The original La Folia Magnetostatic Loudspeaker recipe (plus modifications)

In its original design, the loudspeakers are very
ineffective (70-72 dB SPL).

The most complicated part of the whole project is to get the materials and also depends on how much time you use experimenting with magnets,
the membranes, the MDF
wood frame
, 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 Apogee’s 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 as in the “La Folia” project.

Step by Step

1- Cut the wood pieces, one the mirror image of the other.

fastgori2- 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 taking into consideration the magnet’s dimensions and the metal plates that cover the inner side of the hole, and so on.

3- Prepare 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 improvements 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.

design7- 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.

In the case of the woofer conductor, it’s necessary to have an unbroken conductor, that covers the membrane, this way it moves as a single flat surface.

This design has two conductor strips, placed in series. The way the conductors transmits the current is always in the same direction and this creates a force that is either inward or outward, depending on the current direction with respect to the magnetic field. (In my project I used approx. 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 had a 3 ohm resistance.

tweeter-bThe tweeter conductor was originally built with four conductor coils, 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%. Corrugating it will certainly avoid rattling and it tends to move too when the woofer is reproducing lots of bass.

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 approx. 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 approx. 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 analyzer, 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 makeshift, but so is the
whole loudspeaker project!! :-).


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