Linkwitz Lab LX521 Dipole Loudspeaker Project - Page 2


Construction log - continued from page one

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Click to enlarge thumbnails below...

 

Drivers Arrive [January 7, 2013]

UPS delivered the package from Madisound this morning. Because I already owned the woofers, they were not part of the shipment. I could actually lift this modest sized box!

The tweeter does have a steel back. I had assumed that it would be aluminum, but there's no doubt that it's a turned steel piece. It will take a little more determination to shorten the stub on the back to fit the baffle if I need to do that.

I was also curious about how steel screws might affect inductance of the coil. I had pre-drilled holes for screws to clamp the vertical sides of the bracket to the foot while glue dried (or maybe not even use glue). The screws would be positioned into the wood near the coil, which can be problematic.

I performed an experiment. With no screw near the coil, my meter read 4.17mH. With a steel screw placed over the opening of the coil, the inductance rose to 4.22mH. Brass screws caused no change (4.17 mH), so the solution is easy. Use brass screws near the inductor.

<--click any picture to enlarge

 

Test Routing Baffles [January 11, 2013]

I'm waiting for my cherry panel blanks to arrive for my "official" baffles. For practice (or to make idle hands busy) I used some left-over MDF in the shop to test the template/routing steps to find a good workflow.

I created 3 MDF clones of the template I that made earlier. I may use 2 of these to use temporarily to listen to the speaker before finishing it. Who knows?

I did discover a sequence of steps that made work a little easier - rather I learned what NOT to do. It didn't affect the outcome, just production speed. What I learned will be useful when the cherry panels arrive.

While I used 1/8" metal pins to attach the template to the stock for routing, one could just as easily use double-stick tape.

If I were using tape to hold the parts together for routing, here's a step sequence that makes sense:

  • Cut the stock to be a little larger than the baffle.
  • Draw a vertical centerline on it. Position the template to align with it, and trace around the template.
  • Using a jig-saw or a band saw, cut to within 1/16" (or tighter if you're good!) of the traced line. Cut rough holes for the driver openings (which can't be done on a band saw).
  • Attach the template with double-stick tape to the stock using the center lines and traced outline for positioning.
  • If you have a router table, you can probably figure the rest out from the third picture in this group. Use a flush trim bit. If you don't have a router table, you will route from the top with the work positioned above the template using a flush trim bit. Support the work well, and pay attention to the proper routing direction with either case.

There are other possible scenarios, but these two approaches will get the job done.

I want more time to continue work on these, but that doesn't pay the mortgage. My day-job beckons.

Look for only sporadic updates until I manage to get more free time.

 

Fabricating the Passive Crossover Board [January 21, 2013]

Martin Luther King day was a holiday from my job, so I went into the [cold!] shop to do a little work. I tackled how I was going to make a "cradle" for the passive crossover components and their connections.

I arrived at a sub-assembly that would nest inside the bracket, held at the bottom by Velcro. The 4-connector lead from the Speakon would be inserted into the cable clamp, and the leads pass through a false-bottom into a sub-chamber where all the messy soldering would be. That way it's hidden from view.

It's a little hard to explain, so I'll let these three pictures hint at how it will be configured eventually. There's still a lot of work to be done.

I've been seeing some nicely executed LX521 baffles where the wiring is almost entirely hidden. I hadn't planned to go to that extent - especially if I use a hardwood baffle - but it's becoming a temptation. I don't NEED to have the wires hidden, but a challenge like that is sometimes fun.

 

Minor Update [February 19, 2013]

Progress is slow because it's been a very busy time in my life. When I had a free hour or two in the past two weeks, I worked on stuffing the ASP circuit boards. Another delay occurred because I ordered some additional capacitors for stuffing the boards. I measure everything before soldering, and I felt that some of the tolerances were a bit sloppy. With extra capacitors available, I'm able to sort them and pick those closest to the design values. Unfortunately, the extra Mouser order took about a week to arrive using their economy shipping choice.

