QED Reference XT40i Speaker Cables

As QED Cable Brand rapidly closing in on its fifth decades at the forefront of speaker cable design, QED’s brand new Reference XT40i speaker cable builds on the formidable reputation of the XT40 speaker cable it replaces and offers a decisive step up in sound quality. This pure oxygen-free copper cable uses an evolution of QED’s established X-Tube technology to guarantee superb timing and stereo imaging, and adds the brand new and highly innovative Air Gap dielectric to bring unheard-of rhythmic accuracy and musicality to a cable of this modest price. Very low DC resistance adds unrivalled frequency response to XT40i’s already potent audio capability. The result is a cable that advances on the class-leading standards set by its predecessor.

The journey to XT40i speaker cable.....

QED XT40 was first introduced in 2014 and was voted the Best Speaker Cable by What Hi-fi? in the same year. Instead of resting on our laurels, we have decided to improve upon this winning formula and introduce a cable improvement in the form of XT40i.

In common with all our cables, the design of XT40i speaker cable is informed by the results of our exhaustive research into loudspeaker cable design, which was begun in 1995. XT40i speaker cable has been so designed that in some aspects it matches or outperforms its more expensive sibling. At the same time, it retains the ergonomics and ease of use of a normal figure-8 cable with its easy-strip jacket and relatively small 6 mm profile.

So what are the features which make XT40i speaker cable so special?

X-Tube™ technology first came to prominence in 2005 with the launch of the world-renowned Silver Anniversary XT, the most highly regarded mid-range cable in the world, winning the What Hi-Fi Product of the Year three years in a row. These design principles also grace the multiple award winning XT40 speaker cable and XT25.

As the frequency of an AC signal increases, electrons are forced to flow more and more towards the periphery of a conductor so that if the frequency is high enough only a very thin layer (or skin) on the outside of the conductor can be used. This ‘skin depth’ can be calculated for different materials at a fixed frequency. In copper it means that if a conductor has larger than 0.66 mm2 cross-sectional area not all of that area is available for an analogue music signal to use. In XT40i the ‘skin effect’ problem is effectively eliminated by the use of X-Tube™ Technology which works by placing all of the conducting material around a central hollow insulating rod. By forming the conductors into a tube-like shape with a hollow centre, current densities at different frequencies are equalised.

Low DC Resistance

At QED we recognise that low DC resistance of the loudspeaker cable is of paramount importance for high-fidelity signal transfer. This is because the speaker presents a frequency dependent load to the amplifier of which the cable forms a variable proportion. If resistance is allowed to be too large then audible changes to the frequency response characteristics of the loudspeaker will be introduced which cannot be corrected for by the amplifier’s negative feedback loop. In order to provide a marked improvement over similarly priced cables in both our own and our competitors’ ranges without increasing the overall size and flexibility of the cable substantially, cross-sectional area of XT40i has been maintained at a large 4 mm2 which instantly gives the new cable a vanishingly low loop resistance of just 8 mΩ/m and therefore a higher fidelity musical delivery. Additionally, the use of long-grain 99.999% oxygen-free copper ensures that XT40i works at 102% of the conductivity of standard copper conductors.

Low-Loss Air Gap Dielectric

It is not generally appreciated that electrical signals moving at or near the speed of light in a wire, do so via the medium of electromagnetic (EM) waveforms, which must travel within the dielectric which surrounds the conductors as well as within the conductors themselves. It is therefore important to ensure that the dielectric material used to insulate and protect the central conductors of the speaker cable is of a type which ‘permits’ the establishment of EM waveforms without appreciable loss.

To make XT40i a user-friendly, easy-to-terminate cable, a soft PVC outer jacket has been used in combination with hollow PE rods which would ordinarily only give average results as far as dielectric performance goes. However, due to the advantages conferred upon it by its X-Tube™ geometry, the increased separation of the conductors results in much more of the space between them being made of air, which has the effect of reducing the dielectric constant of the insulation without even changing the materials.

In this way, using an Air Gap Dielectric both capacitance and dissipation factor have been halved when compared to the old award-winning cable, bringing audible improvements in musical timing, lower distortion and better transient performance.

Just Listen

Unique proprietary technology and trademarked geometries plus our award-winning heritage all combine to create audible sonic improvements when compared to other loudspeaker cables.

Here are some comments from our panel of testers drawn from experts in the industry and amateur enthusiasts alike:

• “…I wanted to keep listening and turn it up…”
• “…very engaging, could follow individual instruments with ease...”
• “Bass under control”
• “Reveals a new level of detail”
• “Good timing, clean highs”
• “Separation of instruments is better with wider dynamics”

So, unless you invest in XT40i with QED X-Tube™ technology you can be losing a lot of the performance of your expensive loudspeakers.

Is there a measurable audible result of all these improvements?

The graph below shows how the resistance part of the impedance of XT40i speaker cable remains effectively unchanged across the entire audible frequency band when compared to an ordinary speaker cable of the same cross-sectional area. Combined with the sonic advantages of a lower-loss dielectric, these improvements place the new speaker cable in a unique position to rival its illustrious predecessor.

During development, we typically design and test at least four variants of each design to see where the extremes of each of the parameters lie. The cable that eventually becomes a product has been measured and listened to exhaustively and represents the best state-of-the-art at any given time. QED XT40i speaker cable is no exception in this regard.