I have a rotel RA-840BX4 amplifier with an imput impedance of 20kohms. I want to connect a digital source to it like a laptop/smartphone. Every time i try it the amplifier, besides playing on a lower volume, after a couple minuets of playback time it suddenly plays extremely distorted sound. I recon that my devices have a very low output impedance, designed to drive headphones. What are my options? I could use my pc that can output 250ohm but that's just not practical. is there some short of relative inexpensive DAC to get the job done?