I’ve recently bought a new headphones preamp, the FiiO Olympus 2-E10K.
Is a very very small (about 2cm x 8 cmx x 2cm h) DAC that uses the PCM5101 chip (seems that the old WM8740 used in previous version of this box was suffering by delays in hi-res reproduction start). The amplifier chip is the quite common LMH6643.
The specifications on the site above claims for very low noise (S/N Ratio >= 105dB) and I have to say that the general circuit noise is very low indeed. A very trivial trial: with my ATH-M50 headphones, nothing in reproduction, all volumes set to max, no noise at all.
The claims to drive from 16 to 150 Ohm should be enough for my AKG K601 (120 Ohm) that are currently my best open headphones.
The setup under Debian Sid has been straightforward: plug it and select it as main ALSA output. I avoid using pulseaudio as possible as software mixing is not what I’ve choosing the external card for, but sometimes I will need to.
I’m very pleased also for the very small dimensions and the metal case, seems very robust. The gain and bass switch probably will never be used, unless some esoteric try: I lke the sound as clean as possible.
Now I only need a cable at least 4 m long to connect it to the Marantz vintage amplifier…