Tip #25: IN PURSUIT OF THE PERFECT CONDUCTOR
by Dick Olsher (March 2001)
Note: The is an abridged and updated version of an article originally published in the January 1996 issue of Fi Magazine. Most of the updates are minor in nature, except for the addition of MAC wireand Kimber Kable Select KS 1030 interconnect to the recommended list.
The notion of cable as a high-end component, so contentious in the 80s, is today rather well accepted by most serious listeners. However, the underlying technical foundation for the sonic character of cable remains nearly as controversial and fuzzy today as it was back then. Well...NOT if you're an electrical engineer. Most of these folks look at the corpus of a cable through entirely materialistic eyes; mainly because they are taught to analyze a circuit in terms of a few fundamental electromagnetic quantities. They idealize and model a cable's behavior as a function of its lumped electrical parameters: resistance (R), inductance (L), and capacitance (C). These parameters are ordained by the cable's dielectric materials and "geometry," by which I mean conductor wire gauge, the number of strands, the spacing between strands in each leg, and the spacing between the "go" and "return" legs. On this basis it is possible to calculate a cable's impedance. And because the calculations agree with the meter readings, this model must be judged as physically valid.
Taking this one step further, it isn't difficult to find instances where a cable's basic electrical parameters do make a sonic difference. Adding the loudspeaker impedance to the model, it is easy to show that since the cable impedance is in series with that of the loudspeaker, a speaker cable can actually modify the loudspeaker's frequency response. It soaks up a greater percentage of the amp's output power as its own impedance rises and that of the load decreases. Imagine a worst case scenario where a 2-ohm dip in the speaker's impedance curve meets up with a 1-ohm cable impedance; the result being about a 3 dB drop in output at that frequency. In general, it's fair to say that cable impedance contributes subtle EQ effects to the overall system sound. In addition, a high-impedance speaker cable increases the apparent output impedance of the power amp. The obvious consequence of that is that the amp's damping factor is reduced. Another not so obvious consequence involves the "de-tuning" of the loudspeaker's crossover network. Crossovers are designed to look into specific source and load impedances, so an increase in the amp's output impedance works to shift the actual crossover point from its intended frequency. All of these effects clearly impact the sound, but because their precise nature is unpredictable and usually deleterious, I would therefore be the last guy on this planet to recommend high-impedance speaker cable. On the other hand, it's important to note that a high-impedance interconnect is normally not a sonic detriment because it operates in a high impedance circuit; a power amp's input impedance typically being on the order of 50,000 ohms. But interconnect capacitance can be an issue if the preamp's output impedance is unusually high. The cable capacitance and output impedance form a low pass network that rolls off the treble. Just how critical is this effect? Lets take a look at a typical scenario. The capacitance of most interconnects measures between 100 and 150 pico Farad (pF) per meter. No serious preamp should have an output impedance greater than 500 ohms, but lets stretch that a bit to 2,500 ohms for the benefit of some tube preamps. Using figures of 150 pF per meter, a 20-foot run of interconnect, and a realistic upper limit of 2,500 ohms for the preamp's output impedance, results in a calculated -3 dB frequency of 73 kHz - and that folks is good enough for me. Undoubtedly, it is a cable's electrical "personality" that gives rise to specific system interactions and much confusion about the merits of any given cable. For example, a high-inductance speaker cable, which would be expected to sound dull in a tonally balanced system, may in fact be hailed as "sonic nirvana" in the context of a bright-sounding system.
A Question of Perception
The decisive mistake engineers make is to ascribe ALL sonic differences to a cable's electrical parameters. It's about as absurd as trying to divine a person's character from his weight, height, and rectal temperature. Card-carrying members of engineering societies by and large tenaciously uphold scientific dogma pounded into their heads during many years of schooling (as a reformed engineer, I should know). Put a gun to their head, and they will maintain till their dying breath that the RLC paradigm is the truth, the whole truth, and nothing but the truth. In the past ten years, a handful of investigators have shown that the RLC paradigm just doesn't go far enough, and that there are other factors that do indeed affect signal transmission. This is also true for other passive parts such as caps and resistors where a simplistic test-bench measurement oriented paradigm has failed to fully account for sonic differences. Turn a Meter Head loose with an Audio Precision System and have him try to differentiate between a mass-market receiver and a Mark Levinson, or for that matter, between a run of #16 awg zip cord and an equal length of high-end cable. It's like trying to judge fine wines on the basis of a chemical analysis. Such measurements are in general not predictive of human perceptions.
