Tonearms 101 – An Introduction

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Hadcock GH228 Super – Unipivot Design

Introduction

Unless your turntable comes with its manufacturers own tonearm you will need to select an arm to suit the turntable and carry your proposed cartridge. The problem for the newcomer is the huge selection and range of costs for these very important vinyl tracing wands.

So what is the function of the tonearm?

The tonearm is of paramount importance in optimizing the performance of your cartridge. The arm’s rigidity, mass, bearing & damping all play a role in extracting your cartridges peak performance.

The tonearm has to support the cartridge in its correct position over the entire record, while allowing it to move inward to the centre of the record and navigate any vinyl surface imperfections. This requires the arm to hold the cartridge at a constant height and angle as it traces the groove towards the records center. While at the same time keeping the downward stylus tracking force and anti-skating bias force constant. (The bias may be reduced a little as the stylus moves to the records center). Finally, it contains the cabling that carries the cartridges electrical audio signal to the pre-amplifier.

Some of the grooves undulations are minute, being smaller than the wavelength of light. So any movement of the arm in relation to the traced groove modulation will impact the final electrical signal. The rigidity of the armtube is of paramount importance, as is no movement or ‘play’ in the arms pivot point. The pivot, ideally, should provide no resistance to the armtube in any direction as it traverses the records surface. The armtube has to be stiff, carrying the cartridge at one end and a heavy counterbalance weight at the other and yet light and strong. Armtube materials include, stainless steel, aluminum and carbon fiber. Remember that the stiffer the arm the more likely it is to resonate or ring when ‘energized’ by the vibrations from the stylus that get transmitted through the cartridge body and into the arm. These need to be absorbed or damped so as not to ‘color’ the original sound by being reflected back into the cartridge. Getting all these mechanical issues just right, and a few more, is what makes a great tonearm.

Tonearm Design

When the analog master is cut on a lathe its cutting head, that is carried on a ‘sled’, travels in a straight line from the outside to the inside of the disc.

Neuman Cutting Lathe

The more popular pivoted arms form an arc as they move across the record so the stylus will always be at a slight angle to the way the groove was originally cut. While careful geometrical design of the arm can reduce this tracing error it cannot remove it except at best at two points, see here. This angular offset will produce a very small amount of tracing distortion but in a geometrically well designed and setup arms it will be minimal.

Parallel Versus Pivoted Arm Vinyl Tracing

Also the stylus has a centripetal force acting on it due to the records circular motion. This causes uneven pressure on each side of the stylus that must be compensated for or uneven wear on the groove walls and tracing distortion will result. An opposite and equal force therefore needs to be applied through a bias or anti-skating mechanism. It is created in various ways based upon the manufacturers preference, from simple threads and weights or levers and weights to more complex magnetic and spring arrangements.

Hadcock GH 228 Unipivot Arm String & Pulley Bias System

As previously mentioned tone arms must be very rigid and geometrically correct. This gives rise to three basic arm shapes:

1. Straight arm with the offset headshell or cartridge mounting holes, such as the Hadcock GH228 shown above.
2. Curved arm that offsets the headshell, such as the SME 3009.
3. ‘S’ shaped arm that offsets the headshell, such as the SME Series III.

SME 3009 Arm – Detachable Headshell & Pulley Bias System

SME Series III – Classic ‘S’ Shaped Arm & Pulley Bias System

In the interests of rigidity some arm manufactures will not provide for the headshell to be easily removed through some form of ‘quick disconnect’. The advantage of this is that, as shown by the GH228, the headshell can be rotated on the armtube; a requirement for correct cartridge alignment. Some companies like Wilson Beseech have created extremely strong and rigid arms using carbon fiber composite technology. These arms are many times stiffer than any metal equivalent. Clearaudio has introduced the use of magnetic bearings that ‘float’ the arm pivot connection with virtually no pivot contact at all.

There are four basic tonearm designs that revolve around the pivoting method used:

1. Parallel
2. Gimbal
3. Unipivot
4. Magnetic

A number of other niche designs do exist each trying to improve on these basic designs. For example the dual arm Dynavector and the tangential arm by ViRa.

Cartridge/Arm Cabling

In all cases the cables that connect to the cartridge provide a challenge in exiting the arm as they generate some degree of resistance and friction. The least problematic approach is to have them exit the arm immediately adjacent to the pivot and drop down a central hollow pivot. Exiting at any other location often requires the wires to be “dressed” in order for them not to impact the arms performance. Often they can be arranged so as to simply work as part of the anti-skating force. In all cases these wires need to be VERY flexible and provide very high conductivity. They are each made from many strands of very fine wire and frequently in higher quality arms made from Litz wire or  silver plated wire and covered with a very light weight and flexible silicon insulation.

