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"dB61TL" 2-Way Transmission Line Monitors Project, Text and Images courtesy of Dave Brown A great sounding, simple, two-way, transmission line loudspeaker that can be built for $200 a pair.

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Before you Begin

Note to potential builders: I've received some feedback and questions recently about the dB61TL. Before you decide to build this speaker, here are a couple of comments:

1. The enclosure is not as easy to construct as it might look at first glance. It is helpful to have access to a table saw/radial arm saw and to have some experience using a router -- or have somebody helping who has. There is also some work involved in getting the stuffing and acoustic foam installed and it may need to be adjusted by listening.

2. The design goal of this, and most transmission line speakers is, to produce bass which is non-resonant and smooth. If you're looking for loud or hard kicking bass, this isn't the correct design.

3. If you have any questions about building the dB61TL, please e-mail me.

A note about transmission line bass: The goal of a t-line system is to produce bass which is totally free of port and box resonances. The result should be a very natural sound which is full but not boomy. You may find that you prefer louder bass with more kick or punch. If so, a vented design may be more to your liking and there are several available which use the same drivers as the dB61TL.

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Driver Selection

I'd been playing around with the Parts Express/Dayton 6 ½ inch woofer for several projects. One was a vented two-way project designed for and built as an employee project for at work. This evolved into a W-T-W two-way floor standing design I built for my son.  These designs also used a new tweeter from Parts Express – a 1 1/8 inch silk dome tweeter.  Both the woofer and tweeter worked extremely well in those speakers and are still much in favor in an almost cult-like following on the Parts Express discussion forum at http://www.Parts-Express.com/.

The dB61TL was built as an off-shoot of those other Dayton driver designs and used the same construction techniques and crossover which was a first order, minimum component design.  Unfortunately, the sound of the T/L version was disappointing.  The bass shelved down below about 400 Hz and the woofer resonances at the upper end of its range gave the speaker a bright sound.  So, the T/L sat in the corner of my garage for several months, gathering dust.  Then, thanks to Wayne Jaeschke, one of expert "gurus" on the Parts Express site, who, within a couple of months had designed, built and posted three designs using these same Dayton drivers, I decided to try some changes.  I used the crossover Wayne designed for his “Dayton II” speaker project. That crossover is essentially the design used here, the only difference being the addition of 6 dB of baffle step compensation and a slight change in the tweeter L-pad attenuation.

Wayne's crossover helps solve problems with the woofer’s resonance in the 4000 to 5000 Hz range. It uses second order slopes and a notch filter that helps with the resonance. I added additional baffle compensation to bring the bass up to where the response is essentially flat down to 25 Hz. With a transmission line speaker, the ability exists to produce extended bass. However, the output is, at least in this line, lower than bass produced in a vented alignment. Additional compensation works well to elevate the bass level and still provides high impedance allowing any amplifier to operate with full power. The result is a speaker that is controlled and stable yet retains a coherent image with full, tuneful bass, a smooth midrange and a very acceptable treble response. A new 275-070 tweeter, which will be supplied for orders from Parts Express starting in September, 2001, incorporates a closed back and added diaphragm damping. It can be used as a direct substitute for the old tweeter utilizing the exitsting crossover, but a revised crossover, designed by Wayne Jaeschke (shown below) has been optimized to maintain constant phase alignment througout the crossover range. I built the revised crossover and installed it at the same time I relplaced the 275-070 with the new tweeter. The new tweeter, with the revised crossover, will sound considerably more clear with an airy quality distinctly different from the old tweeter. The imaging is maintained while the level of detail is increased by an order of magnitude. I should mention, however, that I changed the tweeter and crossover at the same time. I didn't try the new tweeter with the old crossover. It is possible that some of the audible differences I experienced were due to the crossover change and not just the tweeter. I did, however, install the new tweeter in another pair of speakers, without changing the crossover, and there was a noticeable and similar improvement in the high frequency response. I should mention that the new tweeter's output sounds higher that the old version and, for this reason, makes the overall tonal balance of the speaker brighter.

