BR928

Dados to hold the sides are cut.

Next a prefinished blank is secured to the fixture.

The angle calculated from ALK's program for a 7"x20" horn is 23.4 deg. Half of that is 11.7 so that is the angle of the fixture.

With the cabinets about finished and it's time to build the tractrix horns. All of the problems were figured out building the prototypes so this should go pretty fast. I thought I would share the steps I used to build them. The parts are all cut on a CNC router. I first built a fixture to hold the material at a fixed angle while they were cut. This was done to keep from having to cut compound angles, which a 3 axis CNC can't do.

I put some color on the cabinets. Only one coat of tung oil so far. I hope to start assembling the tractrix horns this weekend.

Starting to take shape.

I got the motorboards attached along with the ports. I painted the interior of the ports to get a better contrast when the enclosure is finished. I used threaded 1/4-20 inserts so I can mount the woofer internally for a cleaner look.

Buscuits and pocket screws with butt joints make assembly simple and easy to keep square.

After edge banding all the plywood I can get started on the cabinet assembly.

I have a ShopBot CNC w/ 5HP Colombo Spindle. The sides are oak plywood. The wood is cut, steamed for an hour and bent over a form over night. There is some springback but that is easily overcome when the sides are inserted in dados in the top and bottom.

The sheet was screwed to the spoilboard. The larger pieces that that I could not put holes in were just cut free. Smaller pieces that the dust collector would pick up were cut with tabs to hold in place. The front will be curved that the grill cloth will follow.

Where did you get the dampening material? What material are others using?

I received my ALK crossovers so now the ball is back in my court. I finalized the design and was able to cut the parts for the enclosure out today. It may be next week before I can start assembling them.

I am ready to build my Cornscala enclosures. I have downloaded many versions of Cornscala enclosures. Each one has somewhat different internal volume, port volume and port opening. Using a K-33E or CW1526 what should the internal volume of the enclosure be? Most have around 6.5 cu ft. What should the port volume be? Most have around 650-715 cu in. What should the port opening be? Most have around 48.7-49 sq. in. Tuned to 30- 40 Hz? How critical are the dimensions and what side should you err on?

Another Sherlock Holmes! Very good Evan. You can listen any time you want.

After chatting with a few people I have decided to stay with a standard Cornwall II enclosure with a slotted port. To get the wooden tractrix to fit inside I will have to make the enclosure about 3/8" deeper. That would add about 300cu in to the internal volume. The horn probably will take up that much more space so I figure it's about a wash. How critical is it to keep the original volume?

Thanks Dave. I remember discussing this technique with you early on.

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I have been lurking on the forum on and off for years with special interest the the custom speaker builds. I have been building speakers since the 70’s when I build a set of K-Horn clones from some planes that were published by EV in the 50’s. If I recall, those bass bins had close to 80 pieces in them. My last build was about 8 years ago with some la scala bass bins, EV511B/Atlas squawkers, EV T35 tweeters and ALK Universal crossovers. The bass response was lacking so I augmented that with a RSW 15 subwoofer. That filled it in nicely. These speakers weigh a ton. I moved them once for a party and swore I would never move them again! A friend of mine recently rekindled my interest in new building project. Naturally I came back here for some inspiration. After reading the forum for days the most promising seemed to be the Cornscala configuration with a wooden tractrix horn. The hardest part was the tractrix horn so I started there. Using the forum, I found ALK’s trachorn.exe program to calculate the dimensions. I decided to go with a mouth of 7” x 20” and 1” driver since that seemed to be one of his proven standard horns. After looking at the others construction technique, I decided I would try something a little different. I have a CNC router. I cut the pieces for la scala I build with this 8 years ago. Using the ability of the CNC to cut in 3D, I cut the top and bottom plates with a dado to accept the flared sides. By placing the top and bottom plates on the CNC at the proper angle, cutting the compound angles is eliminated. By using the 3D cutting ability, the round to square transition piece between the driver and mouth was made. At this time I discovered that the exit of the Atlas PD-5VH & the EV K-55 drivers were ¾” and not 1”. The throat of the K-400 and the adaptor for my 511B horns are also ¾”. To use one of these drivers I would have to make a round to square adaptor almost 2 ¾” thick or redesign the horn from scratch. A suggestion to use a true 1” driver was taken and I bought the JBL 2426H. The next step is the bass bin. I like the Cornscala idea if I can get good bass response from it. The consensus on this forum says it will. I want to keep these as small as possible, for a large speaker, so I want to stay away from the split design. By staying with the original Cornwall dimensions, or close to it, I can mount the tractrix horn lens through the cabinet and mount the driver externally. The round to square transition piece of the horn would actually be part of the rear wall of the enclosure. If the speaker does not need to be flush against the wall I see no problem with this approach.

I just acquired a 4 way MCM 1900 system. Can you tell me the date of manufacture? The serial numbers are as follows: MWM S 1001,1002,1003,1004 MSSM 1137,1138 MSM 1137,1138 (Optional Gauss HF-4000 Driver) MTM 1411,1412 CROSSOVER 1003,1004 The date on a woofer I checked was 67-8607. Figure that is 1986. Have the MCM's always used piezoelectric tweeters? The tweeter arrays were disassembled and one of the 8MFD caps has been changed. Are all of the tweeters paralleled? The barrier strip only has a spot to terminate 4 tweeters. (i.e. 8 point terminal strip) Do I assume you just double up on one them? Is a replacement 8 MFD 100V oil filled cap available? Thanks for your help. Stan