I just got two of those audio transformers (http://www.parts-express.com/70v-15w-line-matching-transformer--300-039
) and they seem to be working correctly. I wired up the 5W/GND to my input to the speakers and have the 8ohm/gnd going out, which lowers the voltage nicely and gives me the ground I need.
Now I need to implement automatic level/gain control. I've been thinking about not using the SA575 Low Voltage Compander as I think I've come up with a code-based method using the Arduino and because I'm not sure what would happen with the ALC on quieter parts of songs, which I may not want to be extended to the same voltage range as louder parts.
So, my idea is this: I use a digital potentiometer in a voltage divider at the output of the transformer. I'll have clamping diodes after the voltage divider which will limit to ±5V and then programmatically I can see if the output is being clipped by the diodes. If it is, I can adjust the potentiometer accordingly. This would allow me to decide whether or not to shift levels mid-song and whatnot. The output of the voltage divider would also be fed into the MSGEQ7 (maybe through a buffer) to be split into different channels and read into the Arduino.
Does anyone see anything flawed about this design? It gives me extra flexibility and would not be too hard to implement.
EDIT: This is the pot I am thinking of using:http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en531252