The transmission line is 94 inches long -- from the woofer to the terminus at the rear. If we include another 12” from the rear to the front it makes the total acoustic line distance 106”. The line tapers from 35 sq. in to 26 sq. in. -- a taper ratio of 75%.

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The response shown here was measured with a Real Time Analyzer (RTA) and an inexpensive mike. I believe the response is valid below about 2 KHz, but above that the roll off is likely due to the mike. The measurements were taken in my living room, so there are certainly some other errors.

The lower limit of the bass response shown in the graph is 25 Hz because that’s the lower limit of the RTA used for the measurements. The response extends below 25 Hz, but, using my test setup, how low it goes is unknown.

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The following tables and drawings show the parts list, enclosure part dimensions, and crossover layout and parts list. The drivers, crossover components and accessories are available from www.Parts-Express.com.

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Construction

Construction is easy, but there are a couple of tools and techniques that will make it easier.  A router with a setup for cutting holes for the drivers, i.e. a circle cutting jig, makes for a very smooth cutout, although I used a 3 1/2" hole saw for the tweeter. 1/2 inch and 3/8 inch rabbeting bits will help to cut the flush mount rabbets. The technique for that is to cut a hole 1 inch or ¾ inches smaller than the diameter of the overall diameter of the speaker flange, then use the rabbeting bit to cut the flush mount portion to the flange diameter. Set the depth to correspond to the thickness of the speaker flange and the gasket or sealant you'll use to mount the drivers.

Another technique for cutting holes that I like best of all involves the use of templates. I cut the template so that it looks just like the hole I'm trying to make in the speaker. Use 1/8" or 1/4" hardboard and a jigsaw to cut the shape. Make sure to cut carefully and sand smooth since the hole you cut in the speaker will be exactly like the template. Attach the template to the baffle with a couple of screws and, using a 1/4" router bit with bearing on the end, cut the hole. The bearing rides on the edge of the template. Note, that this must be done before the speaker is assembled. Also, it's almost a necessity to use a router table for this since there won't be much surface for the router to ride on near the edge of the baffle.

Another very useful technique is to cut the front, back , top, and bottom slightly oversize (about 1/8" on each edge), then trim the overhang after assembly. For this, a straight bit with bearing is needed.

I assembled the box using drywall screws and glue, but since have gone to using finishing nails instead of screws.  It takes a little getting used to, but you don’t have to pre-drill and counter sink and you don’t have the big holes to fill.

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dB61TL Schematics

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The Main Cabinet
Click Here for Step-by-Step Cabinet Construction

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The Cabinet Base
Click Here for Step-by-Step Base Construction

The base is very important. Combining the speaker's narrow width with its height of 4 feet, it doesn't take much to knock it over, especially on carpet. The base adds the needed width for stability. It is also a place for the crossover. Use of the spikes is also recommended.
 

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Step by Step Cabinet Construction

1. Cut all pieces to the dimensions shown.

2. Cut driver cutouts in front baffle and rabbet for flush mount.

3. Cut hole for terminal cup in rear panel.

4. Cut 45 degree bevels on interior reflectors.

5. Cut the two terminus trim pieces with 45 degree bevels.

6. Lay out in pencil the locations of the four reflectors and the divider on both side panels.

7. Glue the divider, top, and bottom in place between the two sides.

8. Glue the four reflectors in place.

9. Install foam absorption material to the top reflectors and divider.

10. Install foam to the inside of the front and rear panels.

11. Install triangular piece "G" to top of reflector "C".

12. Install piece "H" onto rear panel centered horizontally and at a point which will be 4 1/4" below the lower edge of piece "G". Note, these last two steps are done now so that the terminus can be cutout with a router using "G", "H", and the sides as guides.

