This is a review and detailed measurements of Aune X1S DAC and headphone amplifier. It is a kind loan from a member who has waiting for its review since last fall! The X1S seems to be have been out for a while, having found a review of it going back to 2015. It is still available though and costs USD $259 from Amazon including prime shipping. This is nice since US customers at least don't have to wait for it to arrive from China and early customer support issues can be resolved by just returning it to Amazon.Aune has its own "design language" which I like. It shouts "high-end" even though the price is in budget category:The unit has the matt/fine textured finish that is popular these days and picks up smudges easily. I had to touch up the above in photoshop fair bit even though I cleaned the unit prior to taking the picture with a microfiber.As you see, the input is rather plain with an analog volume control and an input button. If you hold that button down for a bit you enter the setup and you get to decode morse code to figure out how to change settings. Fortunately there are not that many things you can change so you can get through it.Somewhat unusual in this price range is the ability to just use the headphone amplifier by itself using rear RCA inputs.We also have our usual three digital inputs of USB, Toslink optical and S/PDIF coax. But in another nice twist, there is also coax output so you can use this as a USB to S/PDIF bridge if you don't like the rest of the unit.Power is provided through an external, large transformer/linear that provides both positive and negative voltages (12 volts) through a multi-pin DIN connector. This makes the internal design simpler (and potentially better) because the negative supply does not need to be generated by a switching power supply. I don't recommend trying to replace it anyway but if you want to pair another power supply with it, your choices will be highly limited.The power supply cabling is a bit of pain though. It has two 3 to 4 feet cables to AC and the DAC. The DAC side is too short to connect to the DAC if you leave it on the floor. And if you put it on the desk, you need to have an extension cord as it may not reach your AC outlet either. Given how ugly and big it is, it would have been nice to have a longer DC cord so that you could hide it on the floor.The X1S is UAC2 compliant and I needed no drivers to get it working on Windows 10 Creators edition.As I noted the volume control is analog so no remote control is provided.The internal DAC chip uses is ESS 9018K2M.Let's see how it does on the bench.As usual with these combo units I start by testing the DAC portion alone. So I connected the RCA out which have a fixed output to my Audio Precision analyzer:As we see the unit runs a bit "hot" with 2.2 volts instead of 2 volts (shouldn't be an issue though).Distortion is quite low with the third harmonic being dominant at -112 dB or so which naturally sets the THD+N/SINAD value to 111 dB. This is a bit better than the specified 110 dB. Not surprising since the manual comes with a set of measurements performed with Audio Precision! This strong SINAD value puts the unit in the top tier of all DACs tested:All DACs that do better than it are considerably more expensive. Yes, the slightly higher output voltage may be giving the X1S some advantage here so don't go nuts on this point.Dynamic range is excellent too:Would jitter test let us down or will the good news continue?The noise floor is very low so it reveals tiny spikes but they are way, way below threshold of hearing at -135 dB and lower. There are some power supply related spikes visible on the left but again, levels are at -130 dB and lower. And I could have probably reduced them by messing with grounding but I did not bother.How about linearity? Surely this budget DAC will do something wrong there, no?Nope. It nails that too. How about multitone?Nothing out of ordinary here either.Come on, we need something to complain about!And this it:We have our classic "ESS Hump" where distortion rises at mid levels. The noise level is very low so it looks even more pronounced than previously tested ESS based DACs. The X1S makes up for it though by keeping its distortion below noise level until we get to absolute max level. In contrast, the Topping DX3 Pro which is its direct competitor starts to distort some (hence the reason its SINAD is not as good). Being based on AKM dac chip though, the Topping DX3 Pro has no mid-level distortion.I tested the filter response using the first setting (sharp?) and got this:I like to see a sharper filter that truncates out of band frequencies (above 22.05 kHz for 44.1 kHz sampling) a lot more than this. This seems to be a typical choice these days.The King of measurements here is distortion+noise relative to output power. So let's start there with 300 ohm load:I like to see more than 100 milliwatts of power and the Aune X1S gets there easily with 140 milliwatts with no distortion! It bests the DX3 Pro in high gain in that regard but loses in overall noise level to DX3 Pro especially in low gain mode.Let's switch to 33 ohm load and see how much current the unit has to drive such low impedance loads:Here, the X1s falls behind DX3 Pro, starting to clip earlier at 270 milliwatts.Channel balance unfortunately is off at low volume levels:Given lack of low gain setting, this may be more of a problem with sensitive IEMs than it would be otherwise. The Topping DX3 Pro has digital volume control so has perfect channel matching.Last bit of not so good news in this department is output impedance:5.9 Ohm is not end of the world and should be good for many headphones but ideally we would want to see 2 ohms or less.Listening TestsAs usual I started my listening tests using my Sennheiser HD-650 headphones. Here, there was plenty of power to drive it to thundering level. Not much margin was left though and I could detect some distortion at max volume.Same situation existed with Hifiman HE-400i which sounded excellent through the Aune X1S. It however got distorted earlier and sounded more harsh during the last 20% of the volume range.Really though, the performance was still excellent with both of these headphones which tells me you will have similar experience with a wide range of headphones.Despite its age, the Aune X1S provides a very strong showing. You can tell it is well engineered with proper design verification as indicated by my measurements but also inclusion of measurements graphs right in the manual (much like Topping). What holds it back a bit are design choices such as ESS DAC chip, no digital volume control, and less than ideal output impedance.The direct competitor to Aune X1S is the Topping DX3 Pro which has far more features (LED display, variable RCA out, Bluetooth). Alas, as of this writing the DX3 Pro there are reports of reliability issues causing many to turn away. For those people, and really in general for other preferences, the Aune X1S provides an excellent alternative.Needless to say,It is proof yet again that going by such things as chip specs is a fool's path. What you want is excellence in execution Aune brings that in spades here.------------As always, questions, comments, recommendations, etc. are welcome.Please considerfunds using: https://www.patreon.com/audiosciencereview ), or https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).