[Armaegis]

Neurochrome HP-1
 

 
 
Full product info, specs, and measurements:
http://www.neurochrome.com/hp-1-ultra-high-end-headphone-amp/
 
 
Introduction
 
So I've had on my desk these past couple weeks a lovely little amp built in Canada by a one-man show under the brand of Neurochrome. Fairly well known in the DIY world for his fantastically well measuring power amp and preamp modules, this is designer Tom Christiansen's take on a headphone amp design. It frankly doesn't look like much at first glance, but man oh man does it sing.
 
Ok so let's talk about Tom for a moment, but to draw comparison I'm going to talk about myself first (how narcissistic!). My background is in metallurgical engineering, and I've got a whole bunch of letters after my name to show for it. I walk into threads where people are talking about wires and single crystals and grain boundaries and all that hooplah, and to me it's like watching kids talk about fractions. Sure, a picture of that copper crystal structure is worth a thousand words, but could you actually write a thousand word dissertation on it and understand what you were saying?
 
So now back to Tom. He's an electrical engineer. Ok fine, that in of itself doesn't mean anything. How about the fact that he worked at Texas Instruments and designed opamps? In his own words: "Precision is my life." That was his job. He's not some amateur enthusiast who managed to run some simulations in SPICE and designed a pretty pcb. Speaking as a former rocket scientist, let me tell you that what Tom does with circuits is equivalently the same. Sure, the circuit is a composite amp topology with an OPA1611 and LME49600, whopdee doo; back in the day I built an OPA627+BUF634 with multiloop feedback and you know what? I knew just enough to put the parts together but not enough to make a truly high performing amp. I mean, I read at least a dozen good webpages on what were the fanciest parts, how to design a power supply, I could quote Walt Jung on some of the fiddlier bits of opamp theory... but at the end of the day I was just some guy mucking around with parts on a breadboard for poops and giggles. I did not have the tools nor the expertise to truly know what I was doing, nor did I have the means to objectively test and evaluate my creations. I could make a pretty picture... but Tom is the man who can actually write the thousand words.
 
All this is just a roundabout way to tell you that Tom knows his stuff, and he has built something with monstrously good specifications and has the measurements to back him up. For me to attempt to explain his design would be a disservice, so just go check out his own words here: http://www.neurochrome.com/hp-1-ultra-high-end-headphone-amp/
 
The HP-1 is available both as a complete product or as PCB/chassis for those who wish to build it themselves. Do note however that it is predominantly SMD soldering, which is a different beast if you've only done through-hole work before.
 
 
Build/Design
 
Originally intended as a DIY project, the chassis itself is not some bespectacled jewel. As a fellow engineer I can appreciate the functional aesthetic, but others may desire something with more bling. That said, this is not some off-the-self Hammond case; the chassis are custom ordered and feel quite sturdy once assembled. The board is a complex 4-layer PCB and the entire layout is quite dense giving you a very compact amp.
 
On the matter of the guiding design principles behind the amp, I was originally going to write up some stuff but then decided why reinvent the wheel when I can simply copy Tom verbatim either from emails or posting on other forums. So here we go...
 
 
On the overall design topology:

The HP-1 uses the OPA1612 for the differential receiver and input buffer. Input selection and gain selection is accomplished by Kemet relays with gold plated switch contacts. The headphone output stage is an LME49600 in a composite loop with an OPA1611. The composite amp has a built-in compensation network which ensures stellar performance even with heavy capacitive loads (long headphone cables). It'll drive up to 22 nF || 300 Ω. That's close to 1 km of headphone cable. That aught to do it... I have seen OPA1611+LME49600 amps from other vendors. I wonder if they noticed that it'll oscillate at load capacitances in the 100-500 pF range (common cable capacitance). You need that compensation circuit... To minimize DC on the output of the amp, an OPA2140 provides a third order DC servo for an output DC offset below 100 µV. I chose a 3rd order servo to ensure a fast settling time while not impacting the THD at 20 Hz. The servo settles in about 30 seconds. The HP-1 needs no further warm-up than that.

There are no capacitors in the signal path as the HP-1 is DC coupled. Should any part of the amp or any circuitry upstream of the amp fail resulting in DC on the output, the built-in protection circuit will disable the output. Also, in the event of power supply failure where one rail collapses, the other rail will be shut off. The final feature of the protection circuit is that it provides a turn-on delay, thus, eliminating turn-on and turn-off thumps in your headphones.

