4.1 The Skin Effect

In order to understand the importance of the insulating material used in a hifi audio cable, we must first examine the distribution of alternating current within a conductor.

Different frequencies occupy different (radial) positions within the conductor. The low frequency signal occupies the central part of the conductor, while the high frequency signals are confined to the surface of the conductor. High-frequency signals are then forced to flow within a cross-section of the conductor's cross-sectional area smaller than the low-frequency frequencies, so that the effective resistance of the cable, seen from the point of view of high-frequency signals, is greater than that seen from low-frequency signals. Cable losses are therefore frequency-dependent, with the high frequencies subject to greater loss.

This phenomenon is known as the "Skin Effect". The subject is a source of considerable controversy in audiophile circles, where many argue that it is only relevant to those high frequencies that are already beyond the reach of human hearing. However, this is not entirely true, because the resistance of the conductor starts to increase, due to the skin effect, already around 20 kHz.

It's the HIGH frequencies that create timbre, ambience and defined highs.

Look at the figures below from left to right.

1. Radial positions inside the conductor according to frequency.

2. Area of occupancy of the High Frequency in a multi-wire conductor.

3. Area of occupancy of the High Frequency in a solid-structure conductor.

Effetto Pelle dei cavi audioThe low and medium frequencies occupy the central part of the conductor. In power cables, in particular, the optimisation of the low-frequency signal component is particularly important. Extensive testing suggests that the conductor should have a cross-section of the transverse area between 3.00 and 4.5 mm/q in order to provide the maximum possible amount of clean bass frequencies. In addition, large cables should be constructed using a high quality dielectric configuration such as expanded polyethylene, PTFE or Microporous PTFE.

In addition, other factors, which we cannot measure, influence quality.

Projects using more insulated wires would claim to overcome the problems of increased resistance due to the skin effect, but these low inductance schemes tend to have a higher capacitance. Low capacitance and low resistance cables do not affect the components to which they are connected, as very capacitive cables do; speaker cables must have low resistance to avoid signal loss, while signal cables must have low capacity to improve signal transmission speed.

Amplification systems that sound brighter than others within the audio frequency range may actually work unsteadily, due to the use of particularly capacitive cables. Brightness is often mistaken for improved dynamics, but 'improvements' in dynamics should never be at the expense of low frequency information, such as when an amplifier becomes unstable. Excessive brilliance is often also caused by the use of silver cables; these can become tiring for listeners after a certain period of time. Atlas does not use silver plated cables or cables made of two different metals with different resistance characteristics.

As explained above, the three drawings above, from left to right, represent the radial area of occupation within the conductor as a function of frequency. The low frequencies occupy the centre of the conductor. It follows that a large conductor offers a lower resistance at low frequencies and will be able to better reproduce the low range. This is why Atlas speaker cables are available in a variety of sizes, for example in the case of Hyper speaker cables, 1.5 mm/q, 2.0 mm/q and 3.0 mm/q, etc. Systems that have a 'bully bass' could be wired with a smaller diameter power cable, while to increase the low frequencies it is advisable to use a larger size cable. Even if the power cables have to cover longer distances, it is advisable to use a cable with larger diameter conductors.

The second drawing illustrates the area of occupation for the high frequencies within a multi-wire conductor.

The third drawing shows the area of occupation of the high frequencies in a solid conductor cable. The area of occupancy in a solid conductor is greater than that in a multi-wire conductor, so the high frequency signals will encounter less resistance in a solid conductor that can reproduce better, non-strident highs. All Atlas bi wire speaker cables use multi-wire conductors for low frequencies (low) and solid conductors for high frequencies (high). Well, the answer is that if we used a 3.00 mm/q solid conductor, for example, it would be too rigid, and would break easily if it were bent, hence the reason why we use a multistrand conductor.

The approximate optimum size for a solid conductor is 1.5 mm/q. Atlas bi-wire cables have four ends on the driver side, of different lengths. The two longest ends are those that connect to the high-frequency (H.F.) connectors on the drivers (obviously taking into account drivers that we have a bi-wiring configuration!), while the two shortest ends are those for the low-frequency (L.F.) connectors.

4.2 Propagation velocity (VOP) and types of insulators. (speed is important)

High frequency signals occupy the periphery of the conductor (see above). Low dielectric quality (insulating material) reduces the speed of this signal, resulting in a sound that is unbalanced towards the low and medium frequency regions of the audio spectrum. Therefore, poor quality sound is often accompanied by the use of a low quality dielectric cable.