With the new order, I was also able to incorporate some changes suggested on the Orion/Pluto/LX521 owners web site. For example, the circuit board has 15mm pin spacing for the C7 & C100 caps, but the 10/15/2012 Bill of Materials specifies a smaller 5mm capacitor. I didn't want to bother with soldering extension legs onto any capacitors to fit the boards, so the re-order gave me an opportunity to choose alternative parts that fit better. The published tolerance range of the new caps is better too.

The capacitors that are attached to a paper strip allow easy labeling once measured. I find that useful when trying to match capacitor values between left and right channel circuit boards.

 

Bridge Musings [February 22, 2013]

While unable to get into the shop to do some real work, I've been pondering what I can do to make the bridge stronger side-to-side and perhaps to make it more attractive too.

In early ideas, I entertained the idea of using 2x2 lumber legs with 2x2 cross bracing to support the upper parts. Because the 2x2 is thicker than the 3/4" plywood specified for the sides, it would require a bit wider upper platform to mount to. The increase in side-to-side strength would be sizable. I'm not convinced that this can be made to look attractive, although visually the 45-degree braces would echo the 45 degree woofer baffles inside the box. I've "sketched" it in several different color combinations, some with very thin panels located behind the braces to hide the box somewhat.

Another idea was to use PVC pipes with a finish similar to my Pluto tweeter tubes. I was able to make a very attractive audio rack/table using these tubes finished in the way that I used for my Plutos. The nicely textured lime-wash surface, applied in a spiral fashion, completely hides humble PVC water pipe origins. It looks mysteriously high-tech in its own way.

The audio table uses 1/4" x 20 all-thread rod through the center of the PVC tubes to pull things tightly together. It is stronger that I'd expect. I'd do the same thing if I use this approach on the LX521 bridge. This approach too would require a slightly larger upper platform on the bridge, but I might be able to incorporate gentle arcs in the platform between pipe attachment points.

I attached a picture of my audio table so that the end result could be more easily visualized.

On another front, the woodworker who is gluing the cherry-wood baffle blanks said that he'd deliver them by the weekend. He says that it took three attempts to get a warp-free glue-up for the baffles. Perhaps the weather swings we've had affected the process. In any case, it will be good to get back to some woodworking, and to finish the front baffles.

It's still way too cold to do any spray-painting though!

 

Baffle Stock Arrives! [February 23, 2013]

The woodworker delivered the glued-up baffle panels today. He had trouble with flatness in two previous attempts. It appears that the problem was one of heat. Because he was drum sanding them to thickness without letting them cool, the heat likely changed the wood's moisture content and they warped. Usually, he is in a production mode making a number of cutting boards, and does a bunch at a time. The cutting boards can cool as the next one is drum sanded, then they are reversed for sanding the other side. Cooling between sanding the front and the back appears to be key.

I was happy to get the cherry panels, and spent about an hour on the project this afternoon. I began by marking a centerline on the panels, positioning the template to align with the centerline, and marking the top registration hole with a transfer punch. After the 1/8" hole was drilled through, I inserted a 1/8" steel dowel pin through both pieces to register the top half. Then I carefully aligned the bottom of the template with the centerline. Once aligned, I drilled through an existing hole in the template into the cherry panel so that I know that registration will be perfect. I'll put a steel dowel pin in the lower hole to hold the pieces together for the routing steps instead of the double-stick tape. It will keep the surface of the cherry neater - no tape residue.

More work tomorrow, I'm sure!

 

More Baffle Work [February 24, 2013]

I began the morning by accomplishing a lot in two hours. I was convinced that I could get these finished today. But then I was called upstairs because our 30-year old clothes washing machine leaked sudsy water onto the laundry room floor. Why can't I get a whole day to work on anything?! Just ONE whole day? Please!

