How to build an omnidirectional, fullrange speaker (HxWxD=100x18,4x22,4cm) without crossover-network?. This project is inspired by the Walsh-driver from the seventies and especially by the DDD-driver from German Physiks. (see links elsewhere on the Internet). This driver is very good, but expensive (about 2000$ per driver). Conus 1 will not ruin you since it will cost you 500 Dkr (85$) a pair to build- cheaper if you use recycled materials. Hence the "folk". To make Conus 1 a fullrange speaker I have built it into a Voigt quarterwave "horn" after an article on "tapered pipes" by David Weems in the American magazine "Speaker Builder" # 2, 1987.Other horns (like the Ace-horn) or transmission-lines (see for examble John Cockroft's "Freeline" in SB # 5, 95) are possible, or you could use Conus as trebble/midrange-driver in a boxsystem. Or you might ad a trebbledriver if you need more air.
Anyway, Conus 1 has now been playing for a couple of months at my home and has been OK'ed by my wife and my teenage son. This doesn't mean that the project is finished. I hope others will see a challenge in attributing to this speaker. Once before I have tried this type of project: around '88 I suggested a kitchen-bench ribbonspeaker a la Apogee called La Folia. Quite a few people worked on this speaker and shared their ideas for mutual benefit. Today I have a good ribbonspeaker, but all the same, I felt the time had come to try some new ideas. The Conus 1 isn't a better speaker to my ear. It's different and has some strong points mixed with weaker points. Its smaller, it's more efficient than the ribbons which is good. Its better in the timedomain and soundstagingwise which contributes to an exiting feeling of "being there". It does not have quite the same myriad of microdetails as the ribbons. It dosn't play the last octave. You can't play very loud music because of the full-range concept; it will rattle if you incist. Its no partyspeaker, but it plays loud enough for my 15 m2 room.
1. The Conus driver:
Rebuilding a driver. Take a full-range driver. Maybe one that has been thrown away for some reason other than a fault in the voicecoil. I guess the best is a Lowther driver because of the sophisticated magnetdesign. Or a Coral. Or a Fostex. Fullrange drivers are rare today, because of the fall in popularity of the horn-speakers. I used a Monacor SP-155X. Mostly because it is inexpensive and I didn't know where my experimenting would lead. I don't know how much difference the quality of the magnetsystem makes compared to the quality of the rest of the speakersystem: diaphragm, spider, sorround etc. Probably a stronger more accurate system like Lowthers' is better. Well, it sounds good as it is so I've stuck to it. So far. (some spec. from the factory SP-155X: z= 8 ohm; 50-20kHz -I wonder; SPL= 93 dB; voicecoil induct.= 0,4 mH)
Cut off the diaphragm as close as posible to the voicecoil and free the surround from the metallic frame. Use a very sharp knife. The cables glued/painted on the diaphragm is easy to remove if you heat it with a soldering iron. Beware of toxic fumes, open windows etc. The flexible cables from the speakerterminals must be soldered as close to the voicecoil as possible. Otherwise they tend to rattle with pianomusic, guitar etc. You now have the "motor" for conus 1.
How to make the cone. Use the draft above: draw a line, define a center C on this line, draw a circle (r=8 cm). Draw another circle (r= 24 cm). Now draw an angle of 56 degrees centered in "C" using the line as the left leg. The right leg of the angle crosses the circles as seen on the draft. It's not as much trouble as it sounds. Maybe you want to experiment with another size; another shape of the cone. These measures gives a rather narrow cone ( 9 degrees from vertical). If you use an angle of 94 degrees and r=4,8cm for the first circle the cone will be wider - more like the DDD-driver - 14 degrees from vertical. I've used the narrow cone. The height of the cone made in this way is 16 cm. A note: if you insist on inches divide cm with 2,5.
The material: I used metalic paper like the one you use at X-mas. (Exp. with other materials). It is a laminate of aluminiumfoil and paper. Buy it, say at the bookstore. First you want to make a template in cardboard or thick paper. Then I suggest you practise making a cone of paper. This papercone is handy to protect the alucone when you handle it and it is also good when you repair dents in the cone. Then you are ready to use the template with the alupaper: it's quite easy to cut it with a pair of scissors.
Glue the straight sides of the conedraft together. It ought to look like the picture above. Overlapping just 2 mm. Be precise. Ordinary paperglue will do. Now glue the cone to the voicecoil. Use contact-glue. Be very precise. It i s a little delicate to make it stand straight up and you only have one try if you use contact-glue. Well you can always peel it off if it didn't work, remove the old glue and do it again. Be careful and make sure the cone is fastened well. Otherwise it might rattle. Make sure it doesn't hit anything "inside" the voice-coil, when it moves.