PVC (Polyvinyl Chloride) is cheap to produce and, as such, is the insulation most frequently used in AV cables. However, PVC is the worst quality insulation for Hi-Fi or AV applications and the signal may experience a high loss and a significant reduction in its speed. PVC is best used in power cables and should be avoided in Hi-Fi and AV signal cables.

Other dielectrics commonly used are Polyethylene, Polypropylene and Polytetrafluoride Epoxy (better known as PTFE or Teflon and the new and unique Atlas Microporous PTFE).

Teflon has a high melting point (327°C); excellent for use in a non-stick pan as a coating; not as good when used to coat processed copper - at the high temperatures involved, OFC and OCC would revert back to their initial highly granular state, destroying the integrity of low-grained or monocrystalline structures. But over the past few years, Atlas cables, in collaboration with its suppliers, has been studying a method for coating copper with Teflon, without the deleterious effects mentioned above.

Finally, after a major research effort, we are now able to coat copper with a particular type of Teflon called Fluorinated Ethylene Propylene (FEP) - with a melting point of 275°C - by cooling the copper during the coating process.

FEP allows the user to obtain few losses, typically associated with this type of dielectric, allowing instead to still have all the advantages of low-grained copper conductors. Teflon FEP is used in all Atlas products of the Atlas Ascent line, Atlas Superior, Hyper power cables, etc..

4.3 Microporous dielectrics.

Further research has led to the introduction of Microporous PTFE. The first Atlas products to introduce the new dielectric were the Mavros and Asimi signal cables and their corresponding power cables.

Microporous PTFE is unique, a low-density dielectric material that offers significant performance improvements over solid PTFE dielectrics. Microporous PTFE contains a higher percentage of air than solid PTFE, a quality achieved by introducing small air voids (less than half a micron in diameter) into the material. The result is a lower dielectric constant between 1.5 and 1.3 (typically Teflon, the second best option for dielectrics, has values between 2.1 and 2.3). The propagation speed is typically increased between 72% and 80% compared to traditional cables and about 30% compared to cables using standard Teflon dielectrics.

4.4 Phase Stability - less signal cancellation.

Microporous PTFE has improved phase stability with respect to temperature variation, because this depends on the thermal expansion coefficient of both the cable dielectric and its conductors. Since Microporous PTFE has a lower thermal expansion coefficient than PTFE, the use of a microporous dielectric results in a lower dielectric expansion and therefore a better phase response with respect to temperature.

While maintaining the same external diameter, cables using Microporous PTFE show a lower signal loss compared to those using solid PTFE. This happens firstly because the low dissipation factor of the dielectric itself reduces attenuation, especially at higher frequencies, and secondly because the low dielectric constant of a microporous insulator allows the use of a larger diameter conductor.

For example, in the Mavros speaker cables, the excellent bass reproduction and low-frequency information qualities have been obtained thanks to the adoption of larger conductors and Microporous PTFE.

The thermal expansion of solid PTFE can have deleterious mechanical effects on the cable, because when the PTFE expands with heat, the air space between the dielectric of the cable and the contact with the connector is reduced, thus degrading the impedance of the termination. But since the microporous dielectric expands minimally with heat, these effects are insignificant.

From a certain point of view, the differences highlighted above, between microporous dielectric and solid PTFE, could be considered as minor, but in reality the effects determined by these small changes can cause a degradation of the audio signal. Removing the layer of this degradation, made of non-correlated and non-linear harmonics, reveals more details of the musical recording.

The table below shows the properties of a selection of dielectrics. Although not used in our cables, we have included PVC for comparison purposes.

 

 Dielectric material

Property

Polyvinyl chloride PVC

Foamed polyethylene PEF

Polypropylene P.P.

Teflon (FEP) o PTFE

Mircoporos PTFE

Dielectric constant
(@ 50 - 106 Hz)
4-82.32.252.11.3
Dielectric Strength 
(kV mm-1)
23-3030-5030-5020-25n.a
Tangent loss
(% @ 50 - 106 Hz)
8-150.02-0.050.02 - 0.06
(@ 106 Hz)
0.02-0.07n.a
Resistivity volume
(ohms.cm @ 20°C)
1012-15> 10176.5 x 1014> 1016n.a
Traction resistance
(kg mm-2)
1.0-2.51.0-2.03.0-4.01.9-2.21.0
Melting point
(°C)
-130112-120155-160275275
Max. temperature of continuous operation
(°C)
607590200260
Min. operating temperature
(°C)
-15 to -40<-60-5 to -45<-60-250