Originally Posted by lukejs
OK.. thanks ! I'm starting to get it now. I read where they say that my amp shoud give 20 percent more watts than I need , in order to make up for losses from the cable runs and the transformers... I like the idea of transformers with mulitple taps, so I could install a switch on each speaker to select from, for example, 3 different volume settings .. anybody know where I should look for such a transformer ? one that I could get about 50 watts to the speaker at maximum volume ? thanks !
You probably don't need 20% extra power. I'm sure some installs have no extra capacity but 10% is an adequately conservative target. Of course too much power is better than not enough. When I was working with these 300W was about the highest popular power point for dedicated 70-100V amps, and they ranged all the way down to 5W mixer-amps still with 70V outputs.
There are many speakers sold for this instal market with the transformer already built in. If you use a conventional amp and step it up to 70V, some of the add on step up modules use an autoformer instead of a true isolated output winding transformer to reduce cost. This is not a huge deal but professional installers prefer fully floating 70v lines since they are more tolerant of inadvertent shorts to one side or the other.
There are many companies making these products so the models to choose from is probably larger than you think.