The WA7d Fireflies Duo is immensely flexible with multiple inputs: optical (TOSLINK), asynchronous USB, and stereo RCA. Use the optical input to connect to your DVD/Blu-ray player, XBOX/PS3 game system, or other digital sources. The USB input is best for computer audio to enjoy streaming music services such as Spotify, Apple Music, and TIDAL. For those spinning vinyl, connect your favorite phono preamp to WA7d's RCA input to enjoy your records collection. There’s a ¼-inch output for full-size headphones and an ⅛-inch output for IEMs. Another important feature is a rear-facing impedance switch, which you can toggle to synergize with everything from sensitive IEMs to power-hungry dynamics and planar magnetics. To keep this amplifier small while isolating the audio circuit from electromagnetic interference, the WA7d has a separate power supply (choose solid state or tube).