Besides the importance of shielding techniques, grounding configurations and insulation materials, a cable manufacturer should equally take into account a number of parameters that will help in controlling the electrical characteristics of conductors in order to achieve optimum performance in any given application.
One of the determinant parameters for the effectiveness of a cable is the electrical conductivity of signal conductors. Electrical conductivity is the ratio of the current density to the electric field strength and it measures how well a material can accommodate the movement of an electric charge. The values of electrical conductivity are measured in units of SI (Siemens per meter), but they are also reported as a percentage of IACS (International Annealed Copper Standard). The conductivity of annealed copper (annealed copper standards: density of 8.89 g/cm3, length of 1 meter, weight of 1 gram and resistance of 0.15328 ohms) is the benchmark and it is defined as 100% IACS at the temperature of 20°C. The conductivity values of all other materials are measured as a percentage of IACS compared to the value of annealed copper. The below table presents a classification of metals according to their conductivity values as a percentage of IACS:
CONDUCTIVITY (as % of IACS at 20oC)
Tin & Ph. Bronze 15%
Ni Al. Bronze 7%
Steel 3 to 15%
By reading the data of the table, we can assume that copper, silver and gold are excellent conductors of both electricity and heat and the best way to increase their electrical and thermal conductivity is to decrease their impurity levels. Nowadays, the cable industry has amazingly improved the processing techniques of these metals, offering purity levels that exceed standard values reported on the list of IACS. A good example of that could be a copper conductor of 99.99997% (6N) purity that is able to provide a greater conductivity levels than the benchmark value of the annealed copper, approaching even the IACS value of silver.
Electrical resistivity is the reciprocal of conductivity and it measures the opposition of a material to the flow of an electrical current that passes through it, resulting in change of electrical energy into heat, light, or other forms of energy. The electrical resistivity is measured in ohms per meter (Ω/m) and the resistance values of a conductor depend upon four main variables:
1. The first variable is the length of the conductor which will affect the total amount of resistance, where the longer the conductor is, the higher the resistance will be.
2. The second variable is the cross-sectional area of the conductor which will affect the amount of resistance, where the higher the cross-sectional area of a wire is, the less resistance to the flow of electric charge will be.
3. The third variable is related to the type of metal (or metals) that the conductor consists of. As stated on the “Electrical Conductivity”, some materials are better conductors than others and therefore can offer less resistance to the flow of charge.
4. The fourth variable is related to the purity of the metal and is able to affect the resistance in a manner where the higher the purity of the conductor is, the less the resistance to the flow of the electric charge will be.
A very interesting research conducted by Nakane, H. Watanabe, T. Nagata, C. Fujiwara, S. and Yoshizawa, S. at the Science University of Tokyo, analysed how the reduction of metal’s impurities resulted in a reduction of electrical resistivity. The four scientists based their research on high-purity copper and they measured the resistivity of this metal by implementing a method that estimates resistivity using the difference in the impedance of a circular multilayer solenoid coil with a cylindrical copper core and an identical coil without a copper core (SRPM method). The residual resistivity ratio (RRR) of high-purity copper measured at 100Hz, which is a level that correlates well with the values measured by the DC four-probe method. The results showed the existence of frequency dependences at very low resistivity values for high-purity copper and as the frequency was raised, the skin depth seemed to affect the surface resistivity due to the oxide coating and dirt on the surface of the samples. In addition to that, they discovered that below liquid N2 temperature (77.3K), the resistivity of the copper sample with high purity such as 99.9999% (6N) or 99.99999% (7N) fluctuated at significantly lower levels than that of the copper sample with 99.99% (4N) purity.
