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This is a review, detailed measurements and comparison of two battery operated portable headphone amplifiers: then JDS Labs implementation of O2 and the Topping NX3s. The former is one that I own and it retails for USD which I purchased for about $140 including free Prime shipping. The Topping NX3s is on kind loan from a member and is a lot cheaper at just USD $59 on Amazon with free shipping. I believe it went on sale on Massdrop for $45???Size wise, the Topping MX3s is much, much less chunky than the O2:The NX3s is definitely pocketable and could I guess be strapped to the back of your mobile phone. No such prayer for the O2 amplifier. The NX3 feels quite slick while the O2 is like many DIY type products.The NX3s uses any USB charge source which is very convenient. The O2 on the other hand uses a small but rather heavy AC mains transformer. AC can be converted to the positive and negative supplies easily for amplification and hence the choice of that by its designer, the aloof NWAVGUY.Both units come with high and low gain and 3.5 mm headphone and input jacks as seen in the photo.There is a bass boost on the NX3s which I did not test. There is no such feature on the O2.Let's get into measurements and see how they compare.My first experience with the Topping NX3s was not good after I fired off the dashboard with the amp put in "unity gain" mode meaning it generates the same voltage output as what is fed input:We see a rather large mains spike at 60 Hz and tons of smaller ones limiting ultimate rating of THD+N/SINAD. Specs online were a lot better so I disconnected the USB power from it and performance substantially improved:This is a remarkable improvement of nearly 20 dB in channel 2! Performance was even worse when the battery was almost empty. I tested with an external USB power supply and it was even worse than what I got above which was from my computer's USB 3.0 capable port. Clearly the NX3s is made to charge and go rather than mostly desktop use while plugged in.The JDS Labs O2 did not have much variation to speak of with or without its AC wall-wart:Its performance falls short of when the NX3s is running on batteries but better when the NX3 is USB powered.Note that there is fair bit of variability in performance of O2. Every time I put it on the bench, it generates slightly different results. Even sitting there the SINAD numbers jump up and down. There is some kind of instability inside there (at micro/small level).I measured signal to noise ratio but the data is junk because both devices severely clip at maximum level which is what is used for determining SNR:I need to devise better test parameters here.The key test here is distortion versus power level when loaded down so let's look at that at 300 Ohms:For reference I have shown the JDS Labs Atom desktop amplifier which is distortion-free until max power which exceeds both units. So if you need a desktop unit, don't look at either the O2 or the NX3s.Both NX3s and O2 generate the same power prior to clipping. At that point, the distortion in NX3s is lower but the O2 keeps going after clipping so likely can get louder during momentary peaks than the NX3s.In case you are interested in low gain performance, here is the data on that:Switching to a much more stressful 33 ohm load we get this:Both units put out far more power as is typical. The results are similar as 300 ohm with NX3s having lower distortion at clipping than O2 but not able to max out as well as O2 can. The desktop JDS Labs Atom leaves them both in dust though with double the power prior to clipping.Both of these units naturally use analog potentiometers for level adjustment which can be subject to channel imbalance. Here is how the O2 does when I manually change the volume and measure the differential:Worst case imbalance is around 0.5 dB or so.NX3s does worse at 0.75 dB of error:Finally, here is how the output impedance compares:The NX3s has an impedance of 5.5 ohm which is well above 1 ohm or so I like to see. The O2 does meet that criteria at 1.3 ohm. This means that the NX3s can modify the frequency response of headphones below 50 ohm which have variable frequency response (for good or bad).I used my Topping DX3 Pro as the source DAC and set its digital level to max/0 dB. I then used a split RCA cable to feed both amplifiers at the same time. I then used an AB switcher to switch back and forth between the two amplifiers.As usual I start with my Sennheiser HD-650 to see how the product performs with high impedance headphones. Here, the O2 stole the show right away. It could get louder than NX3s with better bass performance.Switching to Hifiman HE-400i to represent low but flat impedance headphone, the O2 was louder but you could hear its distortion easier. Prior to getting to that point though, it did better than NX3s just as well.From cost, looks and portability the Topping NX3s leaves the JDS Labs O2 in the dust. While I did not test it, it will likely have much longer battery life too.Unfortunately in actual performance the JDS Labs O2 simply sounds better with the headphones I tested. It has much lower output impedance and has no variability between desktop and battery operation so you could use it as a desktop amplifier just as well.I don't have a clear cut answer or recommendation here. Personally I would never lug an O2 around with me and for desktop use, I would get the JDS Labs Atom which is cheaper and higher performance. Maybe the right answer is to use the NX3s with a more efficient headphone. I don't really know.So I say the battle is unfinished. Hopefully we will find more clear cut choices in portable, battery operated headphone amplifiers.------------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 ).