HP-1: Ultra-High End Headphone Amp

2019/09/16: Those looking for an HP-1 may be interested in my upcoming retail headphone amp, the Tom Christiansen Audio HPA-1. It is currently available on pre-order here: www.tomchr.com.

2019/03/24: The HP-1 has been discontinued. It remains a phenomenal headphone amp and I plan for its reincarnation in the form of an improved commercial/retail product available this fall. Hopefully in time for Rocky Mountain Audio Fest (RMAF).

The Neurochrome HP-1 is an ultra-high end headphone amplifier capable of producing up to 3 W into a 20 Ω load at vanishingly low distortion. The HP-1 will drive even the most challenging headphones with the utmost fidelity and natural sonic profile. It is designed to be as close to a straight wire with gain as technically possible. In addition to providing the ultimate technical performance, the HP-1 also provides peace of mind as it contains advanced protection circuitry which will protect your headphones in case of various fault conditions.

Key Features

  • Fully optimized 4-layer circuit board.
  • Total harmonic distortion: 0.000017 % (200 mW, 600 Ω).
  • Total harmonic distortion: 0.000032 % (200 mW, 300 Ω).
  • Total harmonic distortion: 0.000050 % (200 mW, 32 Ω).
  • Total integrated output noise: 1.0 µV RMS (A-weighted, 20 Hz – 20 kHz).
  • Output power: 3.0 W (32 Ω).
  • Output power: 450 mW (300 Ω).
  • Gain selectable (+6, +12, +20 dB) by front panel switch to accommodate a wide range of sources, including smartphones, tablets, DACs, and CD players.
  • 1/4″ phone and 4-pin XLR output connectors.
  • Balanced (XLR) and unbalanced (RCA) inputs.
  • ±0.1 % tolerance Susumu RG-series resistors used in critical parts of the circuit to ensure the best possible performance.
  • All signal switching is accomplished using relays with gold plated switch contacts.
  • ALPS RK097 volume control potentiometer.
  • Well filtered switching power supplies followed by onboard low noise linear post-regulators ensuring the highest power supply performance.
  • Protection against DC voltage on the amplifier output.
  • Integrated mains soft start and 320 Joule surge suppressor.
  • ON indicator emitting a subtle and pleasant blue glow.
  • International mains voltage compliant (85 – 264 VAC @ 47 – 440 Hz).
  • All aluminum chassis made in Canada.

HP-1 reviewers and listeners are saying:

I’ll be frank; coming into this review, I expected this to be a tremendously clean amp, but I didn’t expect it to sound as incredible as it does. I thought this drop in THD would be noticeable, but not spectacular, and I was wrong there too. The best way I can put it, even if this is a little bit sensationalist, is that it has effectively put a veil on every other amp I’ve used since.
– Sam Braverman, hearfidelity. Read his full review here: Neurochrome HP-1 Review (hearfidelity).

The HP-1 is a super nice and competent solid state amp. I’m surprised it’s so clean and powerful in such a small and light box.
– ‘brencho’

Good bass control. Very involving. Highs are well presented and with awesome delineation. Great instrument separation, as in no mush. […] As an audio amp, this is a great accomplishment. And IMO it delivers the goods like a champ. It is not going to win display time at Tiffany’s, but it definitively delivers reference level resolution and superior sonic qualities to the table.” Read the full review here: SBAF HP-1 Thread.
– ‘ultrabike’

As far as SMD projects go, this one is pretty straightforward with easy soldering pitches, logical PCB layout, and excellent documentation. […] I quite like the quality, colors, and utilitarian aesthetic of the chassis Tom offers. The PCB and board-mount components fit the chassis perfectly.
– Corey Warner, Solderworks Audio. Read his full build experience here: Solderworks Audio – Neurochrome HP-1.

The HP-1 is available in various options ranging from a bare circuit board (no components included) for DIY assembly to a fully and assembled and tested product. A fully assembled and tested HP-1 is shown below.



The full set of specifications for the HP-1 are tabulated below.