By definition, skin effect is the tendency of alternating currents to flow near the surface of a solid conductor, thus restricting the area that they use to a significantly smaller part of the total cross-sectional area of the conductor. This is causing an increase in the resistance. The major cause of skin effect is the self-inductance of the conductor, which increases the inductive reactance at the relatively higher frequencies of the current, forcing electrons toward the surface of the conductor. Even if skin effect appears mostly in some very high frequency applications like RF power and transmission, it can also attenuate within the frequency range of audio signals, affecting mostly the upper bands. The use of Litz conductors is considered as one of the most effective solutions to the problem. A Litz conductor consists of individually insulated wires (the number of wire and the twist pattern varies depending on the design of each manufacturer), where each of them is less than a skin-depth and therefore they do not suffer from appreciable skin effect losses. Additionally, the fact that the individually insulated wires do not follow the same radial position across the bundle prevents electromagnetic effects that are causing skin effect, helping the conductor to provide lower resistance.
This effect is defined as the tendency of a current to flow in loops or concentrated distributions due to the presence of magnetic fields that are generated by nearby conductors.Proximity effect produces an apparent increase in the resistance, especially with high-frequency alternating currents and it is the main cause of imbalanced load distribution. The main symptom of proximity effect in audio, is that it raises the frequency curve around the lower bands, as if someone has turned the “bass knob” clockwise by a quarter of a turn. A solution to this problem can be provided by the use of proper conductor sizes and insulation materials for each application, in combination with an appropriate spacing and an accurate twisting. For some cable applications, the use of Litz conductors may also be a solution to proximity effect.
The conductors of Signal Projects
By wanting to create the optimum cable connections for any type of audio and video device, we have focused our research on the development of hybrid type conductors that can provide increased conductivity and lower resistance, not only due to their metal structure, purity, thickness and cross-sectional area, but also due to their geometry that helps in the further reduction of losses from skin effect and proximity effect.
Hybrid in metal structure
In theory, the use of pure silver would be the optimum choice for any type of conductor, since it is the metal with the highest value of electrical conductivity, which means that it would provide the lowest resistance than any other material at a given thickness, type and length. But in the real world of reproduction electronics where the signal transferring process is so sensitive to numerous parameters, the “recipe” for the optimum signal conductor cannot be as simple as implied by the above – theoretically stable – assumption. At times, numerous equipment reviewers and many more audiophiles have described their own practical experiences of cables based on high purity conductors of a single-metal structure, reporting that even if they realised improvements in many key areas of reproduction, they still did not acquire the anticipated results from the overall performance of their devices. A very frequent example refers to cables with silver conductors, which on the one hand help systems expand their response (mainly at higher frequencies), but at the same time they make them sound slightly emphatic on the upper bands, as if someone has turned the “treble” knob clockwise by a quarter of a turn or even more. Similar impressions are left by many other examples of units connected with copper cables, with users expressing their satisfaction for the performance in the low and medium bands, while at the same time complain about the lack of “air” and “openness” at the higher frequencies.
All these practical examples and a number of tests and scientific investigations made on the properties of copper and silver conductors lead us to the assumption that no conductor of a single metal structure is able to excel in all areas of audio and video reproduction. Challenged by these references and of course by our experiences and measurement results, we focused our efforts on the development of conductors with a hybrid metal-structure from copper, silver and gold. With the proper containment and the lowest possible level of impurities, our conductors will exploit and combine all the advantages of the metals they are made of, creating ideal signal paths for any type of connection.
The relatively lower resistance values at any given thickness and the flat response across the full frequency range, are the major advantages of our conductors that will assist electronic devices to reveal their original characteristics without adding any unwanted coloration.
Hybrid in geometry
There is a large number of home users, reviewers and professionals that show their preference towards cables made of solid wires, believing that this type of conductor is the ideal one for any kind of connection. Many others support that the optimum performance can only be achieved by cables using stranded wires while others insist on using cables based on Litz conductors, aiming to reduce losses caused by skin effect and proximity effect. Aiming to create the ideal signal paths for any type of connection, we invested on the development of conductors with hybrid geometry, which is based on both solid-core wires and stranded wires that are winded with specific patterns (depending on the application) and they are insulated according to the Litz configuration.
The unique geometry of our conductors in combination with their hybrid metal structure produces the optimum signal paths for any type of connection that practically will create this very desirable feeling of coherence, reproducing musical instruments with impressive accuracy and simulating the three dimensions of the stage with outstanding precision.