This is a review and detailed measurements of Monoprice Portable Headphone amplifier and DAC with "THX AAA" technology. I purchased this earlier in the week for USD $279.99. With tax it came to little over $300.For those of you who don't know, THX has come up with amplification module with feed forward error correction which serves to lower distortion in amplifiers. My review of Massdrop THX Headphone Amplifier showed it to have exceptional performance, far and above anything I have measured before. So excitement has been built up for the same technology (but not identical part) used in Monoprice version. Here, we are treated to a portable headphone amplifier and DAC all in one.The Monoprice Monolith THX has a very serious, business-line appearance:It is similar to a modern smartphone but much thicker and heavier. It has dual USB sockets as you see, one for charging and one for data access.There are buttons on the side for selecting menus options. A low resolution display does the job of showing what is going on. It is lacking in style but it is functional and much better than not having one.The volume control is analog but controls the level digitally as you will see later. Here, I ran into a bug in its implementation. I was testing the unit when I realized the output level was jumping up and down. I looked a the display and it was rapidly switching between 9.5 and 10.0 dB. By chance the level was set between those two steps, causing it to randomly be read as either 9.5 or 10.0. This should be fixed. Fortunately I could not repeat the issue so may require precise positioning of the volume pot to make it happen.The unit is plug and play and that is how I tested it using USB input. The line-in and Toslink input testing will have to wait for a later installment. As does coverage of a handful of DSP functions such as filters (both shelf and parametric), dynamic range compression and crossfeed. In other words, I am focusing this testing to just a normal USB DAC and headphone amplifier in the interest of getting this review out as fast as possible.So let's get into measurements and see if this is a groundbreaking portable DAC and amp as its much larger desktop amp was.I used the volume control to roughly get the output to 2 volts as I normally do for dashboard measurements of DACs and this is what I got:Ah shoot! Yes, this is competent performance but SINAD of 105 (signal over noise+distortion) is not state of the art at all. Indeed, it puts the unit solidly in the middle of second tier of many products I have tested recently:It falls in the same category of Topping NX4 DSD for example, but that unit retails for just $160 or nearly half the price.Ignoring distortion and just measuring dynamic range, we get similar performance:To put this in context at the risk of creating another golden cow as I have done with my SINAD graph, here is how the last few products with this measurement rank:Let's look at intermodulation distortion:Noise is disappointingly higher in the part of the graph where it dominates (downward sloping). It is worse than recently reviewed Topping DX3Pro for example.Let's look at noise and jitter:I was unhappy to see that parade of spikes below our test frequency of 12 kHz. It seems to be a constant and shows up in other test such as the spectrum of this 1 kHz tone:As we see, none of it exists in Topping NX4 DSD. This could be from the DSP or the controller running the display. Either way, it should have been better isolated from the sensitive analog output of the DAC/amplifier.Fortunately linearity is quite good:Switching to headphone amplifier part (together with DAC), we see a nice low output impedance of 0.7 ohm:Power output at 300 Ohm is rather low in my book:And here is output power at 33 ohm:For a unit with such strong emphasis on its amplification technology (i.e. THX), I would have thought they would push the envelope on power availability. Such is not the case. We have higher noise too until we get to limit of power before clipping.Channel imbalance is non-existent due to digitization of volume control pot and using that to control gain:Finally for you geak heads, here is the filter characteristics for the default setting:As an aside, I got a kick out of it rejecting out of band signal better than Chord Mojo whose claim to fame is filtering!Anyway, we can stop here as the performance characteristics of the unit are quite clear.I started my testing Sennheiser HD-650. Unfortunately I was easily able to push the unit into clipping distortion with my standard test tracks. That said, there is nice headroom here with respect to gain allowing me to get pretty loud levels out of my "audiophile tracks" that are recorded at much less than max. There, music performance was fine.Things improved a lot with Hifiman HE-400i where I had much more power. I could still get into distortion category but overall, it was a more enjoyable experience.So other than lack of power, not much at issue here audibly.The Monoprice Monolith THX Portable DAC and headphone review reminds me of the famous line from Voltaire:Here, I would paraphrase that to:I went into this review thinking I would see incredible measurements but that was not the case. What is there is instead good performance with some flaws such as excess noise bleeding in the middle of audio band. No angels sang in my head as I was expecting.Power is rather anemic and missed opportunity to showcase this amplification technology. There are better choices for less money both for desktop and portable use.Overall, the Monoprice Monolith THX AAA misses the mark created by the hype and anticipation for the product. On the other hand, it is a fine unit if you don't have high expectations of volume (as I do). Normally I would recommend such a product but on the basis of what we all expected, I am left lukewarm in that regard.-------------As always, questions, comments, recommendations, etc. are welcome.If you like this review, please consider donating funds 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 ).