Astral OFC 4.4mm
A creation driven by obsession and passion.
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From exploring the underlying materials (Brass, Copper, Silver) to analyzing the plating options (Silver, Gold, Rhodium, Platinum, Palladium), we left no stone unturned. Even seemingly small choices, such as plating thickness, were scrutinized to ensure the best possible sonic performance.
1. Base Metal
Brass, Copper, & Silver
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Brass:
Brass is harder and much more resistant to wear and scratches than softer metals like copper or silver. This makes it an attractive option for durability, especially in use cases where connectors are frequently plugged and unplugged. However, brass has significantly lower electrical conductivity—roughly 28% of copper’s conductivity—which makes it less ideal for high-performance audio applications.
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Silver:
Silver has the highest electrical conductivity of any metal, making it theoretically the optimal choice for signal transfer. However, silver has two major drawbacks:
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Softness: Silver is much softer than brass or copper, which means connectors made from silver are far more prone to scratches and deformities, especially in tighter 4.4mm sockets where clamp force is higher. Over time, this can degrade the physical integrity of the connector.
Cost: The price of silver has skyrocketed in recent years, increasing over 2.5 times since the launch of our Reference Silver cable. This makes it a less practical choice for a product balancing performance, durability, and affordability.
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Copper:
Thus comes copper, the perfect balance between conductivity, cost, and durability. While not as hard as brass, it is significantly more conductive (roughly 97% of silver’s conductivity) and much harder than silver, making it less prone to scratching in tight sockets. Given these factors, copper emerged as the best option for our 4.4mm connectors.
2. Plating
Silver, Gold, Rhodium, Platinum, & Palladium
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Plating is the outermost layer of the connector, and it plays a crucial role in determining the connector’s surface properties, including corrosion resistance, hardness, and conductivity. Each plating material has its strengths and weaknesses:
Silver:
As with the base metal, silver offers the highest conductivity of all plating materials. However, its softness and tendency to tarnish quickly (forming silver sulfide, which is non-conductive), and silver cleaning cloth would often simlpy wipe off the entire silver plating layer over time, this is no ideal for long-term performance.
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Gold:
Gold strikes an excellent balance between conductivity and long-term reliability. While its conductivity is slightly lower than silver's (~70% of copper’s conductivity), gold is highly resistant to corrosion and maintains its performance over time. Its softness is mitigated by the use of thin plating layers, making it a practical choice for high-quality connectors.
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Rhodium:
Rhodium is extremely hard and resistant to wear, making it an attractive choice for durability. However, its conductivity is significantly lower than copper or gold, which makes it less ideal for this application.
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Platinum:
Platinum offers excellent corrosion resistance and durability, but is even less conductive than rhodium. This could be a "purpose-specific" consideration in the future.
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Palladium:
Palladium offers good corrosion resistance and conductivity, but it is softer than rhodium and has higher costs than gold without providing significant advantages in performance.
3. Unplated vs. Plated
One of the most important considerations in the design process was the choice between plated copper and bare copper connectors. This decision directly affects the connector's performance, longevity, and the subjective listening experience.
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Bare Copper:
Bare copper offers the best possible conductivity, as there are no additional layers to introduce resistance. During our "ears-on" testing sessions, participants consistently described the bare copper version as delivering a more direct and transparent sound, with a sense of immediacy that appeals to purists.
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However, bare copper has a significant drawback: it oxidizes quickly when exposed to air. This oxidation forms a non-conductive layer of copper oxide, which increases resistance and degrades performance. These effects are particularly pronounced in environments with high humidity or pollution, where oxidation occurs more rapidly. Over time, this can compromise the connector's ability to maintain consistent signal transmission, leading to degraded audio fidelity.
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Plated Copper:
Plated connectors solve the problem of oxidation by adding a protective layer over the copper base. In this case, the plating shields the copper from exposure to air and moisture, preventing the copper from oxidizing.
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However, plating introduces a new challenge: additional resistance. When electricity flows through the connector, current must pass through the bonding layer and the plating material before reaching the copper base. Each layer has its own resistivity, and this cumulative resistance can slightly reduce overall conductivity. This is why the conductivity of the plating material itself becomes critical—materials with higher conductivity, like gold or silver, minimize the impact of these additional layers.