 
 
On the matter of why Tom choose to use a SMPS (switch mode power supply):

Switchers provide state of the art performance. They are available world wide. Because I use a switcher, I can offer an ultra-high end amp that will work on international mains voltages. You really should reverse the question: Why would anybody use a linear power supply? To provide sappy marketing copy? Nostalgia? Maybe reliability. Maybe... Switchers are pretty darn reliable.

With my DG300B I painted myself into a corner pretty badly. That design requires a lot of different voltages and I wanted to use just one (1) mains transformer. There is exactly one (1) transformer in the world that can power the DG300B. Guess how much it is to ship to Europe? So basically my sales are limited to North America. There are a few European builders of the circuit. They've either had a custom transformer made for the amp or eaten the shipping cost of the transformer from the US to the EU. I didn't want to go down that rabbit hole with the HP-1.

In summary: I chose an SMPS for reasons of performance, parts availability, and ease of use.
****
It's just that there aren't really any technical advantages of a linear supply. The switchers are less expensive too. There may be a marketing advantage of the linear supply. Some fear switchers - and for a good reason. There are horrible switching implementations out there. Mine is not one of them. I hope my measurements will put any doubt about the capabilities of switching supplies when in the right hands to bed once and for all. You can also see my measurements of the MiniDSP 4x10HD. They use an external switching power brick and a couple of switching power regulators inside the chassis. No issues there either.

 
 
On why he choose that particular potentiometer:

The RK097 is specified to ±3 dB (-40 -> 0 dB). Mine measures way better than that. The RK271 "Blue Velvet" pot is ±2 dB -60 - > 0 dB and ±3 dB -70 -> -60 dB. The RK271 is a slightly better pot, though I'm not sure it's meaningfully better. Unlike the RK271, the RK097 is commonly available in large quantities in 10 kΩ, audio taper. The RK271 is often only available in 100 kΩ (at least at Mouser). The 10 kΩ pot is already the dominant noise source in the HP-1, so I'm definitely not going with a 100 kΩ pot. I'd use a 2 kΩ pot if anybody stocked it.

There's another advantage of the RK097: The pins have little leaf springs on them so the pot snaps in place. That's very handy from an electromechanical standpoint as pots tend to be torqued loose from the PCB with years of use. That's much less likely to happen with the RK097 than with the RK271.

 
 
On single ended vs balanced:
(do note the HP-1 is a single-ended amp, the XLR4 output is simply provided as a convenience)

I see no advantage of differential drive of headphones. In theory, you should get cancellation of the even order harmonics (assuming the two channels of the differential circuit are perfectly matched). That'll lower THD, which would look good on the spec sheet, but leaves the odd order harmonics that tend to be favoured the least in listening tests. I'd also point out that I'm measuring -135 dB THD already. I'd be a bigger proponent of the harmonic cancellation if my circuit actually created any harmonics worth cancelling. Also note that most cancellation circuits look great on paper, but are mediocre if not downright detrimental to performance in reality. At least that's been my experience designing precision circuits for a living for over a decade.

I do see some advantage of using the 4-pin XLR over the 3-pin phone connector, however. That little bit of shared ground impedance in the 1/4" phone jack actually degrades the channel separation from 115 dB to 90ish dB. That's physics for you. There's no magic voodoo here. It's all Ohm's Law.

 
 
On discrete vs integrated circuits:

IC design has a lot in common with discrete design. The fundamental topologies used are identical in many cases. The main difference is that in IC design, the matching between devices can be stellar (if you make a good layout) but the absolute component tolerance is not as good. Thus, IC circuits depend on matching (among other things) to achieve good performance. In discrete design, the component matching is usually rather poor (compared with the matching you can get on an IC anyway), but the absolute tolerances are stellar. These days ±0.5 % resistors cost the same as ±1 % resistors and ±0.1 % resistors are only about $1 each. So discrete circuits rely on the absolute component values to a greater extent.

The stellar matching and the possibility of having high-impedance nodes on an IC without destroying the signal integrity makes it possible to design an amp with very few gain stages that has stellar linearity and an incredible amount of gain. In discrete circuits, the nodes are long and leaky, so you really want to avoid high-impedance nodes. On top, the devices are slow and have relatively low gain. Thus, you end up with many gain stages to get even above 60 dB of loop gain. Compensation becomes a challenge.

I have yet to design a discrete amp which outperforms an IC amp.

 
 
On SMD:

the exciting new parts are all SMD. We'll have to deal with it. The HP-1 mostly uses 0805 and larger parts, which are *huge* by modern SMD standards. There are a few (12-14) tiny diodes in SOD-323 package (aka SC-79 if I recall correctly). They're damn tiny. If you can deal with those, the rest of the board should be no biggie.
 
