Line magnetic LM-755 Alnico

Why Field Coils?
Almost all modern speakers are permanent magnet speakers. That is, they have a fixed magnetic field created by a magnet. Early on, they were typically Alnico but most often now they are ceramic. With a fixed magnet speaker the voice coil is suspended in a gap between the poles of the magnet. The voltage that is applied to the voice coil causes it to move in and out.

A field coil speaker uses two coils; the voice coil and a field coil. Instead of a magnet, DC is applied to the field coil creating a magnetic field. This takes the place of the permanent magnet in creating the fixed magnetic field. Field coil-based drivers done well, which is a no easy job, are claimed to drastically reduce distortion levels, and are able to control the driver much more accurately. Drivers in speakers vibrate up to thousands of times per second. Some claim that permanent magnets actually lose strength slightly with each vibration, causing a loss of low-level information and a blurring of the signal. Field coil drivers, with their own power supplies should not lose strength and so have much less distortion than their permanent magnet counterparts.

Well, I’m in over my head with all this technical talk, but one thing I know is that they are significantly more expensive to build than permanent magnet speakers, just like Alnico magnets have gotten too expensive for most speakers. The other thing I can say is that at least a couple of the best speakers I have ever heard use field coils. That is not to say only field coils are among the best; I love Lowthers and Quad 57s as well. No, I’m not saying that field coils are the only way to go, but I am saying that cost and difficulty in manufacturing are the reasons most of us had never heard of a field coil speaker until lately. Permanent magnets are always cheaper than a well-built field coil driver and do not need a power supply.

The Power Supply
As you have probably gathered by now, like electrostatic speakers, field coil speakers have to be plugged-in to work. As I have already mentioned, this is done by using some kind of power supply that is plugged into the wall and into the speaker. In the case of the LM 755i, Line Magnetic Audio has chosen to build a power supply named the LM Audio PR-3 that uses 300Bs to power the magnets.