My post on coils, magnets, and compromise certainly has spurred a lot of interest from Vinylphiles. Several correctly pointed out I neglected to mention the very interesting light-based phono cartridge from DS Audio.
My friend, Gary Galo wrote to me with this over-the-top treatise that I share with those of you interested.
“Actually, very few moving-magnet cartridges have the magnets attached directly to the stylus. The old Decca cartridges did, as I recall, but the vast majority have the stylus attached to one end of the cantilever and the magnets attached to the other end, balanced by some kind of rubber or elastomer suspension, forming a “teeter-totter”. The magnets are normally very low in mass, which allows relatively low tracking forces. High output is achieved by using relatively large coils, but the trade-off is that the high coil inductance – normally several hundred mH – limits the high-frequency response and, most importantly, degrades the phase response in the upper audible octave and beyond. The high electrical impedance makes moving-magnet type sensitive to capacitive loading.
Moving coil cartridges must employ relatively small coils – if they’re too large the added mass will limit the cartridges tracking ability and require excessively-high tracking forces. To get any amount of output, large magnets are used. But, there’s a limit to how much output can be achieved with such small coils, even with relatively powerful magnets. The advantage of low coil inductance is extended high-frequency response and, consequently, superior phase response in audible range, giving moving-coil types that sense of air and extension in the top end. But, even relatively small coils have greater mass than the magnets used in moving-magnet types, so MCs typically require tracking forces in the 2 – 2.5 gram range. The low electrical impedances make them insensitive to capacitive loading, but the higher moving mass can put the high-frequency resonance point in the upper octave which, depending on your point of view and your associated equipment, either gives them a wonderful sense of air and space, or makes them sound too bright. Some moving-coil cartridges place the coils directly above the stylus, while other attach them to the back end of a cantilever.
Then there are moving-iron types. Grado has been the best-known proponent of moving-iron cartridges. Both the coils and the magnets are fixed, with a small piece of iron moving in the magnetic gap between them. GE called these “variable reluctance” back in the 1950s (the GE VRII was highly-regarded in the mono era), and Ortofon once made moving-iron cartridges that they described as “variable magnetic shunt” (I was very disappointed when Ortofon abandoned their excellent VMS line decades ago – I owned one and it was excellent). Joe Grado’s implementation of the moving-iron concept allows the cartridges to have much lower impedances than moving-magnet types, and gives them much of the air and dimensionality associated with moving-coil types. Coil inductance is around 35 mH on many of the Grado models, making them insensitive to capacitive loading, while still producing high enough output to feed a moving-magnet phono preamp without any additional step-up. Grado’s moving-iron implementation is a very effective tradeoff. In the old GE cartridges the moving iron was attached directly to the stylus, but it seems that it’s very difficult to make a 45/45 stereo cartridge this way. Grado places the iron at the back end of the cantilever (as I recall, Ortofon did this, as well).
It’s interesting that the moving-magnet cartridge manufacturers have moved away from the ultra-low tracking forces once favored by Shure and other: 3/4 to 1.25 grams was once typical. The best moving-magnet types made by Audio Technica, Ortofon and other now typically have recommended tracking forces in the 1.5 – 2 gram range. Shure placed a high priority on ultra-low tracking forces but, in my view, at the expense of other things. As you note, everything is a compromise. The trick is to find an effective balance.”
