100 V lines

In shops, stations, schools and sports halls, many loudspeakers are connected to one amplifier over long cables. Ordinary low-impedance wiring would waste power in the cable, so these systems use constant-voltage lines, usually 100 V.

The problem with low impedance

To deliver 100 W to an 8 Ω loudspeaker, the amplifier must supply 28.3 V and 3.5 A. A long thin cable has resistance, and the loss is proportional to the square of the current. With dozens of speakers on one cable, the currents are large and the cable losses are substantial.

The idea

Raise the voltage and lower the current. A 100 V line carrying 100 W has a current of only 1 A, so the cable loss is lower by a factor of

(3.5 A / 1 A)² ≈ 12.5

for the same cable. Each loudspeaker is connected through a step-down transformer that converts 100 V to the voltage the driver needs. The amplifier is fitted with a step-up transformer (or has a built-in 100 V output).

Power taps

Each loudspeaker transformer has taps, such as 30 W, 15 W, 7.5 W and 3.75 W. The tap sets how much power the loudspeaker takes from the line at the nominal 100 V. The sum of all tap powers must not exceed the amplifier's rating (usually with about 20 % reserve). A 100 W amplifier can feed twenty 5 W loudspeakers.

For a line at 100 V, a load of power P appears to the amplifier as an impedance of

Z = 100² / P = 10 000 / P

100 W is a load of 100 Ω, and 5 W is 2 kΩ. Loads in parallel add their powers.

Variants

• 100 V is the usual standard in Europe and many other countries.
• 70 V (70.7 V) is common in North America.
• 25 V is used in some installations, particularly where safety rules call for lower voltages.

Advantages

1. Long runs with thin cable.
2. Any number of loudspeakers on one line, adjustable level per speaker with the taps.
3. Easy to add or remove loudspeakers without recalculating impedances.

Drawbacks

• The transformers limit the bass, and are costly for good quality, so these systems suit speech and background music better than high-quality music.
• The transformers saturate at low frequencies, so a high-pass filter at about 50 to 80 Hz protects the amplifier.
• Voltage on the line is dangerous, and installations must follow the local rules.

Calculation example

Eight loudspeakers at 15 W each make 120 W. Choose an amplifier of at least 150 W. The cable current is 1.2 A on the 100 V line. Delivering the same 120 W into a single 8 Ω load would need 3.9 A. Use the Ohm's law tool for such checks.

Draft: written by an AI assistant, awaiting technical review.