Build log: a pair of AutoTubas

Last year I built a THTLP sub for Burning Man. It works great there, but I'm going to a regional burn and it doesn't fit in my car. So, I'm building a pair of AutoTuba subs to take instead. I have some left over plywood, an amp, and the drivers are only $30 each. Plus, I like building stuff that doesn't have to look pretty enough to put in a house. I've got a lot of parts for some new main speakers for my living room, but I think I'm afraid to get started because I want those to be pretty. This build will get me building again, and will be a nice distraction.

Tonight I worked out my cut list, and did enough layout and cutting to get the first panel glued into place. I'm trying to do this fast, but it is my nature to work slowly and methodically. I spent 4 hours tonight.

After I cut the side panels, I used a circular saw (with Eurekazone track) to cut the access panels on one side of each sub.


My jigsaw blade was too wide, so I used a Japanese flush cut saw to finish the cuts in the corners:


Here's the first panel glued up and clamped, and you can see the layout marks on the side panels:


The clamps just need to keep the boards straight while the glue sets. They aren't very tight. A straight piece of angle iron is really useful for gluing joints like this. PL Premium glue is great for this application -- it expands to seal the joints, and can even fill gaps if you get sloppy with the cuts:


I'll keep updating the thread as I do more. I'm trying to get both done in 2 weeks.
Edited by maxcooper - 10/4/12 at 1:23pm

I'm using this 8" driver: http://www.mcmelectronics.com/product/55-2421

The BFM designs site has an SPL chart for one inside a vehicle: http://billfitzmaurice.net/autotuba.html
More graphs: http://billfitzmaurice.info/forum/viewtopic.php?f=12&t=4420

My primary goal is to have bass for dance music in an outdoor environment with low power requirements. It isn't a huge crowd (way less than 100, unless the whole event population ends up at our camp), so massive output is not necessary. And it needs to fit in the car (a Jeep Grand Cherokee). And I don't want to spend a lot, so using wood that I already had and inexpensive drivers was perfect. I have no plans to EQ it outside, but I'll probably end up running them in my garage with EQ.

-Max

I'm waiting for the drivers to arrive before I cut the hole and install the Hurricane nuts in panel #2. So, to make progress anyway, I cut all the panels today. I thought I had another sheet of suitable plywood, but it turned out to be the wrong size. I got another sheet of "outdoor" 5-ply plywood from Lowe's. It isn't Arauco, since that is no longer available, but the stuff they had seemed good enough so I bought it.

I am enjoying the build process, and spent another 3h53m today. I'm tracking the time carefully since people always ask me how long my projects take and I never remember. I can probably glue panel #3 in place, and cut the braces and access panel, too.

Tracking my hours carefully, here's what I've spent so far:
10/03/12 - 1h51m
10/03/12 - 1h26m
10/06/12 - 3h53m
running total = 7h10m

I got the drivers on 10/09/12, so I cut the 7-3/16" driver holes with my router and Jasper jig that night:


Next I marked the holes for the fasteners. I first drew a line through the center of the hole on the board, and then placed the driver face-down and lined up two of the holes. This doesn't really matter, but it keeps the bolt pattern square with the board. I centered the driver on the hole by feel, using my fingertips to ensure that the gasket edge was lines up with the edge of the wood on both axes:


I marked the holes with an awl. Poking with hand pressure is enough for the soft plywood:


I checked the size of my 10-32 Hurricane nuts and used a 1/4" drill. I started in the drill press, but my dinky drill press doesn't have a deep throat so I ended up doing the rest with a hand drill. At this point, I also realized that I cut my driver holes 1/2" off center in the panels -- which I decided was a good thing so as to not anger any gods by creating something too perfect.

I mixed up a little epoxy and put some on each hurricane nut, being careful to keep the epoxy off the threads. I double-checked which side the driver went on, and wrote it on each board to ensure that I was installing the hurricane nuts on the right sides. I then tapped them in with a hammer.


I then temporarily installed both drivers to ensure that the threads were good and that the holes are in the right place. Unfortunately, one of my hand-drilled holes was a little bit off, but it was close enough that I just filed the side of one of the holes a little bit in the driver frame to make it fit. I also spun two hurricane nuts, which was super annoying. I decided to let the epoxy dry and see how it holds. The driver has a thick foam gasket, so you don't really end up tightening the bolts very much, so hopefully I won't be stressing them too much. And I don't really plan to take the subs apart anyway. I removed the drivers to let the epoxy dry -- it would suck to accidentally epoxy the bolts in place.

Then I glued one of the driver panels in place:


That was it for 10/09/12. I spent 2h55m, bringing my total build time to 10h5m

On 10/10/12, I started by inspecting the alignment of the driver panel I glued the night before. Unfortunately, the top of the panel shifted, so it was glued in the wrong place where it met panel 1:


I broke the joint using a clamp to pull the panel, and a chisel to break the joint loose. I kept the chisel in there to open a gap, and I pushed fresh PL Premium into the gap and sort of "caulked" the joint on both sides with my gloved finger. The joint won't be as strong as it would have been if I got it right the first time, but I think it should be strong enough, and it is essential that it is air tight.


