffle with four screws. Use a small amount of rope caulk to give it a good seat in the hole.
  • Connect the endcaps to the tube. On the first pass, I used rope caulk to seal the endcap/tube interface. Later I glued the tube to the endcaps making it effectively impossible to disassemble the unit without destroying something. Add a flat washer, lock washer and tighten the nuts on the rods.
• Feet
  • To raise the unit off the ground (after all, this is a downward firing sub) I made feet out of 4 hardwood balls. These have a flat part and are sold as doll heads in craft stores (they may also be sold for other purposes). The ones I used are 3 inches in diameter. [four wooden "doll's heads" craft balls; 5,022 bytes]
  • To mount the flat part of the ball to the baffle, I made a drilling jig. I drilled a 3/8 inch diameter hole through a block of scrap wood to make the jig. The purpose of the jig is to guide a drill bit perpendicularly into the ball.
  • Locate the center of each ball's flat area. Position the hole of the jig over the center of each foot and using the jig as a drilling guide, drill a 3/8 inch diameter hole about 1-1/2 inch deep hole into each ball.
  • Cut four lengths of 3/8 inch threaded rod from stock left-over from the main conneting rods. These should be long enough to go from the balls, through the baffle, and stick out the other side with clearance for a flat washer, lock washer and nut assembly.[hardware for each foot, plus jig block; 6,764 bytes]
  • Epoxy a piece of 3/8 inch threaded rod into each ball.
  • To locate the four feet accurately on the baffle, I made another jig. This was 2 pieces of wood, half-lapped onto one-another at right angle (makes a large '+' sign). I marked off the radius from the center of the jig and moved the jig about the baffle until all four sides were equi-distant from the center based on the markings.
  • Drill 3/8 inch diameter holes through the baffle for the feet
  • Mount and secure the feet with a flat washer, lock washer and nut on the inside of the tube. Reach in through the driver opening to do this.
• Lastly.
  • Vaccum out the interior in case there's debris still hiding in there.
  • Mount driver to the baffle. The supplied gasket is pretty useless so use rope caulk. Connect the wiring, then screw the driver down.
• Retraced steps.
  • As noted in the results section below, there was excessive vibration on the tube and top endcap. I decided to take the tube assembly apart and try and damp out the vibration without having to rebuild the endcaps. I added a layer of 6 lb carpet padding to the inside surface of the tube. This was glued in place.
  • The tube was then glued to the endcaps with Liquid Nails construction adhesive.

Building the Amplifier

There wasn't much to do on the Parapix amplifier. The modules arrived almost ready to use. Here's what I did :

• Modifications to the amplifier.
  • The best place to obtain infomation on the Parapix amp is on the Apex Jr. web page.
  • I used a 48 VCT 3 Amp transformer, which mounted directly to the component side of the Parapix faceplate on two of the existing mouting holes.
  • For the power wiring harness, I connected five of the six wires (two for the primary, three for the seconday of the transformer). I decided not to wire pin 1.
  • The amp jumper is set to bridge mode.
  • Since the amp is external to the subwoofer, I needed two connectors for the amp's output. I mounted a pair of 5-way binding posts from Radio Shack to the faceplate where the Paramount logo sits. I spaced them 3/4 inch apart for use with standard dual plugs.
  • Next to the Peak LED (between the two knobs), I drilled a 1/4 inch hole and mounted a plastic T1-3/4 LED retainer to hold the Power LED.
  • Next to the LEDs, I mounted a SPDT switch. Three wires from the switch were connected to a pair of headers salvaged from the original metal housing (see below). These headers go to the three pins of the boost circuitry. This allows the user to enable or disable the boost rather than hiding this feature on the amplifier PCB. Make sure to mount the boost switch to make sense - up for boost on, down for boost off.[Added boost switch with wiring headers and power LED; 10,296 bytes]
• Housing for the amplifier.
  • The metal casing is from an old Exabyte 8 mm tape drive whose mechanisms have long ago seen the local land fill. The only parts I kept were the 3 sided metal casing and some wiring (see above).[Exabyte case; 4,786 bytes]
  • For the two ends, I milled endcaps much like the subwoofer (of course !). Each endcap has a solid cherry outer piece and a smaller particle board inner piece. The inner piece is screwed to the outer cap and also to the metal casing.
  • The amplifier module is mouted to the inner particle board pieces with screws.
  • Black spray paint and a clearcoat was applied to the metal casing to match the black Parapix faceplate. The cherry endcaps received a coating of Danish oil and wax; same as the subwoofer.[Metal box spray painted black, with cherry endcaps; 6,850 bytes]

Results

To test the subwoofer, I used the line level output of my PC sound card (Turtle Beach Malibu) to drive a Rotel amplifier. Later I repeated this test with the Parapix amp. This particular sound card has decent low end extension (see test results at the PC AV Tech web site) so I wasn't worried that it would limit the low end test signal.

When I cranked the volume up, I noticed that the tube wall and top endcap vibrated. This should not happen with a tubular enclosure. Adding mass to the top decreased the vibration; sitting on it removed all of it. I concluded that I did not do an adequate job of strengthening the top endcap - there was no bracing, the inner circle was low grade plywood, and as mentioned previously, I had neglected to fill the void where the nuts are hidden.

I tried to do what I could short of ripping things apart. I glued the tube to the endcaps, and added carpet padding to the tube walls. This seemed to help a wee bit, but the walls still vibrated. The real solution was to rebuild the endcap. Since I didn't have the time to do this, I decided to place the subwoofer into service and worry about it some other time - hey, this is DIY, which means things are never really done !

No measurements were ever made. Instead I simply listened, looked,and felt the results. It was pretty obvious that the frequency response cutoff was in the mid to upper 20's as predicted. As for SPL, it can definitely crank but it didn't take much to reach the driver's excursion limit. Nonetheless, the SPL at that point is more than enough for HT use in a "typical" room.

29-September-2000

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