Like I was saying on the Patreon, this amplifier has a really high output impedance. It shines with Beyerdynamic’s own high impedance dynamic driver headphones, as well as a number of Sennheisers and ZMFs, but it should never be paired with low impedance (i.e., sub-250 Ω) cans or planar headphones. I was pretty disappointed to see Zeos not pull out his HD 600 for this review. The A20 is completely lacking in versatility, but, as someone who loves his DT 1990 Pro, it’s an amazing amplifier. I love its simplicity and its build.
As for the above post, I have noticed that the A20 seems highly sensitive to any source noise. It is unusable if connected directly to my PC with an RCA to 3.5mm cable and as useless connected to a number of DACs. Somehow though, it sounds perfect for me when connected to my Schiit Gungnir Multibit w/ Gen 5 USB or to the Schiit Bifrost Multibit w/ Gen 5 USB. I figure it has to be something with Schiit’s newer USB implementation, but I really don’t know. Picking up a Topping D70 from Massdrop and I’ll see how that does in a month or so.