I kept it at 0805 for human solderability. The board is limited by the size of the connectors on the rear and by the length of the power supply bricks in the other direction. You wouldn't really save anything by going with smaller passives. Also note that some area is required to keep the LME49600s running cool. The board acts as a heat sink for those.

 
 
 
How It Sounds
 
Associated equipment:
The majority of my listening was done with my modded Hifiman HE-6. However, I did also spend considerable time with a Sennheiser HD650. Other headphones used were a modded Fostex T50rp, Nuforce HEM8, and a Sennheiser HD598. I also hosted a meet during my time with the HP-1, and tried it out with a Hifiman HE-X and 1Kv2, Audioquest Nighthawk, Oppo PM-2, Focal Utopia, Mr Speakers Ether C Flow, etc. Sources were predominantly a NuPrime uDSD and Schiit Modi Multibit, but others crossed my path. Other amps on the table were predominantly the Sjostrom QRV08 (another high end DIY offering), Wolf Ear Makoyi, and Bryston 2B; others included the Schiit Ragnarok, Chord Dave, etc. Also of note is that I typically left the volume pot at max on the HP-1 and controlled volume through my preamp/mixer/dac/etc.
 
 
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. Blah blah blah. Some might argue there's a dryness to the sound. I won't refute that, but I will counter with the thought experiment that if an amp sounds dry, what does wet mean? Snarf.
 
The bottom end is likewise very clean. At first I thought it was actually lacking in thump, but I was comparing back and forth with a very buttery tube amp at the time. Once I volume matched with tones and returned to music, I realized it wasn't a lack of thump but it was a lack of the extraneous stuff that the other amp was putting in.
 
There is a great clarity in the mids... wait did I already use that word? How about there is a superb precision to the... oh wait I already used that word too? Screw it, superlatives do not a review make.
 
When I was reviewing the HP-1, I had a couple other amps on the table. In a odd turn of events, during the flip flopping of amps, I felt myself "enjoying" others more, yet somehow the HP-1 became my reference point in switching back and forth. It was the neutral one. It became my reference point. Most usefully, it served as a palate cleaner to help clean up any of the aftertaste left behind by the other amps. It wasn't until I shipped the HP-1 back out that I realized how much I was using it to recalibrate my own ears. That should tell you something.
 
 
 
Comparisons vs Sjostrom QRV08
I spent a lot of time comparing these two directly as they are both purportedly world class DIY projects (although the HP-1 is offered as a completed project as well). Overall, they are both good performers but the HP-1 is easily a step above. The HP-1 runs at dead neutral, while the QRV08 has a bass tilt to the sound; it is a bit thicker in the bottom end and feels rather grey up top and lacking definition.
 
From my preliminary notes, this was my original "three point summary" between the amps:
QRV08 - taupe, lacks sparkle, good solid kick
HP-1 - mauve, super clarity, surgical
 
Perhaps the best mark of agility was when I was switching around dacs and added in a Schiit Modi Multibit. Using a preamp to feed multiple sources into both amps, all the sources sounded the same on the QRV08 but I could tell the difference with the HP-1.
 
Ultimately, the HP-1 resolves much better than the QRV08 which felt flat especially in the top end. A bit akin to taking a layer of thin damping paper away from the front of a driver. One very specific example, the song Royals by Lorde. There are constant snaps throughout the song, making it easier to pinpoint. In the background of each snap, you can hear it echo. Delve a little deeper into it and you'll hear it resonating with a metallic tint. With the Mimby and the HP-1 it was like I could start to hear/feel the shape of the resonator that was indistinguishable before.
 
 
Comparisons vs Cavalli Liquid Carbon
This will be brief as it was from memory, but I thought to mention it since these amps are roughly the same size. I did not care for the LC. It was a very *shrug* reaction as nothing really stood out, and there were background noise issues. I did not get an immediate sense of prowess or power like I did with the HP-1.
 
 
Comparison vs Bryston 2B
Only valid with the HE-6, and not many others will have this combo, but it is the one I am most familiar with. The Bryston carries much more body in the bass, but isn't as clean as the HP-1. That's a clear repeating theme for most of this review actually. Staging on the 2B was a little more closed in but rounded in feel, while the HP-1 was a bit wider but shallower. The HP-1 is a clear step above in resolving power, which makes me a bit sad as the Bryston is my primary amp and I really don't want a reason to start hunting for upgrades again.
 
