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I am starting to build a modified pair of Altec 821A cabinets. These are the cabinets for the Altec 820A systems. They will be used as surround speakers for my home theater. I will be building them at the local technical college. This way I will be able to work on them during the winter. I will try to document the project with photos. Todays photo shows the horns bottoms, tops, centers, sides and the baffles. Saturday I will start assembling the horns. I will use Gorilla Glue and nails. The sides of the horns are 3/8" bending plywood. The 2 layers of the bending plywood run the same direction making it easy to bend. The other plywood is 11 layer 23/32" Baltic Birch.
http://home.new.rr.com/timetraveler/...affelparts.jpg
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Please keep us informed as you progress. I have been wanting to build a set of these cabs for years but feel my skills and tools would make a poor cab. I just let let go of two sets of A7 cabs that I had stacked in my living room (pushed into the corners AKA "The Poor Mans 821A"). I miss the sound already,Duh.
Good luck DwayneB
PS ? What are the mods you are making? any drawings?
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The main modification that I am going to make is to move the bass reflex port holes from the bottom of the cabinet up to the horn area. The cabinets will be in the rear corners of my home theater and some loveseats will be about 5' below and in front the cabinets. The cabinets will be located on an 8" riser with the rear seating. I don't want the reflex portion of the sound to be blocked. I don't have the exact dimensions for this modification yet but will figure it out as I go. I am also going to do something different with the grill and also with the top and base of the cabinets. I will add pictures as I proceed. I got the plans from an old book by Don Davis named "How to Build Speaker Enclosures". 821A Plans Page1 and 821A Plans Page2.
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These are the Altec 817's that I built for the front channels in my home theater.
Altec 817's
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Reed, I wonder if relocating the bass port higher in the cabinet might negatively affect the low end response? At the floor, the port will see something like 1/8th space loading, while higher up it would be closer to 1/4 space.
Haven't studied the 821A in detail, but I suspect they were doing something quite deliberate with the low frequency loading. Those drilled holes in the bottom of the reflex volume combined with the exit path likely result in a carefully worked out ducted port design. In the catalog Altec refers to the design as a "...modified bass reflex principle."
When it comes to low frequency performance, a driver benefits from any loading it sees as a way to gain purchase on the air it is trying to move. This is why bass horns, though large, work extremely well. I wouldn't regard the 821A port as "blocked", but rather as designed to increase the loading at low frequencies.
This cabinet was designed long before Thiele/Small, and before ducted port loading was well understood. It may have been a trail blazing design. There was a lot of experimentation going on during this period, including the EV reflex boxes with short horns on the port and the Jensen Ultraflex. All were seeking greater low frequency extension from a given box volume.
Before committing your obviously fine craftsmanship to a finished pair of modified boxes, you might want to knock together quick examples of both stock and modified designs to test and listen to.
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Steve, Thanks for your thoughts. I like the shape of the cabinets and they will work well in my situation. I did not mean that the port is blocked by the design but by nearby furniture and things such as blankets on the floor. I am in no need of bass as I have 4 (four) 25 cu. ft. cabinets in the front of my theater with 4 Altec reconed 3182 18" sub woofers. Bill in OK City reconed them. He did a great job. I believe that I have 12 db of attenuation on them to keep them under control. I will be using speakers that I have on hand which will be 2 Altec 515E speaker, 1 Altec 288K driver, 1 511E horn for each system. Remember that these are surround only. If they were not partially blocked by furniture, I would probably stick with the original design.
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Hi Reed,
Upon rereading your earlier post I realize that I hadn't studied it enough, and thought you were trying to improve on the design. In light of your application it seems to me that your proposed changes should work very well.
I think that you will be pleased with the performance of your new surround speakers. The audition of a pair of 820Cs in the home of an ex Altec employee about twenty years ago knocked my socks off and set me on a course of exploring all the great old Altec and other industrial sound equipment. It has had a tenacious grip on me ever since.
