For a headphone driver, the STELLAR.45’s voice coil is remarkably long, and is the first to be given a symmetrical overhang. This means that the voice coil runs through the complete magnetic gap and protrudes to the same degree above and below it. Consequently, the coil can take advantage of the full range of magnetic flux in the gap. The driving force remains constant even with longer diaphragm strokes so that distortions are reduced to an absolute minimum.
An extremely thin copper-clad aluminium wire is used for the voice coil. With a diameter of just 60 µm, this wire is as thin as a human hair. To improve its durability, the wire is also given a special coating. The density of the composite material is only half that of pure copper, but still boasts excellent electric conductivity. Weighing around 80 mg, the weight of the 3.2 mm long voice coil is considerably less than that of the coils used by other driver manufacturers.
We also use powerful NdFeB magnets in our Tesla drivers, but these are arranged in a ring shape outside the voice coil. Due to the larger surface area of these ring magnets, with Tesla drivers a higher magnetic flux is coupled into the magnetic circuit, and this is a factor affecting the characteristics of the magnetic field in the gap. The optimum characteristics for the transducer depend on the type of voice coil used. In contrast to the STELLAR.45 driver, the Tesla driver has an underhung voice coil (fig.1). This voice coil is located on a collar that positions it in the middle of the magnetic gap. With a height of 0.7 mm, the height of this underhung voice coil is significantly less than that of an overhung voice coil, such as that in the STELLAR.45. Owing to this, the delicate voice coil in the Tesla driver weighs just 20 mg. It is generally true that this underhung arrangement permits ultra-light coils with low winding height and low weight.
What is interesting, however, is that our voice coils are consistently significantly lighter than those of other driver manufacturers –and this has an impact on the sound. Because the mass of the voice coil is less, the diaphragm is able to follow the excitation signal more quickly, and this makes for excellent reproduction of transients.