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70V audio speaker systems

General information

70 volt systems are generally used in commercial applications where many speakers needs to be run from one amplifier. It is also called high impedence speaker system. The advantage is that, being high impedence, long cable runs of relatively small gauge (usually 20-24 gauge) do not significantly affect the output as they would in a common low impedence speaker system.

The speakers themselves are commonly 4 or 8 ohms, but there's a transformer at each speaker that matches that low impedence, to a high impedence, which is on the line side. Typically those transformer have multiple output taps so the sensitivity of the speaker system (output volume) can be adjusted as needed.

Amplifiers designed for 70 volt operation often have an output transformer as well for matching purposes. Typically a 70V line can be driven with normal audio amplifiers if that matching transformer is added. If you have powerful enough amplifier (high output voltage), you might be able to run the line directly (for example some powerful PA amplifiers can be used to drive a 70V line directly in bridged mode).

Where 70 volt system is used ?

A 70 volt system is used in restaurants, small bars, department stores etc. You would want to use this type of system if you plan on powering say 10 speakers with one amp. If the amp produces 100 watts then each speaker would get 10 watts. The speakers have a tranformer inside to prevent them from blowing up. You couldn't really connect 10 normal speakers to an average amp because the impedance would be too low, and it's not the best idea to parellal and series your speakers.

Advantages

To an extent, speakers can be added or removed from a 70 volt system without regard for impedence matching. Something you simply can't do with a low impedence system.

Disadvantages

There are two primary disadvantages to a 70 volt system.

• 1. You are limited to a max amplifier power of 250 watts. Beyond that the transformers saturate and your signal goes to hell.
• 2. Frequency response is limited, unless Serious mony is spent on transformers (and it usually isn't). I don't have hard numbers off the top of my head, but a range of 250-10k is in the ballpark. This means it works well for voice application (hence where it's usually seen), but poorly for music applications.

Other similar systems

As an aside, 70 volt is common in the US, but in other countries, and other times, various other high impedence system designations have been used, including 25 and 100 volt.