It may seem odd to build yet another portable HP amp when I only recently finished the "retro-2308" compact HP amp. I have a number of Lithium batteries over from my many Oatley K272xx of the past and built this primarily to use those batteries. I happen to have a couple of genuine OPA2134 chips also and what better way to use them but in a CMoy style amp. Once I have used the Lithium batteries I may just use high quality alkaline.
In the past my Oatley HP amps have been a little big and heavy though 100% portable. Travelling over seas a lot made me think it was time for a lighter more compact amp. The "retro-2308" definitely fits that bill and running on 4 X 1.5V AAA batteries made it long lasting and cheap to run. Now I have a choice of two compact portable HP amp. Both sound fantastic, I'm spoilt for choice.
With space at a premium I had to build the whole chip amp and differential PS on a tiny slither of vera-brd. Two lithium batteries are held in place by glued foam strips and once more my "safe on" technique for insuring amp is not on when not in use. Only input caps are needed and I used inexpensive polies. A mil-spec chip socket, metal film resistors, quality switching phone 6.5m sockets are used and fine wire-wrap silver plated wire is used as hookup.
With two 9V lithium batteries I should get 120hours of play between changes. The batteries are rated 1200mAh. Current draw is 9.5mA.I never use rechargeable batteries at all because they are initially dearer and generally lower capacity and lower voltage.
The OPA2134 has distortion figures of 0.00008% with a FET frontend and a very capable drive backend. In this and other devices I have used the chip in, always sounds excellent. For $9 or so each they are good value and are recommended for audio applications with diminishing noise figures.