Constant Voltage Audio Systems
Why 70V/100V Distribution?
Traditional 8Ω low-impedance speaker systems are limited to short cable runs and few speakers per amplifier. Constant voltage distribution (70V in North America, 100V in Europe/Asia) solves this by stepping up the voltage at the amplifier output, allowing:
- Long cable runs: Up to 300m+ without significant voltage drop
- Multiple speakers: 50-100+ speakers on a single amplifier circuit
- Individual control: Each speaker has a transformer with selectable power taps (0.5W-32W)
- Simplified wiring: Parallel connection, no impedance matching required
System Design Principles
Voltage Drop Limits
Industry standard: 3% maximum voltage drop from amplifier to farthest speaker. Exceeding this causes reduced volume and poor frequency response. Use larger gauge wire or higher system voltage (100V vs 70V) for long runs.
Amplifier Sizing
Total speaker load + 20% headroom for dynamic peaks. Example: 200W total speaker load requires 240W amplifier. Never exceed amplifier rated power to avoid clipping and distortion.
Tap Selection
Speaker transformers have multiple taps (e.g., 8W, 4W, 2W, 1W). Select based on coverage area and ambient noise. Typical: 2-4W for quiet offices, 8-16W for noisy industrial areas, 32W for outdoor paging.
Common Applications
Commercial Paging & Background Music
Retail stores, offices, warehouses. Typical: 70V system, 16 AWG cable, 2-4W ceiling speakers spaced 6-8m apart. Amplifier: 120-240W for 30-60 speakers.
Life Safety / Emergency Voice Alarm (EVAC)
Fire alarm voice evacuation per NFPA 72. Requires FPLP (plenum) or FPLR (riser) rated cable. Supervised circuits with end-of-line resistors. Backup battery for 24-hour standby + 15 min alarm.
Industrial/Outdoor Paging
Factories, construction sites, parking lots. Use 100V system for longer runs. Horn speakers rated 16-32W. 14 AWG or 12 AWG cable. Weatherproof enclosures (IP65+).
Cable Selection Guide
| Cable Gauge | Max Run (70V, 3% drop) | Max Run (100V, 3% drop) | Typical Use |
|---|---|---|---|
| 16 AWG (1.31mm²) | ~150m | ~300m | Standard commercial paging |
| 14 AWG (2.08mm²) | ~240m | ~480m | Long runs, high-power systems |
| 12 AWG (3.31mm²) | ~380m | ~760m | Industrial, outdoor, very long runs |