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820 on Ebay with a good picture of the vent ports on this auction. uses 811.
I perdict it will go for about $1,300
http://cgi.ebay.com/ALTEC-LANSING-IC...QQcmdZViewItem
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Thanks for the link to the photos. Pictures are worth 1,000's of words. This will help me with my project.
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At last I have taken a couple of more photos. It was -11 degrees this morning when I took the pictures. The project is stored in the garage. The bending plywood works very well. It bends in one direction only and when glued and nailed into place the horn assemblies are as a molded unit. The lower photo shows where the Gorilla Glue seeped out of the joint. The glue is so hard that I will not clean it off. It adds additional strength. If anyone ever uses Gorilla Glue follow the instructions. You are suppose to wet the wood before you apply the glue. I tried it both ways (with wetting the wood and without). If you wet the wood the glue seems to really work its way into the wood. I cut dado groves in the sides of the horn so the top, center divider and bottom are inserted into the horn about 1/8 inch. I cut the braces between the horn and baffle slightly over-sized and then milled them smooth with the back of the horn using a router. The baffle then is perfectly flush with the back of the horn. This Saturday I will be attaching the bottom and cutting the holes for the divider that will be on the top of the horn. Remember that I will not be making the port at the bottom of the wooden horn assembly but at the top of the wooden horn assembly. The cabinets with the bottoms attached will be too big to put into my 4 door Saturn so I will probably have to tie them to the top of my car. I live about a mile from the school where I am building them.
http://home.new.rr.com/timetraveler/images/821A3.jpg
http://home.new.rr.com/timetraveler/images/821A4.jpg
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Those are lookin' good. Keep up the nice work, looks like fun.
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Q: What kind of saw did you use to cut the curves with?
Did you do the curves free hand or did you make and use a jig?
When I get the time I really want to make a set myself.
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Reed, I feel your pain. I have found that it is very difficult to cut curved panels on edge so that they are square to the straight sides, and also to glue up an assembly so that it comes out square and not a parallelogram.
When glueing up square or rectangular mouth horns, I have found it helpful to first make up a jig to ensure squareness of the mouth. The mouth dimensions are marked out on a flat board (hard enough to find!) and wooden strips located just outside the lines here and there. Then the assembly is glued up mouth down, and the jig forces it to be square.
In your pictures it appears that your curved panels (vertical) extend past the horizontal pieces. Perhaps you could leave them a bit long, and do the final trimming on them with a router after the glueing is finished.
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Hi there...
For the curved pieces try making a template. then use a router to cut the curves. Take your time, do it right and you will not have to put the ribs together and sand them to an even size. You can make the jig with your band saw or by setting up your router so it will swing and cut the radius you need. You can cut all the curved pieces that way or do one and then use it as a guide to cut the others with the router. Take you time. Measure three time, cut once.
To assemble the horns. Make a jig to hold the "ribs" on their front edges. You can use spacer pieces to keep everything aligned. Assemble the curved pieces to the ribs after they are in the jig and so the will not shift. When making the jig for the ribs, remember to make it so the curved pieces go past the edges. You can use a router to clean up the edges before attaching the baffle board.
There are many ways to skin the perverbial cat. My way is only one of them. Good luck.
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I used a large band saw to cut the curves freehand. After they were cut, I taped the six of them together with double sided tape and sanded them on an oscillating spindle sander with a 3" sanding sleeve. They turned out really nice. Now the bad news. I was not careful enough putting the horns together. I just noticed that they were slightly twisted and the front was not square to the top and bottom. I tried to work around the problem with shims but things only got worse. I finally realized that I was going to be off about 1/2". I thought about this for a couple of days and then went and bought more wood and am starting over. I set the failed attempt out for the trash man and they are gone. I am doing this as a hobby and I have learned a bunch. I will not make the same mistakes twice. In the long run I will be happy that I started over. I will replace the pictures when I get to that point. The only time that I have to work on the project is Saturday morning at school from 8:00 AM to about 1:30 PM. It takes a long time to build templates and jigs to assemble the project. I will spend even more time on this attempt. I will try to post photos of the jigs and templates that I make.