I used my brad nailer to pin panel 2 to panel 1 in the correct position this time.


I forgot to take a picture of this for the first sub, but it is really helpful to draw a line before you glue, so you can easily line things up visually:


Then I glued panel 2 in place on the second sub, this time being much more careful to keep everything aligned. I used the brad nailer for the panel 2/1 joint, and I remembered to use wax paper to keep the glue from sticking to my angle iron guide, too.


I spent 1h5m tonight, bringing my total build time to 11h10m at this point. That may seem like a long time, but I like to be sort of meticulous (even though I still make mistakes!), and recall that I have already cut all the main panels (but not yet the braces) for both subs.

-Max

Hey Andreas!

The boards are 15.75" wide so that I could get 3 big strips from a 48" wide sheet of plywood. That leaves a little slack for saw kerf, and so I could have the store do one of the cuts (with low precision), thus allowing me to fit the wood in my car. :-) The total width of the sub with the 1/2" sides added on will be about 16.75".

-Max

A Behringer A500. I won't actually be using mine in a car, but rather outdoors in the wilderness, so I'm using the A500 that I already own, and powering it with a generator.

I did consider double-loading, but figured that I would get more output using two 16"ers than one minimum-width double-loaded AT. And I get more flexibility if I ever just want to use one (like to put in my car, or if we overpack and end up only having space for one ). I do plan to V couple them, or whatever configuration works with the AT, at the event. I've seen how to V-couple some of the other subs, but I still need to research what works for the ATs.

I know my requirements are unusual, but for purposes of running subs outside on small generator power, sensitivity is paramount. Getting bigger generators gets expensive very quickly, and transporting gas is a pain. The environment is hard on electronics, too, so using inexpensive, low power amps is also a goal. Having the sub driver deep in the enclosure makes the sub rugged and easy to transport, too. I also think that horn subs are interesting and fun to build.

My total spend so far is about $107 for both (drivers, shipping, plywood, glue), not counting stuff I already had. I don't think I need anything else.

-Max

Build log update:

I glued panels 3 and 4. This pic shows one of each:


I glued panel 5:


And installed the 2/5 panel brace. It's hard to keep the glue where you want it while sliding the brace into place. The glue looks gray rather than beige because I am trying PL Premium Fast Grab rather than normal PL Premium, since I'm trying to glue up lots of panels this weekend. It is nice that it grabs better than PL Premium, which, for instance, makes installing some of the braces simpler because you can just glue it to the first panel and then put the second panel in place. The brace will stay put, and won't wander as it dries. The Fast Grab is thicker, too, so the glue and the panel slide around less, but you can definitely still reposition as needed. Fast Grab doesn't squirt out of the tube when you want it to stop as much as the normal stuff, either, which is really nice. It is very easy to make a nice bead and then stop when you want to stop. Fast Grab is stronger than normal PL Premium, but the normal stuff is plenty strong, and that means Fast Grab might be more trouble if you need to break a joint and reglue. I think normal PL Premium might expand more, and it is also cheaper. I'm sort of undecided which one I like better. I still have a big tube of normal PL Premium left over from my last project, and the glue inside is still good, but it is a pain having to poke the nozzle all the time, and getting inconsistent feed rates -- so I bought another tube, and this time I got a smaller one.



Next I glued panel 6:




My total time spent so far is now up to 16h05m. I have until Wednesday night to finish.

-Max
Edited by maxcooper - 10/14/12 at 5:25pm

Wow, two weeks went by just like that!

My plans changed when we realized that we weren't going to have enough room, nor any real need, for a sound system. So I didn't finish the AutoTubas and take them to the event. The event (Youtopia in San Diego) was really fun and crazy. And we were camped near enough to a massive sound trailer that I went to sleep each night with bass shaking my tent quite thoroughly.

But now I'm back, and I'm going finish the AutoTubas since I ended up putting the THTLP I built in my living room rather than the garage as I originally intended. I can use the AutoTubas in my garage! And it would be nice to have have some subs that I could fit in my car and take to an event just in case I need to throw a dance party somewhere. It could happen!

I glued up panel 7 on both, which went fairly smoothly. I like to use brads to tack the edge in place, tack a scrap across the top to keep the panels square, and then squeeze the long edge together gently with the angle iron and light clamp pressure.


And I glued up panel 8, too, using a similar technique: brads to keep the bottom in place, temporarily squeeze the panel down with light clamp pressure, and then put brads in the other edge. With the brads all in place, I then clamped a square brace on the corner, and used some scraps and more clamps to gently squeeze the rest of the edges together:


Total time spent so far: 17h29m, and enjoying every bit of it. I can't wait to fire these guys up and see what the little 8" drivers can do.

