Zaph Audio ZDT3.5 Loudspeaker project

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Build Log

The following steps outline the design, fabrication, and assembly of this set of Zaph ZDT3.5 speakers. I began in the summer of 2008, and finished in May, 2011. I obviously took my time with this project, working on and off.

Click any picture below to enlarge...

Cutting Large Panels

I had Lowe's cut down 4x8 sheets of MDF so they fit into my Toyota Matrix. At my store, they will make two cuts on their panel saw at no charge to the customer.

Back home, these panels were further cut down outdoors using a circular saw and straightedge. I cut across sawhorses, but don't worry about the saw cuts in the top of them. The tops of the sawhorses are common 2x4s, and easy to replace when they get too cut up.

After that, the pieces were brought indoors for trimming on the table saw. One challenge was to cut each side panel to the exact same length, so I fabricated an outboard stop on my table saw for the long pieces being cut. The last picture in this section shows how the workpiece butts against an adjustable stop to cut all the side panels to exactly the same length.  To provide fine adjustment, a 3/8-16 carriage bolt threads into a brass insert mounted in a sliding wood sled. The head of the carriage bolt serves as the stop.

Aligning Biscuit Cuts
I used biscuits to align the glued-up edges of the cabinet and some of the bracing. The mating edges of the cabinet are easily cut with the help of a jig that aligns them. For the braces, a straightedge clamped across opposite sides of the vertical panels allowed me to cut slots for biscuits in proper alignment. These slots, cut into the face of the cabinet and the ends of the braces, properly aligned the cabinet bracing and shelving. They are "keyed" into place when gluing up, and don't slip out of position. This little extra work up-front made later work much easier and more accurate.
This photo shows how the braces and shelves are keyed into the biscuit slots. I can snap the pieces into position for a dry fit without glue. Further, I can glue the cabinet together in sections. The unattached side panel is used as a positioning template during glue-up to support the braces and shelves in alignment. When the glue was dry on the bottom part, the unglued panel was removed to provide access to the inside.
Baffle Fabrication

After drawing the cuts for the baffle, I used a Jasper circle routing jig on my Bosch plunge router to cut the recesses for the drivers and the through-holes on the front surface. The baffle material was 1-1/8" thick MDF stair tread from Lowe's (Ipdate 2014: sadly, it's no longer carried by Lowe's).

I used a 3/4" flat-bottom router bit to cut the recesses for the speaker mounting flanges, then cut the center away with a 1/4" spiral upcut bit. Note the use of a backer board behind the baffle. It serves two purposes - it prevents the bit from cutting into the work table surface, and it also holds the centering pin securely for when the final cut is made through the baffle. For this build, I used double-stick tape to hold the baffle to the backing board. In the past, I've used countersunk deck screws in each circular cutout to secure them. Obviously if you use screws, they must be positioned outside of any cutting area. Having a carefully drawn cut plan on each baffle helps prevent mistakes - including the most common mistake, forgetting to mirror-image baffles that have offset drivers.

A drill press and a Forstner bit cut the recesses for mounting the threaded Hurricane nuts used to fasten drivers.

Later on (but not pictured here), I roughed-up the seating surfaces of the steel Hurricane nuts with a triangular file for better grip in the MDF, then use a drop of Gorilla Glue to make sure they don't spin loose later.

A 45 degree chamfer bit cut mounted in an inexpensive router table was used to chamfer the back side of the baffle. This chamfer allows the drivers to "breathe" better without obstruction around the edges It's especially important on thicker baffles. I lightly chamfered even the tweeter hole to provide a little extra finger room for attaching wires later on.
Intermediate Assembly
The cabinet, partly assembled with biscuits and glue, is able to stand on its own. The remaining side was glued in place after acoustic foam and wiring were installed.

Because the non-glued side panel was clamped in place for gluing the other pieces, then removed, its fit is accurate. It essentially snaps in place when the time finally comes to glue it onto the rest of the cabinet.

Fitting Sonic Barrier Acoustic Damping Material

While the speaker cabinet was still open to work on the inside, I cut Sonic Barrier acoustic foam into shapes that would nest among the braces and shelves. Once I had traced the outline of the shape onto the foam sheet, I'd take it to my bandsaw to cut it. A bandsaw makes very quick work of this task, but the blade needs to be cleaned afterward. The sticky pressure-sensitive adhesive on the back of the Sonic Barrier sticks to it.

