Moving Parts – Part 1

By John Strohbeen • Ohm Speakers • Monday, January 26, 2015


Sales of “Black Vinyl” records are having a resurgence. They are a bright spot in the otherwise dim area of sales of recorded music. In fact, reports are that Black Friday sales of Black Vinyl were the second best since records (pun intended) started being kept in 1991. Please hold your applause. While the percentage increases are huge, they are working from a small base and the sales of other recorded media, mostly CD, dwarf the large black spinners.

Nonetheless, there is clearly a renewal of interest that can be seen not only in sales, but also in the interest generated by our pair of postings on CD vs. Black Vinyl last year.

So, if you’re interested in Black Vinyl, you’re going to need the gear to play the platters. And there are a plethora of products out there. Let’s take a look at the component closest to the bone, or groove: the phono cartridge.

Like speakers, cartridges are transducers. That is, they change one form of energy to another. In the case of speakers, it’s electrical energy to sound waves and in the case of phono cartridges, it’s motion stimulated by the undulations of the groove into electrical energy.

Today, the most common types of phono cartridges are Moving Magnet (MM) and Moving Coil (MC) designs. In many ways, they are mirror images of each other and both have their adherents and detractors. The cartridge structure is similar for both designs.

In the MM design, a cantilever is fitted with a stylus at one end and tiny magnet at the other. On four sides of the magnet are electric coils. The system basically creates a generator. As the stylus tracks the groove and its undulations, the cantilever moves and varies the magnetic field in the coils – generating electricity. The voltage, millivoltage actually, then is transmitted to the pre-amp or amplifier.

The advantages of a Moving Magnet design are that the stylus/cantilever can be replaced and that it provides a voltage suitable for direct connection to most pre-amps. MM cartridges are widely available in virtually all price ranges and from a variety of manufacturers.

The Moving Coil design inverts this system. Again, a stylus is located on one end of the cantilever. But instead of magnets on the other, an MC design has electric coils. There are magnets in the cartridge body. The undulations of the groove are tracked by the stylus, moving the cantilever and varying the electrical output of the generator.

In the Golden Age of Audio, Moving Coil adherents felt that the lower mass required allowed the stylus to negotiate the groove more quickly and deliver a more accurate and musical representation. Then, AlNiCo was the most powerful magnetic material at 7 million Gauss-Oersteds (MGO), moving magnet assemblies were heavier than moving coil assemblies. Today, with Neo magnets readily available with over 50 MGO energy level, the weight advantage is going away and there are moving magnet designs with lower mass that most moving coil designs. Still, some vinylphiles believe that MM cartridges are, by their nature, less musical and less realistic than other designs and many favor the MC.

Drawings Courtesy of Ortofon Inc.

Detractors feel that the trade-off of higher price, lower voltage output – requiring some kind of step-up amplifier or transformer – are not justified by the perceived performance.

There are many other design considerations than just the generator geometry that have dramatic effects on tracking, frequency response, channel separation and overall performance.

Both MM and MC cartridges can and do deliver extraordinary performance. In a future post we will examine some variations on these two basic designs including Moving Iron and Moving MicroCross, as well as some more esoteric solutions, such as “StrainGauge” and optical.

For now…

Enjoy & Good Listening!

John

PS: For those of you in the NYC area on February 27, I hope some of you can come to the live performance of the Ohm Quartet at NYU’s Skirball Hall with the FLUX string quartet playing Spencer Topel’s, as part of “Spatialized 2015.” (http://nyuskirball.org/calendar/spatialized2015)

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