Construction Article

dB515

A bass reflex MTM small floor standing speaker. Primarily designed for music, but sufficiently dynamic for home theater main speakers. Excellent bass extension, natural mid-range and detailed highs. Uses Dayton 295-300 or 295-301 Shielded 5 1/4" mid-woofers and the infamous 275-070 silk dome tweeter. May not be safe for use with amplifiers that aren't 4 ohm stable.
 
The woofers are magnetically shielded. The tweeter is not shielded, however, it can become shielded with the addition of a bucking magnet, although this may not even be necessary unless you place the speakers very close to your TV/video monitor.
 

 

Drivers

 

Dayton 275-070 Silk Dome Tweeter

Dayton 295-300 Shielded Mid-Woofer

Dayton 295-301 Shielded Mid-Woofer

 

Enclosure

 
I used BassBox 6.0 and the Parts Express Catalog Thiele-Small parameters to simulate the box design. Since my goal was to produce a full range speaker for primarily music listening, a vented enclosure was called for. I also wanted the box to be a small floor stander -- approximatley 40" tall with a narrow baffle. The optimum box volume calculated by BassBox was 1.2 cu.ft. In order to keep the size down, I traded off some low frequency extension and went to a 0.88 cu.ft. volume. Tuned to 42Hz the 3dB down point was 46Hz. I opted for two 1.5" diameter ports (5" long). I chose this vent arrangement because it keeps the needed length short enough to fit in the box and keeps the vent velocity down to 0.06 Mach. A single 3" port would have been too long, and a 2" port would have had a vent velocity too high.
 
The box dimensions were chosen by starting with the 6 3/4" baffle width (external) I wanted and an overall height of 40". That resulted in a depth of 9 5/8" (8 1/8" internal) to arrive at 0.98 cu.ft. which, after accounting for the volume of the drivers and braces, comes out to 0.88 cu.ft. net volume.
 
The enclosure construction is the same as I've used on several other speakers. There are four internal braces. 3/4" MDF is used all around. I considered a thicker front baffle, but, considering the extensive bracing around the drivers, I didn't feel it was necessary. All the joints are butt joints and I used the overlap method described in my Links and Tips page.
 

 

Some Basic Assembly

 
If you use the Parts Express 1 1/2" ports, you will need to lengthen them to 5 1/8" (5" plus 1/8" to account for the flared end). You can use some cardboard and epoxy or, like I did, take a 2" long piece of 1 1/2" I.D. PVC pipe and enlarge the inside using a hole saw, 1" deep into the pipe. This was a messy operation because it's not easy to keep the hole saw centered. But, it did work. Once I had the extensions made, I used PVC primer and cement.

Or...you can use 1 1/2" PVC pipe for the ports...
 
The only part some people may find tricky in the enclosure construction is flush mounting the "pincushion" shaped woofers. I already had a template that I'd made for an earlier project with this woofer, so it was easy for me. But, making a template for this is not that difficult.
 

1. Place a woofer face down on a 12" x 12" piece of paper and very accurately trace the outline of the basket.

2. Set a compass to a width equal to 1/2 the diameter of your router base minus 1/2 the diameter of your straight cutting router bit. In my case, this came to 2 13/16. I added 1/16" to this for clearance around the driver, so the final compass setting I used was 2 7/8".

3. With the point of the compass on the outline of the basket, draw a multitude of arcs from a multitude of points on the outline. Connect the outermost sections of the arcs to create the shape of the template.

4. Transfer this odd-looking shape onto a piece of 1/4" hardboard.

5. Use a jig saw to cut out the template.

6. Draw a centerline on the template running from the top to the bottom. This will aid alignment of the template on the baffle.

 

Hot News! Really Hot News!!!

Parts Express is now shipping a round version of this woofer. The outside diameter of the woofer is reported to be 5 11/16".
A drawing showing a modification for these drivers is shown below. Basically, an extra 1/8" is added between the drivers.

 

 
 
But, continuting with the pincushion driver challenge,... you can download this file: flushmount.doc. It's a quarter segment of a drawing of the template traced from my template then converted to a CAD drawing and inserted into Microsoft Word. Print four of them, tape them together, and use it as a guide to cut a template. This is a Word document and hopefully, the scale will be maintained. Make sure to check the printout against the dimensions shown on the drawing and use the outside of the line since I traced it with a pencil along the inside of my template. Definitely do a test run on some scrap before doing your baffle!!!!!
 

