While streaming music is the current fashion – and record companies, artists, and retailers lament the diminishing sales of Compact Discs – sales of vinyl records are maintaining their resurgence. In fact, the format – presumably given a death blow 40 years ago – is now the dominant physical media format for music. This has spawned renewed interest and increased sales in turntables and cartridges – the items needed to unlock the music stored in the grooves of a vinyl record.
Turntables are a pretty obvious necessity in the vinyl world and are easily understood. To play records, you need a platter revolving at a constant 33-1/3 rpm with a minimum of mechanical vibration transference. On the other hand, cartridges are not as easy to understand, and their costs – particularly to the non-audiophile – can seem absurd. However, a cartridge, like other parts of your audio system, can have a profound effect on the sound of your vinyl.
So, what are phono cartridges, and how do they work?
Cartridges belong to the class of devices called "transducers." They convert one form of energy to another. If you're reading this, you're undoubtedly familiar with transducers, even if you don't recognize the term. Speakers, headphones, and earbuds are probably the most common transducers in audio. They convert electrical energy – received from the amplifier – into sound waves. Cartridges work in almost exactly the opposite way: they convert the motion of the stylus – our grandfathers called it a needle – into electrical impulses as it jiggles around in the record groove.
The two most popular phono cartridge designs are Moving Magnet (MM) and Moving Coil (MC). They can be considered mirror images of each other. Both approaches have loyal adherents – and both have benefits and shortcomings.
In the Moving Magnet design, the light metal strip cantilever is affixed to the stylus at one end and a tiny magnet at the other. Electrical coils surround the magnet on four sides so that a generator is formed. As the stylus tracks the groove undulations, the cantilever, and thus the magnet, is moved relative to the coils – the same way that action at one end of a see-saw creates action at the other. The magnet’s motion alters the magnetic field and the electrical energy generated by the system. The varying voltage produced is relayed to the preamp, the amp, and finally to the speakers. The speakers then reverse the process, turning electrical energy into sound waves.
The principal advantages of Moving Magnet cartridges are greater output than Moving Coil designs and more easily replaceable stylus/cantilever assemblies. The higher voltage output means that MM cartridges can be connected directly to most preamps – no intermediate stage cartridge amp is required. The easier stylus/cantilever replacement allows the audio enthusiast the opportunity of replacing those parts at home – no shipping back to the manufacturer or going to a specialty shop.
Popular and well-regarded MM cartridges include:
The Moving Coil design reverses the cartridge's topography. The coil is mounted on the cantilever, and the magnets are housed in the cartridge body. The electrical process is the same: the stylus tracks the groove, and the coils move relative to the magnets, creating a varying millivoltage stream.
The benefits of the MC approach historically included lower mass, allowing the stylus to negotiate the groove faster and more accurately. In theory, the MC design provides a more accurate, and some feel a more musical vinyl ride. However, technology moveth on, and magnets have become lighter and more powerful. In what has been termed the Golden Age of Audio, the most powerful magnets were Al-NiCo. They produced about 7 million Gauss-Oersteds. Today, Neodymium magnets can easily provide 50 million Gauss-Oersteds. Some MM cartridges exhibit lower mass than many MC designs. Scoring for the other side, MC models are now on the market with output high enough to go directly into a preamp and models that offer user-replaceable stylus/cantilever assemblies.
Some MC cartridges to consider are:
While, as noted, MM and MC are the most popular cartridge designs, there have been and still are others. In the 1950s, General Electric developed what it called a Variable Reluctance design. Today, we call it Moving Iron. The cantilever, as expected, has a stylus at one end, but instead of a magnet or coil, a small piece of iron or other ferrous material is mounted. Magnetic coils flank each side. Since the iron could be lighter than the magnet, the design allows mass reduction. The development of lighter magnetic materials and other advances largely negates benefits today, but for the 1950s, Variable Reluctance or Moving Iron was a clever and elegant solution.
Moving Micro-Cross, a term used by Bang & Olufsen’s legendary engineer, S.K. Pramanik, is a variation of Moving Iron. A miniature ferrous cross moves between two fixed magnets and coils as the stylus at the other end of the cantilever tracks the record groove. According to Pramanik, the design further reduces mass and increases channel separation.
Moving Magnet, Moving Coil, Moving Iron, and Moving Micro-Cross cartridges all depend on magnetism to generate electricity. There are other approaches, though, namely electret cartridges, which flex a piezoelectric material to generate voltage. Although rarely used now, this design was popular in the early days of audio. It generated enough voltage, so that relatively little amplification was necessary. A drawback, however, was mass.
“Moving lights” conjures up images of LED, LCD, or other large billboards along Broadway, the Ginza, or Hong Kong. But in the 1970s – before lasers became commonplace – Toshiba offered audio enthusiasts an “optical” cartridge, which relied on the variable intensity of a laser light beam.
While the Toshiba technology was innovative, the transduction process was familiar. The stylus tracked the groove and moved the cantilever, which modulated the beam of light. The benefits of the Toshiba system were lower mass and more accurate tracking. A drawback was that it required a special auxiliary preamp. Although a curiosity, it disappeared quickly.
