QED XT40i X-Tube Speaker Cable (3m Pair)

QED Reference XT40i — X-Tube Speaker Cable

The XT40i sits within QED's Reference tier, positioned above the entry-level XT25 and below the flagship Supremus in the company's speaker cable hierarchy. It is built around QED's X-Tube technology, in which conductors are formed into a tube shape around a hollow insulating core. This geometry equalises current density across the conductor cross-section, addressing the skin effect that causes conventional round conductors to concentrate high-frequency signals at the surface. The result is very low inductance and consistent amplifier loading across the audible frequency range.

The 4 mm² (12 AWG) conductor uses 99.999% oxygen-free copper with a minimum stranded diameter of 2.75 mm. An Air Gap dielectric surrounds the conductors, using air as the primary insulating medium rather than solid polymer. Air has a relative permittivity (εr) of approximately 1.0; the XT40i's effective dielectric constant is 2.4, roughly half that of conventional solid-insulation designs. This reduces both dissipation and parallel capacitance, which QED attributes to improved timing accuracy and rhythmic performance.

Construction and Termination

The XT40i is factory pre-terminated with Airloc Heat Shrink 4 mm banana plugs, which use a collet-grip mechanism with a heat-shrink sleeve for a secure, low-resistance connection. Each cable pair carries red and black identification rings at both ends. The flat-profile cable jacket measures 6 × 14 mm, making it relatively easy to route along skirting boards or under carpets.

Measured electrical characteristics include a loop resistance of 8.0 mΩ/m, self-inductance of 0.57 μH/m, and parallel capacitance of 35 pF/m. The dissipation factor at 10 kHz is 0.0295.

Design Philosophy

QED's approach with the XT40i focuses on minimising two parameters that affect signal integrity over cable runs: inductance and dielectric loss. The X-Tube geometry addresses inductance by arranging conductors symmetrically around a central void, while the Air Gap dielectric tackles capacitive loss by replacing as much solid insulation as possible with air. The combination is designed to present a benign, predictable load to the amplifier regardless of frequency, which helps maintain the stability of the amplifier's output stage.

The XT40i is available pre-terminated in 2 m, 3 m, and 5 m pair lengths. An unterminated version is also available by the metre for custom installations. Designed and engineered in Great Britain.

What the Press Says

• "You'll struggle to get your system sounding cleaner at this price." — What Hi-Fi? ★★★★★
• The Audiophile Man: 9/10, Deeply Groovy Award
• Hi-Fi Choice: Recommended