To paraphrase Rene Descartes, I hear, therefore I am. Meaning, that the whole is greater than the sum of the parts. While it is possible to dissect a sound field with a variety of frequency and time domain measurements, these meter readings or waterfall plots in no way add up to reflect the emotional reaction I might experience. No wonder science has had such a hard time defining perceptual attributes. Take timbre, for example. The American National Standards Institute defines timbre as "that attribute of a tone by which a listener can judge that two sounds of the same loudness and pitch are dissimilar." Pretty vague if you ask me. The following layman's definition is no better: the perceivable difference between a clarinet middle C and a violin middle C is timbre. To quote Handel (Listening: An Introduction to the Perception of Auditory Events: MIT Press), "timbre is not reducible to an acoustical property that automatically yields a clarinet note or a violin note." Timbre [has] to be judged subjectively. Human vision presents us with a similar perceptual dilemma. The wavelength of visible light can be measured very precisely with a spectrometer. At a wavelength of 520 nm, light is perceived as green; at 470 nm it is blue. But at what wavelength does it change from green to blue? That's a question that measurements cannot settle. There's an infinite number of blue-green shades between these wavelengths, so in the end the answer depends totally on the observer.
It is precisely because sound perception is nearly impossible to predict on the basis of objective measurements that subjective audio reviewing was invented some 40 years ago by J. Gordon Holt. And that's why the ear must be the final arbiter in all things musical.
Beyond the RLC Paradigm
My main thesis is that cable sound is profoundly influenced by its constituent materials. In hindsight, this assertion strikes me as not only sensible but also as self evident. A long-standing audiophile tenet holds that a component's sound is determined to a large extent by its parts quality. Hence, in the case of an active component such as a power amp, we can safely state that identical circuits executed with different ingredients will sound dissimilar. Ditto for passive components such as caps where, for example, the choice of dielectric makes a world of difference. By extension, it is logical to expect that the following factors would impact cable sound: conductor type, its purity, crystal granularity, and choice of cable insulation or dielectric. As with capacitors, the quality of the dielectric is critical in determining the harmonic lucidity of a cable. A fraction of the signal soaks into the dielectric, to be released later in time with a rather slow decay. This has the effect of smearing musical transients and causing textures to turn grainy or gritty. Certainly, dielectrics with small "memory" effects are best for audio applications. The main issue then revolves around the choice of conductor material, which brings to mind questions such as: "is silver superior to copper?" or "is 6N copper audibly better than OFC?"
The Inner View
To understand why factors such as conductor purity are important in signal transmission, it's imperative to examine the conductor's inner world - a microscopic realm inhabited by the ubiquitous electron. Metals are by definition good conductors of electricity. The noblest of these, copper, silver, and gold, are distinguished from insulators by virtue of possessing many more free electrons. These electrons, being detached from atoms in the crystal lattice, are the mobile charge carriers that perform the actual conduction. The poplar view of how a conductor works, has electrons buzzing down Electric Avenue at the speed of light. If that were the case, the cost of a relativistic electron accelerator would be less than a single dollar: simply connect a piece of cable to the terminals of a battery. The truth of the matter is that electrons meander down the length of a conductor at a snail's pace. It is the electromagnetic field associated with the flowing electrons that conducts the signal at the speed of a photon torpedo. This field surrounds the cable, cutting through the space around the conductor. The audio signal travels through the conductor at a fraction of the speed of light in vacuum. The transmission speed increases slightly with signal frequency, so that the bass is slightly delayed relative to the treble. Without the application of an external voltage, free-electron motion is thermal in nature and totally random. In other words, there is no net current flow along the wire. Recall that electric current is defined as the net charge passing a cross section of the wire per second. Thus, in the absence of an applied voltage, there are as many electrons coming and going at any point in the wire, so that the net current is zero. The application of an external voltage to the wire provides a velocity component to the free electron that is parallel to the field. Electrons drift down the crystal lattice, pushed along by the voltage gradient, until they bump into atoms in their path. These atomic collisions stop the forward drift and again deflect the electrons in a random direction. After each collision, the electrons are accelerated again by the potential gradient. The result is random electronic motion characterized by an average drift velocity down the conductor. In a sense it's like a microscopic pin ball game with the electrons (the pin balls) crashing into massive bumpers (the atoms). In both cases, the situation is chaotic; it's impossible to predict the path of either the pin ball or the electron. However, it is possible to calculate the average electronic drift velocity. For a 20-gauge copper conductor and a current of 1 ampere, the actual velocity is on the order of 1 foot per hour. It would in this instance take an electron on average about a day to complete a journey down a 24-foot cable!
