October 12, 2011

The incredible situation of the neodymium cost through last year lead us to develop new ferrite magnet circuits as an alternative to the neodymium ones.

One of the leading characteristics of neodymium magnet circuits is their low weight. Obviously the use of ferrite does not allow for equivalent weight figures, however the use of ferrite magnet circuits with higher magnetic grade and minimal external diameter allowed us to contain the higher weight making it, for some applications, acceptable.
As an example, enclosed herewith is the comparison between the weights of neodymium, traditional ferrite and new ferrite magnet circuits referred to a 75mm voice coil.

While designing the neodymium magnet circuits we decided to use magnets placed inside the voice coils because this allowed for significant improvements in speakers’ parameters.
No doubt, such assembly is not possible with ferrite magnets. Nevertheless, aiming at equivalent performances, we did some mechanical works on all the components of the new ferrite magnet circuit so to make them equivalent to the neodymium ones.
As an example, the comparison among frequency response curves on a 12” assembled with the 3 different magnet circuits.

Another achievement obtained is the better magnetic field distribution in the air gap, as can be better seen on the following graph.

The advent of high power digital amplifiers suggested us to implement also the ventilation of the voice coil, so to reduce the power compression. The air flux inside the magnet circuit was optimized, reducing up to the 20% the voice coil temperature at the same power as the neodymium magnet circuits that already were an improvement if compared to the previous ferrite magnets.

Considering the higher powers available and the following excursions required to the speakers, the maximum mechanical excursion possible was increased to 25mm, so to avoid problems even when used in extreme conditions. 

Considering that these new ferrite magnet circuits will be mainly used on mid-high quality speakers, we have decided to provide them, as standard equipment, with demodulating rings, introducing a further improvement with reference to both the old neodymium and ferrite magnet circuits, as an example the total harmonic distortion reduction.

Comparing ferrite and neodymium magnet circuits, we also have to consider that ferrite magnets have no problems of maximum working power temperatures, hence there are no problems of demagnetization even when driving the voice coils at temperatures higher than 300°C. That is not always true with neodymium, even if it is of high thermal grade.

Finally we believe to have developed some valid alternatives to neodymium magnet circuits, against a higher weight, they offer interesting advantages with reference to the dynamic behavior of the speaker, the cooling of the voice coil and the thermal stability.