Why OCC? And what's OCC?
This section will cover the science background behind DHC's wire and why it matters.
It may be helpful to read this journal article first, while it does get a bit technical, it makes some surprisingly clear conclusions about OCC and high purity metals and the effects of these different qualities on the wire's performance.
The world of cables has many words and classifications to describe the wires and metals in use. There are a lot of different variables in place that decide what makes a high-performing wire. We hear everything from "high-purity" to "oxygen-free" to "7N" to "high conductivity". But instead of worrying about the other wire that's out there, let's focus for now on DHC's stranded OCC copper wire, named "Nucleotide" (in keeping with our science-inspired theme).
There are five important qualities to a wire that should be covered first.
#1, metals purity. This refers to what percentage of the wire is, elements, wise, not copper - stuff like iron, sulfur, silicon, and so on. DHC Nucleotide is at least four nines pure (the limit of all tests short of the most exotic research equipment) and potentially up to seven nines pure of metals impurities (99.99999%) which is independent of the OCC process itself, which we will get to. To get a wire where the metal is highly pure, the copper stock - the base metal that is turned into the wire - must be highly refined in a refinery. Our manufacturer uses an extremely refined stock of copper, as this is the only way to achieve an end product where the metal is highly pure. The OCC journal article mentions that purer copper was more conductive - and that impurities promote more grain boundaries (breaks in the crystal structure - where 2 crystals meet - a perfect OCC wire has none of these breaks, as the crystals are hundreds of meters long). More grain boundaries are found to cause "distortion and attenuation" (Xinhui, 1038).
Part of this means that we will not use alloys or less conductive metals (metals other than copper and silver) out of principle. There is nothing to gain from slowing the audio signal down artificially to produce an intentional coloration. Any time other elements are added to a metal, they are an impurity that will only reduce conductivity and electrical performance. While they may not necessarily sound "bad," and might compensate for a deficiency in one's audio equipment, alloys and less conductive metals are not as neutral as copper. When it comes to picking the right metal, there is no wheel to reinvent - copper and silver are it.
However, this page will continue to explain that conductivity is not the only variable of the equation that makes OCC wire great. OCC is considerably more conductive than traditionally processed metals, but it has other improvements that help give it its refinement and neutrality. However, alloys as seen in the above chart are much less conductive than pure silver or pure copper - a greater magnitude than the difference in conductivity between traditional metals and OCC. It is for this reason that it is just as important to choose OCC metals as it is to choose pure metals in general, and OCC alloys are self-defeating as they violate the original purpose of OCC - to provide a consistent lattice of copper or silver molecules without boundaries. Adding other metals into the lattice is disruptive, and it is questionable how these OCC alloys compare to pure OCC metals on a molecular level.
#2, freedom from gas impurities. Oxygen-free copper was invented a long time ago to combat oxidation in copper, as copper oxides are not good conductors and you do not want them on the inside or outside of your wires. DHC's OCC copper goes beyond typical OFC, OFHC, or CDA101 copper (which is 4N pure oxygen free copper) because the OCC casting process (discussed later) naturally removes gases from the metal during the heating steps. The basic principle behind OCC is that heated molds are used to cast the copper, rather than a cold extrusion, in an oxygen free environment. Heating anything causes the substance to "degas," in other words the gas escapes because of its volatility - this technique is used to prepare oxygen-free liquid solutions in my microbiology lab. This table from Furutech's white paper on OCC technology shows the low gas content of OCC copper. Less gas trapped in the metal means fewer copper oxides forming and a more conductive metal - as well as a physically stronger metal, as gases such as hydrogen can make copper more brittle.
#3 - The OCC process. It has long been known (before the technology existed to produce such a thing) that single crystal copper (copper cast into one long crystal) would be better in every way - stronger, more malleable, more conductive, and able to transmit a signal through it with greater fidelity. This is why it was a goal in the 1980s for materials engineers to come up with such a process. Dr. Atsumi Ohno came up with the theories involving crystallization in metals that helped design a machine that could produce single crystal copper, as described in his book "Solidification: The Separation Theory and its Practical Applications". The key was controlling how crystals form when cast copper is cooled - with some modifications to existing casting techniques and some new machinery, copper wire containing single 100 meter crystals could be made. Ohno Continuous Casting was born (OCC) and Dr. Ohno licensed his patented process a few select industrial metal facilities - Furukawa of Japan, for example - to produce metals with his process. DHC only uses one of these patent-authorized facilities to make their wire - there are many imitators across mainland China, but there exists no authorized facilities there, and plenty of the OCC copper in existence right now is counterfeit. There is no conformation to any standards and the metals purity among other things is highly variable, even if the company is using a variation of the OCC machinery that they have illegally constructed.
