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I think It may be better and easier to simply connect the stereo speaker outputs directly to the Flight Com intercom inputs with an 8 Ohm resistor across each audio output to load down the stereo output properly. The 600 Ohm com input shouldn't matter as long as the driving impedance is at or below 600 Ohms. In this case it would be 8 Ohms. The problem with using an impedance matching transformer is that it is going to step up the voltage 75 times. I think this high voltage could blow your com input! You shouldn't need ground isolation, but if you do, you can put two non polarized 10uF electrolytic capacitors in series. If you don't know whether or not you need ground isolation, try it with the capacitors out, if it squeals or sounds bad, try putting the capacitors in. I put a schematic and parts list on the web:
http://www.testelectronics.com/ftp/comload.gif

Regards,
Ed Armstrong
Watsonville, CA.

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