Last month we discussed the parts that underpin a turntable -- the plinth and platter -- and the ways different types of electrical motors can cause the platter to spin. I also made a comment about turntables being high maintenance that irked an old audio pal (sorry, Reg), but I calls ’em as I sees ’em. Some might argue that the ritual of cleaning an LP and stylus and checking the balance and tracking accuracy of a tonearm is part of the fun of spinning vinyl. And math is part of the fun of doing taxes.
Still, there’s a big place in high-end audio for the record player, and we won’t shirk our responsibility to explain how the things work. Electrically, the record player is a simple device, especially when compared to the compact disc player. The latter operates on the same principle as the record player -- spinning a software-carrying medium and converting the information contained thereon into an electrical signal. But where the disc player uses a laser to read information, the record player makes use of a decidedly primitive device: an electrical generator.
Lower your arched brows, folks. I’m not trying to put one over on you. Take a look at the tonearms pictured here and you’ll see a small box fitted to the front end. That’s a cartridge, and inside that cartridge are the makings of a tiny electrical generator. At its most basic, a generator requires two things to make electricity: a magnet and some coiled wire. Look at that last sentence closely and you’ll make out the origins of the two most common types of cartridge: the moving magnet and the moving coil.
Before we explain those, let’s reconsider how a generator works. First you take a magnet and move it toward some wire. Conversely, you can take some wire and bring it close to a magnet. That’s it. Of course, to make a continuous flow of electricity you’ll want to automate all that movement by, for example, rotating a coil of wire inside a field of magnets. The interaction of the magnetic field and the wire causes a current to form in the wire and, thus, electricity is sparked. Too obvious?
A moving-magnet cartridge generates electricity by -- you guessed it -- moving a magnet within a coil of wire. But what causes the magnet to move? Well, in the tradition of "the foot bone’s connected to the leg bone," the stylus moves the magnet and the ridges within a record’s groove move the stylus, thus beginning the progression: ridges, stylus, magnet, coil, music.
I know what you’re thinking: "There’s not a lot of space inside that cartridge, so the small generator must make only a tiny amount of electricity." Right you are. In fact, the signal is so small that record players need a special type of preamplifier just to bring them up to what’s known as "line level." But we’re getting ahead of ourselves.
A moving-coil cartridge works in the same way as a moving magnet, but it reverses the position of the constituent parts of the generator. Whereas a moving magnet has a magnet at the end of the stylus, a moving coil has a set of coils (two for stereo) that moves within fixed magnets. Now, because you can’t fit a lot of coiled wire on a stylus (but you can comparatively fit a lot more within the cartridge housing) the electrical output of a moving-coil cartridge is much lower than a moving magnet cartridge can manage. This explains why there are two kinds of phono preamplifier, one for each sort of cartridge.
By now everyone has figured out that out that the "stylus" is a synonym for "needle," which is what most non-audiophiles call it. Most styli are diamond tipped, but as any about-to-be-engaged young lady will tell you, there are diamonds and then there are diamonds. There are also many ways to shape them, and this too is reflected in the design of various styli, with the higher-priced ones tending to employ more exotic angles. By way of example, just within the middle-range Aurum series from Clearaudio -- a big name in little tips -- prices for styli range from $165 to $653, so you can see there’s a lot of scope for mucking about with diamond quality and shaping.
A cartridge is of no use unless it’s suspended from some kind of rigid structure -- let’s call it a tonearm -- that keeps the stylus exactly where it needs to be: in the groove. But like so many things in life, it’s not that simple. The tonearm must neither push the stylus into the groove too forcefully -- dulling or even bending it and damaging the fine pits and valleys stamped into the vinyl -- nor allow the stylus to float about within the groove like an untethered astronaut. The tonearm’s job, if you will, is to find the Goldilocks spot, where the pressure is just right.
Returning to the low level of a phono cartridge’s electrical output, we can see why it’s so important that external vibration be kept away from the critical stylus-cartridge-tonearm interface. With so little current being created, it’s easy to imagine how even the smallest unwanted vibration could completely overwhelm that tiny signal. So in addition to having a properly damped platter and plinth, a turntable also has to have a rigid, non-resonant tonearm.
Keeping the tonearm vibration-free isn’t easy, but a well-engineered one will tend to employ shaping and materials to ensure that the only thing being picked up at the phono preamp is the current induced in the stylus by the record. Most arms combine a tapering tube made of metal alloys, or straight arms made from carbon fiber, or some combination of both. Many arms are straight, but some are bent into very wide Ls. Some might be 9" long, while others are 12". Some don’t even look like arms. Cantilever designs look a lot more like miniature heavy-lift cranes than parts of a record player. There are even higher-end turntables that can be fitted with two or three tonearms, each one designed for a different sort of sound.
Tonearm complexity doesn’t end there. The fulcrum, where the arm achieves balance, might be as simple as a very fine point of a very hard metal pivoting in a small indentation on a flat surface, or it could be as complicated as a joint in the Canadarm. And then there’s the counter-weight fitted at the back end. Is it a saddle type, or a ring encircling the back of the arm? Is it held in place with a simple set-screw, or is it fitted with a complex ratcheting system that allows it to move in tiny, measured degrees?
Inside the tonearm (or outside in some cases) reside the leads, which emerge from the cartridge and wend their way back toward whatever is used to output signal from the tonearm -- sometimes RCA jacks, sometimes an interconnect cable hardwired on the tonearm end. If you take a close look at the back of a cartridge, you’ll see that there aren’t two leads, as you might expect in a stereo setup, but four. This is because for a stereo there must be one positive and one negative per channel, thereby necessitating four leads.
Boy howdy, there’s a lot more to tonearms and cartridges than meets the eye! So much, in fact, that we’ll push phono preamps, record clamps, record cleaners, and other accessories to Audio 101 Part 10. See you next month.
. . . Colin Smith
colins@soundstagenetwork.com