OCC wire boasts many improvements over conventional copper that have led to its adoption by the audio industry. Xinhui's paper mentions the following:
"It is evident that grain boundaries, which are perpendicular to the direction of signal transmission, have an evident effect on the signal distortion and attenuation. Since there is no grain boundary in single crystal copper wire, the quality of signal transmission is excellent."
"The resistivity of the copper wires increases with increasing the the number of grain boundaries."
"Impurity can not only result in an increase of the wire's resistivity, but also influences the signal transmission."
OCC is not simply some audiophile voodoo, it is a dramatically different sort of metallurgy that benefits the wire in more ways that one and produces a greatly improved image of the sound when used to build cables. It is more neutral and more convincing of an actual musical performance compared to other wires.
#4 - Annealing. Copper wire is nothing without proper annealing - a heat treatment process which increases conductivity and flexibility. Plenty of copper wire samples that have passed through our doors have simply not felt or sounded right, and the difference is in the annealing process. Our manufacturer takes annealing very seriously as a component of a wire's performance, and as a result Nucleotide wire is exceptionally flexible. Some call a very long annealing process "super annealing" and indeed, Nucleotide wire is annealed for a long time until it has less stress than other wires, making the wire relax internally.
One metallurgy guide says:
"Work-hardened metal can be returned to a soft state by heating, or annealing. During annealing, deformed and highly stressed crystals are transformed into unstressed crystals by recovery, recrystallization and grain growth. In severely deformed metal, recrystallization occurs at lower temperatures than in lightly deformed metal. Also, the grains are smaller and more uniform in size when severely deformed metal is recrystallized. Grain size can be controlled by proper selection of cold working and annealing practices."
Annealing is an important piece of the puzzle and well-annealed wire will sound and feel the best.
#5 - Cryogenic treatment. While cryo treatment for audio applications has been questioned by many over the years, it has legions of fans that swear by it, even cryoing entire pieces of equipment in liquid nitrogen to improve the sound. However, cryo treatment has been used to improve the performance of many different metal items - increasing the longevity of blades, improving the accuracy of guns, or improving the sound of wind instruments. It is unquestionable that cryo treatment changes the molecular structure of metals at a deep level, resulting in a stronger finished product. Cryo treatment is used to prevent "residual stress" in the metal structure - stress that remains after a stressful event such as bending of the metal - with known benefits to electrical properties including "less electrical resistance, reduced coefficient of friction, less creep and walk, improved flatness, and easier machining." Nucleotide wire is cryo-treated by the manufacturer's own proprietary process, immediately after it is manufactured, using deep liquid nitrogen immersion. While cryo treatment is not cheap, it is of a high value to us - the ratio of performance to price is high and thus it is worthwhile in order to make Nucleotide the best OCC copper wire out there.
Besides these major categories, there are a few other traits of Nucleotide wire worth mentioning.
1) DHC's polyethylene (PE) insulation. It was a tough call for us, but PE was decided on as the best material overall. Its performance as a dielectric (material resistant to conduction - an insulator) is very close to that of Teflon, but it is much less hard and so less prone to vibration and "ringing". It has a soft yet firm feel and is very flexible, making it practical for headphone and portable cables. Some teflon insulation likes to slip off of wire or fails to form a tight seal because of how incredibly friction-resistant Teflon is, and this bothered us as well; our PE insulation is able to form an airtight seal on the wire after it is formed onto the wire in a nitrogen environment (oxygen-free). Thus, Nucleotide wire stays shiny and pink perpetually, within its insulation.
2) Gauge of the wire. We have stuck with 24-26 gauge wire over the years for good reason. Plenty of cabling theories state that the smaller the wire, the better the quality of signal transmission - to where thin-wire zealots have made interconnects out of 30+ gauge wire. There are cables going to the opposite extreme, saying that 18 gauge wire or below is necessary to transmit all the sound information. It is no surprise that the majority of headphone and interconnect cables are in a sweet spot, around 24 gauge. Headphone and interconnect cables do carry a fairly potent signal from the output stages of a DAC or amp and thus, while a 32 gauge wire is ideal for a super-low-level, super delicate signal like that from a record player's tonearm, it is less suitable for other applications such as our headphone cables. However, we are not talking about AC power here or the output of a 200 watt speaker amp - these applications may benefit from using a large enough wire to match the speaker's low impedance load. You truly will have losses if you use a 24 gauge wire to connect an amp to floorstanding loudspeakers, that is without a doubt. (see http://www.audioholics.com/education/cables/speaker-cable-gauge) However, headphones are not floorstanding loudspeakers, and using huge wire is going to cause timing errors and skin effect issues as it lacks the refinement for these lower-level audio signals (in short).