Audio Cable for Stadium PA: Balanced, Long and Grounded Once
Quick answer / TL;DR: Short answer: distribute at line level over balanced, shielded twisted-pair cable, ground the shield at the source end of each run and use transformer isolation where two grounded systems meet. Keep audio cable separate from lighting and power, and provide a test point at each amplifier location so a fault can be localised to a zone.
The screening family tree: no shield, overall foil, braid plus pair foils.A stadium public address system distributes audio over distances measured in hundreds of metres, to amplifier locations spread across the structure.
At that scale the cable is a transmission line rather than a patch lead, and the grounding arrangement decides whether the system is quiet.
This article covers the level and cable choice, the grounding topology, and the fault-finding that a distributed system requires.
A stadium has many power sources, many grounded systems and a lighting rig that switches large loads. The audio system has to remain quiet in that environment.
Balanced transmission rejects common-mode noise, but only if the balance is maintained and the shield arrangement does not create a loop between two grounded points.
The most common fault in a large system is a ground loop created by connecting two grounded systems with a cable whose shield is bonded at both ends.
Cable and Level
| Parameter | Practice | Reason |
|---|
| Level | Line level, not microphone | Higher level improves noise ratio |
| Pair | Balanced, twisted | Rejects common-mode noise |
| Shield | Bonded at source end | Avoids ground loops |
| Isolation | Transformer where systems meet | Breaks the loop without unbalancing |
| Routing | Separate from power | Reduces induced noise |
Distributing at line level rather than microphone level is the single largest improvement available, because the signal is far above the noise the environment injects.
Where a microphone must travel a long distance, convert to line level or to digital at the stage end rather than running a low-level signal across the building.
Grounding Topology
Choose a single reference for the audio system and bond each shield at the source end of its run, so that no shield carries current between two grounded points.
Where a run leaves one grounded system and enters another, insert a transformer isolator. It preserves the balanced connection and breaks the loop.
Draw the grounding arrangement and keep it with the system documentation, because a later addition that bonds a shield at the far end reintroduces the loop.
Measure for voltage between chassis before connecting anything. A potential difference is a warning, not something to be absorbed by the cable.
Fault-Finding
- Provide a test point at each amplifier location, not only at the control room
- Label each run by zone so a report identifies a location
- Record the normal level at each test point at commissioning
- Carry a transformer isolator as a diagnostic tool
- Treat a hum in one zone as a local problem before suspecting the source
Real-World Applications
A stadium distributes line level to amplifier rooms over balanced cable with shields bonded at the control end, and the system is quiet.
A new zone is added and bonds its shield at the amplifier end. A hum appears across the system, and the bond is removed.
A lighting change introduces noise in one zone. The audio run shares a tray with a lighting feed, and rerouting restores the previous noise floor.
A zone fails before an event. The test point at the amplifier identifies the run as faulty rather than the amplifier, and the spare run is patched.
Standards and Verification
Balanced audio interconnection follows the wiring conventions for the connector types in use, and cable performance is published by the manufacturer.
Verification is a level and noise measurement at each test point with the lighting and power systems active, plus a documented grounding arrangement.
Measure noise with the building live. A system measured in a quiet building has not been tested against the conditions it will operate in.
Delay Zones and Cable Paths
A delay zone requires its own signal path, and the cable route to that zone is part of the timing design rather than an afterthought.
Keep the path for each zone independent, so that a fault in one zone does not remove the feed to another.
Label the zone at both ends of every run, and record the delay setting alongside the cable record, because the two are meaningless apart.
Where a zone is added later, feed it from the distribution point rather than tapping a nearby run, which would change the loading on the existing path.
Where amplifiers are distributed, keep the mains supply for each room on the same phase group as its audio source, so that a phase difference does not appear as a hum that follows the zones.
Where the control room is at one end of the structure, the runs to the far zones are the longest in the system. Give those zones a distribution point closer to them rather than extending a single path.
Keep a spare isolator at the control position. It is the part most likely to be needed during an event and the least likely to be in stock.
Related reading: Audio cable for live sound — blogs/news/audio-cable-for-live-sound
Related reading: Audio cable for house of worship — blogs/news/audio-cable-for-house-of-worship
Related reading: Balanced vs unbalanced audio — blogs/news/balanced-vs-unbalanced-audio
Bond each shield at the source end. A shield bonded at both ends is a ground loop with a connector on it.
Frequently Asked Questions
Should stadium audio run at line or microphone level?
Line level. A low-level signal over that distance has a poor noise ratio.
Where should the shield be bonded?
At the source end of each run, to avoid current flowing between two grounded points.
How do I connect two grounded systems?
With a transformer isolator, which breaks the loop while keeping the balanced connection.
Why measure noise with the building live?
Because the noise sources are the lighting and power systems, which are not active in a quiet building.
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