Rob
Seventy-volt systems are
encode-decode systems. The 8-ohm impedance of the amplifier is converted (
encoded) to the 70-volt standard at the output of the amplifier, and then individual transformers at each speaker are used to
decode the amplifier output back to 8-ohm impedance, at different wattage settings.
The output of the amplifier transformer (which could be external or internal of the amplifier) will feed a speaker wire pair informally called the
home run. The home run will go to each speaker enclosure, in a parallel, daisy-chain fashion.
Inside each speaker enclosure, the home run pair will attach to the primary winding of the
decode speaker transformer, and then another pair will probably be attached in parallel, which connects to the next speaker in the daisy-chain.
The transformer secondary winding will have a common ground connected to the negative terminal of the speaker, and about 4 positive polarity wires. The positive wires, which typically are different colors, are tapped at different windings of the transformer, at, say, 1/2, 1, 2 and 4 watts. In the best case scenario, each wire, or
transformer tap, out of the transformer has a little paper label on it with the wattage output (1/2W, 1W, 2W, 4W). Simply pull off the positive wire out of the transformer, and change it to a higher wattage number for greater output. In the worst case scenario, you will need to try the different leads by playing music in the system, stripping each tap wire end, and touch each lead to the positive terminal of the speaker to test for relative loudness by trial-and-error. Write down the color for each relative loudness (1=red, 2=blue,3=yellow, etc.).
If the wire with the greatest output still does not have adequate loudness desired, you will need to increase the gain (volume) on the amplifier until the speakers in question are loud enough, and re-wire the transformers on the other speakers so that they are not too loud.
The combined wattage settings of all the speakers should not exceed the output of the amplifier, or you could overload the amplifier. So you should open up a speaker in each of the different zones to calculate how many watts are being drawn from the amplifier.
The advantages of a 70-volt system are that you can use smaller guage (less expensive) wire for the 70-volt home run than for 8-ohm wire home runs, and also you can connect a great quantity of speakers to the output of a single amplifier. For instance, if you had a 100-watt amp, you could power 200 drive-in speakers tapped at 1/2 watt each. The disadvantages of 70-volt systems are that the transformer intrinsically attenuates bass frequencies, wattages are limited at the speaker end, and the insertion loss of the transformer dissipates wattage, for reduced effiency.
Several manufacturers, like
Atlas Sound and
Lowell, make 70-volt speakers with volume control knobs, which eliminates the need to open the enclosure for adjustment. Also, manufacturers make wall-mountable volume controls so that you can vary volume. Savvy consultants design these into church offices, dressing rooms, etc. Some volume controls can have a knob on them, or a screwdriver adjustment. If required, you could probably buy replacement components through a web site.
As you are doing all this research and work, make a little drawing, with a symbol for the amplifier and each speaker connected to the system, and noting the wattages being drawn from each speaker. Leave a photocopy of your drawing next to the amplifier in little binder. So, 5 years from now, you or the next guy will be saved some effort.
