Computing devices (i.e. PC, smartphone, ...) & noises:
Intentional 4G/3G/2G/WiFi/Bluetooth radio frequency emitters and receivers with a high electronic integration density within a very limited small volume like smartphones, i.e. small computing devices, should create a lot of EMI/RFI noises.
A solution: moving the DAC/amp from the noisy Android device
"In the second design, the external audio peripheral C can be designed for higher audio quality and greater power output without impacting the cost of the basic mass market Android device B. Yes, it is a more expensive design, but the cost is absorbed only by those who want it.
Mobile devices are notorious for having high-density circuit boards, which can result in more opportunities for crosstalk that degrades adjacent analog signals. Digital communication is less susceptible to noise, so moving the DAC from the Android device A to an external circuit board C allows the final analog stages to be physically and electrically isolated from the dense and noisy circuit board, resulting in higher fidelity audio." [Android audio team]
A further solution: no 5V voltage bus power line from the noisy Android device (cleaner 5V power from an external 5V battery, no EMI/RFI noise issued by the Android device via the 5V Vbus power line, no cross contamination of the USB data lines by the USB Vbus power line)
"OTG Micro to Micro cable for Chord Hugo [i.e. self-powered USB DAC]
"Mundorf Silver/Gold wire, 0.5mm
D+, D-, Gnd, no 5V
"Our USB products offer the option of clean battery power, whilst isolating the DAC from the noisy 5v power."