Ranting aside, I did put larger counter bores on the rear side of the two tweeter holes per a suggestion on the Orion/Pluto board. Because the two tweeters face 180 degrees from each other, the upper counter bore, about 1/8" deep, is on the front of the baffle, and the lower tweeter's counter bore is on the rear of the baffle. I am hoping that these little extra spaces, made invisible by the tweeter plates, will provide a generous space for tweeter wires. I marked the center of the counter bore by placing the template over the work and using the smaller Forstner bit to mark the spot. Then I removed the template, and drilled the shallow counter bore in the appropriate positions.

Once that was done, I pinned the template to the work once again, and used it to guide the Forstner bits accurately. At this time, both baffles have three holes in them, and what remains is to rough-cut the large midrange hole, rough-cut the outline of the baffle, then route everything flush with the template.

Lots of sawdust to come.

Wish me luck with the clothes washer in the meantime.

 

Boards and Baffles [March 3, 2013]

The LX521 ASP circuit boards are stuffed. That work is easy to do in small chunks of time. It takes the soldering iron about 5 minutes to warm up, and I have a card table set up next to my computer desk to hold the work. I play some music through my Plutos, and work until I have to stop. I need to insert the opamps and do some spot electrical checks. I feel confident that I won't have much trouble. Both the Pluto boards and the wASP subwoofer boards went together without a hitch. There's no reason to believe that my luck will change this time. Famous last words, eh?

I had most of today to do some more work on the cherry baffles. I rough-cut the mid-woofer hole with a jig saw, then used the band saw to rough-cut the outside shape. Deja vu! I still have the MDF baffles done earlier if I completely change course and decide on paint instead of natural wood.

After rough-cutting, I pinned the template to the board, and took the "sandwich" to the router table to trim. I used a Katana 1/2" dia. [#17805]  flush trim bit from MLCS that I had purchased in the distant past. I wanted to get a top quality Whiteside flush trim bit to ensure good results, but couldn't find the time to drive to Woodcraft. However the Katana bit cut very well, even across the grain. I was left with a very smooth cut when done, without much sanding needed. I left a light burn mark on only one spot - completely my fault. It sanded out easily though.

After cutting the cherry boards to shape, I drilled the bracket mounting holes and test-fit the baffles to the brackets. I used 1/4" x 1-1/2" flathead machine screws, and things lined up very well. There was no binding when the screws were inserted.

I left the shop satisfied today.

There's still work to do on these. First, I need to mark and drill pilot holes for the wood screws to mount the drivers and tweeter sub-baffles. I also need to drill, saw, or mill out a slot or hole for the internal wires between the two tweeters. This doesn't have to be very neat because it will be hidden by the tweeter sub-baffle.

I will probably use either Danish Oil or General Finishes Arm-R-Seal Urethane Topcoat on these. The GF finish would go much faster, but builds on the surface more than the Danish Oil. It's also more prone to brush marks and embedded dust, unlike the Danish Oil.

 
More Mockups [March 7, 2013]

It's too cold in the shop. I'm a wimp. Instead of cutting wood, I played at the computer.

As you can see from the picture, I'm narrowing down the choices for the bridge. I mocked up a version in my drawing software (CorelDRAW X6) showing the Pluto-type posts to support the upper assembly, but the front view looked too "stocky". The extra width of the columns added girth to the front view, and proportions looked wrong. I'm glad I took the time to model this before I committed to cutting material.

I settled on cherry sides with extra edge trim for appearance. It will add a little additional side-to-side strength too. I'll might use some 5/4 cherry to add even a bit more width to the side trim than I have shown.

I'm wondering about the top panel of the bridge, and how just a slab of cherry will look. End grain appearance can be a problem*. If I add four mitered trim strips around the periphery of the top panel, it will take a lot of time. The extra trim around the panel won't extend very far out, so the vertical uprights will have to attach to the trim itself, or even span the joint - neither of which is best practice.

Maybe I just think too much and do too little. These choices will have no effect on the sound.

*Edit: In later experiments, this was found to be NO problem. In fact, the finished cherry end-grain is beautiful!

