Listening #16

Consider the fate of Giordano Bruno, a 16th-century astronomer who challenged Ptolemy's notion of Earth being the center of a finite universe—and in doing so went head to head with the church of Rome. Bruno's scholarly diligence and fearlessness were rewarded not with fame, riches, or accolades from his colleagues, but with a hot-lead enema, after which he was burned at the stake. Next heretic in line, step right up, please.

English audio designer Denis Morecroft hasn't fared quite so poorly, although he has yet to become famous or rich from his work. But it's never too late for accolades—so if you appreciate audio amplifiers of intentionally limited power, give Morecroft credit for making and selling them 20 years ago. Similarly, if you think small, simple, solid-core cables make more sense than their monstrous competitors, thank Denis Morecroft for being the first manufacturer to bring them to market. Battery-powered preamplifiers? Morecroft's DNM was the second company in the world to offer one (after Mission), way back in 1981. Slit-foil and T-Network capacitors? Morecroft invented them. Star grounding schemes? Again, DNM was among the first, and their three-dimensional circuit layouts have taken that approach to extreme heights.

Yet for all that, the soft-spoken and very humble Denis Morecroft maintains a professional, even genial view of his more conservative competitors. Regarding his unconventional use of acrylic for all DNM casework, for instance, Morecroft declines to bash the status quo. "I've met quite a few 'traditional' manufacturers who would like to move in that direction," he told me recently, "but they feel they can't, because of consumer resistance." Since 1984, Morecroft's preamps and amps have all been built into small plastic boxes, and although his reasons for doing so are compelling, it remains a tough sell: "I've long heard the comment, 'These amps are so light they feel cheap.' So manufacturers who are worried are right to be worried..."

The distorting fields
To understand why DNM Design tries to minimize the amount of metal in their products, one has to understand Denis Morecroft's design philosophies; in particular, his thoughts on the critical effects of eddy currents (aka Foucault currents).

Theory tells us that in any setting in which a continuously changing magnetic field travels through any electrical conductor or even adjacent to it, that field will coax electrons in the conductor into movement. These electrical eddy currents induce their own magnetic field in the conductor, which will tend to oppose or distort the original field (footnote 1).

Esoterica? Minutia? Weirdness? Not at all. Morecroft suggests a demonstration, some variation on which takes place in science classrooms every day: "Go get a nice, strong magnet and a big bar of aluminum. Put the magnet next to the aluminum: Right, it doesn't stick. Now, arrange the aluminum bar as a ramp and try to slide the magnet down it. You'll find it will move, but only very slowly.

"This is well understood and applied in other fields," Morecroft continues. "In fact, there are eddy-current brakes—using electromagnets—on the propeller shafts of 50,000-lb trucks that travel through the Alps. Not small things at all." Of course, Morecroft's interest in the subject is limited to audio, where the effect is somewhat smaller than in 25-ton trucks—but decidedly unwanted. And in audio, one needn't look far before finding a constantly changing magnetic field traveling through a conductor.

Which brings us to the Morecroft design most audio buffs have encountered: DNM solid-core cable. Introduced in 1984, DNM's interconnects went against the grain by giving the music only a single, small (a cross section of about 1mm) conductor to travel along, in order to minimize eddy currents and the distorting fields they induce. For the same reason, connectors of very low mass were chosen—which, it's safe to say, kept great numbers of audiophiles away by appearing insufficiently chunky and jewel-like—and stranded wires were avoided, not only because of their own propensity to create eddy currents, but also to avoid the smearing that results when one signal travels along many adjacent, uninsulated conductors. (Besides, in a world in which most amplifiers are happy driving a resistive load, why add complexity to a cable merely to make it less resistive?) DNM's speaker cables were similar, using a conductor of slightly larger cross section and, again, connectors of the lowest possible mass.

DNM's cables haven't changed much over the years, although the appearance and durability of their sheathing has been improved. They continue to find homes in an impressive number of non-DNM systems; perhaps more significant, Morecroft's solid-core cables have been imitated by many of his competitors in the past 20 years. But the negative effects of eddy currents extend beyond the ends—literally—of an audio system's cables, and Denis Morecroft has gone to fairly remarkable lengths to design preamps and amps in which these distortions are vanishingly rare.

To his way of thinking, solid-state circuits can perform better than their tube counterparts, but the transistor's potential must be exploited to the fullest—and that means the designer must understand and embrace negative feedback. "Feedback was developed in the early 1940s," Morecroft says, "but what's never been generally understood is that feedback must be applied with incredible accuracy.

"An amplifier is a device that operates at radio frequencies for its control. Somewhere between 900kHz and 1MHz is the time period for the amp to make a correction around its feedback loop. It seems infinitely fast, but it isn't." Interesting enough—but what do conductive metals and eddy currents have to do with the correct application of negative feedback?

"Picture a metal heatsink next to a transistor. Now, a fast-moving feedback signal is induced into the transistor—but because this sets up an eddy current in the electrically conductive heatsink, it generates a field that becomes a magnetic brake against the signal itself. In the case of that transistor and heatsink, the heatsink doesn't move, but the signal and the magnetic field it sets up are always changing."

In other words, the precise moment at which a music signal needs to travel quickly and without impedance is when it is likeliest to be opposed. DNM's solution was to develop a special coupling heatsink made not from aluminum but from aluminum oxide, which is white ceramic. The new part is very conductive to heat but not to electricity—so the signal is not opposed or distorted through electromagnetic inductance.

Around the corner
Apart from active parts, passive parts, and circuit-board traces—themselves specially designed for correct shape and mass, and thus low eddy-current effects—contemporary DNM preamps and amps contain virtually no metal. "There was still a time when we had a metal shaft running through our preamp, on the selector switch," Morecroft says. "That's how the part was supplied to us. And when we finally got rid of it, the improvement was astonishing!"

Footnote 1: Contrast this with the aqueous sort of eddy current, as found in trout streams, which are sometimes associated with feeding lanes: Although they repel the stream's primary direction, they tend to attract anglers.
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