 
Once all the parts were cut out and routered, the box was assembled. I didn't use any nails or screws, just a couple of clamps and weights.
 

   
 
 
 

 The Lefty-Waldron Base

 
 The base is used to add stability -- important if you have kids or large pets. On a carpeted floor, it helps to use spikes. The method shown here is called the "Lefty-Waldron Base". Lefty came up with the idea for using the disks/rounded corners and the truck bed liner paint and it was Barry Waldron's idea for the spacer between the speaker and the base.
 
Made from 3/4" MDF. Before attaching the 5 3/4" x 8 5/8" piece to the 11 5/8" x 8 3/4" piece, glue on the four 2" disks. Then you can use the disks as a guide to router the rounded corners. Mount to the speaker using 2" drywall screws.
 

 
The pics show the base painted with Dupli-Color Truck Bed Coating. $7.95 for a can from Krangen's auto parts store.
 

 

Crossover

 
I began with a crossover configuration done for me by Jason Andreason using SpeakerWorkshop and imported .frd files. Jason's, Dave Burke's, and Wayne J's help was invaluable in keeping me from giving up on this project. I was experiencing a pronounced "glare" in the midrange/upper midrange which I had been unable to get rid of. These folks spent the effort to run my configuration with their systems and make some recommendations. I ended up using Jason's basic crossover. Working with his design and some actual measurements I was finally able to accomplish using the mic, mic preamp, and Wallin jig sent to me by Dave Burke, I made some changes. Eventually, the midrange level was tuned by ear over a period of several weeks. Shown below is the current configuration which sounds great. The problems I initially had with a glaring sound on loud, complex music is gone. The detail and imaging is very good and the bass, although not as low as we might want, is solid and clean.
 
 Here is the crossover schematic. Cn, Rn, and Ln are a notch filter centered around 4800 Hz. Ce2 is a bypass poly cap for the non-polarized 47 uF electrolytic in the Zoebel compensation filter. Important: Note reversed polarity of the tweeter.
 

 
Here is a Speaker Workshop simulation of the response with the final crossover components.
 

 
 Here is the final system SPL response curve using SpeakerWorkshop. There are some obvious differences from the prediction. The predicted response is what it would look like using the component values selected by listening, if that makes any sense...
 

 
And here is the Woofer Tester measured impedance. Note that the impedance dips to around 3.5 ohms at 200Hz. For non-4ohm stable amplifiers, this could be a problem.
 

 
 Another Woofer Tester impedance plot for the lower frequency range. The enclosure tuning frequency can be seen at 41 Hz -- close to the target of 42 Hz. (Each horizontal division is 10 Hz starting at 0 Hz at the far left).
 

 

 
Here's the cost breakdown for one speaker:
 

Item

Parts Express
Part Number

Qty

Cost/ea

Cost

Dayton 5 1/4"" Woofer

295-300

2

$12.15

$24.30

Dayton 1 1/8" Tweeter

275-070

1

15.50

15.50

MDF

-

 1 sheet

18.65

18.65

Port

260-402

2

.40

.80

Acoustic Foam

260-316B

1

2.40

2.40

Terminals

260-311

1

3.95

3.95

Crossover Parts

       

C1 (6.8 mF)

027-424

1

2.25

2.25

C2 (33 mF)

027-350

 1

.85

.85

C4 (8.2 mF)

027-426 

1

2.75

2.75

Cn (3.3 mF)

027-420 

1

1.77

1.77

Ce1 (47 mF)

027-352

1

.90

.90

Ce2 (0.47 uF)

027-458

1

1.38

1.38

L1 (0.33 mH)

266-812

1

2.80

2.80

L2 (1.0 mH)

266-826

1

4.50

4.50

Ln (0.33 mH)

266-812

1

2.80

2.80

 R1 (5.6 ohm)

 004-5.6

1

1.25

1.25

R2 (2 ohm) 

 004-2

1

1.25

1.25

Rn (16 ohm)

 004-16

1

1.25

1.25

Req (4 ohm)

 016-4

1

.39

.39

     

Total

$89.74

 

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