Making The Right Connection
Much of the information such as conductor quality, purity, grain structure and insulation quality etc. is pertinent to both speaker cables and interconnects.
6.1 Black boxes....... a word of warning!
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There is a recent fashion for cables fitted with 'black boxes'. These usually contain a simple and cheap filter, often called a Zobel network, which aims to reduce the deleterious effects of Radio Frequency Interference (RFI) and improve amplifier stability margins. Cables fitted with these Zobel networks are often quite expensive. It should also be noted that amplifier manufacturers often (but don't always) fit Zobel networks to their products as a matter of course, in order to avoid damage if high capacitance speaker cables are used. Where this is the case, extra Zobel networks are not required (unless the cable is so esoteric that it probably should not be used anyway) and could do more harm than good.
Part of the Atlas philosophy is to avoid barriers in the signal path that manipulate the signal or that delete due to the use of materials of differing characteristics. That's why OCC and OFC with their absence or reduction of signal deleting crystal boundaries are preferred. It's also the reason we at Atlas avoid solder contacts for most of our products and it's why we prefer plugs and connectors that are made from copper (All Cu).
6.2 Capacitance
Low capacitance interconnects and speaker cables are preferable to high capacitance ones which may prove a mismatch for some amplifiers. The frequency distortion and amplifier instability which can result from using high capacitance cables is not caused by the cable itself, but rather by using the cable between inappropriate components. Low capacitance cables are safer to use in any system than high capacitance ones.
The capacitance of a cable is determined by conductor geometry and the dielectric constant of the insulation employed. Although the vast majority of cables available use PVC, PVC should be avoided for audio applications. Better alternatives are polyethylene, foamed polyethylene, polypropylene, PTFE or the new Atlas Microporous PTFE. Foaming reduces the dielectric constant, thereby stabilising the frequency characteristics. Any insulation between conductors creates a capacitor which stores and later releases energy. The less energy absorbed, the better the signal velocity (speed) will be and any energy that's absorbed would be better converted into heat than released later. Within the audio range, the magnitude of capacitance, together with conductor resistance, governs the level of attenuation. The lower the attenuation, the less the signal is 'deleted' and the better the cable is!
6.3 Impedance
In the digital domain and in the case of video signals, it is of fundamental importance that the correct type cable is employed in order to match the signal source impedance into the load presented by the receiving equipment.
The appropriate cable is an asymmetrical or co-axial, unbalanced design, usually of 75 Ohms impedance. The cables supplied with a CD player by the manufacturer are usually co-axial 75 Ohm types.
In the digital domain, there should be no deletion of information....... but there is!
Different cables sound different, better or worse, so there is clearly some deletion of information taking place. RCA plugs are most commonly used, though BNC plugs are better devices with regard to impedance matching. The vast majority of RCA plugs (sometimes called phono plugs) grip the cable with a compression gland (collett) or a grub screw in the side of the plug which is screwed into the cable.
The co-axial or asymmetrical cable uses going and return conductors which are not electrically equivalent, giving rise to often used descriptor: 'unbalanced.' The return path acts as a screen and a barrier to radio frequency interference (RFI). The cover afforded by the screen is expressed as a percentage. The higher the percentage, the more effective the screen. To improve the quality of digital and high frequency cables, a second shield and an additional aluminium foil are often employed.
In production, in order to achieve a 75 Ohm impedance, the cable manufacturer selects a diameter for the central conductor which then determines the correct separation between the centre (or going conductor) and the screen (or return conductor). If that separation is reduced by a plug which compresses the cable, the cable's impedance will be altered. If the impedance is altered, reflections take place inside the cable causing signal deformation (the rise or fall time of the square wave signal is changed) and unwanted jitter.
Using a typical RCA plug can reduce a cable's impedance from 75 to between 25 and 35 Ohms! Atlas digital and video interconnects use plugs which don't compress the cable. We will never make an interconnect which purports to be suitable for digital or video signals which employs an RCA plug with a grub screw or which uses a compression method to attach it to the cable.
6.4 Shield
Cables are shielded to protect the signal from external noise.
There are two types of shield, one to the reject electrically induced noise and one to reject magnetically induced noise. The rejection of electrically induced noise requires the shield to be made of a metal with a high conductivity (since RFI rejection requires 'shielding' currents to be able to flow easily in the shield - RFI shielding is inversely proportional to the resistance of the shield). Copper or aluminium foil are suitable for this purpose. In an unbalanced interconnect, the shield also acts as the signal return path while in a pseudo balanced configuration the shield and return paths are parallel, with the shield left open at one end. This makes pseudo balanced interconnects directional (see more on the subject under Appropriate Interconnects). A cable should be wrapped around a ferrite core (iron etc.) to maximize the rejection of magnetic fields. Because interconnects incorporating such a shield are bulky, this method of magnetic field rejection is not generally used on Hi-Fi or AV cables. To reject magnetic interference in Hi-Fi applications, it is usually sufficient to twist the insulated conductors together to form what is known as a twisted pair.