13. Glue the front baffle and rear panel in place.

14. Cut out the terminus opening.

15. Install the two terminus opening trim pieces.

16. Drill wire hole in bottom.

17. Paint or apply veneer as desired.

18. Install wiring from drivers out hole in bottom.

19. Install wiring from terminal cup out hole in bottom.

20. Install drivers and terminal cup.

21. Install base and connect wiring to crossover.

Step by Step Base Construction

1. Glue side pieces to top of base.

2. Drill hole for wires going into speaker.

3. Drill holes for spikes.

4. Drill and countersink holes to attach base to speaker.

5. Use ½” rabbeting bit to cut recess for bottom panel.

6. Cut out bottom panel to fit opening in bottom of base.

7. Pre-drill and countersink holes to attach bottom panel to base.

8. Paint or apply veneer to base.

9. Attach base to bottom of speaker.

10. Install crossover onto bottom panel and connect wires to drivers and terminal cup.

Wiring

It's best to install the wiring after assembly and finishing, but before stuffing the line. Run a pair of wires from each driver down and out through a 1/2" hole in the bottom of the speaker, into the base. Run another pair from the terminal binding posts down and out through the same hole. Leave about 16" extra for each wire coming out the hole. Attach the wires to the crossover and secure the bottom base cover.

Note: If you plan on bi-wiring, i.e., using two sets of binding posts and splitting the crossover into separate tweeter and woofer sections, you will need to include the baffle step filter (R4/L3) in each section. That's because this filter operates on both drivers, over the entire frequency range. However, for the tweeter section, all that is really needed is R4, the 8 ohm resistor. L3, the 2.0 mH inductor, can be omitted since it provides a low pass path at around 500 Hz.

Stuffing

I ended up using a stuffing density of around .75 lbs/cu.ft. of Polyfil pillow stuffing. You may have heard that this material is not the absolute best to use, but it’s inexpensive and readily available.

Finishing

Veneering is an easy and, considering the time and effort involved in an well done painted finish, an inexpensive option.  A 4' x 8' sheet will cost about $60 for common paper-backed veneers such as oak and cherry. 

You can buy them at hardwood (woodworker) supply stores or from online suppliers such as http://www.woodveneers.com/.  Even if you’ve never veneered before, it’s not that hard.  You can then stain and apply a gloss or semi-gloss finish that will look great.

You can also paint the enclosure.  Depending on your experience and the amount of time you want to devote to it, you can do a spray can job or create a piano black gloss finish.

The Grille

Here is a drawing of the grille.  It is made from ¾” MDF and covered with black grille cloth.  Round over the inside and outside edges of the side away from the speaker.  Before installing the grille cloth, drill a small (1/16”) hole where the grille guides will go, place the grille on the speaker, and shove a small finishing nail or brad through the hole to mark the front of the speaker.  Drill out the grille guide holes then install the grille cloth with hot glue or staple it in place.

Crossover
Original Version

This crossover was developed by Wayne Jaeschke for his Dayton II two-way, vented loudspeaker which used the original version of the Dayton 275-070 tweeter. You may use it for the new version of the tweeter also and it will work very well -- or you may want to build the revised crossover which Wayne designed specifically to account for the characteristics of the new tweeter.

Note that both crossover versions include an added 6dB of baffle step compensation to account for the transmission line's shelf response.

Component Values for the Original Crossover

New Crossover
Revised for the new generation 275-070

This alternate crossover was developed by Wayne Jaeschke for the new production version of the Dayton 275-070 tweeter. The new tweeter is damped to provide a lower Fs and has an overall smoother response as long as a response peak near the crossover frequency is dealt with. The revised crossover also contains fewer components.

R2, R3, and R4 provide approximately 3dB of attenuation for the tweeter. You may adjust this L-Pad to your liking or as necessary to account for how the speaker sounds in your listening room. See the L-pad chart below which gives values of Rs and Rp for different desired attenuation values. Note that R2 and R3 in this crossover are 4 ohms and 6 ohms paralleled to arrive at 2.4 ohms for Rs.

Component Values for the Revised Crossover

About the Designer

This fantastic "T-Line" project was submitted by Dave Brown (AKA "dB" on the PE Tech Talk Board) of Placerville, CA. Thanks Dave, we appreciate it!

Dave's home page