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Aaawww! Bummer.
Hey thanks for letting us know what happened.
If I do finally go through with making a set, at least I know one more thing from your experience and the imput of the others here Thanks!!!
DwayneB
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Thank you ------ Thank you very much!
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Good work. I'm sure you'll be pleased with the outcome. As for filling in the holes, why bother? Unless the screw went all the way through to where it will show in the finished product. I would be more concerned with the filler material coming loose and causing a "buzz" sometime in the far distant future.
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Reed, I've followed your work for a good long time and I just gotta second the notion that the build quality looks just beautiful. I bet it's a lot of fun too! http://www.hostboard.com/forums/
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The reason that I am posting the photos is to help others avoid the same mistakes that I made and give them some ideas. I will be cutting the wood for the other horn this Saturday at school. I will also cut some wood for external bracing of the horns. I will post photos as the project progresses.
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I am back. Winter always slows the progress on my projects. We have had quite a bit of snow this winter and it is cold so I cannot work on my cabinets out in the driveway. My space in the basement is very small and my time at the woodworking class at the Tech. School is limited. Remember that I have started over because nothing was square after my first attempt. This time I build forms to stabilize the horn assemblies while gluing. There is a stable base screwed to 2x4's. The forms are mounted to the base with 1/4" bolts into tee nuts. The holes for the bolts are slotted so that they my be adjusted to match the dado joints in the sides of the horns.
http://home.new.rr.com/timetraveler/images/form.jpg
I temporarily screwed the horn ribs to the forms. I chose this method instead of using clamps as the screws hold the pieces together better. I used 4 screws. One in approximately each of the 4 corners of the ribs. I will later fill these holes with wood filler.
http://home.new.rr.com/timetraveler/...ormandribs.jpg
I spread glue on one edge of each horn rib and placed the horn side onto the ribs. I nailed the horn sides every 1". Note that the horn side extends beyond the ribs. I will mill the excess off with a router.
http://home.new.rr.com/timetraveler/images/oneside.jpg
Detail of inside corner.
http://home.new.rr.com/timetraveler/images/corner.jpg
Finally a picture of both sides glued to the 3 ribs.
http://home.new.rr.com/timetraveler/.../bothsides.JPG
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http://home.new.rr.com/timetraveler/.../bothhorns.jpg
<center>Picture of both new horns.</center>
http://home.new.rr.com/timetraveler/images/side.jpg
<center>Picture of side of one horn.</center>
http://home.new.rr.com/timetraveler/...millededge.jpg
<center>Picture of milled edge.</center>
http://home.new.rr.com/timetraveler/images/edge.jpg
<center>Picture of top edge.</center>
The next step will be to secure the bottom of the cabinet and the center divider of the cabinet to the horns. I will then mill the side edges of the horns to be flush with the edge of the cabinets. I will take photos of the milling operation. I mill with a router. Each step is a new learning experience. I am pleased with my second attempt at making the horns. By bolting the pieces to the temporary frame and then gluing, everything is square. Note the excess Gorilla Glue. I am going to leave it there as it adds additional strength and it is on the inside of the cabinet and will not be seen. I added ribs to the rear of the horns to reduce the ringing of the horns. I will also insulate the back side of the horns with fiberglass.
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The weather is now nice again so I have started working on the cabinets outdoors at home. Yes that is snow in the background. I am still making the parts at school and assembling them at home. I glued the bottom of the cabinet to the horn assembly and the divider to the top of the horn assembly. I milled the front corners.
http://home.new.rr.com/timetraveler/...oremilling.jpg
<center>Front corners before milling. Bottom view of cabinet.</center>
http://home.new.rr.com/timetraveler/...llingsetup.jpg
<center>Temporary milling setup with router.</center>
http://home.new.rr.com/timetraveler/...ingcloseup.jpg
<center>Milling setup closeup.</center>
http://home.new.rr.com/timetraveler/...termilling.jpg
<center>After milling the corners.</center>
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How are they coming along. In your last set of photos the cabs looked like they were going to be great.
