This is a review and detailed measurements of the Gustard H10 Headphone Amplifier with balanced input. It is on kind loan from a member. Seems like the H10 was released back in 2015 and is no longer available. It is listed for US $399 on some sites with no stock.The H10 is quite large and heavy for a headphone amplifier:The volume control is large which is nice. What is not so nice is its stiff feel despite the size of that knob. As shipped, the knob would slide past the max and min so I had to readjust and tighten the screw. Alas, the volume control shaft is round instead of notched so this problem will remain.There is only a power switch and a 1/4 inch headphone jack.Here is the back panel:As you see, there is a set of XLR inputs in addition to RCA. There is a dip switch to seemingly select between them but it doesn't do that. Instead, it just sets the gain appropriately -- both inputs are live at once. The rest of the dip switches adjust the gain and require mastery in morse code to figure out what they do.I could not find a manual. I left the switches all down for "low" gain and right one indicated by +12 as "high" gain. The switches interact with each other to provide half a dozen or more combination of gain. While this flexibility is nice, I much rather have a control in front than reaching in the back to mess with the switches.For my measurements, I used the XLR input exclusively. Here is our dashboard view for 4 volt in, 4 volt out with the lowest gain:We are missing the rated THD+N a bit but given the so many gain and volume knob setting, one can probably get to the number they have. SINAD (signal over noise and distortion) shows good design but not class leading in this day and age:Signal to noise ratio likewise lands in "competent" but not the best class:With respect to very sensitive IEMs, the SNR lands in the middle as well:Frequency response is flat until you get to low frequencies:The roll off indicates that there is a capacitor to isolate the output stage and headphone connection. Capacitors act like shorts at high frequencies but at lower one, present an increasingly higher impedance and hence that roll off.Intermodulation versus output power shows higher baseline noise level but plenty of power at very low distortion:Most important test of a headphone amplifier is its power output at different load impedances. Let's start with 300 ohm which tests its ability to pump out voltage to drive high impedance headphones:As we already could tell from IMD charge, there is tons and tons of power here. We also have more noise though especially in one channel that shows the "T" symbol.Switching to 33 ohm, the performance drops some but still more than good enough:Now higher gain doesn't give you more output because the limitation is current delivery, not gain/output voltage.Output impedance is a very good 1.9 ohm:Edit: original value was incorrect.I started testing with Sennheiser HD-650. Man, there is so much power available here to not only shake out the cobwebs from the transducers but your eardrum as well!As a result there is plenty of bass, detail and impact even in lowest gain setting.Switching to Hifiman HE-400i showed another excellent performance with abundant amount of power and fidelity. It was so nice that I kept listening after the testing was over!Mind you, there is no "class A" magic here. Just good old high-fidelity and transparent sound.From measurement point of view, the Gustard H10 shows competence with no gross errors. Times have changed though since it came out and other products have claimed the top of the kingdom (Benchmark HPA4, Massdrop/drop THX 789 AAA and JDS Labs Atom). So the match is no longer even and the H10 falls behind.The Gustard H10 though takes its revenge due to copious amount of power it delivers at exceedingly low distortion with no need to mess around with balanced headphone jacks and such. Its balanced input terminals help eliminate nasty ground loops and that is all you need.Overally, based on good measurements and very impressive subjective listening, I am happy to recommend the Gustard H10. See if you can pick one up in the used market before folks read this review.-----------As always, questions, comments, recommendations, etc. are welcome.I make this simple: I need more money!!! Pleaseusing: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/