(Hindsight Note: the damping material can have an affect on how much bass is present. I sometimes wonder if my enthusiastic application of very thick Sonic Barrier affected the amount of deep bass these are capable of. See the listening notes at the end.)

This photo shows the Sonic Barrier in place, and wiring has been run from the bottom chamber where the crossovers will be housed.
Final Cabinet Glue-up

After fitting the acoustic foam to the cabinet walls and running wire for the drivers, it was time to glue on the remaining cabinet side panel. Because the biscuits had indexed this panel during the previous glue-up, it was almost a "snap fit". Alignment was already perfect.

Because of the size of this cabinet, glue-up required almost every large clamp that I had in the shop.

Baffle Fitting

After the cabinets were completely glued together, it was time to fit the baffles onto them. I had previously penciled center marks on the top and bottom mating surfaces so that I could find the desired position when doing this. The mounting holes for connector bolts were already drilled in the baffles, and I had to mark the spots were I would need to drill for the threaded inserts. A tight-fitting transfer punch was inserted into the baffle holes to mark the exact spot.

Not shown was some painter's tape that I used to keep the baffle from shifting position as I punched the locator marks.

Drilling and Inserting Threaded Inserts for the Baffle

Using the dimples left by the transfer punches, I drilled holes for the threaded brass inserts used to attach the baffle. The threaded brass inserts were purchased from Woodcraft.

Connector bolts with 1/4-20 threads from Woodcraft hold the baffle to the cabinet. The broad heads of the connector bolts spread out the clamping load on soft MDF. They look nice too.

When threading the brass inserts into edge-on MDF, I lightly tighten a C-clamp on the sides of the MDF to prevent splitting of the material.

Base Attachment Gussets

Some 45 degree gussets were cut from MDF, and were glued in place in the bottom of the cabinet. Their purpose is to provide a place to fasten the base plinth for the cabinet. They were all recessed about 1/8" inside the cabinet so that the attachment plate on the plinth would nestle inside a small amount. This provides centering of the plinths and prevents sideways motion.

To provide an equal 1/8" depth for all gussets, a simple jig was used during glue-up.

The Crossover Boards

I chose to use separate boards for tweeter, midrange, and woofers. This simplified the task of building and fitting them into the cabinet.

Crossover component mounting plans were done in CorelDraw software after measuring the shapes of each of them, and drawing the shapes to scale. I find it easy to move around the parts, drawn life-size, in software. This permits rapid optimization of space for the crossover, and eliminates confusion during assembly.

One handy extra benefit of drawing to scale is that I use a printout of it positioned on the crossover board to directly mark holes and locations using an awl.

These crossovers could have been squeezed into smaller boards, but there was ample space in the bottom of the cabinet for them as they are.

Pay attention to the reverse polarity required of the tweeter and midrange when wiring. It isn't noted on the crossovers shown here. I simply wire the red driver leads to Ground, and the black driver leads to "Out", which is the reverse of a normal polarity configuration.

I also included all the crossover adjustment options listed by John Krutke on his ZDF3.5 page. To choose one, I merely move the input wire from one input to another. This beats having to unsolder a component, remove it, and solder in a replacement.


The crossover boards were mounted on some 1/8" MDF material that I found in the scrap bin at a local lumber store. It was already painted white on one side.

Note the strips of 1/2" pine that serve as mounting rails on the bottom of the crossover boards. I used small brass inserts in the wood rails and mating machine screws to attach the crossover boards to them. The completed assemblies are attached to the cabinet by simply gluing the pine rails to the cabinet walls. It's strong and easy, yet the crossover boards are still removable using the machine screws.

Attaching Crossover Boards to the Cabinet

The crossover boards mount into the empty space located at the bottom of the cabinets. The crossover mounting rails were wood-glued into position with the crossover boards already attached to them with removable machine screws. The mounting rails also buttress the mounting blocks at the corners of the cabinets to improve strength.
Base Plinth
I used 1-1/8" MDF stair tread (same material as the front baffles) for the bases. I rough-cut the curved arc on a bandsaw, then used a circle jig on my router to clean up the cut.

The MDF gets a couple coats of Zinsser Seal Coat sanding sealer. This keeps it from absorbing excessive primer and speeds finishing. After it's been sealed and lightly sanded, Rustoleum Sandable Auto Primer (in a rattle can) was applied. This primer builds quickly so it fills to a smooth surface quickly. Liberal sanding, often with sanding sponges, is done between coats. If there are especially deep divots in the MDF, I use 3M body spot putty (green paste in a squeeze tube) to fill them. It air dries quickly and sands easily.