Output Power3.0 W20 Ω, THD+N < 0.005 %
Output Power3.0 W32 Ω, THD+N < 0.005 %
Output Power450 mW300 Ω, THD+N < 0.001 %
Output Power230 mW600 Ω, THD+N < 0.001 %
THD0.000017 %1 kHz, 200 mW, 600 Ω
THD0.000032 %1 kHz, 200 mW, 300 Ω
THD0.000050 %1 kHz, 200 mW, 32 Ω
THD+N0.00061 %1 kHz, 200 mW, 300 Ω
IMD: SMPTE 60 Hz + 7 kHz @ 4:10.00063 %200 mW, 300 Ω
IMD: DFD 18 kHz + 19 kHz @ 1:10.00013 %200 mW, 300 Ω
Multi-Tone IMD Residual< -127 dBVAP 32-tone, 200 mW, 300 Ω
Channel Separation115 dB1 kHz
Channel Separation> 105 dB20 Hz – 20 kHz
Gain+6/+12/+20 dBSwitch selectable
Gain Variation±0.02 dB20 Hz – 20 kHz
Input Sensitivity3.7/1.9/0.74 V RMS200 mW, 300 Ω
Bandwidth0.10 Hz – 290 kHz
Full-Power Bandwidth71 kHz
Slew Rate7.6 V/µs300 Ω || 220 pF load
Total Integrated Noise and Residual Mains Hum1.00 µV RMS20 Hz – 20 kHz, A-weighted, 6 dB gain, min. volume
Total Integrated Noise and Residual Mains Hum1.26 µV RMS20 Hz – 20 kHz, Unweighted, 6 dB gain, min. volume
Output Impedance0.5 Ω20 Hz – 20 kHz
Residual Mains Hum< -136 dBV
Dynamic Range (AES17)128 dB1 kHz
Common-Mode Rejection Ratio > 80 dBTyp., 1 kHz
Mains Voltage85 – 264 VAC47 – 440 Hz
All parameters measured at +6 dB gain and using the balanced inputs unless otherwise noted.

The internal circuitry is shown below. The 4-layer PCB measures 6.000 x 7.750 inches. All components except the RCA connectors and the volume knob are available from Mouser Electronics. The bill of materials cost is $250, not including the RCA connectors and volume knob. With a few exceptions, all components are surface mounted. Most are 0805 size or larger. There is a handful of diodes in SOD-323 packages. Those diodes are the smallest components on the board.

To watch an HP-1 built in twelve seconds, watch this video:


Circuit Topology

The simplified circuit schematic of the HP-1 is illustrated below.

The HP-1 uses two LME49600 headphone driver ICs in parallel to provide 500 mA (peak) of output current. An OPA1611 linearizes the output stage and is, thus, primarily responsible for the stellar performance of the HP-1. A frequency compensation network within the output section ensures that the HP-1 remains stable even into highly capacitive loads, such as long headphone cables. The HP-1 can drive capacitive loads in excess of 10 nF corresponding to approximately 300 m of headphone cable.

An OPA2140 is the centre of a high-performance DC servo, which ensures that the DC offset of the HP-1 is well below 100 µV within ten seconds of power-up. The DC servo uses third order filtering to provide a fast settling time with no audible (or measurable) impact within the audio spectrum. Two OPA1612 high-end audio opamps are used for the differential input receiver and the gain stage. The gain stage offers gains of +0, +6, and +14 dB. The output section adds +6 dB of gain for a total system gain of +6, +12, and +20 dB.

The protection circuit provides a turn-on delay of approximately ten seconds to avoid turn-on thumps in your headphones. It is also responsible for disconnecting the headphones immediately upon power-down. Furthermore, the protection circuit continuously monitors the output DC voltage of the HP-1. If excessive DC voltage is found on the output of the driver ICs, the output is immediately disconnected to protect your headphones.


