Build the Mini Line Array

Build a Mini Line Array

Along with all my other interests in life, audio and speaker building is near the top. The first set of speakers I ever built had drivers that came from Radio Shack in 1978. The tweeters had a 10uF capacitor on the + lead and at the time, I thought that was all you needed for a crossover... Over the next 30 years I learned a quite a bit more. I was also very interested in line arrays as the concept always seemed like a good idea. A line array is a group of audio drivers (speakers) mounted in a row. It does a couple things for you. First, it allows each individual driver to use less power thus, in theory minimizing distortion as the driver does not have to work as hard. Second, all the acoustic energy couples together to provide a sound field that does not drop off at the same rate as single driver, which acts as a point source. For more information see this  and this.  This project started out like a lot of mine. Parts Express had a sale on 3inch audio drivers and I said “I always wanted to build a line array, let me order 16 of them!” three years later I actually got around to building them. I am actually glad I waited because there was more information I needed to learn before I built them.

There is a lot of debate around how good they are and what there issues are. On the down side, multiple drivers can cause interference and acoustic artifacts. Interestingly, the same argument is the same for why an orchestra sounds so good live. Next time you go to a concert look at the PA speakers... They are most likely set up as a line array. If you really want to hear how they sound lets build one!

Step 1: The Design

The Design:

I found a speaker building buddy who happened to live in my neighborhood. The first time I went to his house he had a line array using 9 three inch drivers per side and 9 small tweeters. This reminded me about the 16 I had in my garage. After listening to his I knew I needed to put my drivers to good use. Dave suggested I pair them up with a different tweeter than he used. He used a ¾ inch dome tweeter and recommended the same driver that had a 5/8 inch dome. He had used this tweeter in a couple other projects and really liked them.

I had 16 of these: 

and ordered 16 of these for use as my tweeter: 

Dave had substantial experience with speaker design, after measuring my little woofers designed a great crossover. With 8 drivers I chose to wire them 4 in Parallel and then the two sets of paralleled drivers in series giving me roughly a 4 ohm load. This was taken into account for the crossover design. See the theoretical frequency response graph.

Parts List:

Crossover parts (for two speakers:)

(2) 2.4 Ohm Resistor

(2) 3.3 uF Capacitor

(2) 7.5 uF Capacitor

(2) 10 uF Capacitor

(2) .70 mH Inductor

(2).15 mH Inductor

(2) Speaker terminals

Cabinet/Enclosure Parts  (for two speakers:)

½ ” Birch plywood

(2) Front Panel 7” X 34”
(2) Rear Panel 7” X 34”

¾ ” Birch Plywood

(4) Top and Bottom 3” X 5.5”
(4) Side panels 3” X 34”

Miscellaneous

20 feet of Black 18 gauge wire
20 feet of Red 18 gauge wire
Semi Gloss Polyurethane
Wood Stain
Sandpaper
Mounting Screws for the 3” drivers 
One bag of Poly Stuffing to act as acoustic insulation in the cabinet 
Mounting screws for the rear panel.
Electrical Tape or Heat Shrink Tubing

Tools

Drill Press
Soldering Iron
Solder
Orbital Sander
Hot Glue Gun (with a bunch of glue sticks!)
Wood Clamps
Wood Glue
Screw Driver

For the cabinet I chose 13 ply birch plywood. In the Dallas area you can get it here. I wanted the line array to be small and compact. And, easy to build with the tools I have. I drew the front panel in inkscape  (Great Open Source drawing program!) Because the tweeters were so small I decided to just drill the right size hole and then glue the driver in flush to the front panel. I experimented with a 1 ¼” hole and a 1 3/8” hole and ended ordering a 1 5/16” forstner bit from here  that was just right. 

Step 2: The Enclosure

Build the Enclosure:

The most difficult part was drilling the holes for the front panel. This is where a CNC router would really come in handy! I had the small tweeter holes covered with my forsnter bit but I needed to figure out the best way to cut 3” holes in the wood. I had tried hole saws before and they really dont work well for multiple cuts in plywood. Then I found this type of tool: it allows you to adjust it and it makes very nice circular cutouts with a drill press.

After cutting all the pieces out with a table saw I set up my drill press to allow me to move the front panel around while maintaining the correct distance from the side of the front panel. Couple of clamps and some scrap wood are all you need. This way if I was off a little bit the drivers would all line up and it would look good. Then I measured off all the centers of the holes and proceeded to drill away. After finishing the holes I rounded over the front panel with a router and then sanded all the parts. Before you put the drill press away, you need to cutout a hole hole in the back panel for the speaker jack. The one I used conveniently needs a 3” hole cutout. Also, drill pilot holes in the rear panel for the mounting screws you will use to mount it to the enclosure before sanding, staining and varnishing.

