LA FOLIA MAGNETOSTATIC RIBBON LOUDSPEAKER DEVELOPMENT PROJECTThe original building recipe, modified by Mogens Gallardo© 1996-2004. All rights reserved on text, photos, diagrammes, etc, unless otherwise stated. |
In its original design, the loudspeakers are very ineffective (70-72 dB SPL). The hardest part is to get the materials, depending on how much time you use on experiments with the magnets, the membranes, the MDF wood plate design, the membrane design, etc. The basic idea behind the project is a concept used by other very famous speakers in the Hi-Fi World, Stratshearn, Fostex, Magnepan & mostly Apogee. It is a simplified version, in many aspects, of Apogees two-way speakers. Carver together with a friend of the original danish writer of the project, Ole Thofte, have produced a speaker with the same principles in the "La Folia" project. Step by Step1- Cut the wood pieces, one the mirror image of the other. 2- Resonance damping of the woofer hole of the MDF plates, just the top and bottom side of the hole, don't choose a very thick sponge, it'll be very hard to stretch the membrane if its to thick, and wrinkles will be an unavoidable part of your design, not so nice... Cut the tweeter hole considering the magnets dimensions, the metal plates that cover the inner side of the hole, etc. 3- Arrange the metal plate design, cut a piece either to fit the woofer hole or to be screwed directly to the back of your MDF Plate, you find out what is best for your particular case. The tweeter hole needs a piece on the back side too. 4- Put the magnets on in the desired design, all with due consideration to the conductor design you've chosen, the length of conductor you need, the resistance, the impedance (something I never considered, I just measured the Ohm resistance, and calculated it to be 3 ohm). If you're not as unlucky as I to have very weak magnets, there should be no need to stick them on to the plate, their magnetic strength should be more than enough. 5- Design and create the membranes, more about that in the membrane section, and the possible betterings of the design. SWEAT, SWEAT, this is the tough part, so I feel... 6- The row of magnets in the tweeter section closest to the woofer must have the same polarity as the top row of magnets in the woofer section (important!!). The tweeter magnets are double, boosting the magnetic field strength. 7- The conductor design: This speaker system uses the collective principle. The current must flow in the same direction for all conductors in the same magnetic field. With respect to the woofer conductor, it's necessary to have an unbroken conductor, that covers the membrane so it moves as a single flat surface. This design has two conductor strips, in a serial disposition. The way the conductors transmit the current is always in the same direction and this creates a force that is either inward or outward, deppending on the current direction with respect to the magnetic field. (In my project I used aprox. 68 windings of the conductor). The total length of the aluminium conductor in the original project was 18 meters x 9 mm. (30 micrometers in width) and having a 3 ohm resistance. The tweeter conductor was originally built with four conductor coils, as shown in the figure, this makes the resistance larger, the conductor is 9 meters long and half the width, 4.5 mm, thus 3 ohm resistance. The membrane should be corrugated, (now that I think about it I don't know why...) thus the length shrinks aprox. 10%. 8- Crossover and speaker connections: The woofer and tweeter are connected in parallel. Serially connected to the woofer you put a coil (0.95 mH), and serially connected to the tweeter a capacitor, 105 microFarad. The speakers impedance is aprox. 3-4 ohms, very frequency linear. 9- The speaker feet, build them either out of the same wood, held by strong shelve angles or build a more sophisticated design. 10- Check if things are alright. Check out if there should be any short circuits with the multimeter, check if there are any missing connections too. the resistance measured at the speaker connections to the amplifier should be aprox. 3,5 ohm. Now connect the amp............ See what can happen, if all is well I assure you you'll have a hard time erasing the wide and shiny smile you'll have on your face, once you've constructed such a beauty out of such lowly materials, you'll need to do a number of adjustments to the sound balance, I had no spectrum analyser, I just used my ears, the best instruments I have at my disposition, adjust the balance between the woofer and the tweeter by putting more or less resistance on the tweeter, this will crave either more or less current from the amp, thus having more or less treble, balancing the sound image accordingly, I know this is quite artisanal but so is the whole loudspeaker project!! :-). 11- HAVE LOTS OF FUN, I DID, AND STILL HAVE 8 YEARS LATER. |
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