A Guide To: Turntables

A Guide To: Turntables

By: George

If you are a newcomer to the world of high quality audio in the home, the mysterious world of record decks may seem a bit daunting. With the myriad of turntables, tonearms and cartridges to choose from and which combination to use, it may seem too complicated to become involved with. But the resurgence of interest in vinyl has occurred for good reason. It's certainly not the most convenient format to use as discs require storage space, can be damaged and the deck itself must be correctly set-up, but the rewards can be incredibly satisfying. With a bit of care and consideration, the results can be stunning – not unlike listening to a master musician playing a musical instrument in your living room. Why this is the case, especially with a medium that's been around in its current form for over half a century, is difficult to pinpoint, but many argue it's the most satisfying source of music in the home. The scientific approach would be to say that analogue replay in theory, allows an infinite “sampling rate”, but other limits are imposed such as record surface noise. Whatever the reason, LP discs can sound powerful and exciting, so without doubt it's a format worth pursuing.

At Hi-Fi Corner, we will do what we always do in the way we know best – listen to your requirements, ask about your existing system and guide you through to an informed and rewarding choice. Nevertheless, we thought you may like to have a few pointers to the design and workings of record decks, and of the different approaches taken by manufacturers to achieve the very best from the format. We cover below the three main parts of a record deck – the rotating platter, the tonearm and the cartridge. Many, if not most record decks are available with a tonearm already fitted by the manufacturer and possibly a cartridge, but it's still worth knowing about each separate bit.

The Turntable



The function of the turntable is to rotate the disc at exactly 33 ⅓ rpm (revolutions per minute), or 45 rpm for single 12” or 7” records. Any wavering from these speeds is known as “wow” for slower fluctuations and “flutter” for faster variations. The audible effect of wow is akin to listening to constant tone, say a piano note which is changing pitch – as if the record is off centre or the piano is the local pub's ol' Joanna. Specifications for wow and flutter should be given by the manufacturer and should typically be less than 0.05% for a quality deck.
Turntable platter weights vary wildly, not only between manufacturers but between models available from each. A range between 1 kg and 40 kg is entirely feasible. It's safe to say that, especially in the case of belt-driven turntables, a heavy platter is a distinct advantage. The greater the weight, the greater the momentum, the greater the speed stability. Of course, a heavy platter requires a motor of sufficient torque to allow it to reach and maintain the required speed, so the cost inevitably rises. A weighty, thick platter will also help dampen down any “stylus chatter”. Imagine holding up a piece of card (the record) and scratching it with a sewing needle. This is a similar effect to that of stylus chatter – you only wish to hear what's in the groove, not the noise a sharp object makes with friction on a thin surface. If you were to glue a thick piece of steel (a heavy platter) to the cardboard and repeat the exercise, the unwanted noise would be considerably reduced.
The turntable drive motor should not physically vibrate to the point where this noise is transmitted to the turntable platter. This is usually via the belt which links the two (assuming it is a belt-driven turntable) or vibration of the top plate which supports the bearing. The turntable bearing itself should be of sufficient quality that it doesn't inhibit the rotation of the platter or cause undue bearing noise, which would ultimately be transmitted through the stylus. This type of low frequency noise is known as “rumble” and again, the quoted figure should be of a low order, say – 60dB. As this is a negative figure, - 80dB would be a better specification. With some world class turntables, the figure is below the measuring capability of the lab instruments.

Some turntables, such as the excellent Technics SL 1200 range, utilize a direct drive motor system whereby the platter is effectively mounted on the motor spindle. There is no belt linkage but this design calls for a motor unit of the highest quality and close tolerances to avoid any audible rumble. The figure quoted for this deck is an excellent -75dB. Incidentally, the wow and flutter figures for this deck are exemplary too.
In the case of belt-driven turntables, some manufacturers such as SME have an outboard power supply to drive the motor. The function of this, other than as a housing for speed change control, is to refine the AC supply to the motor. AC synchronous motors rely on an accurate 50 Hz supply to maintain correct speed. If a DC motor is fitted, the electronics will provide the appropriate supply which is not dependant on the mains frequency. The important point is that the more “refined” the power feed to the motor, the greater the likelihood of a high stability, smooth and noiseless rotation.