I am using my Behringer A500 amp for my THTLP in the living room. I need another amp. My THTLP uses an MFW-15 driver, which I think could use up to 300 watts, where the A500 can only make ~240w @4ohms or ~130w at 8ohms. So I might power the AutoTubas with the A500 and get a new amp for the THTLP. The MFW-15 is a 4-ohm driver, but I think the horn makes the impedance higher, or something like that, so it is only getting somewhere between 130w and 240w. What amp can do about 300w @8ohm and is efficient so it can run off my little gas generator? I'd like to get a stereo full-range amp, rather than a mono sub plate amp, for flexibility.

-Max

Hey Andreas. Glad you got started.

I need to update my time tally and post some more pics, but I am almost done. I just need to glue the side on, do a little surface prep, and then paint. I expect to make some noise with them before the weekend is through.

-Max

Wow, you move fast. But are you really enjoying the work? I think my working style is heavily toward the stop-and-smell-the-roses end of the spectrum.

Okay, picking up where I left off...

On 10/28, I spent 1h46m cutting the remaining braces (1/8, 5/9, 6/9) and glued panel 8 on the second sub. Later that afternoon, I spent another 1h15m gluing panel 9 on both subs, bringing total time to 20h30m. I used the brad nailer to keep the edges in line, and a few plywood scraps to hold the panels in the right place at the free end. Fortunately the boards were pretty straight, so this all went together pretty easily. I ran out of glue, so I switched to a tube of regular PL Premium that I had. I think regular PL Premium might be better for sealing up the edges, as it stays more flexible than Fast Grab. So, having used both regular PL Premium and Fast Grab, I would recommend regular PL Premium as the better all-around choice for this kind of work. However, Fast Grab is really nice for gluing the braces, since the parts slide around much less (though you can reposition if you want to). One other tip about the braces is that it is smart to measure and draw a line on the main panels where the brace should be. That way you have a reference to keep it straight and level as you are gluing it. Draw lines on the panel at both ends of the brace so you can get it square. For the 5/9 and 6/9 braces, I just used the glue sparingly and gently wedged them into place. Maybe I put a brad in, too, to keep them from sliding down. I can't remember. Pics of gluing panel 9:

Haha. I'm not sure what I'm going to do yet. I considered painting them pink like pigs and having a curly speaker wire sticking out the end like a tail. But good sense took over before I bought any pink paint. I still have a bunch of white Duratex, so I'm thinking of just painting them white. Maybe I could do some kind of Banksy-style stencil over the white Duratex: http://goo.gl/gBvf9

On 11/2, I cleaned the extra glue off the joints near the access holes, to prepare for installing flanges. I use way too much glue, but I'm paranoid that I'll put it all together and end up with leaks. The drivers that I am using are not too deep, so I was able to just glue wood around the access hole to create a flange. I used various pieces of scrap wood that was all generally about 3/4" thick and should be able to hold screws well. Pics, including one from the backside, showing the full horn path:





Man, I always use way too much glue!



That all took 2h16m, bringing the grand total to 22h46m.

On 11/3, I used my router with a 1/2" wide square plunge bit to clean the excess glue off the flange edges and gasket surface, which also had the effect of making the gasket surface a precisely even depth all the way around (or at least no high spots). I clamped a piece of wood to use as a guide for the first edge, but ended up just free-handing the rest. It was easy enough to control the router and not cut into the side of the recess.

I checked to make sure that I could fit the driver through the flange, and found that I had to remove a little material with a file to provide clearance.





I used disc sander even up the edges on the access panels. The thin flush cut saw that I used to finish the corners left some material to remove. After sanding, the panel gap was even all the way around, and I rounded the corners ever-so-slightly. After the sanding, the panels fit perfectly into the access holes.



Then I painted the inside of the horns to the first turn. I remember seeing that someone did that with their DIY Sound Group Anarchy Tapped Horn build on here, and I forgot to do it on my THTLP build. It is still pretty hard to paint with the braces in there, but I did my best to smoosh a foam brush around in there enough to put some color on the walls.



That all took 3h18m, bringing the total time to 26h4m.

On 11/4, I realized that I forgot the paint the inside of the side panel that I had yet to install. So I used a pencil to draw lines connecting the marker holes and painted that area of the panels. I forgot to put this in my log book, but it only took a few minutes.

The instructions tell you to consider installing an access panel on the second side of the sub, but I had already verified that I can install the driver through the single access panel, so my subs will only have one access panel.

After the paint had dried for several hours, I glued the final side on both subs. I inspected all the edges and cleaned some glue blobs off with a chisel in a couple of places. Then I laid down a continuous bead of glue on all edges, and carefully lowered the panel into place. I squeezed the panel down into place, and used the brad nailer to keep the edges lined up.



Gluing the sides took 32m, bringing the total to 26h36m.