In theory, the perfect cable is an un-insulated conductor floating in free air. In practice, there’s a bit more to it than that.
In order to understand the importance of the insulation material in a cable, we first need to examine the distribution of alternating current inside a conductor. Different frequencies occupy different (radial) positions within a conductor. Low frequency signals occupy the centre of a conductor, while the higher frequency signals are confined to the conductor’s surface. High frequency signals are therefore constrained to flow within a smaller cross sectional area of the conductor than the low frequency ones so that the effective cable resistance seen by the high frequency signals is greater than that seen by the low frequency signals.
Cable losses are therefore frequency dependent, with the high frequencies undergoing the most loss. This is known as the ‘skin effect.’ It causes a great deal of controversy in audio circles as many argue that it is only relevant at high frequencies which are beyond the range of human hearing. However, that's not entirely true, because conductor resistance starts to increase due to the skin effect at around 20 kHz. It’s high frequencies which create timbre, ambience and a clear treble.
The low and mid frequencies occupy the centre of the conductor. In speaker cables, in particular, optimisation of the low frequency signal component is extremely important. Extensive tests suggest that a conductor should have a cross sectional area of between 3.00 and 4.5 sq mm in order for it to provide the maximum amount of clean bass. Furthermore large cables should be constructed using a good quality rope lay conductor configuration with a high quality dielectric such as foamed polyethylene, PTFE (Teflon™) or PTFE. Thereafter other factors, which we can’t measure, influence quality.
Designs incorporating multiple insulated strands claim to overcome the problems of increased resistance due to the skin effect, but these low inductance designs tend to have higher capacitances. Low capacitance, low resistance cables will not affect the components they're connecting as much as high capacitance cables can; speaker cables need to have lower resistance to avoid signal loss and interconnects need to exhibit low capacitance for improved signal velocity.
Sound systems which sound brighter than others within the audio range may be working on the verge of instability due to the use of high capacitance cables. Brightness is often mistaken for improved dynamics, but ‘improvements’ in the dynamic range should not be at the expense of low frequency information, as is the case when an amplifier becomes unstable. Undue brightness is also a feature of silver plated cables; these can become tiring to listeners over a period of time. Atlas don’t use silver plated cables or cables consisting of two metals of differing resistance and characteristics for analogue audio applications.
The three drawings illustrate, from left to right, the radial occupation areas within the conductor according to frequency. The low frequencies occupy the centre of the conductor. It follows that a large conductor has a lower resistance to low frequencies and will provide more bass. That's why Atlas speaker cables are made available in a variety of sizes, for example in the case of Hyper speaker cables, 1.5, 2.0 and 3.5mm² etc. Systems that are 'bass heavy' may be wired with a smaller speaker cable but in cases where increased bass is desired a larger cable needs to be used. Longer runs of speaker cables should also use larger conductors.
The second drawing illustrates the occupation area for high frequencies within a stranded conductor.
The third drawing illustrates the occupation area for high frequencies in a solid conductor. The occupation area in a solid conductor is greater than that in a stranded conductor, therefore the high frequency signal meets with less resistance in a solid conductor and is able to provide better smear free treble. All Atlas bi-wire speaker cables consist of stranded conductors for L.F. or bass and solid conductors for H.F. or treble.
That begs the question, why not use a solid conductor for both; well if we take an example of a 3.00 sq mm solid conductor, it won't bend, indeed it will break if bent, therefore it's impractical, hence the reason for stranded conductors. The approximate optimum size for a solid conductor is 1.5 sq mm. Atlas bi-wire speaker cables have four tails at the speaker end, of unequal lengths. The two longer tails are those which are connected to the H.F. connectors on the speakers (assuming the speakers are designed to be bi-wired!), and the two shorter tails are those for the L.F. connectors.
High frequency signals occupy the periphery of a conductor (see above). Poor quality dielectrics (insulation materials) reduce the velocity of this signal, resulting in a sound which is biased towards the low and mid frequency regions of the audio spectrum. Thus, a poor quality sound often accompanies the use of a cable with a poor quality dielectric.
