FiiO BTR13 Bluetooth Headphone Amplifier with LDAC, aptX Adaptive, aptX HD, AAC (Dual CS43131 3.5mm Unbalanced & 4.4mm Balanced Output) (Black)

I'm honestly amazed at how good this one seems to be. It can handle my high impedence Sennheiser HD650 surprisingly well from the 3.5mm jack. These headphones have an extremely high impedance, so a lot of amplifiers simply don't produce enough volume. Some audio sources tend to be too low (unboosted ReplayGain is the biggest culprit in this for me—I have to set things to boost ReplayGain by 5dB and lower everything else by 5dB) and even with this I have to boost a bit when I'm using the 3.5mm jack on a few really quiet sources, but on anything with the correct volume level it can get extremely loud even with my 300 ohm HD650! ReplayGain a small boost seems to be the perfect balance. 4.4mm balanced cables still aren't common yet, but I hope they grow as a thing and I think when mine finally arrives I'll have zero problems even with super quiet audio sources on these headphones. I believe some 600 ohm headphones exist, so for those, you likely still need a balanced cable to get sufficient voltage even with this, but my bet is it would handle them if you do. Hopefully 4.4mm balanced cables become commonplace soon, but for now it's a long shipping time to get one. The power is increased even more with one since the amplifiers are decoupled and this likely even increases efficiency, so I do recommend it to those who can. The noise floor is below my hearing threshold. I'm old enough now that this isn't as definitive of a statement as I would like it to be, but I absolutely don't hear any extra noise coming from the amplifiers even at maximum volume. One annoying thing I had with some other USB DACs built with phones in mind was they were a bit overzealous on their power saving. On those you'll get a pop or cutout after a very short time between songs or sounds followed by another when it comes back in (or even worse, it playing only on one side for a few moments before the other kicks in.) This does not do any of that. In "PC mode" it runs off the USB power (and charges) so doesn't have excessive power saving. In phone mode it's able to run off the battery power for a long time so doesn't use excessive power saving (though the manual says it will switch to USB power if the battery completely runs down.) It seems to intentionally limit the battery's charge voltage to maximize the lifetime of the battery, so I have hopes this will actually last (most battery-powered devices these days wear out their batteries after only a few years.) And finally the Bluetooth mode has all the functions it needs to maximize audio quality. It's largely an imperceptible difference from the USB modes for most with fewer limitations than many devices. It's very tiny so it comes with a clip built in for attaching to a pocket or belt and an optional neck lanyard. I'm sure there are some $100 DAC/amp devices that can beat this for those with golden ears, but for the rest of us this is almost unbeatable for its tiny size.