Reference

Sidebar 3: The Jenving Approach

Tommy Jenving has been making special audio cables in Sweden since 1976, beginning with a chunky speaker cable, the Supra 2.5. The idea of Supra Ply came more recently, and laterally, through developing and patenting a shielded mains cable called Supra Safe. The idea was to protect studio equipment and humans alike from 50Hz and 60Hz AC fields. Research into reducing power-line radiation indicated that low inductance was the key, and that high cable capacitance was unimportant. Realizing that the pulsating, high–peak-current-flow conditions in speaker cables are similar to AC wiring into DC power supplies, Jenving was able to ask, "Why are exotic speaker cables made with low capacitance as a main feature, and with consequent high inductance?" The answer seemed to be that such wares are of fundamentally wrong design, even if some second- and third-order details are attended to.

Unlike almost any other cable maker, Jenving has no trouble clearly outlining the logical design philosophy of Supra 2.0 in plain English. A number of other cable makers—eg, Kimber and Goertz in the US—have converged on much the same minimum-inductance approach, but their products often use more exotic, ultra-costly materials (such as over 99.99998% pure silver), and they're apparently unable to explain their approaches so coherently.

Jenving divides relevant parameters into the "dynamic" (stuff that varies with frequency) and "static" (stuff that doesn't). The latter comprises, first of all, resistance. Cable resistance that's very low relative to speaker voice-coil resistance is essential for good damping, but as the test results show, it's only the beginning of the story. Characteristic impedance is also frequency-invariant—but its relevance in the audio range is truly negligible, even at ultrasonic frequencies.

Turning to the "dynamic" parameters—namely, capacitance and inductance—Jenving reminds us that the two work in opposition: minimize one and the other will rear up its head. The frustrations of cable design are hidden until Skin Effect (including the related Proximity Effect) is recognized. This works like extra inductance; ie, an added, rising resistance with frequency. It occurs because locally circulating "eddy" currents in conductors cause the apparent inner-core resistance to increase with frequency, such that the skin of the conductor appears to have the least resistance to current flow. The counter-intuitive outcome is that fat, low-resistance conductors develop unexpectedly high resistance both at high audio frequencies (above 2kHz) and to transients.

Some cable makers try to overcome this by paralleling thin and fat wires. The Jenving approach is to use zoned tin plating to progressively increase the (DC) resistance of the conductor toward the outside. The higher resistivity of tin largely defeats the skin effect (so that a high-CSA conductor can be used without transient and HF losses and errors), while its relative inertness prevents oxidation of the (almost) oxygen-free copper conductors. The cable is completed with a covering of special PVC having low emission of corrosive chloride ions. Although many audio-grade cables use notionally superior insulators and conductors such as PTFE and Silver, such niceties seem irrelevant until basic details are mastered. Previous extensive testing by Martin Colloms (footnote 1) certainly shows that cable sonics have had little corroboration with the mere excellence of the materials.—Ben Duncan

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Footnote 1: Martin Colloms, "Cables, Cables & More Cables," Hi-Fi News & Record Review, June 1985; "Cable Considerations," Hi-Fi News & Record Review, December 1985; "Cable Talk," Hi-Fi News & Record Review, June 1987.