Examples of Cartridge Cable Paths Leaving The Armtube

Parallel Arms

Lets start with Parallel Tracking as it was, believe it or not, the first type of mount for a stylus. Yes those early cylinder players by Edison and the Dictaphone were all parallel trackers. These days they are highly complex and therefore expensive but do overcome a number of geometry issues alluded to above that all other designs have.

The Original Parallel tracker

The parallel tracking arm tries to mimic the straight line path of the cutting head stylus across the entire surface of the record. The pivot that holds the arm is mounted on some form of moving sled. This type of design doesn’t require an anti-skating mechanism and as the arm is much shorter it is correspondingly much stiffer also having a much lower inertia due to its lower mass.

The most challenging part of this design is the horizontal sled. If the arm has no side to side pivot movement, non concentric vinyl will require the sled to continually adjust it’s position as the vinyl rotates. Also the sled has to be extremely low friction using air or magnetic techniques or needs to be controlled by a complex set of motors and sensors. There are a number of variations on this design theme that use the sled to carry a horizontally pivoted arm thereby requiring less sensitive control and allowing easier tracking of non concentric vinyl. There are also some pivoted arm designs that use a lever system that cause the cartridge to track straight across the vinyl rather than creating an arc. Either way, this makes for a complex and fussy tonearm which are expensive. However they are claimed to provide the very best of performances when correctly aligned.

Gimbal Arms

Single & Dual Gimbal Designs

This by far the most common and popular of designs. The pivot contains two sets of bearings (ball, knife edge and/or jeweled) at 90 degrees to each other at the pivots point of balance. Correctly aligned these bearings will provide good rigidity and low friction freedom of movement. Placing the pivots center of gravity level with the stylus maintains a constant force over warps. While placement of the horizontal bearing at stylus level will minimize any back and forth motion that would occur if the bearings were above the stylus.

The Classic SME 3009

These bearings have to provide complete freedom of movement in both planes while at the same time zero play. Clearly zero play would impact the bearings frictionless freedom of movement. Some ‘play’ must therefore exist within the bearing assemblies. This ‘play’ results in ‘bearing chatter’ as the arm moves and vibrations are transferred from the stylus to the arm. It is the optimization and adjustment of these bearings that takes time and is very important.

These arms also have two other significant challenges:

1. The bearings provide a boundary where arm resources will be reflected and/or produced.
2. Over time the bearings wear out and go out of their fine initial adjustment and will require adjustment and/or replacement.

However you look at it, gimbal arms are a compromise, no matter how well they are built, aligned and ‘tuned’.

These arms are however the easiest by far to set up and correctly align to get the best performance out of your cartridge and arm assembly.

Unipivot Arms

Hadcock GH228 Super – Unipivot Design

Four Ball Unipivot Bearing

Seven Ball Unipivot Bearing

At first look unipivots look to fly in the face of all the major requirements for arm stability and rigidity. After all, the unipvot is just that, an arm that rests on a single point that can ‘wobble’ all over the place, having no torsional rigidity at all; twist the cartridge/headshell and the whole arm twists. The pivot is created by a hardened steel point sitting in a metal or precious stone cup or in the case of the Hadcock, the circular ground point resides inside a minature ball race bearing providing significantly more stability while giving up a minute amount of additional frictional loss.

The unipivot gets its stability by arranging for the counterbalance mass to be attached to the arm in such a way that its inertia, at music frequencies, keeps its stable. Also fluid damping is often applied to these arms (see later comments).

The stability is usually achieved by moving the arms center of gravity well below its pivot point by low slung counterweights or plenty of weight under the pivot. This produces an inherent problem for warped records. As the stylus is forced to move up and down it has to lift or drop the arms center of gravity with all its associated inertia, NOT A good thing as this can give raise to significant changes in stylus tracking pressure. Unfortunately the addition of damping fluids while improving resonances can exacerbate this problem.

The second approach is to move the center of gravity to just below the pivot point. While this improves the arms ability to deal with warps by keeping the forces required for both vertical and horizontal movement similar, it also makes the arm more ‘wobbly’ than the first approach and more difficult to cue.