Nowadays technologies such as SACD and DVD-Audio can produce audio signals well beyond the 20kHz value which was typically considered to be an adequate cut-off frequency for Hi Hi systems. In response to this, some loudspeaker producers are meeting the latest demands for high frequency extension with highly sophisticated tweeters capable of reproduction well above 20kHz. A few speaker manufacturers provide a ground contact on the speaker so that a drain wire can be connected to reject RFI which can otherwise become an audible problem at frequencies in excess of about 20kHz.
6.5 Appropriate interconnects
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A large and perplexing variety of audio and video cables are currently on offer to the potential purchaser, yet little is provided by way of design rationale. I advocate the use of high build quality interconnects which compliment (match) the signal and the equipment they are to be used with. The following section deals with the construction of interconnects and their appropriate applications.
Interconnects are classified as unbalanced, fully balanced and pseudo balanced according to their method of construction and wiring scheme.
The unbalanced interconnect, which possesses going and return paths which are electrically non-identical, is the most commonly used type of interconnect. Unbalanced cable is often referred to as a coaxial because of its construction. In the coaxial design, an inner conductor is referenced to 'ground' via a concentric, cylindrical outer conductor (screen). The outer conductor also acts as a shield against unwanted RFI. In general, the resistance of the screen is lower than that of the central conductor. Unbalanced interconnects are used in the analogue domain between a CD, DVD or mini disc player etc. and the pre or integrated amplifier. Video and digital cables are also coaxial, unbalanced types. The output of a digital source contains high frequency information in the leading and trailing edges of the square wave pulse and thus require a much higher frequency coaxial cable than analogue signals.
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A balanced cable, sometimes referred to as a symmetrical cable, has two identical conductors which carry the audio signals - two signals which are in anti-phase and a ground wire which can double as a shield (if coaxial in nature). Twisting the two signal conductors together reduces the signals' susceptibility to external interference and cross-talk. Fully balanced interconnects use an XLR plug with two signal pins and a separate ground pin. A balanced cable is used, for example, to connect between a balanced pre and power amplifier.
A pseudo balanced configuration is essentially a balanced cable in which one of the signal conductors has been electrically connected to the shield.
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This type of cable design minimizes noise interference, while still allowing shielded inter-connection between single-ended components, and provides the signal with electrically equivalent going and return conductors. In choosing between a balanced and pseudo balanced interconnect to connect unbalanced components, you should always listen before making a purchase in order to find the most appropriate cable for your system.
BALANCED INTERCONNECT: (usually terminated with XLR plugs) will only work if the interconnected electronics produce and receive balanced signals.
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PSEUDO BALANCED INTERCONNECT: (usually terminated with RCA plugs) - this interconnect is recommended for analogue interfaces such as that between a turntable and pre-amplifier or between pre and power amplifier where RCA sockets are provided. Pseudo balanced interconnects are always directional - the screen and return path must be connected at the 'source' end for them to work best.
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UNBALANCED INTERCONNECT: (usually terminated with RCA plugs) - this is the most commonly used Hi-Fi and Audio Visual interconnect type. Care should, however, be taken to ensure than the cable specification is appropriate for digital or analogue signals. WE make cables which are optimised for their job. An unbalanced analogue audio cable will not provide the best results in a video or digital environment and the high frequency cables used in video and digital audio interfaces will not provide the best results when used with an analogue signal.
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6.6 Connectors
Speaker cables are usually terminated with 4mm banana plugs, spades or Cam-con plugs. Of these, the most popular (In Britain) is the 4mm banana.
Spades, produced from copper and gold plated to avoid oxidation, compress under the pressure of the screw down connector thereby creating an air tight seal.
Banana plugs are usually gold plated over brass and the Cam-con is a nickel plated over brass connector. It's always best to have the minimum number of barriers between different materials in the signal path.
For interconnects, the most popular connectors are RCA and XLR types for analogue signals and RCA or BNC types for digital signals. Fully balanced analogue audio signals require plugs with three 'terminals': two for the signal and one for the ground shield. RCA plugs only provide two separate connections and are therefore only suitable for use with unbalanced signals and in pseudo balanced wiring schemes. An XLR plug must be used with balanced audio signals.
Cheap plugs are usually nickel or gold plated brass, while the more expensive ones used in Atlas quality systems are gold plated copper (All Cu).
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Our recommended RCA plug is a patented device containing two conjugate brackets which extend over the conductors, thereby shielding them from external noise. The brackets, which act as the return path, are self cleaning via four assymetrical sprung leafs, which when the plug is inserted into an RCA socket spread and wipe the surface of the socket. Tiny teeth on the bracket grip the cable without compressing it. These plugs are designed and
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manufactured with a range of cable entry holes to match various cable diameters. In the section above on impedance, we explained why it is important to maintain a 75 Ohm impedance in certain applications. Most RCA plugs grip the cable with a clamp or grub screw. These compress the cable, causing reflections which distort the signal. Whilst no RCA plug is a 75 Ohm device, the Atlas patented RCA plug grips the cable without compressing it and thereby maintains the integrity of cable at the cable to plug interface.