The continual scattering of electrons by atoms is a form of friction and constitutes the main source of electrical resistance. The amount of scattering is a function of temperature. As temperature is increased, the atoms vibrate more strongly about their mean position in the crystal lattice and scatter electrons more often. It's a well known fact that the resistivity of a conductor increases with temperature. And as copper, for example, is cooled toward absolute zero its resistance decreases. Ideally, at absolute zero where all thermal motion comes to a screeching halt, its resistance would be zero. I said ideally, because for this to happen requires a perfectly ordered crystal lattice. Quantum mechanics predicts that electrons will not be scattered by a perfect crystal.
Unfortunately a conductor wire is far form a perfect crystal. Numerous lattice defects are introduced during the casting of the metal and drawing of the wire, and these create resistance by interfering with electron motion. A wire, though it may look like a homogeneous mass to the naked eye, is in reality made of a multitude of small crystal grains. Because each grain boundary acts to scatter electrons, metallurgists over the years have invented numerous heat treatments and even a continuous casting process all designed to grow larger grains and thus minimize the number of boundaries.
Just as important an impediment to electron conduction are impurity atoms (mostly oxygen) which lodge in the crystal lattice or precipitate along grain boundaries. The level of impurity is a function of the grade of copper and is often given in parts per million (ppm). Copper as a raw material is available in a number of purity grades. Ordinary Tough Pitch Copper (TPC) is only 99.5% pure. Electrolytically refined copper is 99.9% pure and may be designated as three-nines or 3N in purity. This is the stuff of which common electrical conductors are made of. The next level up is 4N or 99.99% pure. The most significant impurity in 3N copper consists of oxygen atoms at a level of over 200 ppm. Being electronegative, oxygen is adept at latching on to free electrons.
The problems of ordinary zip cord are multiplied by its multi-strand construction. First, there's the problem of strand interaction. Second, by greatly increasing the surface area of the conductor, oxidation of the copper over time is significantly increased over a solid-core design of equivalent gauge. Because copper oxide is a semiconductor material, it behaves as a microscopic diode to rectify low-level audio signals. In terms of purity, Oxygen Free Copper (OFC) represents, in my opinion, the minimum starting purity for audio use. This is a 4N pure material, whose oxygen contamination level is only about a fifth (40 - 60 ppm) of that of TPC.
As an audiophile, the absolutely first question that should come to mind when considering a prospective cable purchase is: what grade is the conductor material?
In 1987 the Nippon Mining company, Ltd., succeeded in implementing copper purification technology suitable for commercial scale production of 6N high-purity copper. The 6N designation, of course, refers to the six nines in its level of purity: 99.99997%. It is a factor of 100 more pure than TPC, with less than 20 ppm oxygen content at crystal grain boundaries. Nippon Mining has also developed the technology for drawing various gauges of 6N copper and of heat-treating or annealing the wire to make it suitable for audio applications. Sold under the trade name of Stressfree 6N, and available for the first time in large quantities and in a variety of forms, the 6N grade has found its way into a diverse cross section of audio products: speaker cable, interconnects, phono cartridge wire, internal component wiring, voice coils, crossover coils, and even power cords. Acrotec's high-purity speaker cable and interconnects remain, to this day, in my stable of reference products.
