There’s a lot to know about turntables. Here, Stephen Dawson explains some of the things that used to be second nature to high fidelity enthusiasts.

For many years very few home theatre receivers – even the truly high end ones – carried phono inputs. There was no real need. The only people who listened to vinyl were seriously high-end folk who would use specialised stereo gear, or those people who weren’t inclined to keep up with the times.

That has changed, with the turntable making a limited comeback for a variety of reasons. Most receivers at the higher end now sport phono inputs, in reflection of this market trend. And the market for turntables – after hollowing out, for years with only horrible cheap models and super high end ones available – has filled out with a range of reasonably economical yet high quality turntables.

In the years, though, in which the turntable was largely missing, a great deal of knowledge once second nature to the audiophile has also been lost. Let’s recall some of it.

Why phono inputs?
Despite the continuing move to fully digital systems, just about all amplifiers and receivers still have normal analogue inputs. So why are special phono ones required for turntables?

The label ‘phono’ on certain inputs on a home theatre receiver or amplifier are not just so you know which input to turn the knob to. Even though they use standard RCA sockets, just like those you might use for a CD player or a cassette deck, they are significantly different to the other analogue audio inputs on the device in three ways.

First they are far more sensitive. Regular analogue inputs are designed to allow full volume to be delivered by the amplifier with an input signal of between 0.2V and 0.5V. That’s perfectly suitable for a CD player, cassette deck, VCR and so on. But phono outputs are much lower. So phono inputs will typically allow the unit to go full volume with a signal of between two and five thousandths of a volt (0.002V to 0.005V, or two to five millivolts). That means the input has to provide one hundred times as much gain as regular analogue inputs. If, that is, the turntable is using a ‘moving magnet’ cartridge.

The cartridge is the signal producing part of the vinyl playback system. It sits on the end of the playback arm (tone arm) and has a diamond stylus on a thin cantilever. The diamond sits in the record groove, moving the cantilever. At its other is a magnet. Surrounding that is a coil of wire. Moving a magnet in a coil generates a voltage. A low one, as we’ve seen.

Some high end cartridges are moving coil designs, in which the coil is on the cantilever and the magnets are fixed. Their output is typically ten times lower (0.2 to 0.5 millivolts). Very few home theatre receivers support these. Generally you should use an external high quality phono pre-amp and plug the output of that into an auxiliary analogue audio input on your amp/receiver.

The second difference is the need for an earth connection. Because of the low voltages, special attention needs to be paid to electrical shielding. A phono’s signal wires can easily generate hum and noise from stray electrical fields. It is the tone arm that needs to be earthed to provide the shielding, so in addition to the signal lead a wire needs to be connected to an earthing point on your receiver/amplifier. A special point is almost always provided for the purpose. The other end of that wire is connected, somewhere inside the turntable, to the tone arm.

Wiggly Grooves
The third reason special phono inputs are required is because the signal has to be EQed, otherwise it will sound very thin and reedy.

Records, in their earliest forms (say, before the mid 1920s) were entirely analogue. There were no electronics involved at all. A singer would direct his or her voice into a large horn, which had a stylus connected to its end. That would cut a vibrating groove into wax or some other material. A cast would be taken of this with a harder material, and this would be used to press replicas. The groove was a reasonably close representation of the vibrations in the air which constitute sound.

Someone would buy one of the records, place it on a turntable and insert a stylus into the groove. As the record rotated the stylus would vibrate and a horn would couple it reasonably efficiently to the air of the room, producing an approximate rendition of the original singer’s voice.

In the years after that electronics provided improved quality, the ‘long playing’ (i.e. LP) version was introduced with a 300mm record in vinyl rather than a 250mm record in shellac, rotating at 33 1/3 rpm rather than 78 rpm, and with a much finer groove. And then stereo, which had two channels of sound matrixed into two dimensions of vibration (side to side and up and down) instead of just one. By the late 1950s all that was in place, and that was what remained the dominant consumer music carrier, unchanged in essence, although gradually improved in detail, for three decades (I’m ignoring, as should all readers, the abortive attempts at multichannel sound in the early 1970s). Around the same time the other very important record format came in: the single/EP. This was a 175mm vinyl disc running at 45rpm.

These changes had practical implications. A stylus designed for the ‘microgrooves’ of an LP would not work in a 78 record, and vice versa. A stylus designed for mono playback would wreck a stereo record (which is why both mono and stereo versions were sold for many years).

 
Inaccuracies, intentional and otherwise
You may have noticed me using words like ‘close representation’ and ‘approximate rendition’. Analogue looks good on paper, but physics doesn’t always lend a helping hand.

If the recording system were recorded truly, with a 1:1 correspondence between the groove and the sound, very little music would fit on a disk. For any given volume level, a bass sound has a longer wavelength than a treble sound. That means that when translated to a groove, the wobbles in the groove would be wider for bass sounds than treble sounds.

Much, much wider. The width of the wobbles would double for each octave down in tone. For a given volume level, a 100Hz groove would be twice as wide as a 200Hz groove.

But it’s even worse than that. In the real world most sound, and most music, is actually louder in the bass than the treble. A rough rule of thumb is that, again, going down one octave doubles the amplitude. Combining that with the wavelength issue and you’ve got the 100Hz groove four times the width of the 200Hz groove.

A truly accurate vinyl LP would be lucky to fit a couple of minutes of music on its surface, and the grooves would wiggle so widely that the stylus wouldn’t be able to track it.

All that was nicely dealt with in the early days by the fact that with analogue recording the efficiency of translating sound to grooves fell off rapidly as frequency lowered. In other words, the bass was recorded at a much lower level than the treble. This was only partially corrected with analogue playback systems, so they sounded pretty tinny.

As systems went electronic and improved, the sound had to be artificially adjusted to approximate that analogue imperfection, since it was useful anyway. There were a multitude of clashing standards for many years, but finally in 1954 the Record Industry Association of America settled on one, which most labels soon adopted. RIAA equalisation cuts the bass by up to 20dB (at 20Hz) and boosts the treble (at 20,000Hz) by the same amount when the record is cut. On playback the bass is boosted and the treble cut to reverse this.

In addition to allowing bass to fit on the surface of the vinyl without consuming massive amounts of space, this also acts as a de facto noise reduction, since vinyl hiss would otherwise be objectionable.