With my wife occasionally working from home and the fact she is an early to bed/early to rise person, I have decided to be a good husband and cut back on my loud/late night listening and build up a nice little headphone system. I never was much of a headphone person in the past, but I recently picked up a pair of HiFiMan Sundara planar magnetic headphones that were clearly so much better than the Beyer DT880 Pro headphones I have had for years. These sounded great on the Chord DAVE DAC, and really got me thinking seriously about the potential of headphone listening. I wanted to not be tethered to my equipment rack though and be able to enjoy music in a more comfortable situation. So it led me to build up a high quality system that I can setup in another room (really any room in the house).
DIYAudio.com has a nice Wayne Colburn (PASS Labs) designed headphone amp kit (WHAMMY) for around $300 with case, board, and parts that I built that sounds pretty nice with the new headphones.
https://www.diyaudio.com/community/threads/whammy-pass-diy-headphone-amp-guide.317803/The HiFiMan Sundara have a low output impedance (35 ohms) that needs a good amount of drive, so this design with a complementary pair MOSFET output buffer provides plenty of current drive. It does use an opamp as the input device, and the output buffer is inside the feedback loop. I am currently trying out a pair of OPA627AU in a Browndog single to dual DIP adapter, but it really sounds good with an OPA2604 (dual) and similar eight legged bugs. The amp is a pretty easy build, I had it finished in a day. Sounds pretty nice, and being able to tailor the sound by switching opamps is a plus.
Using a RPi4 as a Roon endpoint (bridge) running RoPieee connected via USB to a Chord Qutest DAC. I am using the Roon DSP crossfeed implementation. BTW, having the RPI4 on an inexpensive LPS really makes a difference in sound quality.
I probably should look into building a transformer coupled tube headphone amp since the headphones have such a low impedance, but that is another project.
Anyway, nice sounding setup to keep the peace at home.