Constant-voltage speaker system

Constant voltage speaker systems refer to networks of loudspeakers which are connected to an audio amplifier utilizing step-up and step-down transformers to simplify impedance calculations and to minimize power loss over the speaker cables. Constant voltage speaker systems are also commonly referred to as 25-, 70-, 70.7-, or 100-volt speaker systems; Distributed speaker systems; or High impedance speaker systems. In the US, they are most commonly referred to as "70-volt speaker systems".

Constant voltage speaker systems are analogous to electrical power transmission methods employed by power companies. Typically, a power company will step up the voltage of the power being transmitted, which correspondingly reduces the current and therefore also reduces the power loss during transmission. Similarly, in a constant voltage speaker system the amplifier uses a transformer to step up the voltage of the audio signal to reduce power loss over the speaker cable. Each speaker in the system has a step down transformer to convert the voltage back to a usable level.

Advantages

The two main advantages of using a constant voltage speaker system over a conventional low impedance (8-ohm) speaker system are:

Ease of impedance calculation: Typical audio amplifiers are rated to drive loads with an impedance between 4 to 16 ohms (some can go as low as 1 ohm). When using traditional 8-ohm speakers, it quickly becomes obvious that connecting more than 2 or 3 speakers in parallel will exceed the rated impedance of the amplifier. One alternative is to wire the speakers in various combinations of series and parallel, which is more difficult to calculate and more difficult to physically wire. The other alternative is to use constant voltage speaker systems, where dozens of speakers can be wired in parallel, and the only calculations required are simple addition. Also, if multi-tap transformers are used on the speaker side, then varying volume levels can be easily selected per speaker by using different taps on the transformer; a feature that is very difficult to emulate in 8-ohm systems.
Less expensive: Since the voltage of the signal has been stepped up and the current is relatively low, smaller gauge cable can be used without incurring additional power loss. Where a typical 8-ohm speaker system might require 12 gauge cable, a 70-volt system could use 18 gauge or smaller cable.

Disadvantages

Frequency Response: Most inexpensive transformers will act as a high-pass filter when passing audio frequencies. Therefore, low frequency signals will not be reproduced faithfully, resulting in a lack of "bass".

Distortion: Transformers can also add a measurable amount of distortion to the audio signal.