Finally, a word about the plinth. It's important that the record is isolated from external (and internal) vibration. This isn't just from party folk slightly worse for wear jumping around, but from vibrations caused by the loudspeakers and even other units on the Hi-Fi stand. This isolation can be achieved in either of two ways. Either the platter is mounted on a sprung sub-chassis which isolates it from the main base such as with the Linn, or some form of dampening material is used in the design and construction of the plinth and/or the turntable platter. The latter is the method most commonly employed on direct-drive decks. An innovative approach by Pro-Ject is to employ a magnetic floating sub-chassis on their Signature 10 turntable to achieve the necessary isolation.

No one method is necessarily better than the other. The important point is that the manufacturer achieves an excellent standard of isolation. Failure to do this will lead to a “muddying” of the stereo image, an ill-defined bass response and an unexciting, uninteresting sound. It will also affect the tracking ability of the stylus to faithfully follow the record groove.

 

The Tonearm

In a perfect world, a tonearm would support the cartridge/stylus at exactly the right position anywhere on the disc, allow for entirely uninhibited movement in the vertical and horizontal planes and entirely absorb any extraneous vibration from the cartridge. This is a tall order, but with a few decades to refine the product, there are good tone-arms available and a few very fine ones.
A basic problem that's been known about virtually from the outset is caused by a tone-arm being pivoted at one end. A record master disc is produced by a tangential cutter working from the edge to the centre of the record in a radial line across the record. With a pivoted arm, in order to mimic the path of the cutter as closely as possible, a slight bend is incorporated to the arm tube, sometimes even an “S” shaped curve. Alternatively, the required angle may be achieved nearer the area of the head-shell. The arm will still not be able to perfectly retrace the exact tangential path of the original cutter, but it's much closer than if there had been no curve or “offset angle” as it's called. Some tonearms are tangential, also known as parallel tracking and here, as the arm tube is straight, no offset angle is needed. It's also worth noting that, because of the geometry of a 12” tone-arm, the tracking path is nearer the desired straight line than is the case with a 9” arm. This is a major advantage of using a 12” SME arm over the 9” model, providing the plinth can accommodate it!
With a tonearm of any reasonable quality, provision is given for fine-tuning the position of the cartridge to for minimal tracking error. Either the cartridge position can be adjusted by sliding it forwards or backwards within the head-shell, or the entire arm can move back and forth to allow for this adjustment. This is known as “overhang adjustment”, and is one of the many line-up procedures we take great care over when setting-up a deck in-store for you. In order to minimize the possibility of tracking distortion, we adjust the overhang for zero tracking error at the point where it is most difficult for the stylus to follow the groove – at the innermost part of the playing surface. Here the grooves are passing the stylus at their slowest and the groove waveforms are most compacted and difficult for the stylus to follow faithfully, especially so at high frequencies. This can give rise to “inner groove distortion” which can be also be largely eliminated by quality arm design such as that of the SME Series V.

Because of the arm offset angle, a new problem arises. The centrifugal action of a rotating disc on the stylus/arm structure with an offset angle along its length, causes an unwanted sideways inward force. If not corrected, the stylus would be pushed against the left-hand side of the record groove wall resulting in poor contact with the right-hand side. In addition to causing more wear on the left wall of the groove, audible distortion would occur frequently during loud passages, especially on vocal sibilance. To counter this effect, a tonearm should have some form of “bias” or “anti-skating” control which applies an outward, right-hand pressure to the arm to oppose the unwanted force to the left. The correct amount of bias is important and is proportional to the tracking weight used for a particular cartridge. Once again, we at Hi-Fi Corner use test discs and listening tests during the set-up procedure to ensure the correct level of bias is applied.

So far, all the adjustments and characteristics outlined will be available on any arm of reasonable quality. The following attributes are the most important for establishing the sonic characteristics of a quality tone-arm– neutrality, clarity, bass definition, tracking ability and so forth. These include controlling arm flexing and resonances as well as the provision of the correct arm mass for the cartridge to work at its best.
In the real world, the cartridge body will to some extent vibrate and resonate when playing a record. Although the desire is only for the stylus following the groove to move the cantilever, inevitably some vibrational movement is transmitted through the cartridge body and on to the arm head-shell and tube. How this energy is dissipated and dealt with by the arm manufacturer is crucial to mitigate the resonances and flexing of the arm.  For instance, the SME series V arm tube is tapered to prevent an organ pipe-type resonance effect of a parallel sided arm tube. It is constructed of a one-piece magnesium alloy, incorporating the head-shell, the large diameter arm tube and the counterweight cantilever which together provides a model light, rigid, non-resonant support for the cartridge. Every detail of tone-arm design has been thought through, and it's worth a visit to the SME website to see just what is involved in a true reference standard product.

