This is a review and detailed measurements of the Matrix Audio HPA-3U DAC and headphone amplifier. It is on kind loan from a member. From what I can tell, the HPA-3U came out around 2015. Matrix website shows a HPA-3U+. Not sure what the difference is. The HPA-3U sold for I think US $420. It seems to be a popular device as I see discussions about it and its balanced headphone amp only version even today. The latter, HPA-3B by the way, is also coming for review.The HPA-3U has a "pro" audio look to it:The handle on the volume control is a bit slippery for my taste but otherwise, the volume control has a solid feel.There is an input selector to choose between the internal USB and analog input. Next to it is a 3-way gain switch.Here is the back panel and connectivity:Nice to see mains power supply included in the unit.The DAC only has USB input which is fine by me but not if you have other types of digital outputs.The enclosure is quite deep by the way.In use the HPA-3U got moderately warm. Nothing that would concern me but it will be a source of heat and power loss.I tested the internal DAC using the RCA outputs. Alas, their level changes with volume and go all the way up to 11+ volt, telling me they are just taps on the headphone jack. As such, which gain you use can make a small difference when measured at our nominal 2 volt output:Gosh, this is disappointing. We are so spoiled by the higher-end and modern offerings from Matrix Audio that this kind of performance is a real let down. Plotting the SINAD (signal over noise and distortion) shows that the HPA-3U barely makes it into our third quadrant of all DACs tested:Given this performance and the fact that RCA out is essentially the same as headphone out, I decided to not run more DAC tests and jump into headphone testing using analog input.Using my Audio Precision as signal generator, and setting the input and output levels to 2 volts, we get this dashboard:That is quite an improvement. Not only is our SINAD much higher, but what is there is mainly dominated by mains hum. The actual distortion products are vanishingly low at less than 125 dB! The DAC is clearly the weak link in HPA-3U.Reading comments online, it seems that with ultra sensitive IEMs someone could hear the hum and proceeded to put shielding around the transformer which got rid of it. So close proximity of the transformer in the case may be the cause here.Indeed, SNR tests with just 50 millivolt of output indicates the same:Frequency response while flat within audible band, has a bit too much roll off in my view:Here is the intermodulation distortion versus output level (NOT input):We see that at low gain, while noise is higher, once we approach maximum power, we match the performance of our reference Massdrop THX AAA 789 (pink line)! WIth highest gain setting, we definitely have higher noise level but also generate more power. Indeed we can see that in our THD+N versus power:With low to medium gain, the unit never clips with 2 volt input which DACs normally produce over RCA outs. Distortion levels are quite low and the disparity with THX AAA 789 is just in noise level.With highest gain, the HPA-3U produces 360 milliwatts of power which is excellent. My criteria here is just 100 milliwatts so you should be perfectly fine driving very high impedance headphones to very high sound levels. THX AAA 789 would need to resort to its balanced output to match the HPA-3U.Performance degrades more with 33 Ohm load which emphasizes current delivery:But still quite respectable. On power side, 2.3 watts is available which should drive just about any low impedance headphone to deafening levels.Output impedance is comfortably low at 1.4 ohm:Channel balance is surprisingly good down to minimum level:I started my testing as usual with Sennheiser HD-650. At high gain, you have incredible amount of power, so much so that I was scared to even approach max volume let alone leaving it there. Bass and dynamics are incredible as a result with no hint of distortion.Switching to the other extreme using my Drop Mrspeakers Ether CX repeated the above experience, we have copious amount of power available. No hint of distortion was there and I could drive them as hard as I wanted.While probably not an audible concern, the measured performance of the DAC in HPA-3U is disappointing. The headphone amplifier though has tons and tons of power with no hint of distortion. The high mains noise though may rule it out of using it with sensitive IEMs.Subjective listening tests which included its DAC by the way, showed that performance there is excellent across the board with non-IEM headphones (I did not test with IEMs).Overall,You can start with it and maybe add another DAC if you are after best measured performance. Or wait for the review of the HPA-3B and see if that is worth getting and pairing with an external DAC.------------As always, questions, comments, recommendations, etc. are welcome.I have to buy some books for the Panthers for their schooling. Yes, they go to school. Textbooks are not cheap so pleaseusing: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/