FutureSharks from github.com has build a very nice HiFi passive preamplifier controlled by an Arduino board.
It has the following features:
- Volume control and input selection via encoder and Apple remote.
- Uses a 256 step R2R relayed based attenuator.
- Completely passive.
- Only 5V power required.
- Adafruit NeoPixel shows volume level with colour.
- Sleep mode with low power consumption: 0.9W.
HiFi Passive Preamp – [Link]