Altec 821A Cabinet Project

[rghinton]

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.

[misterdwayne]

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?

[rghinton]

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.

[rghinton]

These are the Altec 817's that I built for the front channels in my home theater.
Altec 817's

[Steve Schell]

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.

[rghinton]

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.

[Steve Schell]

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.

[misterdwayne]

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

[rghinton]

Thanks for the link to the photos. Pictures are worth 1,000's of words. This will help me with my project.

[rghinton]

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.