 

The Cartridge

The function of the cartridge is to generate reciprocal electrical energy from the mechanical energy generated by the movement of the stylus/cantilever in the record groove. The level of this electrical signal is much smaller than that provided by say, a CD player or music streamer, so additional amplification is required to lift the signal to a similar level. This is partly the reason why a genuine Hi-Fi record deck (as opposed to an all-in-one type intended for vinyl ripping... maybe literally!), cannot be plugged straight into an amplifier line or auxiliary input as you would with a CD player. The output level from the cartridge is just too low. This is applicable to all types of cartridge, whether they be of moving-coil, moving magnet or moving cross (as in B&O cartridges) design.

A question often asked: “Moving-coil or a moving magnet cartridge, which is best for me?” 
Among audiophiles, it's probably a safe bet to say that most regard moving-coil (MC) types more highly. These certainly are capable of producing sound of extraordinary depth, detail and punch but are generally much more expensive than their moving-magnet (MM) counterparts. In addition, moving-coil cartridges require more thought to accommodate properly, as the output is considerably less than their moving-magnetic counterparts. The signal level output of the former is around 0.25 millivolts (mV) or even less, whereas moving-magnet cartridges produce typically 2.5 mV. This demands a pre-amplifier which not only has a phono input for a record deck, but has switchable gain to accommodate a low-output moving-coil cartridge. Usually, amplifiers don't have such an input, so it will be necessary to boost the signal with a “pre-pre-amplifier”, usually known as a phono stage. There are a few moving-coil cartridges which produce an output level equivalent to that of a MM cartridge such as the Ortofon Quintet Red, so this may be a less expensive option if your preference is for a moving-coil cartridge but keeping the cost lower.



A good phono stage will also offer a range of load impedance settings for MC cartridges. We'll cover load impedances in more detail in another article soon, but basically impedance is similar to resistance but includes an element of inductance. This is generated in this case by the cartridge coils, and unlike pure resistance, it's value alters with frequency. This requires a degree of control which is the “loading” aspect. MM cartridges are not nearly as sensitive to load impedance as moving-coil ones because of the way they are constructed, so the load impedance required for MM cartridges is usually a standard 47k ohms.

In the case of moving-coils, the required load impedance can vary from 5 ohms to 1k ohms. The MC cartridge manufacturer should always specify a range of preferential load values, which you should really try to adhere to. The figure required is normally between 5x and 10x the cartridge's own internal resistance, which should also be specified by the manufacturer. It is possible to utilize a MC cartridge transformer to raise the output level, in which case the loading required will be between 1x and 5x the cartridge's resistance, somewhat less than with a phono stage. The advantage of using a transformer is that, because it is a passive device, it doesn't generate any electrical noise itself to the signal path. The disadvantages are that they have fewer loading value options than a phono stage, and can be sensitive to the load that they in turn are plugged into, just to complicate matters further!

The consequence of incorrect MC cartridge loading, or mismatching as it's sometimes called, won't be to cause any physical damage but the sound character of the cartridge will not be as the designer intended. The sound quality may seem too dull or too shrill, the image ill-defined or maybe just very uninteresting and not very dynamic. If you already own a phono stage that you wish to utilize, and are thinking of purchasing a new cartridge, make sure that the recommended load impedance options are available. We at Hi-Fi Corner will, of course, assist you to ensure a winning combination of cartridge and amplifier.

Finally, it's worth mentioning RIAA equalisation at this point. This is a standard which has been set by the Recording Industry Association of America as a specification for the recording and playback of records. Basically, when the record master is produced, low frequencies are reduced and high frequencies are increased. This allows for more grooves to be cut onto the disc and extend playing time, as low bass frequency grooves are wider than higher frequency ones. In addition, the stylus can ride the reduced width grooves with more ease, reducing the likelihood of tracking distortion. In turn, a phono amplifier will boost the bass frequencies on replay by a reciprocal amount, so a “flat” response is achieved as on the original recording.

 

We hope you found the above a useful “map” for your vinyl replay journey. Feel free to get in touch or drop by into our stores for demonstrations and advice on the carefully chosen, quality decks we have available.

Happy spinning!

 

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