Thinking logically, internal USB interface should have lesser chain of connection, shorter I2S connection to DAC without encapsulation on AES/SPDIF and stuff. My friend conduct some listening tests with Eximus DP1 and Stello U3. We found that using Stello U3 as external interface to AES/Coaxial input sounds better than internal USB interface. Both U3 and DP’s internal USB audio should have about the same quality. They even share the same driver and firmware.

After further tests with USB isolator and noise filter on tested computer machine, we found they minimized the difference between internal and external USB interfaces significantly. We could say that the reason why internal USB interface sounds worse is because DAC itself gets pollution from direct contact with computer audio source. By using external USB interface, DAC itself receives signal from encapsulated stream from USB interface box isolating pollution from USB connection.

However, it’s unavoidable for DSD to be played from direct USB connection. I think it’s important to reduce the pollution in computer audio source or suppress its effects with isolator or noise filter with optimal implementation.