However, as we've seen with the Shibata stylus design, good ideas in this arena have a way of reappearing. A few years ago, ELP developed a "contactless" system utilizing five lasers. And today, several companies are producing optical phono cartridges. One of them, DS Audio, is willing to give away its technology to advance the idea.
Regardless of the method used to turn motion into electricity, the attributes to look for in a cartridge remain the same. As with doctors, the cardinal rule is “Do No Harm.” Vinyl is a durable product for windows, floor coverings, and other applications, but as a record medium, it is quite sensitive, with each groove wall carrying one of the two distinct signals – left and right – that the stylus “feels” its way through. The stylus itself is generally a diamond, or has a diamond tip, and diamond will cut virtually anything. So, the idea is to create a cartridge that stresses the groove as little as possible. Thus, a low tracking force – the weight actually applied to the record during play – is desirable. In the early days of stereo, tracking forces of 3 grams delighted audiophiles. Today, tracking forces of less than half that are common.
There are other equally critical cartridge specifications. As we’ve noted, the stylus sits at one end of the cantilever and negotiates the grooves’ undulations. The force needed to move the stylus, the “springiness” of the stylus-assembly system, is called compliance.
Without getting into the deep weeds of compliance and compliance measurement, the springiness of the system and the length of the tonearm will affect the low-frequency resonance of the system. A cartridge with compliance that is too low has a stiff cantilever, requiring the groove to “work” harder to guide the stylus, which can lead to excessive groove-wear. Vinyl discs are often imperfect and can be slightly warped. A cartridge with compliance that is too low may have trouble maintaining contact with the groove or jump the groove altogether.
Compliance is a balancing act, though. If it's too high, the deep bass of the lowest frequencies may be compromised. Generally speaking, a compliance figure below 12 (expressed as 12 x 10-6) is considered low, and 25 (25 x 10-6) is high. Some authorities break it down further into low, moderate, and high compliance ranges.
As with the other audio system components, frequency response is an important parameter of phono cartridge performance. The “20-to-20” (20 Hz to 20,000 Hz) industry benchmark applies here, too. Virtually all high-quality cartridges exceed that range – some by astonishingly large amounts.
Stereo separation is another specification you're likely to see as you run down a phono cartridge's list. As noted above, the left signal is embedded in one groove wall and the right signal in the other. However, since a single stylus captures both signals, there is always some bleed-through (the same way a single microphone placed on one side of an orchestra will inevitably capture some sound from the other end of the ensemble). But the greater the separation figure, the more clearly defined the signal will be reproduced.
Another essential factor in cartridge design and performance is the stylus size and shape. In the early days of LP and into the stereo era, styli were round and tapered to a rounded point. This shape provided good contact to the groove and was relatively inexpensive to manufacture and assemble. Conicals come in two varieties: bonded and “nude.” Trade offs? The bonded type of stylus is less expensive to produce, but adds weight. Conversely, the nude stylus has less mass, but is more expensive.
Needs and inventiveness have developed in the 60+ years since the advent of stereo, and stylus shape has evolved along with other technology. “Elliptical” styli were developed. Rather than the basically round shape of the conicals, the ellipticals appear elongated with a bit “snipped off” the front and rear. The spec sheet for an elliptical stylus cartridge will show two figures – 0.3 x 0.7, for instance – for the stylus size. The larger stylus surface travels the center of the groove, while the narrower radius side surfaces track the groove walls. The result is better tracking and less groove wear.
The elliptical approach spawned a number of offspring. In 1971, the Shibata stylus was introduced. An elliptical design originally developed to handle the technical challenges presented by the CD-4 four-channel disc, the Shibata design was a variation on the elliptical and was capable of carrying the four discrete streams of information embedded in CD-4 disc’s groove. The Shibata shape had a very small horizontal contact surface, so it could handle the 45,000 Hz CD-4 tracks. While CD-4 and the other quad vinyl formats died, the Shibata stylus design lived on. It had a wider contact surface at the vertical level, producing less vinyl wear. In turn, the Shibata design inspired variations known by such names as Contact Line, Ridge Line, MicroLine, and others. Among other things, these elliptical designs tended to more closely mimic the stylus originally used to cut the masters. The idea was that the more closely the playback of the stylus resembled the cutter, the more accurate would be the home reproduction.
One other relevant cartridge specification is voltage output. That’s the voltage initiated by the movement of the stylus in the groove and then converted to electrical energy that is fed to the preamp. It is important to have the cartridge’s output voltage high enough to meet or exceed the voltage input requirements of the preamp. Generally, a preamp will list the phono input’s voltage requirement and the cartridge will list its output. Although there are high output Moving Coil cartridge designs, traditionally, Moving Magnet models deliver greater voltage. Many Moving Coil models may deliver voltage output too low for a given amp or receiver. These generally require a step-up to bring the signal to the desired level.
Despite the industry’s apparent desire to kill it, vinyl is not dead. It is still a vital piece of the audio enthusiast pie and appears to be vibrant. If you’ve got the cash, it’s not hard to spend thousands, tens of thousands, or even hundreds of thousands of dollars on audio components to enhance the playback of the once humble LP record. But today, if you understand the technology and the important specifications, you can protect your record collection and increase your pleasure for a lot less.