Stephen Dawson helps come to terms with turntable mechanics, of equal importance to high performance.

We have talked about the electrical signal produced by turntables: how low it is in level, how its frequency balance needs to be adjusted and how it needs to be protected from stray electrical fields.

But there’s also the issue of creating the signal. We’ve noted that this is done by placing a small stylus on the end of a tiny bar – called the cantilever. The stylus rests in a groove in the rotating LP. The groove wiggles and this pushes the stylus to the left and right. At the other end of the cantilever this motion is translated by a magnet and coil assembly into an electrical signal.

To make all this work well, the stylus must remain secure in the groove, it must pick up only the signal from the recording and it must give the proper weight to the left and right channels.

Isolation
The stylus needs to get all its movements from the content of the grooves and nowhere else. A stylus can’t tell one vibration from another, so a turntable needs to be isolated from non-signal vibrations. These are primarily from three things.

First, there’s the noise of the motor. Within high fidelity circles the preferred drive mechanism is a belt drive. The long rubber belt won’t transmit the motor’s sound to the platter. However some sound may still get through, depending on how the motor is mounted on the body of the turntable.

This can complicated speed changed. Indeed, some belt drive turntables don’t provide a simple switch for changing speed from 33 1/3 rpm to 45 rpm, instead requiring you to lift the platter and move the belt from one position on the drive pulley to another.

Second, there’s noise from the environment. This can range from footstep noises travelling through the floor, into a bench and from thence into the turntable’s chassis, platter and disk, there to be picked up by the stylus. Reducing or eliminating this usually means going either extremely rigid, or highly sprung, perhaps both. If you have a concrete floor with carpet and the turntable on a very solid piece of furniture, most such vibrations won’t get through. Well-designed suspension can also do the trick, and since this built into the bodies of some turntables, they will be well isolated. Note: turntable suspension does not work well when the turntable is placed on wobbly furniture.

Finally, there’s feedback. Sound is also just vibrations. These can be transmitted through solid objects, so that’s best addressed as discussed in the previous paragraph. But it is also vibrations in the air. If you are playing your system loudly and the turntable is near your loudspeakers, the sound can directly vibrate the stylus. Closing the turntable’s lid will get rid of most of that.

Resonance
The stylus and cantilever in the cartridge are a spring-loaded assembly. The tone arm to which the cartridge is attached has mass and is freely pivoted at one end. That makes it a resonant system: a system in which movements at certain frequencies will be magnified, just as the string on a guitar will resonate at a particular frequency.

If your tone arm resonates at the wrong frequency there will be problems. Too high in frequency and, in addition to unpredictable performance result when trying to play back those frequencies in the music, it will also sharply filter out any frequencies below the resonance. So keeping it well under 20Hz is important.

Too low and if the record is warped, even slightly, then the resonance could coincide with and emphasise movements caused by the warp, possibly throwing the stylus from the groove.

The standard industry recommendation is to ensure that resonance is somewhere between eight and twelve hertz. This is determined by means of a chart which is available at any number of websites. A good one is at that for the cartridge maker Ortofon (ortofon.com/support/cartridgetonearm-resonance-frequency). The specifications you need to know are the mass of the tone arm, the mass of the cartridge and its compliance. These figures are published by all reputable manufacturers. A cartridge’s compliance is a measure of its springiness. The lower the compliance, the stiffer the suspension.

 
Weight
One of the most important adjustments is to have the stylus held in the groove with the correct amount of force. Traditionally we don’t use to force units (millinewtons) but weight units (grams). Since the acceleration due to gravity is pretty constant here on Earth, that’s a practical arrangement.

High fidelity ‘tracking weight’, as it’s also called, is typically between 1 and 2 grams. Sometimes lower, sometimes higher. The cartridge maker will specify what’s appropriate, and unless you have very good reason to do so, you should stick to it. Increase the tracking weight beyond what’s specified and in addition to causing unnecessary wear and tear to the groove (remember, the stylus is diamond whereas the groove is cut into mere vinyl), you could run into the mechanical limits on the movement of the cantilever. Imagine if you filled the boot of a car with concrete. Its suspension would bottom-out far more easily than normal.

But neither should you lighten the weight in a misguided attempt to reduce wear and tear. There’s an imperfectly measured characteristic of record playback systems called ‘tracking’, which is how well a stylus can remain in a groove, properly in contact with both walls. A really highly modulated groove could throw the stylus completely out. The tracking ability is in part a function of tracking weight. Reduce the weight and in highly modulated (primarily loud) parts of the recording the stylus may be flung free. Even if not tossed completely out of the groove, it will rattle around within the groove, inflicting random damage to the groove each times it strikes, as well as producing noticeable distortion during playback.

The weight is virtually always applied by means of a counterbalance at the other end of the tone arm. Usually this has markings on it. You first balance the arm by rotating the weight so that it is dead level, with the stylus having zero tracking weight. Then you wind the weight in closer to the arm’s pivot until it reaches the desired weight, as indicated by the calibrations.

Horizontal forces
Even with the proper tracking weight applied, there’s still one more adjustment required for the tone arm. This is called ‘Anti-skating’, and it corrects for the naturally misaligned forces acting on the stylus. This is best explained in a picture. The white dot at the end of the tone arm represents the position of the stylus. The circular arrow A shows the direction in which the LP is rotating. The stylus is resting in a groove, so the force applied on it (aside from the left/right wobbles, which net out at zero over time, and so can be ignored) is in the direction shown by arrow B, the direction being a tangent to the groove at that point. The only other horizontal force acting on the stylus is that from the tone arm, which is preventing the friction with the record drag the stylus and cartridge away. This can be represented by the arrow C. Note the direction of that arrow is from the stylus directly towards the tone arm’s pivot.

While force C mostly counteracts force B, they are still slightly misaligned due to their different directions. The result is a residual force D, which pushes the stylus sideways towards the centre of the record. If you have a test record with a section lacking grooves and you lower the stylus onto that section, the stylus would skip (or ‘skate’) rapidly towards the centre of the record.

This force increases wear on the inner side of the track, increasing the damage for one of the stereo channels, while reducing the effective tracking weight for the other channel, increasing its distortion.

The solution is to apply a small force to counteract this. The easiest way to do that is to drag the tone arm back at its far end (force E in the picture). This is typically done by either a spring mechanism or a small weight. The amount of force required is proportional to the tracking weight. The turntable/tone arm instructions will show you which setting to use if it isn’t clear from the markings on the device.