This is a review and detailed measurements of Fiio K5 headphone amplifier. I purchased this unit last fall and it has been sitting patiently for a review. The K5 costs USD $110 including free shipping from Amazon. What is unique about this product is that it has a dock mechanism to mate with the USB connector on Fiio portable audio players:By flipping the gray door on top of the unit different Fiio players can connect to it. You can then select that input with the front panel switch. In the back there is a USB jack that routes to the portable player, allowing it to be used as DAC. Without it, there is an analog-only amplifier with NO DAC built-in.The case is metal but it has a paint on it that is normally is used to make cheap plastic look like metal. As such, it looks rather cheap to me. The volume control feels nice though. And it is fairly hefty so sits there without the cables pulling on it.Power is provided by a laptop-sized 15 volt switching power supply delivering 1.5 amps from memory. So one would expect plenty of power to be available on tap.There are a set of 3.5 mm jacks in the back that provided balanced output. I did not a 3.5mm to XLR cables so didn't bother testing that. And at any rate, that is more useful with their portable players.Let's get into measurements and see how the K5 performs.My dashboard for headphone amps involves feeding 2 volts in, and getting 2 volts out ("unit gain"). The Fiio K5 conveniently has a "0 dB" low gain switch for this purpose:Distortion is limited by second harmonic at -108 dB or so. SINAD (signal over noise and distortion) is therefore shy of the 114 dB that its competitor, JDS Labs Atom produces. In other words, respectable but not top of the class.I am not a fan of SNR measurements (as it ignores distortion) but you all want it so here it is:This ranks well:Frequency response is rule flat and excellent:Output impedance is likewise 1 ohm which is low enough for just about every headphone load:The most important measurements for headphone amplifiers is power versus distortion+noise. Here it is using 300 ohm load:Noise level dominates the sloping down part of the curve and there, the K5 almost keeps up with the JDS Labs Atom. But then distortion starts to set in around 10 milliwatts and performance degrades some (gracefully). Importantly the power output is half of what the Atom produces. At 121 milliwatts though, it easily clears my requirement of more than 100 milliwatts to provide good power for high impedance headphones.At low test impedance of 33 ohm we are measuring current delivery capability:Here, the Fiio K5 becomes noisier than Atom and again, distorts more. Power output is similar though and very good at > 1 watt.Intermodulation distortion versus output level (and hence power) repeats the same story:Low and mid-gain again show somewhat more noise level than JDS Atom (dashed lines). High gain though was much worse. Not sure what went on there.Channel matching is very good until you get down to low levels where it goes nuts:Given the 0 dB low gain setting, hopefully you don't have to operate it in that region much. FYI I changed the output level to relative dB so that you can tell the amount of attenuation (red) for every imbalance level (blue). Let me know if you like that better than previous graphs that showed the voltage instead.Throwing my bass heavy tracks at the Fiio K5 while driving my Sennheiser HD-650 high impedance headphones resulted in very satisfying sound and high levels of power. I could literally vibrate my ear lobes with it.Alas, while you have ample power at 12 to 1:00 o'clock, going beyond that I could hear the bass distortion in the amplifier prior to driver doing the same.Similar story emerged with Hifiman HE400i low impedance headphone. Lots and lots of power, great dynamics, etc. until you pushed hard past 2:00 to 3:00 where bass distortion in.Note that outside of testing, I would not need to go to the region where distortion was quite evident. So in practical use, there is plenty of distortion-free power available.I can't find anything wrong with Fiio K5. The issue it has though is the impossible battleground created by the JDS Labs Atom with its combination of high power and extraordinary low distortion. Compared to that reference, the Fiio K5 falls behind one step or so.I am happy to recommend the Fiio K5 if you need its dock capability or like its volume control, etc. Good job has been done here by Fiio.This unit is available for sale if anyone is interested.------------As always, questions, comments, recommendations, etc. are welcome.Snow has melted but now I need gas money to go and pick up all the packages piled up at the post office. So please considermoney using: https://www.patreon.com/audiosciencereview ), or https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).