I used wood glue and clamps to assemble the enclosures. I cut a couple small braces that you can see in the pictures. I left the back off on purpose as this is where the crossover goes.

Before mounting the drivers and building the crossover I sanded the enclosure then stained it with a dark stain and put on several coats of polyurethane varnish.  

Step 3: The Crossover

The Crossover:

My buddy Dave Thomas was very helpful and he designed the crossover. I am trying to talk him into writing an Instructable on Crossover Design. Stay tuned!

You may have noticed that the design calls out for a 2.5 Ohm resistor and the actual in use resistor is 2.4 Ohms. There will be very little difference. It is important to avoid wire wound resistors as they will have some inductance associated with them that is not taken into account in the design or measured. 

One of things I learned about speaker building is to always print out a copy of the crossover schematic and tape it to the inside of the speaker. This way if you ever have to work on it later, you have the schematic! It also helps with the wiring.

Wiring the crossover is pure point to point. I hot glued all the components to the back panel and then connected them per the schematic. Set the rear panel aside and let's mount the drivers. 

Step 4: Mounting the Drivers

Mounting the Drivers:

Lets start with the tweeters. Cut 16 six inch red wires and 16 six inch black wires. Strip ¼ ” on on end and about ¾ of an inch on the other end and. For each tweeter, solder a red wire to the “+” terminal of the tweeter and a black wire to the “-” lead. (See the Picture) After prepping the tweeters, I placed the enclosure on a smooth flat surface and lowered the tweeters in from the back of the enclosure to sit flush with the front panel. Now comes the fun part, using the hot glue gun, glue in eight tweeters per cabinet. Let them cool before moving anything. Now for the woofers...

In a similar fashion to the tweeters, cut wires for the each woofer and prep and solder them to each woofer. Mount them using four screws for each one. Be careful here, one slip of the screw driver can ruin a driver. I had this happen to me and had to order another one! After all the drivers are mounted, lets wire them up.

Step 5: Wiring It All Together

Wiring it all together and final assembly:

Look at the wiring diagram. We are wiring four drivers in parallel then wire the two sets of drivers in series. Then using more 18 gauge wire, connect the tweeters to the correct crossover section. Repeat this for the woofers. Double check all the connections. Before placing the rear panel on we want to put in as much fill of the poly fill as possible. The goal here is to prevent resonance and internal reflections inside the enclosure. Some of the more astute of you may have wondered about the volume of the box and how that affects the frequency response. I use these speakers as right and left of my home theater setup and I have a sub-woofer. In a bit of heresy, I did not deign around box volume and am not trying for deep bass response. The crossover theoretical response is very good above 200hz or so and listening tests will tell how it all came out. OK, stuff the box with fiber fill and then place the rear panel on the enclosure, making sure that no wires stick out or are pinched in the edge.

Step 6: Listening and Use

Listening and use:

I hooked these up to my receiver and fired them up. They sound great, crisp and extremely clean. I have them setup so that the tweeter array is at head height while you are seated in my media room. The one issue with them is that the tweeter array really beams. If you are in line with it there is no issue and the stereo field is awesome. If you stand up above the tweeter array you can actually hear it drop off a little. This got me to thinking about what I would do if built another one. I would curve the tweeter array slightly. See the picture of what I would change. Dave came over and listened to them and really liked them too. His with the 3/4” tweeter exhibit the same issue with being above the tweeter array. In the mean time, these things are very low distortion and sound awesome. In the stereo field, it is almost like you have headphones on. My wife loves them. Movies usually have back ground effects, music and foley in the stereo field and these speakers put you right in the middle. One of the other things a line array should do is handle a lot of power while maintaining low distortion. We had an event at work and I took these in with a sub-woofer and the rocked the house. I used an active EQ, crossed them over at 150Hz and ran them with a 225W per channel amp. They projected very well and cleanly filled the room with both people presenting and our after party. I used the same setup at a neighborhood block party and once again, they rocked the hood. They only have eight drivers per side and are only 34” tall. One could present an argument that they are not full line arrays. In my opinion, they certainly sound much larger than they are. I also brought them to a couple local DIY audio events and they were well received.

The price of the drivers I am using went up significantly since I bought them. I am keeping my eyes out for close out sales and want to build an even larger line array. Happy Building!

Jules