70V systems are typically used for installations that require large amounts of speakers to be installed. The 70V transformer on each speaker allows you to have multiple lines of speakers connected to a single amplifier without the need to worry about impedance issues as you would get with an 8 ohm system.
So generally, we use 70V distributed systems when we have a large scale venue such as a shopping mall that requires coverage throughout the facility. We would install an 8 ohm system where high end performance is required, in venues such as a churches or amphitheaters, where we have greater control over speaker runs and amplifier distribution.
1. Resistance based system which uses a typical 8 ohm output from an amplifier and uses multiple resistors on a detent based multi-position switch (generally a wall switch) and the variable resistance resistors in series with the load (speaker) allows for attenuation (volume control) of the speaker.
This works fine on a single pair of speakers where you might have an amp located in another room and you wish to control the volume in a second location without controlling the gain (volume) of the amp. The problem with this design is that when you have more than one pair of speakers hooked to an amp (generally in a home system) when the volume is turned down on one of these controls, the other room’s volume increases, which makes for a system which is difficult to balance the sound levels from room to room.
2. Another method which works much better in home systems is to use volume controls which have transformers for the primary and secondary (input and output) stages of the sound. In the early days of volume controls the Mortronics 816122-2 control was the industry standard. It stood for 8-16 ohms on the input side and 122 was the model and the -2 stood for stereo. These types of controls go back decades and still today work perfectly as long as a stable amp is hooked up to these. Often there might be as many as 3-10 of these hooked up in parallel (all negatives tied in a group to the amp negative out and all positives ties together to the amp positive out). This would result in what is termed extremely low impedance (resistance measurement) of resistance which is presented to the amp. Many amps cannot handle such a load and will blow up once pushed towards 0 DB (rated output).
Today’s standard volume controls have multiple input windings for the primary as well as the step windings which are selected by the detent positions on the front of the control (these control the relative volume 10 is max 0 is off. The very smart advantage in the multiple input windings is that you can preset the switch under the faceplate of the control to match the number of volume controls which you can hook up in parallel now with no damage to the amp because of the higher impedance of the windings with the higher number of controls preselected under the face plate. In short this means that regardless of how many controls are used the load to the amp remains constant.
A word about speaker selector switches used with volume controls… In the old days they sold speaker switches which had a switch for “amplifier protection”. These are a waste due to the fact that what they did was employed an inline power resistor which would raise the apparent resistance to the amp by dumping power into the resistor as heat. This is useless as now you must run more power from the amp to accomplish the same goal.
3. 70 volt systems use a step up step down voltage division method of getting power to the speakers. This type system is generally only used in commercial systems with a large benefit being the use of smaller gage wire to run great distances to the speakers. This is the same method used by the power company where they step the voltage way up to 70k volts and more to run the power over great distances.
Simply stated, in commercial audio systems we use an amp which has a 70 volt output, and run a single wire in parallel to many speakers and preset the volume of the speakers using the multi-position switch on the speakers or in the case of simple ceiling speakers using the appropriate colors on the primary transformer to set the volume of each speaker. .5 watt is quiet, 1 watt is louder, 5 watts is loud, 20 watts is very louder.