 
Test Finish [March 9, 2013]

I wanted to see how the cherry baffles responded to Danish Oil finish. I've had good luck with it before, and it's easy to apply.

I was very, very pleased with the color of the cherry with the natural, uncolored oil. Also, I was astonished at how good the end grain looked with the oil on it. I will not try to hide end grain in other parts of the construction as mentioned above. I'm embracing it!

 

Preparing Cherry Veneer [March 13, 2013]

Because of it's superior engineering properties, I chose to use Baltic birch plywood instead of solid cherry boards for the side panels. Baltic birch is strong, and straight, and doesn't move much with changing humidity. However, it's rather plain looking.

I am going to attach a thin veneer of cherry over the Baltic birch plywood to match the solid cherry used elsewhere in the bridge. Paperbacked veneer is easy to cut with scissors, and attaches using ordinary wood glue. I've used the iron-on method in the past, although sometimes I wish that I had a vacuum bag to do it without heat.

I used one of the Baltic birch panels as a template to determine the size of each piece that I'd need. I merely stepped it along the veneer, and loosely traced lines around the panel with a pencil. Ordinary scissors cut the veneer easily. The veneer sheet is only 0.023" thick, with 0.010" being the paper backing.

Cherry darkens with exposure to light, unfortunately. In the top veneer photo, the far end of the sheet was positioned on the outside of the veneer roll for display in the store, and has darkened. I avoided having a visible demarcation line halfway through a piece of veneer. The cuts I made produced one panel comprised entirely of dark veneer, and three panels of the lighter veneer. The dark veneer piece will be placed on one side of one speaker, and over time, the tones of the other three sides should approach that of the already-darkened piece. At least there won't be a distinct line bisecting one panel into darkened and almost-darkened wood to worry about.

I chose an interesting pattern in the veneer sheet to use. It has a light herringbone appearance to it. The grain and pattern will run vertically.

That's about all that I had time for this evening. There are other chores that I've been asked to do. I have to maintain domestic harmony!

 

Veneering the Bridge Side Panels [March 16, 2013]

I had a few hours after my Saturday morning shopping chores to get some work done. I completed the veneering of the bridge's side panels. They need trimmed, but I will do that tomorrow.

Using a sponge paint roller, Titebond wood glue is rolled onto both surfaces to be mated. The plywood received two coats, and the veneer got just one coat. I use fairly heavy applications of the glue.

The plywood parts are coated easily - just lay them side by side, and roll on the glue.

The veneer requires taping the edges to prevent curling and to prevent glue from reaching the face of the veneer. That preparation takes more time than actually applying the glue.

Once the glue dries, I align the panels and veneer using pencil marks I placed earlier. I use an ordinary clothes iron to tack it all together. The iron moves slowly, and I press down with as much weight as I can muster. I've successfully used this method of veneering for a number of loudspeaker projects, and it works very well for me. Best of all, the required equipment is simple and inexpensive.

Tomorrow I expect that I'll fire up the router table (shown in the last picture here) and trim the excess veneer from the edges of the plywood.

 

Trimming Veneer and Adding Finish [March 17, 2013]

I decided not to use the router table, but instead chose to limit myself to hand tools for the veneer trimming. As they say, I went Neanderthal.

I had purchased a veneer saw years ago, sharpened it to a razor's edge using waterstones (veneer saws ship with fairly dull edges regardless of manufacturer's claims), and hadn't used it much. This was the perfect opportunity to try it again, and it worked very well. A veneer saw isn't really a saw, but a serrated knife that has a flat bottom for cutting flush with an edge. I just score the material with long strokes until it cuts through. I don't use a back-and-forth motion like sawing.

Cutting with the grain took about 5 passes to cut off the overhanging veneer, and across the grain took more - maybe a dozen passes. I used medium-light pressure to avoid tearing out the thin veneer, especially when cutting across the grain.

