A digital audio link that crackles, drops out, or locks intermittently is usually sent to an engineer as a faulty source unit. In most cases the source is fine and the interconnect is the problem. Coaxial audio cables are transmission lines with a defined 75 ohm characteristic impedance, and the signal they carry has fast edges that reflect at any impedance discontinuity, such as a poor RCA plug, a stray adapter or a kinked cable. This guide explains the impedance rule, when coaxial beats optical or balanced connections, and how to specify the right assembly for a rack, a desk or a long subwoofer run.
Quick answer: Use a true 75 ohm coaxial cable for any single-ended digital audio link, whether the connector is RCA, BNC or F-type. Match the impedance end to end, avoid adapting the run with an unrelated plug type in the middle, and keep the length as short as the layout allows. For runs that cross between buildings or between units on different power, a balanced AES/EBU connection over a 110 ohm XLR cable or an optical link avoids the problem altogether.
Coaxial Audio Cables: 75 Ohm Digital Audio Explained
Why 75 ohm matters in a digital audio cable
A coaxial cable is a transmission line, not a simple piece of wire. Its characteristic impedance is set by the ratio of the inner conductor diameter to the inside diameter of the shield and by the dielectric between them. For coaxial digital audio the standard value is 75 ohm, and the source and destination are built to expect it. When the cable, the plug and the receiving socket all present the same impedance, the signal travels down the line, is absorbed at the far end, and the digital receiver sees a clean waveform.
What an impedance mismatch actually does
Anywhere the impedance changes, part of the signal energy reflects back toward the source. A phono-style RCA plug pressed onto a 75 ohm cable, a spliced joint, or a random adapter in the middle of a run creates exactly that kind of discontinuity. The reflections arrive at the receiver slightly late and disturb the timing of the transitions the receiver relies on to read the data. The audible result is not usually dramatic: it appears as dropouts, a loss of lock on the digital signal, or intermittent clicks that get worse as cable length grows.
Why an ordinary shielded audio cable is not a substitute
A standard line-level interconnect has no controlled impedance, and its capacitance per metre is often far higher than a coaxial digital cable. Over a short run it may work by luck. Push the length out and the same cable produces a signal the receiver cannot lock onto, which is why swapping a random pair of cables between a transport and a converter changed nothing until the length was reduced. The connector matters as much as the cable; a genuine 75 ohm plug maintains the geometry right up to the mating surface.
Which coaxial audio cable should you use for a subwoofer or a digital feed?
Both applications use 75 ohm coaxial cable, and that is not a coincidence. A digital audio link between a source and a converter, and a line-level feed from an amplifier or receiver to a powered subwoofer, are both low-level signals carried over a shielded, controlled-impedance line. The difference lies in the connector and the length rather than in the cable family.
For a subwoofer feed, a dedicated coaxial cable for subwoofer use is the correct choice, usually terminated with RCA plugs and often fitted with a splitter or Y-lead when a single output has to feed two inputs. For a digital feed between two units in the same rack, a short 75 ohm coaxial cable with RCA or BNC ends is standard. Where the run is long, the cable geometry and the shield coverage matter far more than the marketing description on the packaging.
Coaxial against optical and balanced AES/EBU
Coaxial is only one of several ways to move digital audio, and each has a specific advantage.
| Interface | Cable impedance | Connector | Signal class | Best fit |
|---|---|---|---|---|
| Coaxial S/PDIF | 75 ohm | RCA or BNC | Stereo PCM and compressed multichannel bitstreams | Short to medium rack and desk runs |
| Optical | Not impedance based | Toslink | Same digital audio formats over light | Breaking ground loops, electrically noisy racks |
| Balanced AES/EBU | 110 ohm | 3-conductor XLR | Professional digital audio with common-mode rejection | Long runs and studio installations |
| HDMI audio return channel | 100 ohm differential pairs | HDMI type A | ARC carries compressed multichannel audio at roughly 1 Mbps class bandwidth; the enhanced eARC mode defined with HDMI 2.1 carries far higher bitrate formats | Television to receiver over a single cable |
Two practical conclusions follow from the table. First, if a coaxial link suffers from hum or buzzing that changes when equipment moves, the problem is a ground loop rather than impedance, and an optical connection is the simplest cure. Second, if the run has to be genuinely long, a balanced 110 ohm XLR connection rejects common-mode noise that a single-ended coaxial line cannot.
