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Neurochrome HP-1: Ultra-High End Headphone Amp

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I thought I'd start a thread dedicated to the new headphone amp design by Tom Christiansen (Neurochrome). This can be ordered pre-built, or as a partial-DIY project where Neurochrome supplies the PCB, chassis, and BOM list. I'd caught wind of this amp a while back, and preordered the PCB from Neurochrome before manufacturing had completed. Now that Tom has notified me that boards are shipping imminently, I thought I'd get this thread started.

Full specs for the Neurochrome HP-1 are available here. Tom seems to have set out to produce the ultimate "wire with gain" in the HP-1, delivering 3W/20Ω, 450mW/300Ω @ 0.000032% THD! He's using the OPA1612 differential receiver and LME49600 output stage in a loop with an OPA1611.

I've never heard this headphone amp, so am taking a flier on it based solely on my admiration for Tom's circuit design skills. It has also been quite a long while since I've done any SMD soldering, so it'll be a fun project. I'll post some pics of the build as I go and listening impressions at project completion. Meanwhile, my box o' Mouser BOM parts is ready to go!

BTW, this proto unit built by Tom is also going to be making the rounds in the US among some hobbyists. And it has been auditioned up in Canada at a local Head-Fi meet, from what I understand:

An externally hosted image should be here but it was not working when we last tested it.
 
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Fantastic! Thank you for starting a build thread. I appreciate it. I'm looking forward to watching your progress.

The board and chassis bits will be underway by Monday and should reach you before Christmas. What you're getting is actually a bit of a "one of a kind" kinda thing. Well. The second of a kind. :) You'll get the latest board revision, Rev. 1.1 with the black chassis. Future chassis will be blue (RAL5010 Genetian Blue to be specific). The texture will be the same.

As you point out, the goal of the HP-1 was to go directly for the high end. 3 W into 20 Ω, 1.0 uV noise floor, sub 1 ppm level THD. Added protection circuits to protect your headphones.

The HP-1 uses the OPA1612 for the input buffer, OPA1611 + LME49600 (with appropriate compensation) for the output drive, and an OPA2140 for the DC servo. All signal switching is done by Kemet relays with gold plated switch contacts.

There's a bit of chatter about the HP-1 on SBAF. It seems it's been discovered on Head-Fi as well, with a listening impression here: Amps that can drive the HiFiMan HE-6.

So far, I have demoed the HP-1 at the Calgary Head-Fi meet back in October and about two weeks later at a local hifi store, The Audio Room, which hosted a Focal headphone event. Two reps from Focal were there and were quite impressed with my amp. It performed really really well with the Focal Utopia. Man... The amp then went to Winnipeg where it was demoed at a Head-Fi meet there. The first prototype is currently underway to Washington DC and will be on a demo tour (arranged through SBAF) across the country until it lands with its final owner in California.

So far, I've heard it with:
  • Focal Utopia
  • Focal Elear
  • Sennheiser HD-650
  • Sennheiser HD-600
  • Sennheiser HD-580
  • HIFIMAN HE6
  • HIFIMAN HE1000
  • HIFIMAN HE560
  • Etymotic ER-4 (IEM)
I'm liking it!

Tom
 
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The FFT plots showing the 1 kHz THD show a bunch of harmonics at multiples of 30 Hz. That is due to a ground issue in the precision oscillator I'm using (Victor's oscillator).

The best graph to see the residual mains hum under load is the multi-tone IMD graph. It shows the tallest mains harmonic (300 Hz) at -120 dBV.

The residual mains hum plot showing < -136 dBV was done with the inputs shorted, output loaded with 300 Ω.

I'm not seeing the -137 dB (no reference) you mention. All the dBs say dBV. What am I missing? You're looking at this page (HP-1: Ultra-High End Headphone Amplifier), right?

Tom
 
Just ignore my ramblings, must be too tired. Sorry. Are those two peaks at -95 dBV and -100 dBV around 60 Hz on the 200 mW into 300 ohms and 32 ohms graphs the oscillator or real mains hum? they seem to be distinct from the 30 Hz peak train.
 
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The OPA1611 is better suited to a composite headphone amp than the OPA1622. The '11 is lower noise and higher GBP. The OPA1688 is even noisier than the OPA1622.

The OPA1622 is in a SON package. No SOIC or DIP version is available. I can't reliably hand-solder the SON package.

So to answer your question: Yes. I did look at the OPA1622 before committing to the OPA1611/12. I wasn't familiar with the OPA1688 at the time, but seeing it now doesn't change anything for me.

Tom
 
The OPA1611 is better suited to a composite headphone amp than the OPA1622. The '11 is lower noise and higher GBP. The OPA1688 is even noisier than the OPA1622.

The OPA1622 is in a SON package. No SOIC or DIP version is available. I can't reliably hand-solder the SON package.

So to answer your question: Yes. I did look at the OPA1622 before committing to the OPA1611/12. I wasn't familiar with the OPA1688 at the time, but seeing it now doesn't change anything for me.

Tom

OPA1611/12 is definitely the better choice for a composite solution.

We developed the OPA1622 to be a single chip high-fidelity headphone solution capable of delivering enough output power for most applications. A major portion of our effort was put into reducing output stage distortion, therefore significantly less loop gain was needed in the OPA1622 to achieve very low overall distortion. Using less loop gain also had the benefits of improving power consumption and stability with capacitive loads.

However, if you're adding your own output stage externally (as is the case here) you're not going to benefit from all that work we did on the OPA1622's output stage. And there will be less loop gain to improve the distortion of the external output stage.

Great design! If you're not constraint by PCB area and power consumption you can certainly make some really cool things :)
 
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OPA1611/12 is definitely the better choice for a composite solution.

Just beware of the phase wobble from the internal compensation in those. Just slapping an LME49600 and OPA1611 together will result in an amp that will oscillate with common headphone cable capacitances. Also, depending on the closed loop gain, using the OPA1611 as a standalone amp may benefit from a little cap across the feedback resistor to tame the transient response for the same reason.

Thankfully all this simulates, so those paying attention will catch those things early in the design phase.

They're indeed nice amps.

Great design! If you're not constraint by PCB area and power consumption you can certainly make some really cool things :)

Thank you.

Tom
 
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Hi. Seem like an interesting DIY project. Can you elaborate a little on the total cost for a DIY built? From the website I can see:
-Bare PCB + RCA + button = $250
-Enclosure = 250$
Total = 500$ without the parts (Is it in U$?)

Do you have an estimate of the BOM total cost? I can see there are a few expensive op-amp and both AC-DC Power modules are 17$ each. Or can we get the BOM so we can estimate the total cost?

Thanks
SB
 
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