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Here is the similar approach undertaken with Altec 288C to the one that I took with my Vitavox

http://chaudioroom.com/CH288287.html

Some critiques. A minor mistake in the text is that the author says that “magnetic fields generated will go down to zero when the power to the field coil is removed”. If it not as the core still get some residual magnetism but it is not so important. I would not also agree that it is possible to moderate sensitivity of the driver with change of electromagnet magnetizing force. The biggest question I would have about the heat dissipation design. They look like did a good job to have a magnetizing coil’s filler to propagate the heat from the coil to the outer wall of the driver. I do not know if it is sufficient as the coil’s filler does not touches the wall of the driver – this is VERY big mistake in my view.  I would like to see the temperature of this thing after 2 hours running at 2A. The core mass is also seems a little too small and I would like to see another coil at the outskirt of the coil’s filler, magnetizing the return pass. That would force to turn a new driver’s can though….

So, I generally feel that such a driver might be a good prototype to learn about a character of field-coil (the same as I did with my S2) but it is not a final solution.

The good part is that the author makes a sonic claim that is not as idiotic and the claims of other who play with field-coils (“more flux” etc…). The author says:

“A permanent magnet driver sounds somehow louder due to the high pitch distortions, the field-coil driver will sound slightly lower due to lack of distortions and with the most natural and sweetest sound.”

I would agree that field-coil is in a softer side but I am not convinced that it is due to “lack of distortions”. I do not say that the guy is wrong I just do not feel that what author said exhausts all differences between the field-coil and perm magnet.  There is much more than just that. I might understand how he deals with the fact that field-coil less loud (perceptual) but how he deals with field-coil lost of transient characteristics? The guys do not talk about it at all.

Here is how I see the game: he needs to load the amp that drives the electromagnet driver much idler.  This would give some transients back at lover distortion but it would also eat output. The increase of current in field-coil with attempt to gain more sensitivity would not work as it will screw the bottom knee of the driver’s response, not to mention the alleged heat buildup in the field-coil. So, as I see it we have a need to have a dedicated amp with higher gain to drive electromagnet driver. It might work and it might not as we will essentially send more voltage to the voice coil…

I have no judgment how it all work all together but the simple generalizing statement that the “permanent magnet have high pitch distortions” does not address the whole picture in my view.

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