Home Theater

Being the geek that I am I decided, due to the extremely high prices of home theater speaker systems, to design and build my own. I am of the belief that if I am going to take the time to do it I might as well try to do it right. Below is my chronical of that experience.

SUBWOOFER

In my home theater setup I have mixed and matched speakers and have always been somewhat unhappy with the situation. The subwoofer that I was using was a JBL 8" 100 watt that I had borrowed from my brother while he was living in an apartment and couldn't’t really use it (mostly because his 5.1 receiver died and he was waiting to buy a new one). When he moved into a house I knew it would only be a matter of time before I would lose the powered sub and have to go back to the weak passive sub that’s mine. The JBL was only 8” but I was amazed at how much that little thing could put out. It could be overpowered pretty easily but it would certainly do pretty well in most cases. So losing the powered sub for the passive sub was not an acceptable idea to me. So I started shopping for subs.

The first thing I noticed is that powered subs are way overpriced. To get anything even moderately mediocre is $200 and you could easily spend $1000 or more on a good one. There’s no way I could pull that off. So, being a DIY kind of guy who has built speakers before, I started looking into building a powered sub. I even looked at building the amp myself but the electronics stuff is not something I have experience with. So I started checking prices on home theater sub amps and found they are pretty expensive, too. 

I came across a couple of eBay listings for amps that would work and finally settled on a 200 watt amp with a band pass choke. I put a bid and waited for about 5 days for the auction to come to an end (auctions should never be more than a day because no one bids until then anyway, Just my opinion). After some last minute bid warring, I won the amp.

My next order of business was the driver. I read a lot of opinions on what was good… 10 inch versus 12 inch and sealed enclosure versus ported and blah, blah, blah. I could not find many drivers built for home theater that weren’t so completely outrageously priced. So I turned to the mobile audio market. Say what you want but the price difference made me what to try it.

I wanted a driver that had wattage handling of 200 watts and peaked higher. I also wanted a driver whose enclosure would be the right size for fitting in the room and not taking up too much space. My other main concern was a frequency response low end of 25Hz or lower. Automotive subs typically have a response frequency that doesn’t quite cover the low end as well as the others but after a bunch of looking and comparing and price shopping I found a SONY XS-L1200B driver that had the specs I was looking for and the price was right. I found several reviews on the driver and they all were favorable. So I bought it for about $50, including shipping. BUT, turns out they were out of stock and it was discontinued so I couldn’t get it. So I ended up getting a much more expensive JBL 12" driver, GT120. It had great specs and I trust JBL from experience I’ve had in the past. It also had great reviews. So, ordered it and awaited its arrival. I think I will be much happier with this driver than I would've been with the SONY as I believe this one to be a much better driver.

A sheet of MDF and some planning for a box that had the JBL recommended enclosure specs was next. I ran the driver through some computer models to make sure the JBL specs for the enclosure would perform as they had said, and it seemed that it would. I compared the sealed enclosure response curve to the vented and the sealed band pass options and the vented seemed to have the curve that most represented what I was looking for. Good strong lows down to about 23Hz with no heavy peaks. A pretty smooth curve (flat response) and that was what I wanted (the dotted curve is the one I used).

I wanted the box to look professional but be inconspicuous. I debated about whether the sub should be down firing or front/side/back firing. I was leaning toward down firing for several reasons. I had read that home theater applications seemed to do better with down firing subs. I also thought it would offer some extra protection to the cone being under the box like that (I have a few kids running around the house).  So I set to designing a box using my 3D modeling skills and Lightwave 8. Ya, I know, that seems a little strange but it is a program and method that I know well and it helps me to visualize the whole thing before I start cutting the materials. I tried several versions before finding what I wanted and then I did an exploded view of it and put the dimensions on the image to use as a cut list and assembly guide. It was one of the most helpful spontaneous ideas I've ever had. It made cutting and assembly so easy.

