Possible explanation for the fragility of NVA Sound Pipes when fitted with Neutrik ProFi connectors




Without wishing in any way to kick the “hornet’s nest” of recent discussion, I would like to propose a possible explanation for the reported fragility of the NVA Sound Pipe interconnects when fitted with Neutrik ProFi connectors, as shown in the photograph above.

According to the NVA site (http://www.nene-valley-audio.com/frames/nva/cables.html), the Sound Pipe interconnects “are revolutionary new cables utilizing military grade materials used in jet fighters.” If one looks at the photograph one can see two copper-coloured coaxial cables about 2mm in diameter. I believe the cable used is 085 semi-rigid coaxial cable. This coaxial cable has an outer diameter of 0.085"(= 2.16mm), hence the designation, with a solid copper sheath. The dielectric is either solid polythene or PTFE and the inner conductor is ‘copperweld’ which may, or may not, be silver-plated. It has a diameter of 0.020" (= 0.51mm). ‘Copperweld’ is the trade name for copper covered steel wire. It is made by an exclusive molten welding process, whereby a thick copper covering is inseparately welded to a steel core.

There are two points to note here: first, the cables are stiff, but malleable - when bent or formed into a shape, the shape remains; and second, the inner conductor is very thin.

In preparing these cables special cable trimming tools are required. To trim back the solid copper outer a special jig is used which prevents the fine-toothed coping saw to cut any deeper than the wall thickness, so as not to damage the dielectric. When the dielectric is trimmed back, again a special jig is used which prevents the inner conductor from being ‘nicked’ and damaged. If the latter jig is not used, it is very difficult to guarantee the inner core has not been nicked and thus weakened.

From the photograph it would appear that the outer conductor and dielectric of the cables have been trimmed back together, exposing several mm of the inner conductor. And where the one of the inner conductors has broken, it is right at the point where both the outer conductor and dielectric have been cut back, suggesting the inner wire had become nicked in the process.

Now despite the weakness, the nicking of the inner conductor would not necessarily lead to failure if the cables were fully and firmly supported. The construction of the Sound Pipes, have the two semi-rigid cables held in contact along their length by a sheath of heat-shrink sleeving, over which a woven polymer braid is used, presumably for cosmetic reasons. (Electrically connecting the two outer conductors together means the interconnect has half the capacitance per unit length, compared with that of a single cable). The two cables clad in the heat-shrink sleeving means the overall assembly is now quite stiff.

NVA offer two options for the connectors fitted to the Sound Pipes. One is the fitting of more conventional style RCA phonos, where one assumes the cable clamping arrangement is sufficient to prevent the cable being ‘flexed’ with respect to the connector. The second option is to fit Neutrik ProFi RCA connectors, and these are the connectors shown in the photograph.

The cable clamping arrangement of the ProFi connectors uses a plastic collet: sufficient for flexible coaxial cables, but not for the pair of semi-rigid cables used here. It is quite likely the compliant plastic collet can allow the cable pair to flex within the collet, and if any of the inner conductors have been weakened they will break at point of weakness.

This is a criticism of neither semi-rigid cable, nor of the ProFi connectors: they just weren’t designed to be used together.

I believe this explains the fragility of these interconnects when terminated with these specific connectors.