DCAudioDIY.com

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Walt

Alnico (Aluminum, nickel, cobalt) is just another choice of magnets with tradeoffs ike any other material. The advantage of the shorting ring is that other materials can be used to get a very much higher field strength for less money with the shorting ring mitigating the other problem. Even the guy who is the Altec guru has admitted the advantage of ceramic and shorting rings. One advantage of Alnico for military applications is that its Curie point where it begins to demagnatize is higher than most. One of the problems pro drivers have is that when using Neo for lighter weight when used in the field, they have to incorporate heat sinks on the magnetic structure to keep the Neo magnets below their much lower Curie point.

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Walt

Please explain why field coils are supposed to be better, at least a few manufacturers want on to beleive

Probably because of the reputation of the Western Electric 555 horn driver and the fact you can get higher more uniform magnetic fields with coils.

I've got field coil 15" woofers at home. One of these days....

Roscoe

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I can explain it to you, but I can’t understand it for you.

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Walt

This is another case for shorting rings. To keep distortion to a minumum in a driver among other things you want to keep the magnetic field constant. You would think that a permanent magnet on a driver would yield a constant magnetic force. That is true to a point but remember that a driver is both a motor and a generator. The combination of the driving current plus the motion of the voice coil through the magentic field can cause modulation of the field. That is where the shorting ring comes in (or to some degree the Alnico magnet), effectively shorting out that generated field that would modulate the steady state field.

Using a field coil instead of a permanent magnet can be either good or bad depending on the ability of the power supply for the coil to not only source current to the coil but also sink induced currents coming from the movement of the voice coil through the gap. The old power supplies were not very good about either sourcing or sinking those currents in a linear fashion. With modern electronics you can do much better. The question is why bother? Looking at the resulting performance of modern drivers using permanent magnets, the distortions are in the case of competent drivers much lower than any older voice coil driver. My advice is to pick the driver you want to use based on actual ultimate performance, not on some theoritical design "magic".

Instead of speculating, why not drive down to Dejavu audio and give a listen. They usually have some high-end field coil concoctions on display.