A New Paradigm
I've already mentioned the use of various heat treatments to increase grain size. However, the efficacy of the various treatments depends on the level of impurities present. Grain growing conditions are optimum when the impurity level is lowest. Hence, OFC wires have much finer grains compared with Stressfree 6N copper. The crystal grains in 6N copper are considerably larger, so that the total number of boundaries is about 80 to 100 times smaller than in 4N copper for similar gauge wire.
It is known that impurity atoms precipitate preferentially at grain boundary sites. Ono and Kato, of Nippon Mining Company, discuss in a 1989 paper (87th AES Convention, October 1989, Preprint 2865) several ideas as to how impurity atoms at such sites may compromise audio signals. First of all, the precipitate impurities act as a nonconductive "wall" impeding current flow by scattering electrons. They also postulate the formation of microscopic "capacitors" at grain boundaries which cause signal phase shifts. Along the same lines, since these impurities attract and trap free electrons, it is possible that trapped electrons are released fractionally later in time and thus in a manner similar to the memory effect of a dielectric act to smear transient detail. At least such theories correlate with my own observations of the sonic impact of decreasing crystalline granularity and increasing conductor purity.
I can still recall Acrotec's demo many years ago at a Winter CES, where they compared 6N copper to 3N or plain vanilla copper. The cables were otherwise identical, as far as geometry and dielectrics. The richer, smoother, and more detailed presentation of the 6N cable still resonates in my memory banks. Over the years, I've had the chance to get to know Acrotec's 6N cable quite intimately, and then the 8N cables arrived at my door step. With the only variable being copper purity and grain size, I was surprised to find that as good as the 6N stuff was, the 8N cable was even more delicate and refined sounding. After all, we're contrasting 99.9999% pure copper with copper that's 99.999999% pure. While those last two nines represent a mere parts per billion improvement in purity, they were sonically audible.
by Dick Olsher (March 2001)
Note: The is an abridged and updated version of an article originally published in the January 1996 issue of Fi Magazine. Most of the updates are minor in nature, except for the addition of MAC wireand Kimber Kable Select KS 1030 interconnect to the recommended list.
The notion of cable as a high-end component, so contentious in the 80s, is today rather well accepted by most serious listeners. However, the underlying technical foundation for the sonic character of cable remains nearly as controversial and fuzzy today as it was back then. Well...NOT if you're an electrical engineer. Most of these folks look at the corpus of a cable through entirely materialistic eyes; mainly because they are taught to analyze a circuit in terms of a few fundamental electromagnetic quantities. They idealize and model a cable's behavior as a function of its lumped electrical parameters: resistance (R), inductance (L), and capacitance (C). These parameters are ordained by the cable's dielectric materials and "geometry," by which I mean conductor wire gauge, the number of strands, the spacing between strands in each leg, and the spacing between the "go" and "return" legs. On this basis it is possible to calculate a cable's impedance. And because the calculations agree with the meter readings, this model must be judged as physically valid.
