STAX SRE-750H extension cable (5m)

A STAX electrostatic headphone always remains connected to its dedicated amplifier, and that amplifier often sits in the hi-fi rack, alongside the sources. As soon as the listening chair is moved farther away, the original 2.5 m cable becomes too short. This 5-metre extension extends the connection with the same type of flat HiFC copper cable used on the brand’s headphones, and plugs into any five-pin Pro socket.

Why cable capacitance matters more than resistance

Electrostatic headphones do not work like conventional headphones. Their very thin diaphragm is suspended between two grids called stators, and the amplifier sends them a signal of several hundred volts with a tiny current. From the amplifier’s perspective, the headphones behave like a capacitor: an SR-L500 MK2, including its cable, has a capacitance of 110 pF and an impedance of 145 kΩ at 10 kHz. Each metre of cable adds its own capacitance, which the amplifier must charge and discharge with every oscillation of the signal, and the task becomes more demanding as the frequency rises. With so little current involved, copper resistance takes a back seat, and capacitance is what needs to be kept in check.

To achieve this, the SRE-750H uses the low-capacitance parallel flat cable that STAX uses on its headphones: the conductors run side by side in a wide ribbon, spaced apart rather than tightly twisted, which limits electrical coupling between them. An extension still adds capacitance to the connection, however, and good practice is to choose only the length actually needed.

A HiFC conductor instead of PC-OCC copper

The SRE-750H arrived in July 2017 to replace the SRE-750, whose cable used PC-OCC copper, production of which had ceased at its Japanese manufacturer. The new conductor, HiFC, was developed by Hitachi Metals: it is pure copper in which a minute amount of titanium binds impurities, making continuous casting possible. STAX credits it with good conductivity, low degradation over time and flexibility that makes cable routing easier. It is the same conductor used in the cables supplied with the brand’s current headphones, so the extension lengthens the connection without changing materials along the way.

A five-pin Pro connector, and nothing else

The extension has a five-pin Pro male plug at one end and a matching female socket at the other. This is the format used by all current STAX headphones, with a 580-volt bias: one pin carries the bias voltage, while the other four drive the stators in the two earcups. Much older models, with a 230-volt “Normal” bias and a six-pin plug, are not compatible. On the amplifier side, any Pro output is suitable, regardless of the STAX model. The finish is black, like the cables in the range.

Moving the amplifier away from the listening position

The most common use is to leave the amplifier in the hi-fi rack, near the source and its mains power supply, to listen from a sofa or desk farther away. It is also a way to keep a bulky, heat-producing tube amplifier, such as the SRM-007tII or SRM-T8000, at a distance. Added to the original cable, the extension brings the total distance to approximately 7.5 m. The flat ribbon is easy to lay along a skirting board, but it does not tolerate being pinched under a furniture leg or in a doorway, where the sheath would eventually become damaged.

For a shorter distance, the SRE-725H uses the same HiFC cable in a 2.5 m length. The SRE-925S and SRE-950S, in 2.5 and 5 m lengths, are the next tier up in the STAX extension cable catalogue, with a silver-coated 6N-purity OFC copper conductor.

Technical Specifications

Dimensions

• Length: 5 m
• Diameter: 9 mm

Materials

• Composition: pure copper