100V Line Speakers and Amplifiers Explained

A 100V line system lets one amplifier run many speakers over long cable runs. Each speaker has a small transformer with power settings (tappings); you choose a setting for each speaker, add them up, and keep the total within the amplifier's output. It is the standard way to wire background music and announcements in shops, offices, schools and warehouses.

We supply 100V line amplifiers and 100V line speakers from Adastra. This page explains how the system works so you can buy the right parts.

100V line vs low impedance (8 ohm)

	100V line	Low impedance (4 or 8 ohm)
Number of speakers	Many: 4 to 40 or more on one amplifier	Usually 2 to 4
Cable runs	Long runs in thin cable with little loss	Short runs; long runs need thick cable
Volume per speaker	Set on each speaker with its tapping	Same for every speaker
Wiring	All speakers in parallel on one pair of wires	Must match the amplifier's minimum impedance
Typical use	Shops, offices, schools, warehouses, multiple rooms	Bars, meeting rooms and homes where music quality matters most

How tappings work

A 100V speaker has a selector or a set of wires for each power setting. For example, the Adastra metal quick-fit ceiling speaker has 1.5W, 3W and 6W tappings, and the BC4V wall speaker has 20W, 10W, 5W and 2.5W tappings plus an 8 ohm setting. A lower tapping makes that speaker quieter and draws less power from the amplifier.

Sizing the amplifier

Choose a tapping for each speaker.
Add the tappings together.
Choose an amplifier rated at least 20% above that total.

Example: 12 ceiling speakers at 6W = 72W. 72W x 1.2 = about 86W, so a 90W amplifier such as the Adastra UA90 or UM90 fits. If you set four of those speakers in a quiet corridor to 3W, the total drops to 60W and leaves room to add more later.

Never connect more speaker watts than the amplifier is rated for: it can overheat or shut down. For very large systems, add a slave amplifier for a second set of speakers.

Wiring tips

Connect every speaker in parallel: + to +, and - to -, on the same pair of cores.
Keep the polarity the same on every speaker, or the sound will be thin.
Use a 100V volume control in areas that need to be quieter. Choose one rated at least as high as the total watts of the speakers it feeds.
Mains-voltage work and fixed wiring should be done by a competent electrician.

Common questions

Is 100V line dangerous?

The speaker line carries up to about 100 volts AC at full volume, so terminals should not be touched while the system is playing and connections should be made with the amplifier off. It is a normal, safe system when installed properly.

Can I use 8 ohm speakers on a 100V amplifier?

Only on an 8 ohm output if the amplifier has one. Many Adastra mixer-amplifiers have both 100V and 8 ohm terminals. Never connect an 8 ohm speaker without a transformer to a 100V output.

What cable do I need?

Ordinary twin speaker cable or fire-resistant cable for voice alarm systems. Because 100V lines carry little current, a modest cable size works over long runs.