Neurochrome HP-2 ultra-low distortion headphone amplifier

[tomchr]

World, I present to you the All-DIY Neurochrome HP-2 ultra-low distortion headphone amp.

I will be demoing it tomorrow (Sunday Nov. 10, 2019) at the Burning Amp Fest at Ft. Mason. If you're in San Francisco, please stop by for a demo and chat.

Key features of the HP-2:

• 4-layer PCB fully optimized for performance while being relatively easy to assemble.
• Output power: 440 mW (32 Ω), 660 mW (50 Ω), 260 mW (300 Ω) at vanishingly low distortion.
• Inputs: Balanced/differential (XLR) and unbalanced/single-ended (RCA).
• Alps "Blue Velvet" volume potentiometer.
• +5 dB and +15 dB gain settings (jumper selectable).
• Integrated headphone protection circuit, which disconnects your headphones in the case of DC voltage on the output of the amplifier.
• Integrated output muting circuit ensures that the HP-2 is completely absent of turn-on and turn-off clicks and pops.
• Optimized on-board RLC power supply filter allowing for stellar performance with the recommended switching power supply.
• External 28 VDC switching power supply accepting 85 – 264 VAC mains for use world wide.
• The PCB features elaborate use of planes and copper pours to maximize circuit performance by minimizing supply and ground impedances.
• On-board EMI/RFI input filter to prevent wifi and switch transients from interfering with the sound reproduction.
• With the exception of two SOIC-08 surface mounted ICs, the HP-2 is all through-hole construction with socketed ICs and is easy to solder.
• Bill-of-materials includes a Mouser Electronics Project Link for ease of parts ordering.
• Gold plated PCB, fully electrically tested by the manufacturer.
• PCB is designed and manufactured in Canada.
• Custom designed chassis manufactured in Italy.

Build budget:

• Cost of parts for the PCB: ~$160 (including power supply)
• Cost of a full build using my custom chassis: ~$500

Spec. highlights:

• Output power: 440 mW (32 Ω); 660 mW (50 Ω); 260 mW (300 Ω)
• THD: < -130 dBc (50 mW, 300 Ω)
• Multi-tone IMD residual: -144 dBr (AP 32-tone, ref. 100 mW, 300 Ω)
• Output noise: 1.0 µV RMS (A-wt, 20 Hz - 20 kHz)
• Dynamic range: 125 dB
• Gain: +5 dB, +15 dB (jumper selectable)
• Input sensitivity: 2.90 V, 865 mW (gains +5 dB, +15 dB, respectively; 100 mW, 300 Ω)

You can find the circuit description and full specifications on my website: Neurochrome HP-2.

But, but... What does it sound like? I like it, obviously. Ever since I heard my first precision amp, I've been hooked on the precise (i.e. ultra-low distortion) sound. I prefer that the amp makes as little contribution to the sound as possible, as I believe this allows for experiencing the music the way the artist and recording engineers intended it. The precision of the HP-2 allows it to provide realistic and natural rendition of the source material, with a wide and open sound stage (even in headphones!).

The HP-2 will only be available as a DIY product, i.e. I will not be offering it in a fully assembled option. The PCB features two surface mounted components. They're relatively easy to solder 8-pin SOICs. Should you wish to have me solder those ICs for you, I can do that for a small fee.
In addition to the PCB, I offer a nice aluminum chassis, which has been custom designed for the HP-2. The chassis is a very nice option for DIYers who would like to showcase their builds in the form of a professional-looking product.

I am offering the HP-2 at pre-order prices until they arrive in stock or until the first ten HP-2s have sold (whichever occurs first). You can buy your HP-2 here: Neurochrome HP-2.

Thanks,

Tom

[poseidonsvoice]

Superb! Can’t wait to listen to it!

Best,
Anand.

[spikestabber]

Measures better than a "The Wire BAL-BAL", but given the transparency of the BAL-BAL already I doubt I would ever notice a difference with existing direct coupled HD800.

One area I would suggest in the next version is a true differential output, many who would spend that much for a HP amp would already have an existing 4 pin rig.

[tomchr]

You can build the HP-2 with the following outputs:

• 1/4" phone connector
• 3.5 mm phone connector
• 4-pin XLR connector

Naturally, fitting the amp with a 4-pin connector does not make the amp balanced, but it does interface nicely with phones that use such a connector.

There's only a minuscule benefit of using the 4-pin connector. You get ever so slightly better channel separation as you do not have the shared ground connection in the 4-pin connector.
Some will argue that a balanced amp will provide lower distortion due to the cancellation of harmonics. That's true in a high-distortion design. The cancellation doesn't work well for low-distortion amplifiers. In fact, any sort of cancellation scheme is much more likely to degrade the performance of a low-distortion amp than to improve it. Even in the cases where the THD is reduced by such cancellation, the cancellation mostly affects the even order harmonics – the ones most find pleasant sounding – while leaving the odd order harmonics (that many find to sound harsh) in place.

Tom

[RickTH]

No block diagram (yet) on your site, like the HP1 .
HP-2 – Neurochrome

How do you go from the 28Vdc to a + and - supply ?
It's LME49720 + 2 paralleled OPA1656 with an AC coupled output, so a capacitor at the output . mmm...
Output Impedance 2.6 Ω ? Pretty high...

[BrianL]

You would be speculating based on no data?

[tomchr]

Originally Posted by RickTH

No block diagram (yet) on your site, like the HP1 .

Correct. I need to proofread about half the verbiage as well. 24 hours/day one person kinda thing. I just got caught up after returning from BAF.

Originally Posted by RickTH

How do you go from the 28Vdc to a + and - supply ?

I don't. Hence, the output cap. I use nested feedback to eliminate the THD of the output cap. It works rather well.

Originally Posted by RickTH

Output Impedance 2.6 Ω ? Pretty high...

Meh... Does it matter? I agree that it won't look as good on Amir's ranking plots on Audio Science Review, but it will certainly work well (as my data show).

Tom

[tomchr]

And now... A block diagram.

I have one PCB left at the intro price.

Tom

[spiggs]

Would there be any issues using HP-2 as a preamp as well?

[tomchr]

That will be fine. If you use a switched output connector, you can just take the preamp output on the switched side. That way, the preamp out is turned off when you plug in the headphones.

Tom