Originally Posted by FraGGleR
The amp runs at full power and the phone reduces the volume of the music digitally. So noise and a loss of sound quality.
According to HRT, the microStreamer has a digitally-controlled analog volume control:
"Dual purpose built, it incorporates both a high–performance headphone amplifier with an analog volume control that allows you to connect the microStreamer between your computer and headphones; and a fixed-level, low impedance 2.25V line output that allows you to connect between your computer and home stereo, other entertainment system or to powered loudspeakers.
Volume control of microStreamer’s OCL (output capacitor-less) headphone amplifier is accomplished by using the digital ‘slide’ control on the host computer’s music playback software to adjust microStreamers’s internal analog volume attenuator. This clever analogue attenuation arrangement allows the full audio quality of the microStreamer to remain pure, clean and full-ranged, avoiding the known sonic degradation of digital attenuators."
Such an analog volume control of the USB DAC/amp is adjusted by a virtual or physical up/down knob of the PC which has an implementation of USB HID (Human Interface Device).
The digital volume control of the USB DAC/amp is adjusted by a virtual up/down knob of a music player running on the PC.
I don’t have a USB DAC/amp which has a digitally-controlled analog volume control, I don’t know if Android device has an implementation of USB HID.