Started in 1976 with a pair PM2 (largest ferrite I ever came across) from LSV Hamburg I learned very much about full range speaker drivers and owned a lot of historic stuff like the original Voigt mains energised drive unit (field-coil) or some early 'Acousta' speakers and a TP1D with 'tractrix' contour for the used horn.
Paul Voigt was a well-known personality in British audio in the 20's and 30's.
His British patent 238310, on moving coil loudspeakers was applied for only weeks after a similar patent in the US had been registered on March 27th 1924. So was laid the good fortune of the American radio industry that were thus able to claim world wide royalties for many years from all other manufacturers of equipment using moving coil speakers.
After World War II, with the introduction of new magnetic materials, the design of a permanent magnet drive unit was given priority.
Before Paul Voigt left for Canada in 1950 he sold his company to Donald Chave (Lowther) who had also been working on designing a permanent magnet drive unit, culminating in the PM1. This drive unit utilised a Voigt cone assembly.
Donald Chave gradually expanded the range drive units available, all products being hand-made. Possibly the most famous "classics" of these being the PM6, PM2 and PM4 drive units, the TP1, Audiovector and Acousta loudspeakers.
Today Lowther are still producing high quality full-range drive units and loudspeakers which are exported all over the world. There are a few others producing similar and more constant quality for sure but I tested recent cone assemblies and I stick on Lowther even with their shortcomings in precision regarding glueing angles of cones and voice coils.
To get uncompromising results out of the cone assemblies I developed a modern field-coil motor named AL2. Flux is homogenous within the voice-coil gap depth of 8 mm. Maximum homogeneity and linearity is achieved making sure the voice coil never leaves this field even with the rumble of analogue records under normal circumstances.
AL2 field-coil @ 12V DC is guaranteed to deliver 19,000 Gauss, typical 19,750 Gauss. This is ideal for every direct radiating full range unit.
Comment (excerpt) from P. Wallander about the prototype KS-AL2, 09.04.2010:
Now to the field coils. What about the sound? This is not an upgrade. It is not more of something that already is there. This is something entirely different. There is only one possibility to describe the sound: You are listening to THE REAL THING! When listening to piano music on the DX2 or PM5A there is the tone itself with the reverberation from the concert hall that can be heard between the tones. But there is also something else, a type of noise or something which is absent in live piano performances, and this something is what I call “loudspeaker sound”. This something is missing when listening to the field coils. The space between the tones is “black but not dead”. The reverberation of the room is there. The feeling you get when entering a large hall with your eyes closed is there. And this is what you hear from the Field Coil speakers, and no more! The sound is extremely “clean” in a way that you can not imagine. It must be heard. The field coil speakers created the single most important improvement in my music system. I am very thankful to Wolf that he offered me his prototype field coils.
Dimensions without cone assembly.