Euraudio Charisma nearfield monitor loudspeaker building instructions
WARNING
The loudspeaker design and building directions presented below were published in good faith. Nevertheless the publisher assumes no liability either for the accuracy of information contained herein or for any loss or damages arising from the use of these pieces of information.
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THIS IS PRELIMINARY DESIGN INFORMATION!
The Charisma loudspeaker has not been built yet. The plans are based on manufacturer data, and simulation with measurement data of similar drivers, therefore the crossover may need fine tuning. The first builder of this loudspeaker may get free help from the designer. Further information: help for DIY speaker builders.
Building documentation
Caution: The mechanical guidelines is not a detailed mechanical drawing. For example, only the outside lines of the enclosure walls are drawn, the wall thickness is not shown.
Building advice
Please keep to the plan exactly.
Making the enclosure
The front panel is two 18 mm MDFs glued together, the other panels are 22 mm MDFs. It's expediential to have the front panel prepared by using a CNC woodworking machine. It's rather difficult to make the countersinks for the speaker baskets at home. Of course it can be made using a router, provided you have the proper router bits, and a circle cutting jig. The internal edge of the midwoofer cutouts must be chamfered, at approx. 22 mm depth, as exemplified in the next image.
The front panel bevels at the sides are at 30 degrees relative to the front panel, the bevel at the top is cut at 45 degrees. If you can't use a CNC machine, you can make the side cuts using a table saw and the top 45 degree milling using a router. The following images (depicting woodworking on other speakers) may help. A few millimeters' difference from those shown in the mechanical drawings is of no importance.
PVA wood glue, D3 type is recommended for gluing.
The internal cross braces are 22 mm MDF elements according to plan.
Internal damping
Cover the internal walls of the enclosure with elastic adhesive that sticks well to wood, using a notched trowel in approximately 2.5 mm thickness. For example, the Mapei Adesilex G19 or a similar high-viscosity adhesive that dries to a rubber-like layer, can be used. If you are unsure, first test the glue on a piece of MDF. Push 0.8 cm felt sheets into the wet adhesive. As an alternative instead of the elastic glue, you can use a 2.5...3 mm thick self-adhesive bitumen damping sheets such as the Hanno-Protecto PF 830, the Amortson Bi 5A or the Sinuslive ADM. The back panel of the enclosure should be lined with the 0.8 cm thick felt sheets, plus 3 cm thick polyester damping sheets (Acoustilux). Available from Jantzen Audio. Cover the sides with two layers of 0.8 cm thick felt. The damping materials must be fitted before gluing on the last enclosure side.
Crossover
Please mind the polarity of the drivers.
The crossover can expediently be mounted on the bottom panel of the cabinet. Components with 2% or 3% tolerance are recommended. The best solution for fastening components seems to be cable ties. Tie them to a thin (e.g. 5 mm thick) HDF plate and screw that plate to the inside of the cabinet. I haven't made a component layout or PCB plan, just solder components together across their leads or use terminal strips.
Adding the R2, C2 components is optional, but I recommend including them. C2 is specified as a 250V capacitor but in addition to being rated at least at 250V, another important parameter is that it should be able to withstand at least 10VAC RMS at 1 MHz. For example, Wima FKP2 250V capacitors are suitable. Please consult the data sheet of the capacitor you intend to use. R2 should be low inductance type, e.g. metal oxide or metal film. 120 Ohms is an approximate value only. The value of R2 can be adjusted more accurately depending on the exact geometry and dielectric of the speaker cable by using the following calculator. Add 5% to the value of characteristic impedance provided by the calculator and choose the nearest standard E24 resistor value!
All inductors are air core. The L4 (13uH) coil can be made by unwinding wire from a bigger coil while measuring it; it should be within +/-8%. Don't fasten coils with ferromagnetic metal screws (e.g. steel), because they may cause some distortion. Don't place inductors close to the ferromagnetic metal parts of the speakers. To prevent crosstalk, place coils at least 10 cm away from each other or - if they are closer - direct their axes perpendicular and skewed:
The hard-to-come-by capacitor values can be attained by connecting more common values in parallel. By parallel connection, the capacitances are summed. In the circuit diagram of the crossover, the shown voltage of capacitors is the minimum suitable rating, you can also use capacitors with higher withstanding voltage. I recommend the 400V DC rated Jantzen Standard Z-Caps for the capacitors in the crossover.
Internal cabling
Copper speaker cable of at least 0.75 mm2 cross-section or CCAW speaker cable of at least 1 mm2 cross-section is recommended.