Taking this one step further, it isn't difficult to find instances where a cable's basic electrical parameters do make a sonic difference. Adding the loudspeaker impedance to the model, it is easy to show that since the cable impedance is in series with that of the loudspeaker, a speaker cable can actually modify the loudspeaker's frequency response. It soaks up a greater percentage of the amp's output power as its own impedance rises and that of the load decreases. Imagine a worst case scenario where a 2-ohm dip in the speaker's impedance curve meets up with a 1-ohm cable impedance; the result being about a 3 dB drop in output at that frequency. In general, it's fair to say that cable impedance contributes subtle EQ effects to the overall system sound. In addition, a high-impedance speaker cable increases the apparent output impedance of the power amp. The obvious consequence of that is that the amp's damping factor is reduced. Another not so obvious consequence involves the "de-tuning" of the loudspeaker's crossover network. Crossovers are designed to look into specific source and load impedances, so an increase in the amp's output impedance works to shift the actual crossover point from its intended frequency. All of these effects clearly impact the sound, but because their precise nature is unpredictable and usually deleterious, I would therefore be the last guy on this planet to recommend high-impedance speaker cable. On the other hand, it's important to note that a high-impedance interconnect is normally not a sonic detriment because it operates in a high impedance circuit; a power amp's input impedance typically being on the order of 50,000 ohms. But interconnect capacitance can be an issue if the preamp's output impedance is unusually high. The cable capacitance and output impedance form a low pass network that rolls off the treble. Just how critical is this effect? Lets take a look at a typical scenario. The capacitance of most interconnects measures between 100 and 150 pico Farad (pF) per meter. No serious preamp should have an output impedance greater than 500 ohms, but lets stretch that a bit to 2,500 ohms for the benefit of some tube preamps. Using figures of 150 pF per meter, a 20-foot run of interconnect, and a realistic upper limit of 2,500 ohms for the preamp's output impedance, results in a calculated -3 dB frequency of 73 kHz - and that folks is good enough for me. Undoubtedly, it is a cable's electrical "personality" that gives rise to specific system interactions and much confusion about the merits of any given cable. For example, a high-inductance speaker cable, which would be expected to sound dull in a tonally balanced system, may in fact be hailed as "sonic nirvana" in the context of a bright-sounding system.
A Question of Perception
The decisive mistake engineers make is to ascribe ALL sonic differences to a cable's electrical parameters. It's about as absurd as trying to divine a person's character from his weight, height, and rectal temperature. Card-carrying members of engineering societies by and large tenaciously uphold scientific dogma pounded into their heads during many years of schooling (as a reformed engineer, I should know). Put a gun to their head, and they will maintain till their dying breath that the RLC paradigm is the truth, the whole truth, and nothing but the truth. In the past ten years, a handful of investigators have shown that the RLC paradigm just doesn't go far enough, and that there are other factors that do indeed affect signal transmission. This is also true for other passive parts such as caps and resistors where a simplistic test-bench measurement oriented paradigm has failed to fully account for sonic differences. Turn a Meter Head loose with an Audio Precision System and have him try to differentiate between a mass-market receiver and a Mark Levinson, or for that matter, between a run of #16 awg zip cord and an equal length of high-end cable. It's like trying to judge fine wines on the basis of a chemical analysis. Such measurements are in general not predictive of human perceptions.
To paraphrase Rene Descartes, I hear, therefore I am. Meaning, that the whole is greater than the sum of the parts. While it is possible to dissect a sound field with a variety of frequency and time domain measurements, these meter readings or waterfall plots in no way add up to reflect the emotional reaction I might experience. No wonder science has had such a hard time defining perceptual attributes. Take timbre, for example. The American National Standards Institute defines timbre as "that attribute of a tone by which a listener can judge that two sounds of the same loudness and pitch are dissimilar." Pretty vague if you ask me. The following layman's definition is no better: the perceivable difference between a clarinet middle C and a violin middle C is timbre. To quote Handel (Listening: An Introduction to the Perception of Auditory Events: MIT Press), "timbre is not reducible to an acoustical property that automatically yields a clarinet note or a violin note." Timbre [has] to be judged subjectively. Human vision presents us with a similar perceptual dilemma. The wavelength of visible light can be measured very precisely with a spectrometer. At a wavelength of 520 nm, light is perceived as green; at 470 nm it is blue. But at what wavelength does it change from green to blue? That's a question that measurements cannot settle. There's an infinite number of blue-green shades between these wavelengths, so in the end the answer depends totally on the observer.
It is precisely because sound perception is nearly impossible to predict on the basis of objective measurements that subjective audio reviewing was invented some 40 years ago by J. Gordon Holt. And that's why the ear must be the final arbiter in all things musical.
