Just a headphone amp

I apologize in advance for the long-winded post. The short version is that I'm looking for a battery powered headphone amp. Not a DAC/AMP, just the amp. Line in, headphone out. Ideally it should be as transparent as possible, adding no coloration of its own to the sound.
The longer version:
I'm a field recordist. I make a habit of hauling a bag full of sound gear way out in the middle of nowhere to record... the middle of nowhere. Consumer level portable recorders have come a long way in the last decade, offering P48 power, gobs of clean gain, lots of bit depth, and wonderful sampling rates. The one area they tend to lack is their headphone amps.
Two ways to fix this: Option one is to switch to a pro level recorder from a company like Sound Devices or Zaxcom. Only problem? The cost is about $1k per channel and I routinely use four channels. Option two is to use the line out jack on my existing recorder and feed it into a dedicated headphone amp. Hence the question.
I see lots of DAC/AMP combos here on Massdrop, but I really don't need the DAC to make this work. Any recommendations for a battery powered headphone amp I could squeeze in my bag next to my recorder?
100% upvoted

Sep 22, 2017
The Pico Power is an option. Currently on drop here for US$250 plus shipping. Uses two 9 volt batteries. I got some rechargeable Li-Ion ones.
Sep 18, 2017
Something smaller: FiiO A5
The O2 is a little too large IMO to be a portable device.
Sep 18, 2017
The original O2 amp design is battery powered. I think it sounds like the best bet for what you're trying to do. Just make sure you get the original instead of the desktop version.
Many of the DAC/amp combos have line-in capability as well though, so don't discount them based on having a DAC. Just make sure you look at the specs and get the right thing
Sep 18, 2017
Thanks for the tip on the O2, Heefty. I did some poking around on it and came across a lot of good information that was well worth reading. (I'm pretty thoroughly convinced this is what I'm after, so thanks again!)
For anyone else interested, here's a good starting point on the O2:
It gives a lot of insight into the design of the O2 and some of the trade-offs involved with making a battery powered amplifier. The section on the range of op-amps that can be used is excellent. Since I'll only be using these with one set of headphones, I can tailor the amplifier stage to those cans and maximize battery life in the field.
Sep 18, 2017
No problem. I hope it works out well for you.