Thanks to Matt for building and then sending this very interesting DIY amp kit to me for listening and review.
The Neurochrome HP-1 is a single-ended, solid state amplifier designed by Tom Christiansen that boasts a 3W output into 20 Ohms and 0.000050% THD (worst case) which equates to a signal-to-noise ratio of about 126dB. Needless to say, this amp measures extremely well and Tom has provided extensive measurements and analysis on the product description page on his website (https://www.neurochrome.com/hp-1-ultra-high-end-headphone-amp/). The PCB, connectors and chassis can be bought from his website for about $500 which only leaves you to buy the rest of the BoM from a parts seller like Mouser, bringing the grand total for building this amp to under $600 assuming you already have all the tools necessary. At the back, the HP-1 accepts both balanced and-single ended style inputs with a pair of XLR and RCA jacks and has a power switch. Tom opted for using balanced connectors summed at the input to take advantage of the lower cross-talk when compared to single-ended inputs. At the front, ¼” single ended and balanced 4-pin XLR outputs, an input selector for balanced or single-ended and a three-way gain switch. The amp has three gain settings (+6, +12, +20 dB) so it can be used with a wide range of input sources. One of the more unique features of this amplifier is that it uses a switching power supply, similar to those used in things like laptop brick and is international mains input voltage compliant (85 – 264 VAC @ 47 – 440 Hz). This has long been thought to be a compromised power supply design for audio applications due to noise concerns, but Tom appears to believe that switching power supplies have improved and have such low noise that it should not be an issue, and the proof is in his measured SNR for this amplifier. The amplifier is a very nice size for fitting on a desk, with a footprint similar to Schiit’s Jotunheim. The build I’ve received as a loaner uses the blue chassis, and uses a simple, boxy design and blue color that has me referring to it as “the Netgear amp” because of its resemblance to many of their networking products.
The Neurocrome HP-1 sounds like a very clean and balanced amplifier with no noticeable tonal deviations or clarity issues.
In the bass, I think the HP-1 gives an accurate volume of bass, however I don’t feel as though the HP-1 gives me the texture, impact and punch that I’m normally used to. Things are presented more flatly here, the way I’d expect someone in a studio that’s monitoring or mixing might want things to sound. I don’t feel a sense of engagement from the music here, nor do I feel that the movement of air is palpable the way it is through other amplifiers. Decays seem unnaturally quick and overdamped, giving a very dry, flat, and untextured presentation to bass notes. I can clearly hear the bass through this amplifier, and there is still some impact, but it’s not rich or full, it does not give me physical sensation of impact or the nuances of the textures of different instruments.
In the mids I find that the HP-1 continues this trend of presenting a very flat and dry presentation with no texture. Again I can hear the breaths as a vocalist is singing, but I can’t feel the air being breathed in. In music with brass instruments, I’m not able to feel the force of the air that the player is pushing into the instrument from their lungs as they play. I also find that with strings, the smaller impacts and vibrations that I normally pick up are lost here, again from what sounds like an overdamping effect that doesn’t allow for natural sustain and resonance of instruments. I find that the HP-1 sounds overly sanitized in this regard, removing depth and low level detail from instruments, and presenting a very simplified presentation of the notes.
Up to the treble, the thinner presentation of the Neurochrome HP-1, while not to my preference, does not specifically present a problem to me. In the treble, the thinner presentation leads to a more brittle presentation that sounds overly sharpened and crisp. Cymbals tend to highlight the lack of decay and overly sharpened character of the amp here.
The staging of the amp seems appropriately wide, but lacks significant depth. Things seem arranged in a sort of bowed line in front of the listener, with the front center image being set the furthest back and everything else moving to the sides but also coming forward a bit. Imaging lacks definition, and so different sounds can sometimes sound like they are coming from roughly the same blobby area. Separation of sounds is good in that you’re still able to separate and focus on different instruments, but separation of individual attacks and reproduction of low level details and texture is noticeably lacking in the HP-1.
In terms of being able to power headphones, I found that the HP-1 was sufficient to get most headphones very loud, including my Hifiman HE-6, which is notoriously inefficient and difficult to drive. I did also find however, that the amplifier was not able to sustain the same level of clarity and definition when attempting to push the HE-6 to even my normal listening level (about 80dB on average) and believe that this indicates where the amp may be failing to deliver sufficient quality of output. I think there is plenty of power here and more than enough to drive anything but the most insensitive of headphones. At no point did I feel the HE-6 had as much physical impact as on my other source, but I think that is due to the amp’s presentation rather than its power output.
The Neurochrome HP-1 is a story that reminds me in a lot of ways of NwAvGuy’s Objective2 amplifier. It’s a piece of equipment that’s objectively measured as an excellent performer and yet somehow that objective excellence does not translate to similar subjective excellence. Coincidentally, I actually find that the Neurochrome HP-1 shares many of the subjective characteristics of the Objective2, while being slightly more refined. I think the staging between the two amplifiers is more similar than different, as well as the presentation of dynamics and overall thinner character to the sound. Where the Objective2 subjectively has a grainy texture to its presentation that feels entirely artificial, the Neurochrome removes that grainy presentation but maintains the same very flat presentation that lacks sustain, texture, and physical impact overall. I also think that, like the Objective2, this amp, in part due to the impressive set of measurements published about it, is going to divide the community like the Objective2 has done, where some will believe the measurements are the only believable indication of performance, while those with more subjective leanings may disagree about the performance of this amplifier based on what they hear.
Update (February 20, 2017):
After publishing the review I spoke further with Matt to correct some of the inaccuracies in my description of the amps topology which have since been corrected. In discussion with him, I learned that the amp is in fact a single ended amplifier, using balanced connections only for the convenience and lower crosstalk. This also means that when fed balanced inputs they have to be summed to a single-ended signal before entering the single-ended gain stage, where as the single-ended inputs avoids this extra summing stage. In my listening and going back and forth between balanced and single ended inputs out of the same DAC, I find that the single-ended actually solves some of the issues I had with the amplifiers lack of dynamic impact and low level detail retrieval. It also improves with regard to the aforementioned crispiness in the treble, smoothing it out noticeably.