This is a review, detailed measurements and comparison of Neurochrome HP-1 high performance headphone amplifier to Massdrop THX AAA 789. It is on kind loan from company designer and owner, Tom Christiansen. I learned about Tom in chasing a review he had done of a passive volume control. I was very impressed with his depth of knowledge and thorough analysis using an Audio Precision analyzer. Tom is an analog designer, having worked for famous and large leaders in the industry (National Semiconductor and Texas Instrument). He is quite active in DIY audio forum, sharing his knowledge of engineering and audio. Needless to say, I was excited to get my hands on his products to test starting with his HP-1 headphone amplifier given its superlative measured specifications.The HP-1 comes in all forms from bare PCB to fully populated board and finished package. I received the latter:The retail price for this version is $1,249. Bare board with the knob and RCA jacks is $225. And there are options in between. See: https://www.neurochrome.com/product/hp-1-ultra-high-end-headphone-amp-2/ The package feels hefty and dense due to inclusion of AC power supply inside the unit (unlike the Massdrop THX 789 and its external switching supply).Naturally there are both balanced and unbalanced inputs. There are three gain levels (+6, +12 and +20 dB). On output side we have 1/4 inch and XLR headphone jack. As the measurements show later, the power doeschange by using the XLR connector.Let's get into measurements and see how HP-1 compares to its stiff competition, the massdrop THX AAA 789. Given the state of the art that these two products represent, I re-ran all the measurements for the Massdrop again. We are talking about some small performance aberrations that seemingly change in the weather might impact them! I kid of course but it is partly true.All tests are performed with the XLR input. Output is 1/4 single-ended headphone output unless otherwise noted.As usual, we start with our dashboard. Given three gain levels and infinite choices of input and output levels, you get fairly wide variation in performance. I usually play some to get close to the best performance possible and here is that outcome (same output level as my review of Massdrop THX 789: Looking to the right side of the 1 kHz FFT (frequency) spectrum we see essentially no harmonic distortion! This is in the "high-performance" mode of my Audio Precision APX555 which uses dual ADCs to essentially suppress the noise and distortions of the signal capture (so don't try it with any hobby level sound card).The reason SINAD is not some 10 dB higher (limit of my analyzer) is the spikes with have to the left which are from the power supply (harmonics of 60 Hz). Fortunately our threshold of hearing is extremely high in low frequencies so no concern whatsoever. I am left wondering though what a statement this device would have made if those weren't there. As it is, the HP-1 takes a fine position in our ranking of headphone amplifiers when it comes to SINAD (signal over power of noise and distortion):I am not happy with my signal to noise ratio measurements as test conditions make a big impact on what value you get. Here is a conservative sample that is close to what I used for massdrop THX:If I played with input and output levels, I could get above 127 dB. Going with what we have as is, here is how it compares:For the next graph, we have a new measurement that is SMPTE intermodulation test (60 Hz and 7 kHz tones) versus output power level (300 ohm load). IMD tests like this are not bandwidth dependent like THD measurements and hence can be more revealing:I have shown the Massdrop THX 789 performance in dashed lines. The 789 is very well behaved and changing the gain only increases the output power and doesn't impact noise or distortion levels. Highest power is achieved by using high-gain mode which eventually clips as see in dashed green color.In lowest gain mode the HP-1 matches the noise and distortion levels of 789 but produces even more power the latter outputs in highest gain mode. No clipping occurs here so to get max power, you need to use medium or high gain. Once there, you get over twice as much power as Massdrop THX 789. This is done without any increase in distortion or noise over 789.The high gain mode in HP-1 doesn't produce more power so it is there for convenience of dealing with lower input levels (either in source hardware or music being played).Switching to our traditional THD+N versus power at 300 ohm we get our usual power ratings:The HP-1 produces nearly 0.4 watts at this very high load impedance which is impressive. As noted in the IMD test, this is 2.4 times more power than Massdrop THX 789 musters.Running the same test with 33 ohm load we get:The HP-1 delivers twice as much power as THX 789.The THX 789 gets its revenge though in balanced mode where it produces much more power and HP-1 does not:Output impedance is comfortably low on HP-1 at 0.8 ohm:The volume pot is analog so we get some channel imbalance:If this is an issue for you, get a DAC with digital volume control and change the level there. A quick test of changing levels in my analyzer instead of the volume control on HP-1 showed same performance. I plan to do a more exhaustive study of volume control impact on performance in the future.The Massdrop THX 789 uses "forward error correct." This is in contrast to negative feedback commonly used. The advantage of forward error correction is supposed to be in higher frequencies where negative feedback can be weak (due to need to reduce gain to avoid oscillation). So I thought I run a frequency sweep of THD versus frequency at same output levels:Interestingly, one channel in HP-1 matches the THX 789 in high frequencies. The other (in red) does nose up though.In reverse, negative feedback works the best for low frequencies and we see the HP-1 pull ahead there.Note that these are astonishingly low noise and distortion levels so we are nitpicking. And other factors may be impacting the measurements of error this low.As always, I started my listening tests with my Sennheiser HD-650. Ah, you have not heard clean bass until you power this headphone with this class of amplification. It is authoritative and at high volumes, it has no trouble making your ear lobes resonate with the music!Here I started with medium gain but then switched to high-gain for inconvenience for my "audiophile" recordings that have lower levels.Switching to Hifiman HE-400i of course brings more power due to their lower impedance. You lose some of the bass due to design of these headphones but you get that airy sound that is distortion free until the headphone drivers start to cry (and you do too with ear piercing SPLs).I recently bought an AKG K92 closed back headphones so I thought I test with those too. Man, I hit play and it scared the hell out of me! These headphones seem more efficient. While not in the same league as the above two headphones, I like them for better isolation. I checked for level level channel balance and it was fine.I can't emphasize enough how revealing these amplifiers are. As I noted, it obsoleted a lot of "audiophile" tracks I had collected with my speaker system. So many of them have distortions that become so obvious to hear now. For this reason, I hope more recording and mastering engineers invest in this class of headphone amplifiers to better hear (and hopefully fix) the amount of distortion that can exist in their work product.Boy, are headphone users spoiled with these superlative headphone amplifiers. We are talking objective performance that strains measurement gear. The HP-1 delivers that with superb engineering and design using classical negative feedback method. Using standard headphones (i.e. not balanced), the HP-1 produces more power and as such as a lot more headroom than Massdrop THX 789. Any distortion you hear is the recording or your headphones. It is extremely liberating to have such transparent and powerful amplification in the chain. If you have less efficient, high impedance headphones with unbalanced connections, the HP-1 is the perfect companion.The down side is the cost. Fully packaged, we are talking four times higher cost than Massdrop THX 789 which in my opinion, is priced too low for what it does. Tom has given up his day job to build this product in very low volumes so can't compete on price. You can reduce the impact here by either building the unit yourself or getting the assembled board and putting it in your own case. You will have a pride of ownership and uniqueness which users of massdrop THX 789 won't have.Personally and it is of course subjective, I like us as a community to support designs and efforts such as what Tom is doing. Just as you donate money to support ASR Forum, I think it makes sense to spend extra money to make sure we have options in the marketplace. This is especially so given Tom's willingness to help the community by providing knowledge and kits.Needless to say, on performance side, the-----As always, any questions, concerns, 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 ). It will enable me to pay for gear that is not offered for loan.