The 1/4" jack on the digital option card is an input (but not into the preamp or processing of the Eureka). Its purpose is to send another channel of audio to the other channel on the S/PDIF line. Because one Eureka has one channel and S/PDIF is two channel (stereo), there is an empty channel. This input simply runs straight into the other channel of S/PDIF and is ideally used with another Eureka.