[jh4db536]

The bias on my b22 i believe is set too high given my heatsinking (which isn't that ineffective). IMO the optimal bias is the one that doesn't cross back into AB with the headphones used and doesn't allow mosfets to overheat. There's a point of diminishing marginal returns and i probably went past it.
 
And now i present to you, the finished Netgear Amp...jking.
 

 
 

 

[tomchr]

NetGear Amp. <snort> Love it!
 
Tom

 

[Armaegis]

Oh man, we just need some sort of bluetooth antenna dongle...

 

[tomchr]

Yeah, baby. Or some sort of mean looking MIMO antenna array.
 
Tom

 

[bluemonkeyflyer]

Neurochrome HP-1 Headphone Amplifier
Build: February 2017
BMF Flyer
Pictures, below...
 
Prelude
 

I like a lot of different amps. I enjoy building DIY amps and it’s fun “rolling” amps, so to speak…solid state, all tube, and hybrids. So, which one sounds best? Well, for me it depends… on the day, the music genre, the mood, the quality of the recording, and the headphones to mention some of the variables involved in producing “sound quality” and audio pleasure.

 

What one person hears and describes may be heard and described very differently by others. Objective measurements can help but don’t guarantee you’ll like what you hear. It comes down to opinions, personal preferences and simply, “what sounds good, sounds good.”

 

I had the opportunity to participate in S**F’s Neurochrome HP-1 tour. Before my week with HP-1 was up, I knew I wanted to build one. I ordered the PCB, custom enclosure, volume knob, and RCA jacks from Tom Christiansen, an electrical engineer and designer of HP-1. The HP-1 components arrived within a few days of receiving the BOM from mouser.com I had ample time to sort and label all parts corresponding to Tom’s excellent build guide and BOM. I completed my HP-1 build over the course of two weekends. I really took my time and proceeded carefully. I double-checked each component against the BOM and schematic and measured each and every one both before and after soldering them in place.  Patience and attention to details paid off…when I flicked the On switch, the soft blue LED lit up…and there was no smoke! I ran it through the setup and testing procedure recommended by Tom. The DMM showed 200 mW RMS and 0.00 VDC offset on the outputs. OK, time to take HP-1 for a test drive.

 

 

Gear Chain

 

Lossless and FLAC files >  iMac 7 > OPPO HA-1 DAC > HP-1 > HD-650, HD-600, AKG 7xx, Beyer 990, OPPO PM-1 and PM3, Yamaha YHD-1

 

 

“A Few” Eclectic Test Tunes I know very well put amps through their paces requiring complex bass, sweet acoustic/vocal mids, and treble details:

 

Anna Maria Jopek and Pat Metheny – Are You Going With Me?

Beck – Blackbird Chain

Bill Evans – Piece Peace

Cake – I Will Survive

Charlie Hunter Trio – Wizard Sleeve

Donald Fagen – Maxine

Gorillaz – Dare

John Mayer – Slow Dancing in a Burning Room

Jon McConnell – Sober

kd lang – Save Me

Smetana – Ma Vlast mvt II

Shostakovich Symphony No 5 mvt iv

Lucinda Williams – Essence

Mana – Mariposa Traicionera

Marcus Miller – Redemption

Metallica – Whiskey in the Jar

Miles Davis – So What

Muse – Hysteria

Norah Jones – I Wouldn’t Need You

Patricia Barber – Too Rich for my Blood

Perlman – Bach Double Violin Concerto

Punch Brothers – Missy

Radiohead – Jigsaw Falling Into Place

Steely Dan – Babylon Sisters

The Tao of Groove – Multatica Mia

Vlatko Stefanovski & Miroslav Tadic – Kajesh Anjo

Willie Nelson – Vous et Moi

Wynton Marsalis – The Magic Hour

Yes – Long Distance Runaround

Yo-Yo Ma – Quarter Chicken Dark

 

 

Impressions

 

The background is Black and dead quiet. There’s no hum, buzz, or other noises when not playing music, and none during silent passages of songs at any volume pot setting.

 

I agree with Tom, Armaegis, fishski13, and others that HP-1 is “a straight wire with gain.” Parsimony Rules: HP-1 delivers a flat FR. No part of the FR is emphasized or cheated.  HP-1 is balanced from bass, through the midrange, and into the higher frequencies.

 

Bass is fast, textured, and well extended. I really like how HP-1 “does” acoustic double bass harmonic layering and decay.  Kick drums have punch. Tympanis have authority.

 

The midrange is fantastic and neither forward nor recessed. Vocals and acoustic instruments sound beautifully rich while remaining clear.  I love HP-1’s rendition of jazz trumpet, tenor sax, and female vocals.   

