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Arms

THE PICKUP ARM

 

Our simple guide to pickup arms

 

 

 

armheadshot

 

 

The arm  carries the cartridge and tracks it across a disc. There are two basic types: pivoted and parallel tracking. Most are pivoted, for simplicity and effectiveness. Parallel tracking arms are notionally perfect because they follow the path of the original disc cutter, eliminating distortion from tracking error. However, they are complex and introduce a host of other difficulties.


pivoted--parallel-2


 

The role of the pickup arm is to allow free movement across a disc, yet hold the cartridge steady in relation to it. Pickup cartridges are, in effect,  highly developed and extremely sensitive mechanical vibration sensors. The arm, turntable and outside environment can all affect it, through vibration and noise, degrading sound quality. The challenge is to prevent this and for the arm to do so it must not ring like a tuning fork, a tendency of the metal tubes commonly used, it must have friction free bearings that also lack slack, and it must not be too ‘heavy’ (effective mass) so the cartridge can lift it over warps.

 


Here are the main features of the inexpensive and popular Rega RB301arm -


annotated-captioned-rb301


The headshell, arm tube and bearing hub are all incorporated within a one-piece casting. Absence of mechanical joints improves rigidity and eliminates sonic reflections within the structure.


Light headshell area reduces effective mass, to improve warp tracking and reduce warp signal output.


Small tungsten counterweight is heavy and sited close to the arm bearing hub to reduce rotational inertia and, therefore, effective mass at the headshell.


Bias adjuster sets outward bias force to optimise dynamic conditions, providing balanced contact with left and right groove walls to give best tracking.


A finger lift is fitted for hand cueing.


A lift / lower lever operates an oil damped lowering platform.


New three screw cartridge fixing on RB301 improves cartridge to arm  coupling, improving low level resolution. Suits Rega cartridges only at present.

 


ARM  LENGTH

Most popular are 9in (229mm) long arms because they  are reasonably compact, do a good job and don;t demand a huge plinth. Longer 12in arms were originally needed for playing old 16in studio acetates, but their lowering tracking error appealed to some enthusiasts. 12in arms are making something of a comeback: they look good and sound good, but demand a very large plinth.

 


PARALLEL TRACKING ARMS

A parallel tracking arm moves inward on a straight line passing through disc centre, following the path of the original disc lathe cutter. This eliminates tracking error distortion and also the need for bias force correction. It raises many other difficulties however.

 

Traditionally, parallel tracking arms have taken the form of straight, pivoted arms sitting on driven sled. The elegant but extremely complex B&O 4000 is a well known example. Sensors on the arm would read its angular error to the sled and issue correction signals to keep it positioned correctly. Later, Pioneer and others produced their own parallel tracking turntables based on this working method.

 

Another method is to mount the arm on an air-suspended, frictionless sled and let the arm pull it inward. Tilting the arm track slightly can make the arm  slide inward naturally, so it is possible to nearly eliminate sideways force on the arm by this method. The Cartridge Man’s ‘The Conductor ‘ is one example. Both work well in practice, but are for enthusiasts.

 

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