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The Role of 75-Ohm Coaxial Cable in Hi-Fi Audio Systems
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posted by Vincent Zhang
Published by IWISTAO
In high-fidelity audio systems, signal integrity is just as important as circuit topology, component quality, or power-supply design. Among all interconnect options, 75-ohm coaxial cable holds a special position due to its extensive use in digital audio transmission, broadcast video, and measurement systems.
Despite its popularity, 75-ohm coaxial cable is often misunderstood—especially when it is applied indiscriminately to analog audio connections. This article explains what 75-ohm coaxial cable is, why its characteristic impedance matters, and where it is technically justified—or unnecessary—in Hi-Fi audio systems.
1. What Is a 75-Ohm Coaxial Cable?
A coaxial cable consists of four fundamental elements:
- A central conductor that carries the signal
- A dielectric insulator that defines geometry and capacitance
- A cylindrical outer shield that serves as the return path
- An external protective jacket
The term “75 ohms” does not describe DC resistance. Instead, it refers to the characteristic impedance (Z0) of the cable, which is determined by the ratio between conductor diameters and the dielectric constant of the insulation.
The characteristic impedance of a coaxial cable can be approximated by:
Z0 = (60 / √εr) · ln(D / d)
Where:
- εr = relative permittivity of the dielectric material
- D = inner diameter of the outer shield
- d = diameter of the center conductor
For common polyethylene-based dielectrics, a 75-ohm design offers an excellent balance between low attenuation and wide bandwidth, which explains its dominance in broadcast and digital signal transmission.
2. Why 75 Ohms Matters in Digital Audio
2.1 S/PDIF Coaxial Transmission
The most important Hi-Fi application of 75-ohm coaxial cable is S/PDIF (Sony/Philips Digital Interface Format) over coaxial connection.
Although S/PDIF carries audio data, electrically it is a high-speed digital signal with fast rise and fall times. As a result, its behavior is governed by transmission-line theory rather than low-frequency analog audio rules.
Using a true 75-ohm coaxial cable minimizes:
- Signal reflections caused by impedance mismatch
- Edge distortion and ringing
- Interface-induced jitter at the DAC input
Even short cable runs benefit from proper impedance control, because signal rise time—not cable length—determines the severity of reflections.
2.2 Cable and Connector as a System
A common misconception is that “any RCA cable works for coaxial digital audio.” In reality, most standard RCA connectors do not maintain a precise 75-ohm impedance.
However, a well-designed 75-ohm coaxial cable assembly still offers significantly better performance than generic analog interconnects, particularly when cable geometry and shielding are properly controlled.
While professional systems often use BNC connectors for superior impedance accuracy, consumer S/PDIF systems still benefit greatly from true 75-ohm coaxial cabling.
3. 75-Ohm Coaxial Cable in Analog Audio
3.1 Line-Level Analog Signals
For analog line-level audio signals (20 Hz to 20 kHz), characteristic impedance matching is not required. At these frequencies, signal wavelengths are measured in kilometers.
Therefore:
- 75-ohm impedance provides no intrinsic sonic advantage
- Shielding effectiveness becomes more relevant than impedance
- Cable capacitance may have greater impact than Z0
In short, characteristic impedance is largely irrelevant for analog interconnects.
3.2 Phono and High-Impedance Circuits
In phono systems, cable capacitance and shielding quality are critical, while characteristic impedance remains irrelevant.
Using a 75-ohm coaxial cable without considering its capacitance may disturb cartridge loading and frequency response, particularly with MM cartridges.
As a result, 75-ohm coaxial cable is not automatically suitable for turntable applications.
4. Shielding and Noise Rejection
One genuine advantage of coaxial cable—regardless of impedance—is its shielding geometry. A coaxial structure provides:
- 360-degree electrostatic shielding
- Excellent rejection of EMI and RFI
- A predictable and low-impedance ground return path
This makes coaxial cable particularly effective in digitally noisy environments and mixed-signal Hi-Fi systems.
5. Common Myths About 75-Ohm Coaxial Cable
- “75 ohms improves analog sound quality” — False
- “Any RCA cable works for digital coax” — False
- “Cable length must be long to matter” — False
6. Practical Recommendations
Use 75-ohm coaxial cable when:
- Connecting a CD transport or streamer to a DAC via S/PDIF
- Building DIY digital coaxial interconnects
- Working with digital audio or measurement equipment
IWISTAO HIFI 75-ohm Digital Coaxial Cable DAC Belden 1694A Cold Press Self-locking Budweiser RCA
Do not prioritize 75 ohms when:
- Selecting analog RCA interconnects
- Wiring phono cartridges (capacitance is more important)
Conclusion
75-ohm coaxial cable is not a universal Hi-Fi upgrade, but it is technically essential for coaxial digital audio transmission.
When used in the correct context—especially for S/PDIF links—it provides measurable, engineering-based benefits. When misapplied to analog audio, its impedance specification offers little relevance.
Understanding where impedance matters—and where it does not—is fundamental to rational Hi-Fi system design.
References
- IEC 60958 – Digital Audio Interface Standard
https://webstore.iec.ch/publication/6006 - Howard Johnson, High-Speed Digital Design: A Handbook of Black Magic
https://www.pearson.com - Rane Corporation – Impedance Matching in Audio
https://www.ranecommercial.com - Belden – Coaxial Cable Technical Papers
https://www.belden.com
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