 
 
Specific headphone pairings:
 
The one headphone that truly stood out (and hence the first half of this review's title) above the rest as a particularly good match was the venerable Sennheiser HD650. All the stereotypes about the Sennheiser slowness or the dreaded veil were completely obliterated here. There were times even after volume matching with other amps I had to doublecheck that the HP-1 wasn't louder, because the sharpness I was hearing was so much clearer than the other amps. It's like someone took the best crispness aspects of the AKGs and transplanted them in.
 
Now the real stress test was with my Hifiman HE-6. Surprisingly, I could get to quite adequate volumes before topping out. Certainly loud enough for most normal listening, but I'm one of those nuts that used to have speaker monoblocks for his HE-6 so I am not the right candidate here. That said, the sound was very strong and controlled, and if I were not married to my monstrous power amps and wanted something smaller that could play well with other headphones, the HP-1 would be a high contender for that spot.
 
 
Other headphones:
 
Nighthawk - I'd call the Nighthawk a cousin to the HD650. It played very well, though I'm not sure if the Nighthawk really scales as well and I feel it hits the ceiling well before the Sennheiser. In any event, the Nighthawk felt very quick and nimble on the HP-1. Not particularly body-full, which isn't bad as sometimes the Nighthawk gets a but flubby down below.
 
HE-1000v2 - This is a much easier matchup than the HE-6 and sounds excellent. That said, the HP-1 is like a magnifying lens and will shine a light on your headphone's particular traits. In the case of Hifiman, they have a certain treble presentation that might be a bit etched to some. If you like the Hifiman house sound or have mods applied (like I do), then go right ahead and enjoy the super clarity coming your way. If you're on the fence with Hifiman, I would say this is not the match for you.
 
HE-Xv2 - Personally, I think I might actually prefer the X over the 1000. The match up with the HP-1 was virtually the same in performance here.
 
Audeze LCD-2 - I've always felt the Audeze headphones were the "night" to the Hifiman "day". Lots and lots of thump that bordered on too much for me. Well here the HP-1 comes in and cleans it right up, maintaining that sense of body without becoming a beer belly.
 
Oppo PM-2 - Very lively and... alive? I'm repeating myself here. I actually quite like the Oppo family of headphones, and while they Oppo typically sound laid back the pairing here seemed to bring it up a notch on the energy meter.
 
Fostex T50rp variants - Okay okay I am deep into planar land, that's just how it is sometimes. The Fostex drivers play marvellously though. There is more than enough power here for them, and they sing through loud and clear.
 
Focal Utopia - I like the Utopia but don't love it (especially for the price) as there's something in the treble that feels a bit papery to me. As I mentioned before, the HP-1 tends to magnify the characteristic of the headphone, and for me personally this didn't mesh and I would want something with a warmer/slower characteristic to balance it out (like the QRV08 or maybe a Violectric). That said, if you love that Utopia sound and want to push it even further, this would certainly do it.
 
Mr Speakers Ether-C Flow - I'm not to crazy about this latest offering from Mr Speakers, but it sounded quite capable out of the HP-1 and I felt the transient edges were fast and clean.
 
Sennheiser HD800 - more of a bunt than a homerun for me; technically good, but all the stereotypes of the HD800 just get pushed moreso and it's polarizing
 
 
 
Conclusion
 
While I had it, I took it for granted. Now that it's gone, I miss it.
 
I had a lot of gear that all arrived simultaneously and was in a bit of a "new toys" flurry for a while, so the HP-1 unfortunately took a bit of a back seat to everything. Other gear had much more "fun" factor for better or for worse, but what was really notable was that I kept coming back to the HP-1 as a palette cleanser of sorts. I don't mean that in any sort of negative way. It's just that it was so clean and "in the middle" that it reset my ears before moving onto the next piece of equipment.
 
It was also a fantastic razor's edge, with enough precision to allow me to evaluate dacs which other amps could not do. Seriously. My favourite amp on the table (the Bryston 2B) does not have the resolution for me to easily pick out those minute differences, which is actually rather frustrating now.
 
Some might niggle on the price for something that looks so small, but the performance is big beyond it's size and it holds its own and more easily more against the behemoths. Some might think to themselves "oh, it's just a board and parts, why so expensive?"... well let me ask you: can you design and produce that tv in your living room for the parts cost? do you pay your mechanic for just parts and devalue his years of experience that do the work in a fraction of the time it'd take you? do you know how much more a complex 4-layer board costs compared to a simple 2-sided etch job? do you have any idea how much a custom chassis (even a simple looking one) will cost? a builder recently reported 30 hours to assemble; how much do you value your time?
 
