Here I am introducing one of my DIY favourites from the past, the Cavalli-Kan Kumisa III headphone amplifier. The CKKIII (or CK²III) is a joint collaboration between AMB and Alex Cavalli DIY designers.
The amp is in the Headamp GS-1 range of quality, it’s extremely trasparent sounding and has a smooth top end. The tone of this amplifier is very “right “, there are no incoherences in the treble (no spikes, zero harshness), and no roll off at either extreme.
This amplifier is one of the best bargains in solid state DIY designs, provided your headphones are not too power demanding. When owning mine, I had the best results with low impedance headphones like the Audio-Technica AD2000, Grado RS-1, orthos like the Thunderpants (one of the many custom versions derived from the Fostex T50RP), the Yamaha HP-3, Yamaha YH-1 and YH-100. It even sounded exceptional with Etymotic ER4P and ER4S, and Jerry Harvey JH-13.
Where it falls short, is with headphones like the Sennheiser HD580, HD600, HD650, AKG headphones, or Hifiman HE6. Those are headphones demanding more power than the CKKIII can provide.
It can be built DIY for slightly over 100 Euro, or have built by a DIY’er for around 200 Euro.

To find more info about it, you can visit its official webpage: http://www.amb.org/audio/ck2/