DwayneB
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The cabinet with all of the bracing installed. Note the cabinet is upside down in this picture.
http://home.new.rr.com/timetraveler/...upsidedown.jpg
This is a photo of one of the walnut sides being glued onto the cabinet. Guess what is in the cardboard boxes that I am using for weights.
http://home.new.rr.com/timetraveler/...walnutside.jpg
This is a photo of the tops.
http://home.new.rr.com/timetraveler/images/tops.jpg
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Lookin Good!!
Whats in the boxes?
A: 288's http://www.hostboard.com/forums/ Y/N
DwayneB
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Correct. 288K's. They are the heaviest things that I could think of to hold the wood down as it was being glued.
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I work on the cabinets every day. I will upgrade pictures in a week or so. The walnut tops are on with the walnut edging. Walnut is tricky to work with. It is tough using a router on the end grain. Keep the router moving in a forward direction only. Do not pull it back. If you pull the router back it really chews the wood. I only did this for a short distance. I found a good wood filler to fix the problem. I am working on the horn and driver mount. Every thing is coming together and I am pleased with the project. I have learned so much building these 2 cabinets. Thanks for asking.
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I am working on the grills which are not in the picture. They are cut out of 1" plywood. Actually the plywood is 3/4" thick with 1/8" particle board on each side. Anyway, here is a picture of the cabinet as of today. I still have to build a base.
http://home.new.rr.com/timetraveler/...rontwalnut.jpg
A closeup of the top edge.
http://home.new.rr.com/timetraveler/images/topedge.jpg
The backs are built but have to be finished.
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Sweet! Looks like you have a good start on a second career!
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Drool....slobber....sigh http://www.hostboard.com/forums/
I find myself dreaming of another day and time, where our factories reliably turn out such craftsmanship, and at a price that we can afford...sigh
Beauty!
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OK, I am finished. I slid each cabinet down the stairs to the home theater. I had to remove the trim from a door to get them in. I took the 4 backs and 2 grills down the stairs. I got the 4 515E's, 2 511E's, 2 288K's and 2 crossovers and put them all together. I can say that by having basically the same systems for surround in my 5.1 system that everything seems very well blended.
http://home.new.rr.com/timetraveler/.../FullFront.jpg
http://home.new.rr.com/timetraveler/images/FullBack.jpg
http://home.new.rr.com/timetraveler/...houtbaffle.jpg
http://home.new.rr.com/timetraveler/...rillCorner.jpg
http://home.new.rr.com/timetraveler/...ckofbaffle.jpg
http://home.new.rr.com/timetraveler/...utspeakers.jpg
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Only one thing to say ......WOW!!!!!
(only thing wrong with them is that they are not sitting in my music room)
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Very nice work Reed.
Makes me ashamed of my "boilerplate" looking stuff. My stuff works, but pretty it ain't.
Kinda like me I guess. http://www.hostboard.com/forums/
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Outstanding, Reed!!! The walnut is gorgeous. I'm planning on a similar style cabinet which will look a bit more like a Tannoy GFR corner, but modified to be a 9 cu-ft bass reflex design using the GPA 604-8H driver instead of a rear-horn loaded design. It will look very similar to yours, but with some detail (dummy vents) on the vertical side panels.
http://www.44bx.com/tannoy/grf.html
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Reed, your finished cabinets look outstanding! I even had my dad, who is visiting, looking at them, and he complimented your woodwork, and he doesnt compliment many peoples work.
Enjoy, they look incredible!
http://www.hostboard.com/forums/