Because the primer is oil-based, I give it plenty of time to dry before painting with water-based General Finishes Milk Paint. I'm a little concerned about adhesion between the two materials, but so far it's worked nicely on two speaker builds.

Threaded inserts were put into the bottom of the base plinths for attaching conical rubber "spikes" for feet. I used threaded rubber cones from McMaster Carr for them (Search for "Shock Absorbing Bumpers with Threaded Stud"). They allow penetration into carpet to hold the speakers steady without the scratch-causing nature of metal spikes. On a solid floor, they tend to rock a little, but it's acceptable. They prevent vibrations from the cabinet from exciting the floor boards.
Preparation for Painting the Baffle

After cutting the driver openings and attachment holes, I gave them two coats of Zinsser Seal Coat sanding sealer. It's dewaxed shellac. I sanded lightly between coats.

Close inspection in glancing light usually reveals small imperfections in the surface of the MDF. I use 3M automotive spot putty (green paste in a squeeze tube) to fill the minor divots. It applies and dries very quickly, and sands easily.

After filling and smoothing the imperfections, I usually apply one more coat of Seal Coat before priming.

I use Rustoleum Sandable Automotive Primer because it builds quickly, giving me something to sand into. I use fine sanding sponges for the final smoothing before painting. You can see in the photo how smooth it is with just sanded primer. It must be this way for a professional-looking paint job.

General Finishes Lamp Black, diluted 1 oz water to 16 oz. paint, provided the final finish for the baffles. I used a Lowe's small gravity feed spray gun to spray the paint. The finish can be described as semi-flat in appearance.

Both baffles and base plinths were painted using the General Finishes Lamp Black milk paint. This water-based paint is easy to spray and clean-up. It sands nicely between coats too.

If I recall correctly, I painted these using two coats.

The plinth attachment plate was painted with another General Finishes color. It is mostly hidden from view, but if someone is close to the bottom of the speakers, this reddish paint provides a little contrasting color detail.

Cutting Port Tubes Accurately
My concern about cutting the port tubes square and to the right length provided the incentive to make a simple jig for my bandsaw. The jig held the flared end in an upright piece of MDF, and the jig was then pushed along a fence for a nice, straight cut.
Fixing a Mistake

Back in late summer or early fall 2010, I bought some Mahogany paper-backed veneer to use for the speakers. I applied it using my usual iron-on technique, and began sanding it smooth. I made the rookie mistake of sanding one spot too much, and sanded through the veneer. I attempted a fix by applying one coat of finish, then using colored pencils to match the surrounding veneer color, but it didn't work. Real woodgrain changes color with viewing angle, and the matching repair spot became very visible at some angles. I decided to remove the veneer and attach a new piece.

To remove the damaged veneer, I used hand planes. I used a jack plane to take rough cuts, then a smoothing plane afterwards to prepare the surface for new veneer. This went very quickly, and was easy to do.


Sometimes mistakes open new doors, and I consider this to be one of those moments. I have since thought of many speaker-building scenarios where the use of hand planes may be the way to go. Instead of using a router to level the MDF at joints, I'll probably reach for a hand plane. It will produce very little airborne sawdust like the router. Also a large hand plane, like a Bailey #7, will level the side of an enclosure much better than sandpaper.

The only drawback to using a hand plane is the slightly rough surface of planed MDF. The texture is perfect for a veneered finish, but the light texture left on the MDF would have to be filled and sanded before painting.

I haven't experimented with high-angle plane frogs or steeper bevels on bevel-up planes to see if it reduces texture on the MDF. That's something I can experiment with later.


Over a period of a couple weeks, multiple coats of Watco Danish Oil were wiped on using pieces of old T-shirts. After an hour's wait, I would wipe off the excess. I sometimes used some fine abrasive pads to rub in the finishing oil. Drying time between coats was generally several days. About 5 or 6 coats were applied.

After the last coat, I waited about a week, and used #0000 steel wool to rub Johnson's Paste Wax into the surface.

Port Assembly and Installation

From the inside of the enclosure, I pushed one-half of each unassembled port into its opening. The acoustic foam padding held the port halves in position for gluing. I applied plumber's PVC cement on the mating edges, and wrapped vinyl tape around the joints to hold the assemblies in position while they dried.

When they were dry, I applied hot-melt glue on the port flange area before pushing them into final position in the enclosure.