Beyond the RLC Paradigm
My main thesis is that cable sound is profoundly influenced by its constituent materials. In hindsight, this assertion strikes me as not only sensible but also as self evident. A long-standing audiophile tenet holds that a component's sound is determined to a large extent by its parts quality. Hence, in the case of an active component such as a power amp, we can safely state that identical circuits executed with different ingredients will sound dissimilar. Ditto for passive components such as caps where, for example, the choice of dielectric makes a world of difference. By extension, it is logical to expect that the following factors would impact cable sound: conductor type, its purity, crystal granularity, and choice of cable insulation or dielectric. As with capacitors, the quality of the dielectric is critical in determining the harmonic lucidity of a cable. A fraction of the signal soaks into the dielectric, to be released later in time with a rather slow decay. This has the effect of smearing musical transients and causing textures to turn grainy or gritty. Certainly, dielectrics with small "memory" effects are best for audio applications. The main issue then revolves around the choice of conductor material, which brings to mind questions such as: "is silver superior to copper?" or "is 6N copper audibly better than OFC?"
The Inner View
To understand why factors such as conductor purity are important in signal transmission, it's imperative to examine the conductor's inner world - a microscopic realm inhabited by the ubiquitous electron. Metals are by definition good conductors of electricity. The noblest of these, copper, silver, and gold, are distinguished from insulators by virtue of possessing many more free electrons. These electrons, being detached from atoms in the crystal lattice, are the mobile charge carriers that perform the actual conduction. The poplar view of how a conductor works, has electrons buzzing down Electric Avenue at the speed of light. If that were the case, the cost of a relativistic electron accelerator would be less than a single dollar: simply connect a piece of cable to the terminals of a battery. The truth of the matter is that electrons meander down the length of a conductor at a snail's pace. It is the electromagnetic field associated with the flowing electrons that conducts the signal at the speed of a photon torpedo. This field surrounds the cable, cutting through the space around the conductor. The audio signal travels through the conductor at a fraction of the speed of light in vacuum. The transmission speed increases slightly with signal frequency, so that the bass is slightly delayed relative to the treble. Without the application of an external voltage, free-electron motion is thermal in nature and totally random. In other words, there is no net current flow along the wire. Recall that electric current is defined as the net charge passing a cross section of the wire per second. Thus, in the absence of an applied voltage, there are as many electrons coming and going at any point in the wire, so that the net current is zero. The application of an external voltage to the wire provides a velocity component to the free electron that is parallel to the field. Electrons drift down the crystal lattice, pushed along by the voltage gradient, until they bump into atoms in their path. These atomic collisions stop the forward drift and again deflect the electrons in a random direction. After each collision, the electrons are accelerated again by the potential gradient. The result is random electronic motion characterized by an average drift velocity down the conductor. In a sense it's like a microscopic pin ball game with the electrons (the pin balls) crashing into massive bumpers (the atoms). In both cases, the situation is chaotic; it's impossible to predict the path of either the pin ball or the electron. However, it is possible to calculate the average electronic drift velocity. For a 20-gauge copper conductor and a current of 1 ampere, the actual velocity is on the order of 1 foot per hour. It would in this instance take an electron on average about a day to complete a journey down a 24-foot cable!
The continual scattering of electrons by atoms is a form of friction and constitutes the main source of electrical resistance. The amount of scattering is a function of temperature. As temperature is increased, the atoms vibrate more strongly about their mean position in the crystal lattice and scatter electrons more often. It's a well known fact that the resistivity of a conductor increases with temperature. And as copper, for example, is cooled toward absolute zero its resistance decreases. Ideally, at absolute zero where all thermal motion comes to a screeching halt, its resistance would be zero. I said ideally, because for this to happen requires a perfectly ordered crystal lattice. Quantum mechanics predicts that electrons will not be scattered by a perfect crystal.
Unfortunately a conductor wire is far form a perfect crystal. Numerous lattice defects are introduced during the casting of the metal and drawing of the wire, and these create resistance by interfering with electron motion. A wire, though it may look like a homogeneous mass to the naked eye, is in reality made of a multitude of small crystal grains. Because each grain boundary acts to scatter electrons, metallurgists over the years have invented numerous heat treatments and even a continuous casting process all designed to grow larger grains and thus minimize the number of boundaries.