Once cut, there were a few thousandths of an inch veneer overhang remaining, and a small block plane quickly removed the excess. Very easy, no noise, no sawdust flying around! It was satisfying. The hand plane is a Lie Nielsen low-angle block plane. The low-angle blade is ideal for trimming end grain. I love that little plane!

After some light sanding of the veneer surface with 220 grit sandpaper, I arranged the panels side by side and added a coat of Danish Oil. I am trying for a semi-gloss surface, so it will take many applications of finish to get them looking the way I want. It might even require some wet-sanding with the oil.

I also did a little work on the vertical "stiles" for these panels. They needed sanding, and a rabbet cut for receiving these panels. The sanding is done, but the rabbet is only 10% of the depth needed for the panels. I'm setting the router table to take shallow passes to avoid problems, but that means slow work. There are 8 stiles to do, and each one is about 2 feet long.

The stiles will be attached using flathead screws through the panels. The panels will be attached to the top of the bridge and to the feet using pocket screws. All fasteners will be placed out of sight on the inside of the bridge assembly. The stiles will hide the exposed plywood edges.

I'm not gluing parts together because, ever fickle, I might change my mind about the side panels. I keep thinking about how nice it might look with some Ambrosia maple panels contrasting with the cherry found in the rest of the build.

 

Bridge Uprights [March 23, 2013]

The upright panels for the sides of the bridge were given pocket screw holes on the top and bottom ends. Screws will attach the panels to the underside of the bridge top panel, and to the feet. With the Kreg jig, drilling them is straightforward.

Next I needed to shape the stiles into an "L". This became a little tricky on the router table. As the cut progressed across the width of the stile, I was cutting away support. When the piece became strongly "L" shaped, there was little material left on the bottom to contact the table's surface for support. I was concerned that the piece would slip during a cut with damaging results. At some point, I ceased cutting on the router table to avoid such a mistake. However I still had about 0.10" wood remaining to cut away.

(Hindsight alert: Next time I need to make narrow "L" shaped stiles, I will explore the possibility of cutting a wide channel down the center of a wider board, and then rip it in half to get two stiles. It would require less time, and would be safer than the method I used on the router table. I suspect that the wide channel could be cut with either the router table or with a dado blade on the table saw . The dado method would be the fastest. I'd need to buy a dado blade set for my table saw, but I'm overdue for that anyway.)

Another issue was that the 1/2" straight-sided router bit didn't produce a clean cut on two of the boards. I'm sure that grain direction was the culprit. Even with light passes, I had some tearout on the problem boards where they contacted the straight sides of the router bit. There was no easy solution to the grain direction issue on the router table. I desired a 1/2" spiral downcut bit for the possibility of a cleaner cut, but I didn't own one. I ordered a 1/2" spiral downcut bit later in the week, but not until the router table work was done for the stiles. I'll have that router bit on hand for the next time.

To cut the remaining 0.10" material away without risking damage on the router table, I used a medium shoulder plane. Set up to take a medium cut, it still took about 2 hours of work to cut the rabbets for all 8 stiles to final dimensions. Cutting away the remaining material with the plane removed all traces of the router table tearout, as I knew it would. When I encountered minor tearout on the wood surface using the plane, I could flip the board end-for-end and reverse the cutting direction. As a plus, I got quite a workout.

I was pleased with the fit of the stiles. They look as I had envisioned, and will provide a nicely finished appearance to the bridge uprights.

They are not sanded nor finished yet so the color is currently much lighter than the veneered panel itself. The next step will be to drill and countersink screw holes near the edge of the panels for attaching the stiles. I have the hole locations marked, but not drilled. It was time to quit.

 

Final Fabrication and More Finishing [March 24, 2013]

By the conclusion of the shop work today, all the major parts fabrication needed for this project is complete.