PVC (Poly Vinyl Chloride) is cheap to produce and, as such, is the most commonly used insulation in AV cables. However, PVC is the worst quality insulation a Hi-Fi or AV signal can encounter as its high loss causes a significant reduction in signal velocity. PVC is better suited to power cables and should be avoided in Hi-Fi and AV signal cables.
Other dielectrics in common use are Polyethylene, Polypropylene and Polytetrafluoride Epoxy (better known as PTFE (Teflon™) or Teflon,) and the new and unique Atlas (PTFE).
Teflon has a high melting point (327°C) – at the high temperatures involved, OFC and OCC revert back to the high-grain tough pitch state, destroying the integrity of the low grain or mono-crystal structures. But, over the last few years, Atlas has been researching ways of coating processed copper with Teflon, without the deleterious effects detailed above. Finally, after a great deal of research, we can now coat processed copper with a type of Teflon called Fluorinated Ethylene Propylene (FEP) – melting point 275°C – by cooling the copper during coating.
FEP allows the user to gain from the lower losses associated with this dielectric while still enjoying the benefits of low grain copper conductors.
Further research has led to the introduction of PTFE. The first Atlas products that introduce the new dielectric are the Mavros and Asimi interconnects and their matching speaker cables. PTFE (Teflon™) is a unique, low density dielectric material that offers significant performance improvements over solid PTFE (Teflon™) dielectric designs. PTFE (Teflon™) contains a higher percentage of air than solid PTFE; achieved by introducing small voids (less than one-half micron in diameter) of air within the material. The result is a lower dielectric constant of 1.5 to 1.3 (typically Teflon the next best dielectric is 2.1 to 2.3). The velocity of propagation is increased by typically 72% to 80% over ordinary cables and by about 30% over cables using standard Teflon dielectrics.
PTFE (Teflon™) has improved phase stability vs temperature variation because this is dependent upon the coefficient of thermal expansion of both the cable dielectric and conductors. Since PTFE (Teflon™) has a lower coefficient of thermal expansion than PTFE, the use of a microporous dielectric results in less dielectric expansion and so an improved phase vs temperature response.
For the same outer diameter, cables using PTFE (Teflon™) exhibit a lower loss of signal than those using solid PTFE. This is first, because the low dissipation factor of the dielectric itself reduces attenuation, especially at higher frequencies, and second the low dielectric constant of a microporous dielectric allows the use of a larger diameter conductor. With the Mavros speaker cable for example, better bass or low frequency information can be achieved with larger conductors and microporous PTFE.
The thermal expansion of solid PTFE (Teflon™) can have detrimental mechanical effects on cable because as the PTFE (Teflon™) expands with heat, it can decrease the air gap between the cable dielectric and connector contact, thus degrading termination impedance. But since the microporous dielectric expands minimally with heat, these effects are insignificant.
In some respects the differences outlined above between microporous and solid PTFE, might be considered to be small but the accumulated effects of these small changes can cause a degradation of the audio signal. Removing that degradation peels away another layer of non-harmonic related non-linearities to reveal more of the music recording.
The table below shows the properties of a selection of dielectrics. Though not used as a dielectric in our cables, PVC is included for comparison purposes.
|Property||Polyvinyl chloride PVC||Foamed polyethylene PEF||Polypropylene P.P.||Teflon (FEP) or PTFE||PTFE|
|Dielectric constant (@ 50 - 106 Hz)
|Dielectric strength (kV mm-1)||23-30||30-50||30-50||20-25||n.a|
|Loss tangent (% @ 50 - 106 Hz)
||8-15||0.02-0.05||0.02 - 0.06
(@ 106 Hz)
|Volume resistivity (Ohms.cm @ 20°C)||1012-15||> 1017||6.5 x 1014||> 1016||n.a|
|Tensile strength (kg mm-2)||1.0-2.5||1.0-2.0||3.0-4.0||1.9-2.2||1.0|
|Melting point (°C)
|Max. continuous operating temp. (°C)
|Min. operating temp. (°C)||-15 to -40||<-60||-5 to -45||<-60||-250|