Just as important an impediment to electron conduction are impurity atoms (mostly oxygen) which lodge in the crystal lattice or precipitate along grain boundaries. The level of impurity is a function of the grade of copper and is often given in parts per million (ppm). Copper as a raw material is available in a number of purity grades. Ordinary Tough Pitch Copper (TPC) is only 99.5% pure. Electrolytically refined copper is 99.9% pure and may be designated as three-nines or 3N in purity. This is the stuff of which common electrical conductors are made of. The next level up is 4N or 99.99% pure. The most significant impurity in 3N copper consists of oxygen atoms at a level of over 200 ppm. Being electronegative, oxygen is adept at latching on to free electrons.
The problems of ordinary zip cord are multiplied by its multi-strand construction. First, there's the problem of strand interaction. Second, by greatly increasing the surface area of the conductor, oxidation of the copper over time is significantly increased over a solid-core design of equivalent gauge. Because copper oxide is a semiconductor material, it behaves as a microscopic diode to rectify low-level audio signals. In terms of purity, Oxygen Free Copper (OFC) represents, in my opinion, the minimum starting purity for audio use. This is a 4N pure material, whose oxygen contamination level is only about a fifth (40 - 60 ppm) of that of TPC.
As an audiophile, the absolutely first question that should come to mind when considering a prospective cable purchase is: what grade is the conductor material?
In 1987 the Nippon Mining company, Ltd., succeeded in implementing copper purification technology suitable for commercial scale production of 6N high-purity copper. The 6N designation, of course, refers to the six nines in its level of purity: 99.99997%. It is a factor of 100 more pure than TPC, with less than 20 ppm oxygen content at crystal grain boundaries. Nippon Mining has also developed the technology for drawing various gauges of 6N copper and of heat-treating or annealing the wire to make it suitable for audio applications. Sold under the trade name of Stressfree 6N, and available for the first time in large quantities and in a variety of forms, the 6N grade has found its way into a diverse cross section of audio products: speaker cable, interconnects, phono cartridge wire, internal component wiring, voice coils, crossover coils, and even power cords. Acrotec's high-purity speaker cable and interconnects remain, to this day, in my stable of reference products.
A New Paradigm
I've already mentioned the use of various heat treatments to increase grain size. However, the efficacy of the various treatments depends on the level of impurities present. Grain growing conditions are optimum when the impurity level is lowest. Hence, OFC wires have much finer grains compared with Stressfree 6N copper. The crystal grains in 6N copper are considerably larger, so that the total number of boundaries is about 80 to 100 times smaller than in 4N copper for similar gauge wire.
It is known that impurity atoms precipitate preferentially at grain boundary sites. Ono and Kato, of Nippon Mining Company, discuss in a 1989 paper (87th AES Convention, October 1989, Preprint 2865) several ideas as to how impurity atoms at such sites may compromise audio signals. First of all, the precipitate impurities act as a nonconductive "wall" impeding current flow by scattering electrons. They also postulate the formation of microscopic "capacitors" at grain boundaries which cause signal phase shifts. Along the same lines, since these impurities attract and trap free electrons, it is possible that trapped electrons are released fractionally later in time and thus in a manner similar to the memory effect of a dielectric act to smear transient detail. At least such theories correlate with my own observations of the sonic impact of decreasing crystalline granularity and increasing conductor purity.
I can still recall Acrotec's demo many years ago at a Winter CES, where they compared 6N copper to 3N or plain vanilla copper. The cables were otherwise identical, as far as geometry and dielectrics. The richer, smoother, and more detailed presentation of the 6N cable still resonates in my memory banks. Over the years, I've had the chance to get to know Acrotec's 6N cable quite intimately, and then the 8N cables arrived at my door step. With the only variable being copper purity and grain size, I was surprised to find that as good as the 6N stuff was, the 8N cable was even more delicate and refined sounding. After all, we're contrasting 99.9999% pure copper with copper that's 99.999999% pure. While those last two nines represent a mere parts per billion improvement in purity, they were sonically audible.