This is a review and detailed measurements of the Fiio A5 portable headphone amplifier. It is on kind loan from a member. The A5 costs US $130 from Amazon including free shipping. It came out in late 2016 I think but being just a headphone amplifier, it doesn't obsolete like DACs can.Due to inclusion of battery and metal case, the A5 is a hefty unit:The volume control is stiff and sticks on the sample unit. Don't know if it was that way when new. It might be pushed in too far but my light attempt at pulling it out did not work.Most of the controls are on top:There is a Bass boost in the left which I forgot to test.The USB jack for charging it is also on the same side and a red LED indicator blinks saying you have plugged it in (curiously it kept doing that well after being charged).Overall fit and finish is good for a $130 device.As usual we start with our dashboard. For this and the rest of the tests, I disconnected the charging cable. With it connected, it adds fair bit of noise to the measurements:The gain is slightly less than one resulting in the output to be slightly lower than 2 volts that it should be for this test. In actual use that is probably a good thing allowing very sensitive IEMs/headphones to be used at higher volume position.My NAS where I keep the headphone amp comparison is down right now so I can't pull the graph up to update. But you can mentally plug the SINAD of 115 in there:As you can see, the A5 would slot well above average. Of course the above devices are AC powered so they don't get to enjoy battery operation than the A5 uses. Still, that is the mode of operation for A5 so it is not entirely unfair.Frequency response is pretty flat and hence good:Intermodulation distortion versus output power shows a sign of good things to come for A5:The low gain mode is very quiet, almost exiting the graph at the bottom!Using 32 tones as a simulation for "music" we get very low levels of intermodulation distortion yet again:In mid-band frequencies where our hearing is most sensitive, distortion approaches -130 dB which is superb. In other words, there is no audible distortion out of A5 (at this operating level).Once again I don't have a comparison graph but here is the last one I ran for 50 millivolts:At 90 dB, the A5 would again finish near the top so should be quiet with most sensitive IEMs/headphones.Most important test is power into different loads relative to noise and distortion (THD+N). Here it is at 300 ohm:Once again, the low gain mode shows its superiority with very quiet performance and almost no rise in distortion. If you hear distortion in low mode, it is in your music or headphone!Power is quite healthy going past my requirement of 100 milliwatts. The A5 should have little trouble driving high impedance headphones.There is more distortion with 33 ohm load:Once again low gain is excellent. High gain starts to add distortion and noise so is not competitive with state of the art desktop amplifiers above 10 milliwatts. Mind you, at SINAD of 95 it is still very good until it clips.Output impedance is comfortably low meaning it won't cause power loss, or change the tonality of your headphone:Channel matching was surprisingly good:You get about 40 dB of attenuation before you lose matching (and output).I started with my 25 ohm Drop Ether CX inefficient closed back headphones. Performance was excellent with plenty of bass, detail and dynamic range. That said, if you go really, really loud the bass starts to distort severely. Dial it back a bit and it will still be quite loud and above normal listening level.The A5 really shone with the Sennheiser HD-650. The sound was simply superb with no limit in volume. It was so good that I sat there just listening to it rather than my normal setup. The fidelity of the music comes through and makes the HD-650s perform in a way that lower-power/higher-distortion amplifiers cannot replicate.I didn't save the plot because I want to get more mileage on this test. But starting with with USB charger plugged in, showed a lot of random spikes with baseline SINAD of around 95 dB. Unplugged the USB charger cable and performance shot up to 115 dB and was very flat and predictable. So no need to warm up the unit or leaving it on.I tried to listen while I connected and disconnected the charger. I "thought" I heard a difference but could very well be placebo. I should note that I was using a USB connection to my computer for charging. It may be much more quiet with a dedicated charger.The Fiio A5 is a delightful surprise when it comes to both objective measurements and subjective performance. When operated on battery, it rivals the best of the desktop headphone amplifiers. There is plenty of power to drive just about any headphone with very low output impedance. I don't think we have seen this combination before. It runs circles around the venerable O2 headphone amplifier for example.if you want a battery operated portable headphone amplifier.------------As always, questions, comments, recommendations, etc. are welcome.Christmas season is here and I want to give myself a big present. Problem is, I am too cheap to do that out of my own funds so I appreciate your