This is a detailed review and measurements of the Topping NX4 DSD portable headphone amplifier and DAC. It is on a kind loan from a forum member and highly anticipated review. Retail price (shipped from China) on Amazon with free shipping is USD $160 as of this writing. The unit has internal battery so truly portable.Aesthetically from box design and the DAC itself, it oozes quality and nice industrial design. Nothing cheap feeling. The packaging alone would make you think you are buying a luxury product yet the price says otherwise. I have no idea how Chinese companies like Topping pull this off:You want to know why I never, ever want to get in the business of making audio products? This is the proof.Feature wise, this is a rich product with great format support with PCM up to 768 kHz and DSD512. There are two gains: high and low. A bass boost switch. And a jack for that is for both line in and out (!). I did not test this but the unit can be used as a pure headphone amplifier by using this jack and line out for DAC-only use. My testing was with the headphone jack as that is the most common use.Battery life which I did not test is stated to be 7.5 hours.As is with all Topping products, the unit comes with printed documentation that includes measurements from Audio Precision analyzer. Full regulatory certification is included which even some high-end audio equipment lacks. Those of you who are worried about "EMI" should be shopping products that have such certification.My objective testing was with ASIO4ALL ASIO wrapper. For subjective listening I used the USB class driver in Windows 10 creators edition using Roon player for bit-exact playback.Let's start with our usual dashboard with gain setting in low but max volume:We see nice output level of 2 volts but with slight channel imbalance. Distortion is very low. SINAD which is the inverse of that, shows distortion and noise free dynamic range of 104 dB, well in excess of what we need for CD playback (93 dB with dither).Clicking the switch to high gain and max volume, performance degrades substantially:We get super nice boost in output level, nearing 6 volts. But distortion has gone way up (relatively speaking). Fortunately bringing the volume knob down a few clicks gets us back to excellent performance:5.1 volts output with good distortion numbers.Let's look at frequency response:Not ecstatic about the droop at either end of the spectrum although the effect on those of us over 25 years old (ahem), is negligible. Still, I like to see ruler flat response.Channel imbalance in mid-frequencies where our hearing is most sensitive is 0.06 dB so not a concern. More to come on this later at other volume levels.Harmonic distortion and noise is excellent. Here I compare it to the well performing portable Sabaj Da3 and Dragonfly Black:We see that the Topping NX4 DSD is in a different class. Its distortion is so low that it causes my analyzer to have trouble reading it (as indicated by those "T" symbols). I am working on refining the analyzer settings to not do that.Digging into the spectrum of the distortion we see:The Topping NX4 DSD has substantially lower amount of second harmonic compared to Sabaj Da3. It is also well behaved, lacking that "hump" that exists in the output of Sabaj Da3.Same story continues in Jitter test:Note that compared to my older analyzer, these are higher resolution FFTs, showing USB packet noise that was not visible before. At -120 dB these are not audible concerns at all. But like to see them suppressed regardless.Let's get back to channel balance and see what happens when I turn the volume down from max to min manually and plot the difference between left and right (in dB on the right vertical axis):Tracking is good until about -40 dBV (-45 dB attenuation), staying below 0.2 dB of error. From there, it takes a nosedive so better not operate it beyond that region. See my subjective listening tests on the impact of this practice.On output impedance, I expect to see the typical 10 ohm but was pleasantly surprised that it was much lower at just 0.9 ohms:I love to see this in their desktop products! It eliminates any worry of interacting with the headphone to change its frequency response.Let's see the output voltage (and hence power) relative to distortion:For reference I am showing the Sabaj Da3 at 33 ohm. We see that the NX4 DSD sails way past it with lower distortion and higher output voltage/wattage. Measured output power as indicated on the graph is about half of what Topping specs I think. The discrepancy can go away by climbing up the distortion line instead of staying at its minimum level.In the interest of getting this review out quickly and reducing my significant backlog of products to review, I did not perform an AB tests. Instead, I just plugged in different headphones and listened to my music library.On channel imbalance, I only hear that at the extreme low end of the volume level, close to muting. This was with both my Sony headphones and sensitive IEMs. Definitely not a concern in my listening experience.Bass boost is nicely done when listening to mid to lower volume levels. It nicely fills in the low and adds impact without any distortion. It is especially good for compensating for less than good feel of IEMs. Once you start hearing distortion though, adding the bass boost really screws things up. So only use it for less then ear bleeding levels.With my power hungry Sennheiser HD-650 headphones, there was plenty of volume. I always want more headroom with this headphone when I can get it, but otherwise, it is a fine pairing.Other than this, the unit is delightful. I am listening to my music library while composing this review and enjoying it.Let me make a confession before giving you my conclusion: people accuse me so much of being in Topping's pocket based on good reviews I have given them that I secretly wish that some of their products don't do well so that I can say that and get out of jail!!!Alas, that opportunity is not here yet. The Topping NX4 DSD is a superb portable DAC and headphone offering at super attractive pricing.Yeh, channel balance could be better and frequency response droops a bit. But other than that, Topping shows one more time that proper engineering matters. And design verification matters. That attention to quality and execution matters. And bargain prices matter.The DSD NX4 has impressed me greatly. It would be my choice for a portable headphone+DAC or a stand-alone desktop DAC for that matter. All the positive vibes on this unit are indeed true.As always, questions, comments, corrections, etc. are welcome.-----If you like this review,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 ).