Life in the time of Hay...

A cool and rainy spring gave way to a ferocious beginning of summer. The hottest days ever recorded in France. As a result of this sudden change the farmers came out in synchronisation on the same day to get the hay in before it was lost.

While the farmers have been out in the heat cutting, drying and bailing the hay it has been a busy time indoors as well.

Mojo the mad is a constant companion...

constantly chasing everything (real or imagined) ... 

In trying to stay with the real let's get on with business then, shall we?

Our Field coil design has moved forward.

What started as a simplified way to improve on the historic Tungar battery chargers and Selenium rectified supplies and solve the requirement with a sober design created an avenue of discovery. 

It began with a Schottky rectified, choke loaded design which eliminates noise and its harmonics and provides current stability and storage.

Voltage regulation was tried and was the wrong direction.

Current regulation and turned out to be an excellent solution for field coils, improving sonics considerably.

The difference in the type of AC transformers was more pronounced than anticipated. Split bobbin types were used initially to decouple line noise. R - Cores were tested and were quieter in operation. When toroidal types were tested they demonstrated clear superiority in current delivery. While the other steps forward were solid and clear, the difference of the toroid was pronounced. Quite a treat to experience how much better the music moved and flowed with greater weight and agility above the other steps forward.

Field coils deserve better supplies because they make a field coil more of a field coil - meaning the increased stability of the magnetic field in the face of constantly varying load. The difference in quality of supply is easy to hear. 

 The supplies pictured above are heading to the Island of Crete to power WE555 6-7 volts / 1.5 amps with plenty of headroom. 

For those who would like to update their supplies please contact us by email.

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.