I've connected my Sandisk mp3 player from the headphones jack to the audio in jack on the dac. Both are turned on but no sound comes when I plug headphones into the dac. Increasing the volume on the Sandisk and the dac does nothing. I tried this same setup with my tablet and also no sound. I hear sound when the phones are plugged in directly to the Sandisk. Any ideas on how to get it to work? Thanks