This is a pre-amp plus a FDA-2A in one box. It was built in year 2012. I use a old Yamaha DA8X box to house eveythings insides. See the pdf files at bottom for details.
DAC:
FDA-2A with USB, Optical and AES digital input. Push button for digital input selection at the front panel.
DAC direct output is available in RCA sockets.
F5T Pre-amp:
Alps volume control from DAC output and note that there is no other analogy input for the preamp.
The pre-amp is based on Nelson Pass F5T power amp design and striped down to only one pair mosfet and lower bias current at about 200mA. Supplier voltage is at about +/-23V with a small transformer. Power mosfet is using Toshiba J200 / K1529 pair and casing is acted as the heatsink of power mosfet.
A headphone jack is connected in parallel with the pre-amp RCA sockets. It can drive most headphones due to sufficient high bias of the power mosfets.
Details refer to this pdf file: F5T-Pre_DAC
Enjoy!