Installing the Plinth Bases
Four connector bolts from Woodcraft threaded through the bottom plinth to mate with the threaded inserts in the cabinet's corner blocks. The red-painted part nestles inside the cabinet about 1/8" to key the plinth into proper location.

I used conical rubber bumpers from McMaster-Carr as feet for the base. They have 1/4-20 threads, so they can be replaced with something else later, if desired.

The crossover can be accessed by simply removing the base, and unscrewing whatever board (tweeter, midrange, or woofer) is desired.

The cabinet is shown assembled, with ports in place and the binding posts inserted. I used Dayton Audio BPA-38G HD gold-plated binding posts from Parts Express.

Mounting the press-fit tweeter was simple. It was a snug, friction fit, pressed in from the rear of the baffle. After pressing it into place, I added a few drops of hot melt glue on the tweeter flange to make sure it didn't back out.

The extra room milled into the baffle around the tweeter made attaching the electrical leads easier.

Gasketing for the Baffle
Thin weatherstrip material that I purchased from McMaster-Carr served as the gasket around the baffle. It compresses to almost nothing in place, so it doesn't detract much from the appearance. It was 1/8" high x 1/4" wide x 17' Vinyl foam strip with adhesive on one side, McMaster-Carr part number 93675K11. It's very soft material, which is desirable for this application.
Mounting Drivers & Baffle

With the baffle placed in position, the drivers were first wired, and then attached. The midrange driver had countersunk recesses for fasteners, so I used flat-head socket screws. The screw size was 8-32x1, and they are perfectly flush with the flange surface.

For the woofers, I used 10-32x1 button head socket screws. These are attractive fasteners. I used high-quality, USA-made Holo-Krome for this build. The difference between these and generic fasteners is noticeable. I purchased these online from MSC Industrial Supply Co.

Mounting drivers without marring the painted surface requires care. If I set a heavy driver onto the baffle and it slides sideways, the surface will certainly be marred. I sometimes use pieces of cloth under drivers while I attach the wiring.

After the drivers were all mounted, the populated baffles were bolted to the cabinet using 1/4-20 connector bolts purchased from Woodcraft. In the last photo, you can see how the weatherstrip gasket material compresses down to almost nothing when the bolts are tightened.

When tightening driver mounting screws or the baffle to the cabinet, I use a cross-tightening pattern as if I were attaching a head to an automobile motor. One screw gets lightly snugged, then I move to one diagonally opposite, lightly snug it, move to the opposite diagonal, etc. Once they are all lightly engaged, I repeat the pattern again tightening more. It might take three passes to get the screws as tight as I want. This reduces any warping that may occur. It also reduces binding on remaining screws that is often encountered  when one screw is firmly tightened before the others. I try to use USA-made Holo Krome fasteners whenever possible, purchased from MSC Industrial Supply online. they are a cut above the imports.

TIP: Sometimes the connector bolts threads don't properly engage when starting them. If they are forced, they will cross-thread the hurricane nuts.  The solution is to insert the bolt, turn it counterclockwise a revolution or two until you hear (or feel) the thread drop into position, then proceed with the usual tightening.

I was very pleased with the appearance of the finished baffles. They are both mechanically solid and attractive.

They're Finished!

[Click a thumbnail to enlarge]

How do they sound?
I invited some stereo-savvy friends for a listen to the completed ZDT3.5 speakers, and I'll summarize their impressions. First, they thought that the sound was low in distortion and very clean. One listener called them "sweet-sounding". However they all thought that the deepest bass notes were rolled off. Bass is there, and it's clean bass, but it's just not there in sufficient dB to rattle your sternum. In their defense, the speakers were not located near any reflecting wall which would reinforce bass. It was 10 feet further to the front wall. Need more bass? - get a subwoofer!

They also remarked about how the speakers clearly defined each instrument in the recording.

They all agreed that they could listen to the speaker without fatigue. I have the tweeter level reduced per the options provided by John Krutke in his plans. The midrange is set at the design level. These adjustments are specified by John Krutke on his web site as tuning options, and are all done in the crossovers.

In my listening situation, I find that the "sweet spot" is fairly narrow compared to my Plutos. As an example, if I'm not in the sweet spot, the beyond-the-speaker sound effects in Roger Water's Amused to Death collapse to the nearest speaker. My impression about sound stage may be influenced by my listening room, which is long and narrow. I'm also forced by furniture to listen fairly up-close to the speakers - about 6 feet away. I'd prefer the speakers be further apart than my room allows, and to sit further away. Sitting further back would certainly help.

Bill Schneider
May,  2011
Minor edit June 2011

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