Decide if you want to use the frame (basket) of the driver. I did because it was easy, but it doesn't look so good and I'm sure there are refractions from this metallic frame. It is easy on the Monacor to cut it away using cutting nippers. Afterwards you must design some "holdingsystem". You could use a piece of flatiron - larger than the diameter of hole in top of the speaker - with a hole drilled at the ends. Glue the iron to the magnet of the speaker. The rest is obvious if you follow the illustrations, I guess (use longer round sticks).
2. The Voigt Horn.
As I mentioned I have used the Voight quarterwave "horn" described in SB # 2,87. Slightly modified because of the dimension of the 12 mm particle board I had at hand. You can see the original dimensions in Weems article( I used 78 cm in stead of 90 cm). This article also deals with some of the theory and is hereby recomended. The boxbuilding is straightforward if you follow this drawing and the pictures. Notice that "a" covers the whole front, "b" is caught in the middle if you know what I mean. Drill holes for screws to fasten the boards together. Use lots of glue (for wood) to make the box airtight. Drill holes in the round sticks to make it easier to fasten them.
a =18,4 x 72,5cm
a (back) = 18,4 x 78 cm
b = 20 x 78 cm
c= (inside the box) 16 x 63 cm
x (defines where c is fixed to the side)= 10 cm
top = 18,4 x 22,4 cm
bottom = 18,4 x 29 cm
port= 5,5 x 16 cm
hole in top: diameter= 11 cm
2 broomstickpieces each 14 cm (or some thin round sticks).
Alternatively you could make the "box" from a cartboard/millboard tube. Accoustically this is better than the sharpedged box. And it's cheap + there's less work. I haven't tried it yet, though. Diameter d= 20 cm. Height a prox. 80 cm.Fit the tubedivider (63 x 20 cm?) inside the tube from just above the hole in the front to the middle of the tube 10 cm below the hole in the topplate. You'll have to figure out how to form the divider after the tube. Tops with same diameter can be bought.
3. Fitting the driver to the box
The driver should be fitted in an airtight manner to the box. That was a hard one for me to figure out. You have to make some kind of surround. I couldn't get hold of the rubberfoammaterial thats used on most speakers. Instead I have used some thin, airtight nylonmaterial or some other fabric like that (leftover from the sewingbox). The idea is to continue the cone with this softer material into the hole in the topboard and glue it to the side (best results maybe if the hole is coneshaped too. I'm too lazy for that perfectionism). Glue the nyloncone to the alucone using contactglue and then glue it to the tophole as tight as possible. Leave some material loose so that the cone can move freely. If you glue it carefully it shouldn't rattle. You should fasten the topplate with screws, because you might need to work on the cone from the inside of the speaker. (I must admit that I haven't done this airtight fitting yet - probably because it plays well without it. But this must be an improvement).
Trebble-lift:You might want a little more "air" in the sound. I used an RC-circuit in series with one of the speakerterminals: the R=2,5 ohms and the C= 5,6 mikropharads.Use a good quality capacitor. This circuit damps the signal relatively (highpass-filter) by damping higher frequences less from (160 x 1000)/(2,5 x 5,6 )= 11,5 khz. Substitute other values in this formulae and listen. At the moment I use 5 ohms giving a lift from 5,75 khz up.
Stuffing: The box must be damped with stuffing material to get rid of boomy (and other) resonances. The best choice is wool I think. You might be able to buy it inexpensively in a shop for knitting-materials?. Or you could use some polyesterfilling (like the one you use for teddybears and cussions) as I have done. I used aprox. 4 onzes pr. box.
If possible you should use a multimeter in the proces. To see whether there is connection to the voicecoil. And to make sure there is no shortcuts when you connect the speaker to the amplifier.
Rattling sounds can be tricky to get rid of. 1) Maybe the cone isn't glued tightly to the voicecoil. Take it off and glue again. 2) Maybe you have a cold connection to the voicecoil. Clean the cables and solder again - close to the voice coil. 3) Maybe the cone touches something. Clear around it. 4) Maybe the nylonsurround isn't glued tightly to the cone. Redo it.
Eksperiment with the stuffing. The amount depends on the room, on the actual shape of the driver, and the box as you have made it. Also the original Monacordrivers are by no means exactly alike. You can compensate somewhat for this through stuffing, but also through the amount of surroundmaterial glued to the cone.
You might want to change the voice-coil to a lower inductance which should give more air in the top.This means you have to make a new voice-coil. Let me know how you do that. I want to try using narrow ribbons on a voice-coilframe and get a very low inductance. Also you might want to make changes to the spider to lower the suspension.
I would like very much to hear your progress. Send an E-mail to me: firstname.lastname@example.org.