Connectors: RCA plugs, shells and contact quality
The RCA plug is the weak point in most coaxial audio cables. A good connector has a solid centre pin and a shield contact that grips the socket firmly, and its internal geometry keeps the cable’s impedance consistent through the transition. A loose-fitting plug introduces both an impedance step and a variable contact resistance, which is why a system can sound fine until the cable is nudged. Where the equipment offers a BNC socket, use it; the bayonet coupling maintains constant impedance and does not rely on friction alone. Assembly quality matters for the same reason: RCA jack and stereo cable assemblies live or die by how well the shield is bonded and how much of the centre conductor is left exposed.
Three application scenarios
- AV rack feeding a powered subwoofer. A receiver drives a subwoofer across four metres of cable that runs alongside mains leads. The requirement is a 75 ohm coaxial cable with good shield coverage and a solid RCA plug at each end, routed away from power cabling to keep induced hum out of a low-level signal.
- Desktop digital source into a converter. A transport feeds a digital-to-analogue converter sitting on the same shelf. The run is short, so impedance control is easy, but the connector seats and unseats often. Choose a rugged plug with a firm shield grip rather than the thinnest cable available.
- Commercial installation with a long subwoofer feed. A restaurant or studio space places the subwoofer across the room from the amplifier. If the single-ended coaxial run grows long, consider a balanced line, or choose an amplifier with a balanced input, rather than simply buying a thicker coaxial cable and hoping the shield does the work.
Length, shielding and grounding
Coaxial digital audio is not designed for very long single-ended runs. Keep rack interconnects short, and treat anything beyond a typical room-scale distance as a signal to review the architecture rather than to shop for a heavier cable. Shielding coverage matters most where the cable passes near power distribution, dimmer racks or wireless equipment; a foil and braid combination gives better coverage across the frequency range than a sparse braid alone.
Grounding is the other half of the problem. Because a coaxial cable ties the chassis grounds of two units together, any potential difference between them appears as current on the shield. Symptoms include continuous hum, a buzz that changes with lighting, or noise that disappears when the source and the amplifier are plugged into the same outlet. A ground-loop isolator or an optical link solves it cleanly; neither a longer cable nor a thicker one will.
Specifying and ordering
State five things when you order: the characteristic impedance, the connector type and gender at each end, the length, the flexibility the installation demands, and the environment the cable will pass through. For loudspeaker connections, note that a coaxial assembly is not the right product at all; the requirements are different and are covered in the guide to audio speaker cable. Getting the product category right at the quotation stage removes almost every complaint that follows.
FAQ
1. Does a coaxial audio cable have to be exactly 75 ohm?
Yes, for digital audio. The source and receiver are designed around a 75 ohm transmission line, and any other impedance creates reflections that disturb the timing the receiver depends on. A cable with no controlled impedance may work over a metre or two and then fail as the run gets longer.
2. Can I use a standard RCA audio cable for a digital coaxial connection?
It can appear to work over very short distances, but it is not a 75 ohm cable and its capacitance is usually much higher than a coaxial digital cable. Symptoms appear as dropouts and loss of lock, particularly as length increases. Use a cable that is specified as 75 ohm coaxial with a genuine 75 ohm plug.
3. Why does my coaxial audio link hum when an optical link does not?
Hum is a ground loop, not an impedance problem. Coaxial connects the chassis grounds of both units, so any potential difference between them drives current along the shield. Optical cable carries light and provides no electrical path, which breaks the loop instantly.
4. Is coaxial better than optical for home cinema audio?
Neither is inherently better. Both carry the same digital audio formats over short runs. Optical wins where ground loops or electrical noise are present, coaxial wins where the cable will be bent sharply or where a fragile optical connector could be damaged. On modern systems, the HDMI audio return channel often replaces both.
5. How long can a coaxial digital audio cable run?
Keep single-ended coaxial runs short, generally within a normal room-scale installation, and use a quality 75 ohm cable throughout. For longer distances use a balanced AES/EBU connection over 110 ohm XLR cable, which rejects common-mode noise, or move the source closer and carry the audio at line level instead.