The first task today was to drill and countersink holes in the back side of the bridge uprights. The holes are for the screws that fasten the stiles into place. I realized that the engagement of screw threads was a little less than what I'd prefer, so I'm considering a bead of glue on the side of the upright to secure the stiles in place to augment the screws. The glue can be applied to hidden, non-finished edges of both parts to increase strength. I can't realistically apply wood glue to the already-finished surfaces. The finish would greatly reduce the holding power of wood glue.

For the edges of the feet and the bridge top, I milled a combination of curves and chamfers. The edge corners were rounded on a disk sander before routing chamfers. It's fascinating to watch the wood disappear from view when sanding to a line on the disk sander. It's easy and very fast to sand to a pencil line. The corners have a 1/2" radius (1" diameter) arc.

After sanding the corners into arcs, I took the parts to the router table to add the 45-degree chamfer to the top sides. I didn't have much trouble with the edge routing, with only a couple minor spots where light burn marks showed up. These happen more often when going across end-grain, and when I pause slightly in a corner. A few swipes with 180 grit sandpaper removed the blemishes.

It was time to apply finish to the completed parts. I wiped on Danish Oil, as I did for the upright panels. For the the uprights, it was a third coat.

With so many parts and with the complicated geometry, I'm tempted to switch to General Finishes "Arm-R-Seal" Oil and Urethane for the remainder of the finishing. It will build more quickly than the Danish Oil. Properly using the Danish Oil for a very smooth finish requires sanding between each coat, possibly oil-sanding during applications, and substantial drying time between coats. The Danish Oil finished speakers I've done in the past were simple boxes, and oil-sanding didn't consume as much time compared to the multitude of little pieces that I'm working with here.

I remain impressed with the grain and color of the bridge top that has a knot hole. The hole will be covered by the bottom of the baffle bracket, so it won't show or matter. The color and grain that extends beyond the bracket will be a display of wooden beauty though.

I also have one bridge foot that has some interesting spalting in the grain. It's an eye-catching piece of wood. I wish all four feet looked like this one piece.

By the way, the feet and the bridge tops were made from thicker 5/4 cherry. It's about 1.1" thick. The thicker tops should add some mass to the upper assemblies. I have some other ideas to further damp high-frequency propagation into the side panels. I plan to add some felt to the bottom of the bracket assembly before attaching it to the bridge. Maybe. We'll see when the time comes.

The season is nearing to be able to paint the woofer box, and some other assorted parts. I need good weather and big spans of time to spray paint in the garage. While it snowed again today, it won't be much longer before favorable weather returns. For now, I can continue working indoors by applying additional coats of wood finish.

 
Finishing, Finishing, Finishing [March 29, 2013]

Finishing continues. There's nothing interesting to show picture-wise. It's slow work, especially with the penetrating, slow-drying Danish Oil. I haven't yet switched to the General Finishes Arm-R-Seal which dries much faster. Unlike the GF product, the Danish Oil finish won't build to a semi-gloss without a lot of sanding work.

This is where I think the Arm-R-Seal finish will have the edge and I'll switch, but instructions indicate that one should wait at least 72 hours before applying  the Arm-R-Seal product. I will probably wait even longer than that between the two finishes to make sure that they work together. I'm in no hurry.

I did get curious about how a bridge upright would appear, so I held the loose parts together for this snapshot. I like it!

 
Finishing Continues [April 8, 2013]

This is slow work. I did switch to Arm-R-Seal to complete the finishing. While the parts aren't to an acceptable level yet, I thought that I'd post this picture of how the General Finishes semi-gloss Arm-R-Seal worked with the baffles - especially the end grain parts. Semi-gloss Arm-R-Seal has the ideal sheen for my skill and patience.

I do wish for some absolutely lint-less cloth to use for wiping finish. I've opened a roll of Webril Wipes, said to be lint-free, but they do shed cotton fibers in use. When that happens, all you can do is to wait for the finish to dry, then lightly sand the area before recoating.

The trouble is when you add new finish to go over the sanded area, more lint happens. Still, I believe that I'm making headway. The low sheen of semi-gloss looks upscale to me.

