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Sealed Vifa Bookshelf Speakers: Part 2

An easy to build, flat frequency response 2-way bookshelf speaker.

The Crossover

The crossover schematic and parts list.

I got the parts for the crossover, along with the drivers, from Madisound. The 1.8uH inductor isa Sidewinder, the smaller two inductors are standard air-core. The Caps are mostly Carli, and the resistors are all sand-cast types.

Cut a 5" x 8" piece of peg board for the base. Use tye-wraps to hold the coils and resistors down, and to hold capacitors built up from multiple values together. You might also want to use hot-melt glue when you are certain that the crossover won't be changing any more. Use a 5 position terminal strip for input and output connections.

Figure 16. The crossover component mounted on a 5" x 8" piece of peg board.

Mounting the Drivers and Terminal Cups

First, set the drivers and cup in the cabinets and mark the locations of the mounting holes. Use a square to get the driver holes straight. Remove the drivers and center punch the markings with a nail, drywall screw, or hand awl. Do this with hand pressure only, a hammer is not necessary in such soft wood. Drill through using a 5/64" drill bit. We will be using stainless #6 x 3/4" pan head screws to mount everything. Get one screw of this type and manually screw it into all holes before mounting the drivers. This prethreads the holes and makes costly slips later less likely. Magnets like to pull screwdrivers into expensive drivers!

Cut six pieces 16" (400mm) or so long of 18AWG zip cord (RS#278-567 - I like the red and black obvious polarity stuff), then strip and tin both ends. Tin the cup and driver terminals, then solder one of these wires to each cup and driver.

Mount the terminal cups first. I had to remove the sealing foam from them with a screwdriver, wood chisel, and finally scraped with a razor blade. I prefer to use rope caulk (Frost King "Fingertip Caulk") from Home Depot for sealing purposes - NEVER USE CONVENTIONAL CAULK TO MOUNT DRIVERS OR TERMINAL CUPS! Doing so will prevent you from being able to easily disassemble things later.

Remove a single rope strand from the roll and then work it with your fingers to make it longer and skinnier; you don't need much for these small cups. Then apply the rope to the countersink area and flatten it with your fingers. Finally , install the terminal cup using the pan head screws mentioned earlier. Tighten till snug, then back off a quarter turn or so to keep from deforming the plastic.

Next install the tweeter using the same procedure as the connector cup. Keep a firm grip on the screwdriver, and use your other had to keep the tip on the screw. Don't slip!

Solder the driver and cup wires to the crossover terminal strip paying close attention to which wire goes where and to polarity.

Now cut some carpet felt and line the cabinet walls with it, particularly in the woofer section, the back and bottom of the box where the crossover sits. I got a 6' x 9' section of it from Home Depot. Kind of linty, but it seems OK. No need to affix it to the walls, since the poly fluff will hold it in place.

Set the crossover on the bottom inside of the cabinet on the felt, and then use another piece of felt to cover it up (I had some scrap I used here). The poly fluff will hold the felt and crossover in place. Moderately stuff the cabintet with polyfluff (the kind used in pillows).

Finally, place rope caulk in the woofer countersink and install the woofer.

Figure 17. Crossover position within the box. This view is minus felt wall lining and poly fluff stuffing.

Figure 18. Interior walls covered with carpet felt.

Figure 19. Poly fluff added to upper tweeter section.

Figure 20. Remainder of poly fluff added to lower woofer section.

Figure 21. Rubber feet added to the bottom, spaced 1" in from edges.

Testing

I used Speaker Workshop to do verify that the crossover and drivers were working well. I also employed a microphone, preamplifier, and testing jig of my own design. See my audio pages for details.

Figure 22. Testing setup. Speaker is supported on its back above the floor, and microphone hangs from a light fixture 0.5 meters above center line of speaker.

Figure 23. Time domain response, note reflection on right lies outside of the markers.

Figure 24. Gated frequency response, unsmoothed. Good within 3dB or so!

Figure 25. Close-miked frequency about 1 cm from the woofer dust cap, 1/3 octave smoothed. Response is down 3dB at 85 Hz or so.

How do they sound?

These speakers sound great! Very smooth, no resonances or harshness that I could tell. I ran a sine sweep through them, and several very clean-sounding CDs such as Counting Crows and Crowded House. Excellent response! The bass? Not earth shattering, but with some moderate EQ they sound fine. Even though the bottom octave is for all practical purposes MIA, the bass response was actually better than I expected. Some would want a sub, though.

Things I learned building these speakers

The sides of the speaker cabinets are a little rough since the varnish raised the grain. I wish I had sanded after the first several coats, or applied a sanding sealer. It isn't horrible, but it could be better. Then again, finishes could always be better.
I could have made the countersinks for the drivers and the terminal cups a couple of mm deeper. Sanding the cabinets removed some of this depth, and the rope caulk removed more. Again, not catastrophic, but I'll know better next time.

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