A third approach is to partially constrain the arm so as to restrict its movement and wobbling. This can be achieved by adding a vertical bearing sleeve assembly, similar to that of a turntable, that allows the horizontal twisting movement but mounted so as to allow the vertical “nodding” movement of the arm. Supporting virtually no load this vertical bearing can be designed using materials that are almost frictionless.

What all these arms exhibit are very low friction bearings that never need adjustement and generate no bearing chatter. Problems like arm resources still exist and must be dealt with just as with all arm designs.

Typical low center of gravity arms would include: Hadcock, Mayware Formula IV, Kozma, VPI, Moerch and ARO; high center of gravity would include: Audiomeca and Graham.

Magnetic Arms

The above drawings show three basic configurations of how magnetic forces may be used to suspend or support an arm assembly.

In many ways these arms are similar to a unipivot in that the arm, while supported and constrained by a magnetic field, rotates about a point support. The support point is so small that it is virtually frictionless. With no bearing contact as such, there isn’t any bearing chatter, and damping can be added to aid in arm and resonance control.

Several companies manufacture magnetic bearing tonearms. However two companies; Clearaudio and Schroder are well known for their designs.

Tonearm Effective Mass

The mass of the arm assembley is a two pronged sword. In the case of unipivots the arm requires a high mass in order to keep it stable but on the other hand a high mass arm has high inertia that offers resistance to the arms movement controlled through the delicate stylus. So it might be misconstrued that all arms should be as low mass as possible. There was a time when very low mass arms were extremely popular and the heavier unipivots fell out of favor. These low mass arms were very “flimsy” and quite resonant, and in order not to be deflected by the cartridge stylus as it traced the groove the stylus mounting compliance (stiffness) had to be very high (very ‘floppy’). If not they wouldn’t track warped records very well.

It is therefore essential to match the compliance of your cartridge to the effective mass of the arm assembly. Why? The ‘spring’ of the stylus mount in conjunction with the effective mass of the arm creates a resonant frequency. You do not want that frequency to be in the audible range nor do you want it so low so react to warps. Ideally it should be between 7Hz and 15Hz. So you do not mount a high compliance cartridge in a high mass arm or a low compliance cartridge in a low mass arm.

Many arms these days can be considered to be medium mass just as many cartridges are considered to be medium compliance. So matching them is not too big of an issue. Also some arms are very tolerant to a wide range of compliances. I.E Hadcock GH228 Super.

Never the less you should always ensure that the resulting resonance of your arm cartridge pairing is between 7Hz and 15Hz. To calculate it see here at VinylEngine.

Arm Damping

As the stylus traces the vinyl, energy enters the arm through the cartridge body. If not absorbed it can be reflected back to the cartridge, color the sound and cause severe low frequency arm resonances that in an extreme can cause the stylus/arm to jump out of the groove or create miss-tracking. This damping is achieved either magnetically or more commonly by the use of silicon fluid. Arms like unipivots can be made more stable in both planes by adding silicon damping to the arm pivot well assembly. Other arm designs add small paddles to the pivot assembly that are submerged in troughs of silicon fluid.  The degree of damping is often made variable allowing the arm to better support a wider range of cartridge designs. These damping assemblies absorb arm resonance/vibrational energy just like the dampers do on a cars suspension by turning it into heat.

Fluid Damped Pivot Assembly

Geometrical Issues

If the arm and cartridge pairing is to provide its best performance there are a number of critical dimensions and angles that need to be correctly set for all arms. Unless the arm comes pre-mounted with your turntable you will need to mount your selected arm at its optimal pivot to spindle distance and correct arm height. This will ensure that all required offsets, distances and angles can be correctly setup. Follow the arms installation directions exactly. For further information on setting up the cartridge and arm see here.

With enough time, effort, money and patience, all four major design approaches can be made to perform exceptionally well. However, for the novice the gimbaled arm is without doubt the better arm to start of your audiophile life. It is both robust and relatively straightforward to setup. If you have the patience unipvots can provided quite exceptional performance but often require a lot of time and patience to correctly set up. Lastly the parallel tracker is theoretically the best of all but again require some serious expenditure and time and patience to extract its ultimate performance.

In ALL cases it is worth every second of your time to correctly setup the geometry of your arm and cartridge in order to release your cartridges full potential, and of course not damage those pristine vinyl collections. Don’t forget you will also need a suitable analog test LP.

See here for The Basics of Cartridge and Arm Alignment.

See here for the re-build of my Hadcock GH228 Super unipivot arm.

See here for an introduction into Phono Cartridges.

See here for a review of The Ultimate Analog Test LP.