Listening Impressions

So far I have listened to the HP-1 using a variety of headphones, including:

  • Focal: Utopia, Elear
  • HiFiMAN: HE1000, HE560, HE6
  • Sennheiser: HD-650, HD-600, HD-580
  • Etymotic: ER-4 (IEM)

To my ears the HP-1 sounds fantastic. It sounds clear as a bell and has good command of the transducers. The source material is presented as the artist intended it without colouration.

The HP-1 was presented at the November 2016 Head-Fi meet-up in Winnipeg and the organizer of the event had ample play time with the HP-1. He was kind enough to write a full review, which included his impressions of the HP-1, as well as some of the email correspondence he and I had regarding my design philosophy and the design choices made in the HP-1. I have reproduced a snippet of his review below.

The first thing to note about the HP-1 is the absolute clarity in the upper ranges. The terms clinical/surgical often come with a negative connotation, but are not implied here. Precision is the theme. This is quite simply one of the cleanest amps that I have ever heard. How many synonyms do I need to use here to convey that? It’s fast, precise, crystal clear.

– Nathan from Winnipeg. Read his very detailed review here: [Review] Neurochrome HP-1: Making sweet love to your HD650… or, “What the O2 wishes it could be” (Head-Fi).

Others appear to agree:

The HP-1 is dynamic and hits hard. I’ve not heard an amplifier this dynamic and musically engaging before. I think my former Naim gears come close. Subtle dynamic shadings that give clues to real musicians playing in real time and off one another are easily heard. It grips, hits, and grooves hard. It doesn’t do wishy washy or passive. It has complete control over the transducers. It doesn’t [mess] around.

– Corey from Minnesota. Read his full synopsis here: HP-1 (SBAF).


Performance Characterization

The performance of the HP-1 was characterized using an Audio Precision APx525 audio analyzer. The results are show below. The graph below shows the THD+N vs frequency for the HP-1 when driving a 300 Ω load at 200 mW. The THD+N of the amplifier barely makes it above the noise floor of the APx525.


Below is the THD+N vs output power for the HP-1. Note that this is measured using 20 kHz bandwidth, hence, the noise (+N) component is lower and the resulting THD+N therefore better. The jumps in the curve up to 400 mW are caused by the range switching of the APx525. The final jump at 450 mW is caused by the amp entering clipping.


An alternative way of illustrating the vanishingly low THD is to look at the THD+N residual as shown below. Note that the distortion residual had to be magnified by 10000× (80 dB) to even register on the graph. The THD+N residual is entirely noise (maybe with a little bit of THD from the APx525 signal generator itself).


To get a true measurement of the THD of the HP-1, a precision 1 kHz oscillator was used. Unfortunately this oscillator generates a bunch of intermodulation products at multiples of 30 Hz. Its THD is very low, though. The harmonic spectrum for 200 mW into 300 Ω is shown below. The THD measures 0.000032 %.


The THD at 200 mW into 32 Ω was measured in a similar fashion and comes in at 0.000050 % as shown in the harmonic spectrum below.


The intermodulation distortion (IMD) is stellar as well. The standard SMPTE (60 Hz + 7 kHz at 4:1 ratio) is shown below.


An even better test of the IMD is to use 18+19 kHz as this provides a good indicator of the loop gain of the circuit.


Two-tone IMD tests are often criticized for having poor external validity. After all, most people listen to music rather than two-tone signals. A better test of circuit performance with a music signal can be gathered from a multi-tone IMD measurement. The result below was obtained using a test signal with 32 tones logarithmically spaced in frequency. The total power of all the tones was 200 mW into 300 Ω. Note that the worst IMD residual peaks measure -127 dBV. This is superb!


Both channels of the amplifier show stellar gain tracking and only minuscule gain variation across frequency as shown below.


The gain at the three gain settings for both channels is shown below.


The separation between the left and right channel of the HP-1 is shown below.


The residual mains hum is shown below. The tallest mains harmonic measures -136 dBV (160 nV).


The HP-1 ultra-high end headphone amplifier is available in various options ranging from a bare bones DIY option to a fully assembled and tested commercial product. You can buy yours by clicking the Buy It Now button below.