In hindsight, I probably shouldn't have used the Danish Oil initially because of how slowly it builds. It took forever to dry too. It did pop the woodgrain appearance though. However, it probably cost a week of finishing time compared to a simple washcoat of 1# shellac, then to build sheen with the Arm-R-Seal.

One advantage of Danish Oil is that any damage can be repaired with just a little sanding and reapplication, but my overcoat of GF Arm-R-Seal negates that advantage. Danish oil is fine on large surfaces where surface quality can be obtained by wet oil-sanding. I've done it before on box speaker cabinets with excellent results. And you don't get runs and sags. However, it's just not the ideal finish for this project with its intricate parts if a semi-gloss is desired.

Lesson learned.

 

Painting Driver Backs, and I'm Still Finishing Wood [April 14, 2013]

The drudge work continues, and some aspects of finishing have become fairly difficult as the finish builds. With a near-glossy surface, the slightest imperfection shows.

I have some slight surface undulations on the veneered panels that resist work. I used the random orbital sander to sand the surface as flat as possible, but managed to go through the finish into bare veneer in a couple of spots. The newly uncovered areas take finish differently, and it will require building back up to an even sheen over the next few finishing passes. I'm halfway tempted to go to a satin finish instead of semi-gloss. The semi-gloss builds to almost a full gloss and requires extreme care when applying.

Even after all that work, I STILL have some slight undulations on the surface. It looked and felt dead-smooth after the sanding session, but still shows a slight waviness after finish is applied. I'm about to throw in the towel, and just be satisfied that it's as good as I can get it. The first picture of the side panels shows where I am now.

The midrange drivers have silver plates on the driver magnet, and that color was not going to integrate well. I decided to paint them. While waiting for the wood parts to dry, I masked off the back of the mid-range drivers. I loosely inserted some flat-head screws into the vent on the back to avoid getting paint inside. After masking, I used some now-discontinued Krylon semi-flat black in a spray can to paint them. The back of the drivers look much nicer now.

 

Not Much to Report [April 21, 2013]

If you guessed that I'm still finishing parts, you'd be correct. It's a good thing I don't do this for a living or I'd starve.

I have a very smooth coating of semi-gloss on the side panels, and I have a nice shine on the baffles. Even so, there are minor things that require a little more rework - unless I just give up.

After painting the plated-steel end cap of the midrange drivers, I found a little paint leakage under the masking tape onto the annular ring of exposed magnet. I scraped the unwanted paint blobs smooth with the end of a stainless ruler, and then painted the exposed magnet with some General Finishes Lamp Black water-based paint. It's a flat-finish paint, so it will contrast against the satin sprayed paint. Because I used a small artist's brush, I was also able to blacken the ugly white cardboard of the terminal pad. I probably should have painted the red half too, but didn't. If the drivers need a second coat, I'll take care of it. I have this photograph to determine which terminal is the "+" one, so I don't need the red color anymore.

[Edit: I did apply a second coat today, and the red is now covered in black too. It looks better.]

 

Assembly of Bridge Components Begins [April 28, 2013]

Enough wood finishing! With that behind me, I began to assemble the parts that comprise the bridge. First, I placed the 1/4-20 threaded brass inserts in the underside of the bridge top. I had a little trouble with my older T-wrench. It expanded the slotted end of the brass insert because of a chamfer where the shaft meets the threads. It wedges into the opening of the insert, and spreads apart the screwdriver-slot part of the soft brass under load. I bought a new T-wrench from Woodcraft to use, but it came with a chamfer too. It also spread the brass insert. The T-wrench should not have had a chamfer at all. It's a flaw for its intended use.

I worked around the problem by turning the brass insert upside-down when I threaded it into the wood. The bottom of the insert is a solid ring, uninterrupted by any screwdriver slot, and provides more strength.

