Requires Eagle by CadSoft
DOWNLOAD CY-668A SCHEMATIC
The audio enters the 668A via the signal lead and goes through a pre-emphasis circuit before entering the stereo encoder. As these transmitters were designed to be fed from a headphone output, the audio is a little low when fed with a line level input. We'll be looking at how to lessen the input networks attenuation so that it is optimized for a line level input.
Each audio signal goes through an inductor to filter out any RF components, and is then loaded to ground with a 47R resistor which defines the DC input resistance. The signal then goes on through 10K and through the pre-emphasis filter which is a parallel RC network featuring a 47K resistor. It then goes through a DC blocking cap before hitting the stereo encoder.
As yet incomplete and in places erroneous schematic from what I've managed to piece together. The audio input stages are accurate though.
Requires Eagle by CadSoft
DOWNLOAD CY-668A SCHEMATIC