JBL's Differential Drive technology
Apr 1, 1999 12:00 PM, Doug Button
Historically, the pro sound industry has gone to larger diameter voice coils and more massive magnet structures to increase loudspeaker output capability within desired degrees of linearity. AT JBL, we are addressing additional challenges in areas of driver fit and function as we strive to make them smaller and lighter with no compromise in performance. JBL's Differential Drive technology is a step in this direction.
Differential Drive technology uses a pair of separate, reverse-wound voice coils on a single voice-coil former and cone. The two coils operate in opposing magnetic fields to accomplish performance similar to a conventional design but in a considerably smaller and lighter structure. Although the dual-coil approach is not new, JBL has improved on the design through the application of two new features.
I will first explain how Differential Drive works by comparing the new design with the standard approach. For the sake of making an apples-to-apples comparison, assume that both designs have the same total flux density in the gap and that the amount of copper and moving mass is the same in each design. In the traditional JBL structure, magnetic flux B crosses a gap in which a coil of copper has a total electrical resistance of R[e]. These quantities establish the value of the electromechanical coupling coefficient, (Bl)2/Re.
In Differential Drive topology, there are two magnetic gaps with opposing flux. The two voice coils are connected in reverse polarity so that the mechanical forces they produce will add. For the moving mass to remain the same, the two voice coils must have the same height and half the thickness as in the standard design. The value of B will remain the same.
When these changes are made, the total length of the voice coil wire will be doubled, and the resistance per-unit length of wire will be halved. The total resistance of both voice coils in the series will then be four times what it was with the standard approach. Since the length has doubled, the quantity (Bl)2 will now be four times what it was in the standard approach. This results in a coupling coefficient value of 4(Bl)2/Re. Canceling out the fours yields the previous value of (Bl)2/Re.
In other words, we have exactly the same coupling coefficient as before, but we have picked up several important advantages relative to the traditional design. The new voice coil assembly now has twice the surface area of the traditional one, and this means that it will have twice the heat dissipation of the traditional single coil, which translates directly into twice (+/-3 dB) the power input capability for a given operating temperature and observed amount of power compression. The new dual voice coil structure will have less effective inductance than the standard one because the reverse-wound coils will have negative mutual inductance between them. This translates into a flatter impedance curve at higher frequencies, producing more output for a given drive voltage. Finally, the new design is generally more compact, and when used with neodymium magnets, it requires less steel to complete the magnetic circuit assembly. Consequently, it is much lighter. The design, however, is not limited to new magnet materials and can be used with standard ferrite magnets with benefits one and two above still applicable.
The two important design features referred to earlier deal with overall system linearity. First, the two voice coils are not placed at the axial center points of their respective magnetic gaps; they are symmetrically displaced axially outward so that the overall net distribution of flux density in the combined gap space is most linear. This ensures maximum system displacement linearity for the moving system.
In high-level operation, low-frequency high-displacement signals often tend to drive the voice coils out of their linear operating region. While traditional designs rely on progressive suspension designs to constrain this motion mechanically, the Differential Drive transducers make additional use of a shorted electromagnetic braking coil. This is shown in Figure 1. The coil is located mid-way between the two driving coils, and at normal excursions, it is virtually inert. On high excursions, the shorted coil enters each magnetic field alternately, and current is induced into the coil. That induced current, by Lenz's law, acts to oppose the motion that causes it. The result is additional braking on cone motion, resulting in lower distortion.
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