 

HP-1’s treble extends into the upper frequencies providing accurate micro details, “air,” and shimmer without any sibilance or pierce. Harmonics from chimes, triangles, and cymbals add sparkle and nuance to the sound quality. Hearing female singers inhale between phrases and the delicate sound of exhaling on the extended final syllable of the last word enhances the emotional impact of the song.

 

The width of soundstage and instrument placement is in the Goldilocks Zone…not too narrow, not too wide…just right.

 

 

Conclusions

 

Previously, I thought there was no discernable difference between HP-1 and QRV-08. After spending a good deal of time with them head-to-head using a Quick Switch adapter and SPL matching, I have concluded that there is a noticeable difference. HP-1 is rather more resolving, cleaner, and more enjoyable. I can voice this opinion with some authority given I’ve built 3 QRV-08’s And I really do like QRV-08…it’s no slouch to my ears and I like it a lot! It’s just that HP-1 ups the ante. HP-1 also competes with other amps I’ve auditioned, own, and/or have built including, in no particular order:

GS-X mkII

Liquid Carbon

OPPO HA-1

EHHA Rev A

CK2III

Bijou

Kameleon v2

Single Power MPX3

Some other TOTL amps

 

 

Finale

 

I had a lot of fun building HP-1. I’m having more fun listening with HP-1. Many thanks to Tom for creating HP-1, as well as a bit of technical assistance…he’s “on it!”

 
 
 
 

 

[tomchr]

Thank you for taking the time to write a thorough review. I really appreciate it. I'm glad the HP-1 came together nicely for you.
 
Thanks for your kind words.
 
Tom

 

[Armaegis]

A nice clean looking build there BMF! How many hours did it roughly take you to assemble?

 

[bluemonkeyflyer]

A nice clean looking build there BMF! How many hours did it roughly take you to assemble?

25 hours. I used my DMM to verify every component was the correct value and within spec before soldering. I also tested after soldering for resistance and capacitance where possible and continuity with other connected bits. There are over 700 solder joints, IIRC, so testing took longer than soldering. Hard lessons learned from other builds convinced me it's better to go slow and measure on the front end and fix problems as they occur. I've spent more time trying to track down problems after a build was finished than the build itself. HP-1 worked immediately.

 

[dan_can]

Are balanced inputs/outputs for convenience only?

 

[Armaegis]

Balanced input is a proper differential receiver. Balanced output is for convenience.

 

[tomchr]

The main advantage of balanced inputs is that a proper balanced connection moves any ground loop on the input out of the signal path. The result is much, much lower hum and fuzz. The differential input also rejects any interference imposed equally on the two signal wires. So there's definitely a technical advantage there.
 
The 4-pin output allows the ground connection for the each headphone driver to be kept isolated. This results in better channel separation (about 20 dB better when measured on the XLR vs the 1/4" phone connector). That's simply because of the shared ground impedance in the 1/4" connector. While the two measure about 20 dB different, the channel separation is so good anyway that this difference is mostly of academic interest. It is not likely to cause any difference in perceived sound quality. It is quantifiable, though...
 
Tom

 

[Armaegis]

Still waiting on you to make us a balanced amp Tom! Obviously it's just as simple as doubling up the parts and bridging the outputs right?
p.s. and don't forget the VU meters

 

[tomchr]

Exactly! You'll have to wait a while, by the way. I'm not seeing any technical advantages of a differential/balanced amplifier design. Sure, in theory, you'll get cancellation of the even order harmonics. In reality, cancellation schemes only work so well and you're usually better off not generating the harmonics in the first place (which is the approach I took in the HP-1). Also, the even order harmonics are the ones that many people believe sound good, so why would you remove those, leaving only the harsher sounding odd order harmonics behind. /me not understand.
Thus, I suspect the "balanced amplifier" is mostly a feel-good marketing thing. 
 
Balanced inputs, yes. Balanced from input to output ... meh, not so much.
 
VU meters... Wouldn't that be funny.
 
Tom

 

[Rossliew]

Sounds like an interesting amp and possibly a good alternative to the pricier gsx MK ii.

Is it better at driving low or high impedance cans?

 

[tomchr]

Any amp will have an easier time driving a higher impedance. That's the case for the HP-1 as well. That said, you need to get all the way down to 32 Ω before the THD+N vs frequency creeps above the noise floor of the Audio Precision APx525. Even then, it's only towards the high end (>5 kHz) that I see the THD+N emerge from the noise floor.
I'd recommend the HP-1 for driving headphones with an impedance of 32 Ω and above.
 
Tom