You mean like the one shown below? I put it next to one of the Radio Shack ones for comparison purposes. It is from the PFM sound system and was designed specifically to duplicate the sound from the small on-board locomotive speakers into a real audio system. I don't know if they make them anymore. I believe the part number of the transformer itself was PFM-4 and they came in the "Stationary Speaker Filter Set" part number PFM-07.
I am not fully following the dime for pennies comment. The chart which shows wattage vs. reflective impedance indicates the reflected load onto the 70V system of pulling a certain amount of power off for a particular speaker load. A higher power load will reflect a lower impedance than a lower power load. This is typical whether we are talking about constant voltage systems such as this or the transformer outside of your house which delivers lower voltage to your home than what is on the transmission line.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
Others have already responded about the transformer really needing to be an impedance matching device. The 70V transformer you have is a special transformer designed for constant voltage audio distributiuon systems. These are often found in large department stores, warehouses and any other place where a small audio signal (i.e. a couple of watts) needs to be transferred over long distances. Under normal circumstances the resistance of a long piece of copper wire driving an 8 ohm load would be large enough to cause quite a drop in audio signal. Additionally in these types of environments there are many speakers located throughout the building. Simply wiring them in series and parallel would be problematic and along with the wire lengths above, it would become unworkable. This is where the constant voltage system comes in. Here's a useful link with more information on these types of systems.
http://www.ticcorp.com/25v_70v_100v_systems.htm
So for your transformer, it really isn't designed for the type of application you are wanting and the power rating on the transformer you are needing really won't matter.
The transformer in the circuit is being used to match the output impedence of the decoder (8 Ohms) to the input impedence of the amplifier (high). It isn't creating power, it is just transferring it.
Without knowing more about the transformers, and remembering a bunch of stuff I learned too long ago, my first guess is that the circuit wouldn't work. I'd need to know about both sides of the transformer to try to figure out what would happen to the decoder.
Jeff But it's a dry heat!
Uhhhh...it's me again. Just fine tuning my stationary decoder experiment.
I have the system working with the Radio Shack transformer shown in the Soundtraxx circuit.
But I also have some very much more robust transformers (labelled 70volt) on hand. These others have an 8 ohm input, and the following choices for outputs:
5 W, 2.5 W, 1.25 W, .62W, and .31W
A little learning is a dangerous thing, I know.
1. But can any of these output wattages be plugged in to an amplifier AUX input as a line level input?
2. Am I correct in thinking that any disaster on the output side of the transformer in my experiment cannot harm the decoder itself because it is isolated by the use of the transformer?
3. If I plugged any of these wattages in as a line level input, am I going to cook the stereo amplifier?