This is a review of the HIFIMAN EF2A tube DAC and headphone amplifier. It is on a kind loan from a forum member. The retail price is $169 USD but I see it on Amazon for $139 plus shipping. I seem to recall it going on massdrop also. This is a small unit as you can see here, dwarfed not by one but two Audio Precision analyzers;Typical of these "hybrid" headphone amplifiers, the tube is used as the input stage and the output drive is transistors. The see through lid shows a row of power transistors which hints at higher than normal output power.I find the overall look cheesy and DIY type. The volume control though feels good. An external brick powers the unit.Inputs are both digital in the form of USB and analog RCA. I tested the unit both ways.Let's get into the measurements and see how she did. As usual, if you are not familiar with what these graphs mean, refer to my tutorial on understanding audio measurements: https://www.audiosciencereview.com/forum/index.php?threads/understanding-audio-measurements.2351/ Let's start with a test of jitter and noise:As we see, the performance of HIFIMAN EF2A is nothing to write home about. We see numerous distortion and jitter spikes together with elevated noise level (I matched the levels to Fiio Q1).Turns out there are much more serious problems here. Check out this time domain output of a 1 kHz tone at 0 dbFS through USB input:Yes, the output is clipped. And no, this is NOT, let me repeat, NOT caused by the output stage of the amplifier. The output is clipped regardless of volume control position. The USB DAC is overdriving the input stage of the amplifier causing the clipping. I had to reduce the output by 10+ dB to get rid of most of it.Same tests through analog input showed clean output so clearly this is a design problem with the USB DAC implementation.Given the bad state of affairs on USB input, I tested the output power/voltage versus distortion using the analog input at different loads:Distortion+noise curves are rather high even here although within the manufacturer specs. On the positive front, even at 33 ohm load, the output can go as high as 5 volts. At the kneed of clipping point, it was about 3 volts resulting in 0.29 watts.More bad news awaited us when measuring the output impedance:As you see, at 31 ohms, it is worse than anything I have ever measured by far. This means the output of the unit will vary substantially with the headphone load you put on it.Not shown is frequency response which was pretty much flat up to 40+ kHz (using analog input).Given the above issues, and in the interest of getting this review out, I did not perform more tests. I think you get the picture here.We often argue if distortions are audible. Well, this is a case where this is definitely true and not true! It is true in that my son and I heard the problem with USB clipping first in listening test. I was getting ready to do an AB test and to match levels, I played a 1 kHz tone, and to our amazement, it sounded so different on the HIFIMAN EF2A compared the FIIO Q1. The pitch was completely different. As my son nicely put it, "if you can tell they sound different with the headphone on your lap, there is a definitely problem here!"I then set up a comparison between the EF2A and my RME ADI-2 Pro. Surprisingly, it was difficult to hear much if any difference on actual music. The reason is that my reference tracks are the ones I enjoy playing and do not have high amplitude. My son suggested that K-pop music may show the clipping problem better. So we hunted some on Tidal and there, there was some exaggeration of high-frequencies in the EF2A.On the positive front, the EF2A could keep up with the ADI-2 Pro with driving even my power hungry Sennheiser HD-650 with authority. There was plenty of power there to drive it to loud listening levels.As a USB DAC+headphone amplifier, the HIFIMAN EF2A is clearly a broken design. The DAC stage is overdriving the amplifier. There is no excuse for this, nor does it have any positive attribute. Its output impedance is also exceedingly high. For these reason, I can't recommend this unit.That aside, or if you have an external DAC, the unit performs fine. There are no euphonic "tube sound" that either myself or my son could detect so don't buy it because you think the tubes are doing something for you. They are not.As always, questions, comments, corrections, etc. are all welcome.Edit: hardware teardown and pictures here: https://www.audiosciencereview.com/...iman-ef2a-dac-and-headphone-preamplifer.3462/ Edit 2: And horrible linearity results in this post: https://audiosciencereview.com/foru...an-ef2a-dac-and-headphone-amp.3402/post-86238 ----If you like this review,for these types of hardware purchases using Patreon ( https://www.patreon.com/audiosciencereview ), or upgrading your membership here though Paypal ( https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).P.S. Other non-DAC reviews are coming. I promise.It is just that I have a lot of loaned DACs that I am having to review so that I can return them.