How Turntables Work Part 2

Turntables – Part 2

Last time we looked at everything up to the tone arm, now we shall continue starting with the stylus itself. It is often called the needle. Usually made of a very hard precious stone such as diamond, the stylus is the actual part that sits inside the groove and follows the music signal. It is free to move around somewhat, following precisely the undulations in the record. These undulations related directly to the frequency and the volume of the sound. So large “waves’ are low frequencies and small ones are high frequencies. The actual size controls the volume. Each side of the groove is the appropriate channel of the stereo signal. So the stylus tracks along at the set speed and move around in time with the undulations. The size and shape of the stylus is then an issue too. Make it too small and it wont pick up all the signals and be too fragile; too large and it will sit too high. The wrong shape and it can wear out the record quickly. Ultimately there are compromises on the shape of the stylus and most these days are of an elliptical shape.

The stylus is attached to a small arm called the cantilever made from non-magnetic metals or even carbon fibre and this arm has either magnets or coils of wire at the end. Yes you guessed it, respectively moving magnet (MM) or moving coil (MC) cartridges. Moving magnet types have the signal generating coils mount in a fixed position inside the cartridge at 90° to each other and the magnet cause minute electrical currents to be generated in the coils, one coil for left, the other for right. The magnets these days are very powerful so can be made quite small. Moving coil is much the same principal but the magnets are the parts that are fixed inside the cartridge. This type generates a much smaller signal than the MM types and also has wires attached to the stylus’ coils. MC also needs a secondary stage in amplification above that of MM. We’ll look at this shortly. So which is better? A much wiser person than me is required to answer this one, the debate has rage for ages and there is no simple answer. It can be said however that MC setups generally cost more all else being equal due to the extra step-up stage being required.

Styli can often be repaired; the tip can be repaired and replaced if required. Generally the more expensive ones are open to this as the repair cost is generally a lot lower than the new price! Cleaning is important too, as it plays, the stylus also tends to collect dirt from the record so it is best to make sure the record is clean to start with but inevitably the stylus will need cleaning. Brushes are the most common but now you can also use special sticky gels that will form around the shape and remove almost everything.

So now we have our music signal off the record but it is a very small signal, measured in the millivolts (MM) or microvolts (MC). Fine high-grade wires carry the signal down the tone arm and out of the turntable to the phono pre-amplifier. Some systems will just use the phono amp built into the amp or receiver but stand-alone ones are often better quality. Some even have separate power supplies for the ultimate in performance. Phono amps are a special type of amplifier; think of them as pre-pre-amps! They take the very small signal and amplify it to a level that is similar to what is now know as a normal line output level, around 1- 2V. The signal for MC cartridges is a lot smaller than that from MM so they have to have a step-up stage to make it the same level as MM before it gets amplified further. So a lot is going here and the quality of the phono amplifier has a lot to do with the final sound quality.

But there’s more! Due to the physical limitations of vinyl, the recorded signal is adjusted before it is recorded using a process developed by the Recoding Industry Association of America (RIAA), in use since1954. Before that, virtually every record manufacturer used their own style of equalisation to solve the issues. Simply put the low frequencies would cause the stylus to jump out of the groove plus reduce the time playable on the record due to the larger grooves while the high frequencies would be too small to be reliably reproduced. So the response is alter to cut back the low end and boost the high end. This has to be reversed and is done in the phono amp with an electronic circuit, though some manufacturers use transformer-based systems. Again the quality of implementations is crucial as is the tracking of the exact frequency curve. It is regarded as being as important as getting the speed right on the turntable. Finally after this, the signal can continue its way to your normal pre-amp or receiver.

One area we haven’t looked at due to their rarity and cost are laser-based turntables. These systems use a laser beam to track the grooves and the resulting modulation of the laser is amplifier and processed to reproduce the sound. While they do not cause wear and tear, they are still susceptible to dust and dirt plus some are unable to playback coloured vinyl.

Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.