After working through the insertion issue, I tested the fit using connector bolts that I had purchased from Woodcraft. Inserted in the baffle bracket bottom piece and threaded into the inserts in the bridge top, they look nice and "high-tech". The bracket bottom piece, now bare MDF, will be painted a flat or satin black when I reach the spray painting part of this project. I might strip off the flat-black oxide finish from the connector bolts, polish the heads, and use some gun bluing on them to reach a glossy black finish. Maybe.

Thinking about potential problems ahead, I drilled 3/8" diameter blind holes into the bottom of the bridge feet. If I ever need to use conical rubber pads or spikes for use on carpet, I can just install the 1/4-20 threaded brass inserts into them, and screw in the pads. It's far easier to drill the holes now than after the speaker is assembled. If I never need the pads, there's no harm done.

One issue I keep thinking about is the number of mating parts on my bridge. The uprights are each made of three parts - the panel, and two side pieces. They are screwed together. I had intended to insert some very thin polyethylene foam at the junction of the parts to suppress any buzzes or rattles. However it became obvious that it wasn't meant to be. I couldn't juggle all the parts and foam strips while trying to get a clamp into position for marking pilot holes, then repeat again for driving screws. I abandoned the idea. If I encounter buzzes of rattles, I'll run a bead of silicone sealer down the length of the joint. I hope it isn't an issue.

Even though I used some small #6x1 self-threading screws to fasten the sides to the panel, I still drilled very small, shallow pilot holes in the cherry strips to prevent splitting. It took a while to assemble each panel - clamp the pieces together to mark the hole locations with a transfer punch, drill the 1/8" deep pilots, and re-clamp while driving the screws. It took a couple of hours to get them all completed.

I left the vertical side pieces ("stiles") a little long when I fabricated them, planning to trim to fit after assembly. Now was the time. I had registered each panel and its stiles at the bottom end against a flat surface when inserting screws, and that left the stiles a little proud by ~1/16" at the top. It took about 30 seconds per panel on the disk sander to remove the excess for a flush fit. Easy.

They look good overall. I temporarily placed one of the assembled panels onto a bridge foot to preview how they look together. The wide panels have almost a mirror shine to their surface even though a "semi-gloss" finish was used. The stiles remain more satin for contrast.

There is still the occasional piece of dust embedded in the finish, but the flaws are very small and I was anxious to get moving along. I doubt very much if I could have done better with my current capabilities. It still amazes me that a surface this smooth comes from a wiped-on finish. Spraying might be even better, but I refuse to spray solvent-based finishes in my garage. If it's not water-based, I don't spray it for a number of reasons.

One thing concerns me - the panels transmit sound readily. A rap with knuckles delivers a resounding THONK!. I may investigate some sound damping material for the back side of the panels.

 

Bridge Dry Fit [May 4, 2013]

I fastened the feet to the vertical panels, and then dry fitted (stacked) things together to get an idea of how it was looking. I still need to attach the side panels to the top panel, but I will wait until I decide what to do about panel damping, if anything.

Finish on the wooden parts looks good. Painting will begin soon for the woofer box, bracket parts, and other sundry items. I'm at the mercy of the weather from here on out. It's been too windy to paint today and rain is forecast for much of next week. I'll see what I can squeeze in.

 
A Damping Consideration [May 6, 2013]

I have prepared the surfaces of the woofer box and other assorted parts for paint, but it's raining today with 4 more days of rain forecast. I paint in the garage (and still might attempt it), but for now I've been doing "busy work". I have been thinking about how the side panels might "ring" if excited by vibrations, and one obvious approach is to minimize any vibrations that may reach them. I had some 1/16" thick F1 white felt from McMaster-Carr, and decided to cut a pad to fit between the bracket bottom and the top panel of the bridge. It should absorb some of the acoustic energy that may be transmitted from the baffle.

Thicker felt may do an even better job, but at some point, mounting rigidity will be compromised. However, no pad is specified in the original plans. It's really just busy work for an idle mind while waiting for better weather.

 

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