Constant-voltage (70V) distribution allows many loudspeakers to share one amplifier using impedance-transforming taps

In distributed sound systems (airports, retail, offices), many loudspeakers must be driven from a single amplifier over long cable runs. Constant-voltage (CV) distribution uses a 70.7 V (or 100 V) amplifier output with a step-up transformer, allowing loudspeakers to be tapped with a small step-down transformer at each speaker. The transformer tap (25W, 5W, 1W, etc.) determines how much power each speaker receives. All speakers share a common high-impedance, high-voltage bus — the amplifier ‘sees’ the total combined impedance of all taps. Long cable runs are practical because high voltage means low current and low resistive loss for a given power. This system is not used in live performance reinforcement; it is standard in background music and paging systems.

Examples

A conference center installs 40 ceiling speakers on a 70V distribution amplifier. Each speaker transformer tap is set to 2W, drawing 40×2 = 80W total from the amplifier. The long cable runs (up to 300 feet) lose negligible power at 70V.

Assessment

Why does 70V distribution reduce cable losses compared to direct low-impedance connection of 8Ω speakers on long runs? What is the primary tradeoff?