The USB audio is essential for the Evaluation kit to obtain a master clock source and I2S data when evaluating the with a computer as audio playback. Notice the default jumper for the master clock for the MAX98089EVkit in JU12 is 1-4 (see evkit data sheet ). This means that master clock is selected depending on the GUI configuration. If the automatic USB Audio configuration is selected, then the EVkit is configured to obtain the master clock (MCLK) information from the USB-to-I2S IC (U5) and I2S signals into DAI2 pins (BCLKS2, LRCLKS2, SDINS2, and SDOUTS2). If the USB Audio is not connected, then no master clock is generated for the MAX98089 IC to operate. The master clock is necessary for any digital audio signal processing of the IC. If you are testing the optical S/PDIF connection, then you do not need the USB audio connection to be present. The MAX98089 always needs a MCLK signal when processing digital audio information. If you are testing only the analog path (analog input to analog output) no MCLK is necessary.