Saturday, 13 April 2019

The virtue of a Field Coil

I am often asked what the field coil magnet does that other magnets do not in a loudspeaker. The question of magnet type concerns the degree of steadiness against which the voice coil reacts in the process of making music.

Different magnet types have different field characteristics. The motion of the speaker system is determined by two magnetic fields interacting with each other. One is the magnet's field and the other is the voice coil's field when current flows though it. The amplifier only provides the current which the speaker must then turn into music. Without the loudspeaker there is no music.

In theory the magnet's field is constant and the voice coil's field is variable. As the voice coil field varies with the music signal it causes motion by attracting and repulsing the magnet's field. This is theory.

In reality the magnet's field is not constant but behaves like a spring as the two fields interact. The stiffness of this spring is determined, in part, by magnet type. The majority of your expensive amplifiers electrical output does not make it into music - instead it is lost as heat in the voice coil. More than 90% of your amplifier's output does nothing but make heat. This why the loudspeaker is the most critical element in a playback system. This is also why efficiency in a loudspeaker is so important. It should be evident that minimising the over 90% loss of energy is critical to fidelity. 
The best of permanent magnets in regard to a stable field is Alnico. While the Alnico and other permanent magnets have finite magnetic resource the field coil is constantly replenished by the current source feeding it and is adjustable but the amount and type of current provided.

The heat factor also explains why speaker magnets can loose strength over time if they are driven hard and get hot. Heat causes loss of magnetism in permanent magnets. A field coil does not suffer similarly.

The voice coil moves with increasing accuracy in direct proportion to the stability and stiffness of the magnetic field provided for it to work against.

The four classes of permanent magnets are:

    Neodymium Iron Boron (NdFeB or NIB)
    Samarium Cobalt (SmCo)
    Alnico
    Ceramic or Ferrite

This table gives us some of the special characteristics of the four classes of magnets. 
Material
Br
Hc
BHmax
Tcoef of Br
Tmax
Tcurie
NdFeB
12,800
12,300
40
-0.12
150
310
SmCo
10,500
9,200
26
-0.04
300
750
Alnico
12,500
640
5.5
-0.02
540
860
Ceramic or Ferrite
3,900
3,200
3.5
-0.20
300
460

Br is the measure of its residual magnetic flux density in Gauss, which is the maximum flux the magnet is able to produce. (1 Gauss is like 6.45 lines/sq in)

Hc is the measure of the coercive magnetic field strength in Oersted, or the point at which the magnet becomes demagnetized by an external field. (1O ersted is like 2.02 ampere-turns/inch)

BHmax is a term of overall energy density. The higher the number, the more powerful the magnet.

Tcoef of Br is the temperature coefficient of Br in terms of % per degree Centigrade.  This tells you how the magnetic flux changes with respect to temperature. -0.20 means that if the temperature increases by 100 degrees Centigrade, its magnetic flux will decrease by 20%!

Tmax is the maximum temperature the magnet should be operated at.   After the temperature drops below this value, it will still behave as it did before it reached that temperature (it is recoverable).  (degrees Centigrade)

Tcurie is the Curie temperature at which the magnet will become demagnetized.  After the temperature drops below this value, it will not behave as it did before it reached that temperature.  If the magnet is heated between Tmax and Tcurie, it will recover somewhat, but not fully (it is not recoverable).  (degrees Centigrade)

This chart does not directly address the "stiffness" of the field. Tcoef of Br gives a clue as it indicates the stability of field in regard to temperature which can be indirectly correlated to overall stability.

The field coil or electromagnet is not as susceptible to field variations as permanent magnets. When we look at the high degree of loss involved in transducing electrical current into sound it is evident that any minimising of loss is critical to the degree of fidelity.



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