Below are the ideas behind the RP1-XG, as I expressed them in 1983. Most of the concepts are still valid today and would serve as a good starting point for anyone designing an arm. Interestingly, many current arms still have not adopted what might be considered essential features, and are effectively 50-year-old designs.
Description of the RP1-XG design (Written in 1983)
Introduction
Although there would appear to be an enormous number of varying tonearm designs, in actuality there are not really so many, and for each general category, the majority are derived from a common ancestor. However, there are many detail variations within the basic types and these are important in achieving the desired design objectives.
RP1 Origins
The RP1-XG tonearm evolved from an earlier design which was in production from 1981. That arm explored some of the principles which are put into effect in the XG, which is an abreviation for Export Gold. The geometry of the XG is based on the same theory as the earlier arm, and is optimised for LP records.
Objectives
The actual arrangement of the component parts of the RP1 was arrived at with due consideration of the objectives of a tonearm, namely, to let the cartridge maintain a fixed position in relation to the record groove, and to allow it to operate in such a manner that its output is least affected by external influences. These are: vibration coming from the air, from the turntable via the disc and arm base, and from the arm itself. The arm should also preserve the correct attitude of the cartridge, enabling it to function properly and ensuring that tracking distortion, bias compensation and downforce are properly dealt with.
Principles
The RP1 design involves two main principles. The first is to approach matters with a view to making the arm suitable for use with a number of different turntables, both sonically and dimensionally. In other words, for it to have some measure of universality rather than being designed completely without consideration of adjacent equipment, or to suit only one turntable or cartridge. The second principle is that of asymmetry, whereby there is an intention to avoid resonance problems by discouraging their occurrence which depends on symmetrical distributions of material masses.
Viewed in three planes, the RP1 can be seen to be asymmetrical: the arm tube is offset vertically from bearing and counterweight, the vertical moving parts are offset horizontally from the main pillar, and the cartridge and counterweight are displaced from the arm tube axis as viewed from the front. The centre of gravity is substantially below the pivot thus adding a measure of stability due to the self-restoring force.
Materials
In terms of material content, the RP1 uses brass and aluminium alloy, the brass being 24ct hard gold plated, and the aluminium alloy anodised, which hardens the surface of the alloy and helps interface the two materials, which were chosen to present to the arm tube a changing reference mass with non linear transmission properties.
For a tonearm which is liable to be used with turntables with totally disparate mounting board designs and materials including plywood, compressed fibreboard, acrylic, aluminium alloy and other metals, it is essential that a fair proportion of vibrational energy is absorbed in the tonearm in order to reduce the influence of its junction with the turntable. In the RP1 the mechanism for this is the alternation of the transmission properties of the materials in a manner which serves to contain more energy within the arm, but has it dissipate in an irregular way, another instance of asymmetry.
Rigidity
This is a factor which is often confused with zero play in bearings, an aspect which is considered to be of singular importance. However, a tonearm with this particular feature may not be a rigid structure, inasmuch as components other than the bearings contribute to rigidity. Also because our definition of rigidity is concerned with vibration rather than steady state forces, a structure which is rigid at one frequency may not be so at others where resonances take effect.
The perfect example is a bell which, by most standards is a very rigid structure, yet exhibits sonic characteristics which are undesirable for a tonearm. The thrust behind the rigidity theory is that increasing stiffness for a given mass will raise resonant frequencies, so taking advantage of the RIAA equalisation which reduces the effects of resonances as they increase in frequency.
Compromises
Obvoiusly the design process has to involve compromises and in the case of a tonearm, more than most. In a sense, the tonearm is the meat in a sandwich consisting of the turntable on one side and the cartridge on the other.
This leads to many constraints on very basic areas which have direct bearing on performance. For example, if a cartridge has poor damping then excessive vibration is fed into the arm tube, or, if it is too compliant, there is the crazy situation of requiring arms of only a few grams effective mass. There are variations in cartridge weights from 2 to 20 grams which require counterbalancing arrangements of ridiculous range.
As far as turntables are concerned, some have mounting arrangements which allow only 25mm between armboard and record surface which leaves precious little room to incorporate some desirable features. All but a small handful of decks are arranged such that only a nominal 230mm arm length is really suitable, and many have apparently been designed with tonearm termination very much an afterthought, or to suit only one arm.
Damping
There are compromises too in the basic problem area of damping. Ideally, cartridge induced vibration should be kept within the cartridge and dealt with by the cartridge designer, but if it is transmitted, then it could be terminated by clamping it against a high mass which requires a great deal of energy input to excite resonances. The idea of using mass is put to quite effective use in turntables where platter bearings are basically restrained unipivots, free to move vertically, but constrained by their weight. Unfortunately a high mass headshell is not compatible with current stylus suspensions, never mind the difficulties created by tonearms weighing kilos rather than grams.
Another approach to absorbing vibration is to use light weight structures, but incorporate lossy materials in conjunction with them to act as the damping medium. An analogy is with a loudspeaker oudoors where there is a large volume of air which the speaker cannot excite into resonance (high mass) and the small anechoic room, the air in which (low mass) could be excited but is damped by wall panel material and room construction. It is desirable to try and obtain the best of both worlds though this is not a simple matter.
Tonearm Construction
While the main features are covered in the brochure, there are a number of general points worth looking at. In its construction, the RP1 utilises a bonding agent in addition to push or press fits in all joints which are not held by fasteners. In the case of the headshell to armtube joint, there is a small quantity of damping material applied internally, and where the counterweight sleeve is fitted there is viscous material used in the adjustment screw. The important interface between arm tube and its supporting pillar is a strong clamped joint.
The bearings on the RP1 are 1/2 " diameter radial ball races, those for the vertical movement being clamped together as a pair while those for the horizontal movement are spaced apart by some 20mm. The clamping arrangement allows for less chance of small flats being created on the balls and raceways during adjustment (brinelling), which causes permanent notchiness. The offsets and asymmetry of the arm mean that the tolerance for play can be greater without detriment to the sound.
Electrically, the RP1 uses gold plated connectors with four colour 10x0.1mm copper, pvc covered wires within the tube, terminating in four separate gold plated sockets which fit a four pin plug mounted on the arm support. The internal wiring through the bearings consists of four twisted pairs of fine insulated wire which together with an earth lead of the same wire, terminate in a DIN socket wired in a non standard fashion to allow the use of floating phono inputs, without a common signal ground.
Styling
For any product, styling is of considerable importance, and a great deal of time and effort was taken over the layout and appearance of the RP1, in particular to the proportions of the three main masses of the armtube support, main pillar, and lifter. Also, the arm rest arrangement and the proportions of the non-functional parts were given much thought. The result is one which has caused much comment in the past, both favourable and otherwise. Whichever way you look at it, once experienced, the Odyssey RP1-XG is sure to be remembered.
ARMTUBES
A drawing of the original 12" J-shape armtube.
The sketch above shows how a straight 12" tube for the XG needs a headshell with a shallower angle than the standard arm, and also that the arm mount rotates slightly away from the perfect linear offset axis, an amount which in practice is negligible.
Actual shape of the tube can be anything as long as the pivot and cartridge mount remain in the same place. The resonant character varies, and the effective mass. The sketches below show a block headshell with a J shape tube bent to obtain the offset, and how the standard headshell could be used with a variety of shapes, while still maintaining the required offset.
BROCHURES
Below is the first brochure from 1981, with the ideas behind the design.
The XG brochure had a more flowery text...
