Portable USB headphone amps

Specs as supplied by the manufacturer.

iFi iDSD US$189

PowerSource: Battery/USB Bus power
  DAC/Amp always run on battery
Battery Life: ~ 10 Hours playback*
Formats: 44.1/48/88.2/96/176.4/192/384KHz PCM
  2.8/3.1/5.6/6.2MHz DSD
  353/384KHz DXD
  Bit-Perfect DSD & DXD DAC by Burr Brown (1-DAC Chip; 2-Channel; 4-Signals)
Filter: PCM: Standard/Minimum Phase digital (selectable)
  DSD: Standard/Extended Range analogue (selectable)
  DXD: Bit-Perfect Processing, analogue filter (fixed)
Input: USB 3.0 port (2.0 compatible)
  compatible with iPhone,iPod, iPad and Android Devices USB-OTG#
Output SPDIF RCA (only PCM up to 192KHz)
  Audio RCA
  3.5mm Headphone
Dynamic Range: >104dB
THD &N (35mW) < 0.005%
THD &N (Line) < 0.005%
Output Power (16R): > 130mW
Output Voltage: >3.3V (>100Ω)
Output Impedance:(Zout): <1Ω
Power Consumption: < 3W (charging battery and playback together)
Dimensions: 158(l)x68(w)x28(h)mm
Weight: 162g(0.43lbs)