When you get a 70.7 Volt Public Address Transformer, you see that it has connections for different speakers, such as 4, 8, and 16 ohms. On the other winding are various taps for different power outputs such as .625, 1.25, 2.5, 5 and 10 Watts. But there is nothing to tell what the impedance is of the winding taps. I was curious what these impedances were, so I did some simple calculations and found the impedance of each winding.
In a DC circuit, the power is equal to the voltage E squared divided by the resistance. We will assume that the reactance is zero so the answers are the same for DC and AC circuits. Algebraically rearranging the formula, we have the resistance is equal to the voltage squared, divided by the power.
We take the 70.7 voltage and square it and we get 5000. We divide this by the power, .625 watts. We get 8000 ohms, which is the impedance of this winding tap. For the 1.25 way tap we get 4000 ohms. For the 2.5 watt tap we get 2000 ohms for the answer. For the 5 watt tap we get 1000 ohms for the answer. And for the 10 watt tap we get 500 ohms.
So if you need an audio transformer that is 1000 ohms to 4, 8, or 16 ohms, you can use one of these and choose the 5 watt tap, for example. You can also use one for a 5000 to 500 ohm autoformer, but since both taps are on the same winding, they are not isolated from each other. To get isolation, you could put two transformers back-to-back, and contact the 16 ohm windings together. You could also connect two of them together in various series sand/or parallel combinations to get various impedance combinations.
These transformers seem to be much cheaper and more easily obtainable than an audio output transformer. I used a 10 watt transformer in the above examples, but they come in various wattages, both higher and lower. So next time you visit an electronics store, check these out to see what they cost. Sometimes when buildings are demolished there are ceiling speakers left in the rubble, and even if the speaker is damaged, the transformer may have survived. More here: http://en.m.wikipedia.org/wiki/70_volt