I purchased a sheet of 5/8" MDF for the enclosure. I then cut all the pieces and started assembly. I ended up moving the port down lower than in the original exploded view (left) and that made me have to change the brace that the port tube goes through to be a little bit different, also. I also had to move the amp to the side because the port tube was too long and would run into the amp casing if it were on the back. I didn’t want to do that but it became necessary. I didn’t take the depth of the amp into consideration when designing the box. Oops. I used a 4" PVC pipe for the vent tube. 

I used lots and lots of wood filler because I wanted the box to be smooth and without screw holes or seams. I drilled pilot holes for all the screws with a countersink bit to make filling the screw holes easier and neater. I used Liquid Nails and 1.25" coarse drywall screws to assemble the pieces. I put four to five passes of filler on all the holes and seams and sanded in between each pass. I used DAP to seal the inside of the box. I painted the inside of the PVC tube black so it would blend with the box when finished. I used a 5/8" roundover bit on the router to "flange" the port hole on the front of the box. I also used the same bit on the top piece so the top edges would be rounded.

After filling and sanding and filling and sanding (for what seemed like forever) I began the finishing process. I decided to paint the enclosure black and used an oil based black satin finish paint. I used oil based for two reasons: 1) it was the cheapest paint and 2) the finish should be strong and tough.

    
A couple of early pictures of the filler work (left) and the bracing and port (right)

I first lined all the interior walls of the enclosure with R-13 insulation (the pink stuff). I had some laying around so I used it. It's really thick as it's made for putting inside of walls, so I divided the thickness in half. I used 1/2" staples to attach it to the walls. Then I put the top on the enclosure and started painting. After two coats of paint applied with a small sponge roller to avoid brush strokes, I was very pleased with how the enclosure looked. I could see a few minor imperfections in my work but overall I was very happy with my first attempt at this.

I installed the amp and awaited the driver to arrive.

I really had no way to test the amp and make sure it was even working before the driver arrived so I was very nervous about whether this eBay purchased item would actually work. It looked brand new when it arrived as it had some protective packing on it that looked like it was done during manufacture. So that part was good. If the amp didn't work, I would be in a bit of trouble as the hole for the amp in the enclosure was cut specifically for that amp. Plus I'd be really upset if I did all this and had a busted amp.

Well, upset I was. I hooked it all up with my new driver fresh from the UPS truck and the amp was bad. It has something very wrong with it and it just pulses and pops and throbs irratically at very low volume settings. The noise it makes isn't low, just the volume know when the noise starts. I cannot tell you the disappointment I felt when it failed to work. So I called the guy I got the amp from on eBay and explained to him the situation. He originally was trying to say that I blew the amp hooking up bad speakers to it. I am not sure how that could be but I assurred him that it didn't work from the beginning and that nothing ever blew on it. He ended up saying to send it back to him and he would have his technician look at it and fix whatever the issue is. So I had to wait for it to get there, get fixed and get back before I can use my DIY Subwoofer. Not to mention the fact that I had to give up the sub I was using as my brother took it back. So not only is my new one not functioning, but I don't have the old one either. I do have the old, old passive 10 inch woofer from forever ago. Nothing like going backwards with stuff like this.

Since I had this extra waiting time, I decided to do a little cosmetic work. I never liked the texture that the small foam roller put on the enclosure when I painted it. It was not smooth but not really textured much either. Just blah and it left some strokes as it didn't put the paint on very smoothly or evenly. So I sanded (lightly) the painted areas and used a big, fat, thick roller that I originally bought to paint walls that have texture on them. It put a good texture on them. I wanted more texture in the paint layer to cover some of the imperfections that were there.

    
The finished box angle (left) and in its proper place (right)

When the amp came I was very excited and could hardly wait to get  home and  put it all together and give it a test. I left work a little early (ya, I'm pathetic) and put it all together and hooked it up and did a few minutes of testing (all that I could do with dinner being fixed and the kids all over). It sounded great.

After the kids were in bed, I put in the Lord of the Rings, Fellowship of the Ring Extended DVD disc 2, and went to the scene in Moria where they run up against the big fire and shadow demon thing. I played that scene and adjusted levels until I thought it was where I wanted. I can say that I am very happy with the sound coming out of my new sub. It's deep, it's loud and it's exactly what I wanted.