4. Skin Depth vs. Plating
Skin depth remains one of the most debated aspects of wire and connector design, often highlighted as a critical factor in determining performance. However, the nuances of when skin depth becomes significant and how it interacts with other design elements, such as plating materials, are often misunderstood. While skin depth has minimal impact in the audible frequency range of 20 Hz to 20 kHz, the phenomenon still subtly influences current flow. This subtlety may help explain why some listeners perceive differences in sound quality between bare copper and plated connectors, even though the theoretical impact of skin depth at these frequencies is negligible.
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Skin depth is a physical phenomenon tied to the behavior of alternating current (AC) as it flows through a conductor. At higher frequencies, self-induced electromagnetic fields within the conductor push more of the current toward the surface, concentrating it within the region known as the skin depth. For human-audible frequencies, the skin depth in copper is relatively large compared to the dimensions of most connectors or wires. At 20 kHz, for example, the skin depth in copper is approximately 0.46 mm, which is more than sufficient to allow current to flow through nearly the entire cross-section of most conductors. This makes the effect minimal and practically insignificant at these frequencies. However, even at audio frequencies, higher-frequency components of the signal, such as those near 20 kHz, tend to favor the outer layers of the conductor slightly more than the core. While this redistribution of current density is subtle and does not exclude the core entirely, it hints at how the properties of the conductor’s surface may influence performance, even at relatively low frequencies.
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This surface preference becomes particularly relevant when considering the impact of plating materials. The Schottky effect, which refers to the potential barrier at the interface between two different materials, may provide insight into why some listeners describe bare copper connectors as sounding more "direct" or "transparent." In a plated connector, current must pass through the boundary between the plating material and the copper base. This transition introduces a potential resistance or energy loss at the microscopic level. Though this effect is small, it could subtly alter the behavior of higher-frequency components of the signal, particularly those that already show a slight preference for the surface. These small differences in the transmission of higher frequencies may explain the perceived clarity and immediacy of bare copper connectors compared to their plated counterparts.
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Bare copper connectors avoid these issues altogether. With no material interfaces to introduce additional resistance or barriers, bare copper allows for a seamless current flow throughout its surface and core. This is particularly advantageous for higher-frequency components, which are already slightly more concentrated near the surface. As a result, many participants in listening tests have described bare copper connectors as delivering a sound that feels more transparent and direct. In contrast, plated connectors, while offering practical benefits, introduce additional layers and transitions that can affect signal transmission. Even when high-conductivity plating materials such as gold or silver are used, the interface between the plating and the copper base still exists. This interface, though minimized by the excellent conductivity of these materials, may slightly affect the behavior of higher-frequency signals, potentially contributing to a sound that is perceived as less direct.
Despite the negligible impact of skin depth on current distribution at audio frequencies, the conductivity of the plating material remains critical. At lower frequencies, where current flows through both the plating layer and the base material, the plating material’s conductivity directly influences the overall performance of the connector. Poorly conductive plating materials, such as nickel, can introduce additional resistance, degrading the signal and reducing audio fidelity. At higher frequencies, even within the audio range, the slight preference for the surface amplifies the importance of plating quality. High-conductivity materials like gold or silver minimize resistance and ensure consistent signal transmission, preserving the fidelity of higher-frequency components.
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The irony of the relationship between skin depth and plating lies in the fact that, at audio frequencies, where skin depth isn’t a significant limiting factor, the surface material of the connector still plays a pivotal role in performance. The slight preference for higher frequencies towards the surface layers means that the plating material’s conductivity and its interface with the base material can subtly influence the signal. Bare copper avoids these complications entirely, offering a direct and unimpeded signal path that many listeners find superior. Plated connectors, on the other hand, provide advantages in durability and oxidation resistance, but the choice of plating material—whether gold, silver, or nickel—can affect the way higher frequencies are transmitted, shaping the tonal character of the sound. Ultimately, the interaction of skin depth, surface properties, and material interfaces underscores the complex interplay of physical phenomena and subjective perception in audio design.