 
TL/DR;
 
Yes I know my title had a bit of a clickbait ring to it, but I meant it philosophically (in a sense). This is a no holds barred objectively driven amp design, with no compromises for the sake of pedantry, from a guy who has the knowledge and testing ability to back it all up to an extreme degree.
 
 
 
 
 
 
 

 

[Armaegis]

 
Pictures
 
Nope.
 
Sorry, I guess I'm just not a good enough photographer these days to be a legit audio reviewer. What does that tell you about the market?
 
 
 
 
Other amp measurements by Neurochrome:
 
http://www.neurochrome.com/sjostrom-qrv08-headphone-amp/
 
http://www.neurochrome.com/o2-headphone-amp/
I copied some test measurement comparisons into this post: http://www.head-fi.org/t/830987/review-neurochrome-hp-1-making-sweet-love-to-your-hd650-or-what-the-o2-wishes-it-could-be/30#post_13209005

 

[grizzlybeast]

   
Pictures
 
Nope.
 
Sorry, I guess I'm just not a good enough photographer these days to be a legit audio reviewer. What does that tell you about the market?
 

Nothing. Pictures are necessary to some. At least grabbing one from the site (of the unit competed in the chassis) doesn't hurt. 
 
but the review was informative. Thanks for sharing. 

 

[Armaegis]

I figure if people want to see pics, they can just see the ones on Tom's site. The one I chose for the top of the review is the most interesting one to me. All the rest are terribly unflattering. I tried taking a few and figure "meh, these look like crap". I like the amp and would rather not hurt it's value with my subpar camera skills.
 
And really, my statement was more a snark at how I see many modern reviews these days. Lots of pretty pictures, not even a thousand words. It's kinda up there with unboxing videos so people can... vicariously experience the thrill of opening up new toys?

 

[grizzlybeast]

no doubt.... I get you!

---

 
 
The way your review reads is like it is comparable to the Jotunheim in a lot of ways and my Cayin as well. Cant wait to hear it!

 

[swannie007]

Great review, thank you. Like you, I have never understood the hype about the fancy photographs and unboxing BS. Many reviewers spend more time describing the unboxing experience than the actual product on test/review. Go figure, I must be getting old. Perhaps remaining objective and on task are old world values?
It was a pleasure to read an objective and well informed opinion of a piece of gear on review. Bravo. Hope to read more of your reviews in the future. Cheers. 

 

[Armaegis]

I'm glad you enjoyed the review. The hard nosed engineer in me tries to be reasonably straight forward and not beat around the bush as it were. Introduce the product, describe functionality, describe impressions and comparisons with (similar if possible) equipment, closing remarks, maybe a bad joke or two, done. I've done a fair bit of reviewing over the years, and I think I'm developing a progressively more grumpy-ish tone as I go.

 

[swannie007]

Grumpiness, or more accurately, an intolerance for BS, is an age onset condition I believe. I have written numerous technical how-to articles and appreciate your writing style and the importance of staying on task when describing something of a technical nature to others. Cheers.

 

[mindbomb]

This amp looks really good to me. LME49600 is a good buffer imo. I like the xlr output, which is not just for ideal crosstalk but also less contact resistance iirc. I like that he put an emphasis on stability. The measurements look ridiculously good. I wish there was a 1x gain setting for more sensitive headphones. 

 

[ruhenheiM]

informative and not boring. i enjoy read it very much. thanks
 
quick question if you don't mind. do you think it will pair great with grado ps1000e? and since you pair it with he-6, how about the power? i have pioneer se 700 and toshiba hr 810. i don't know which ones more power hungry though.but do you think it will drive them okay? thanks

 

[Armaegis]

  This amp looks really good to me. LME49600 is a good buffer imo. I like the xlr output, which is not just for ideal crosstalk but also less contact resistance iirc. I like that he put an emphasis on stability. The measurements look ridiculously good. I wish there was a 1x gain setting for more sensitive headphones. 

 
I'm not sure if modding is possible, but you can always send an email to Tom and see if it's possible to lower the gain. The pot tracks quite well though, and I was able to use iems on low gain without trouble.
 
 

  informative and not boring. i enjoy read it very much. thanks
 
quick question if you don't mind. do you think it will pair great with grado ps1000e? and since you pair it with he-6, how about the power? i have pioneer se 700 and toshiba hr 810. i don't know which ones more power hungry though.but do you think it will drive them okay? thanks

 
I unfortunately did not get to try the HP-1 with any Grados. I'm admittedly not a fan of the Grado sound, but as I mentioned before with other pairings, the HP-1 is a bit like a microscope that brings out the inherent qualities of the headphone used. If you want "more" of what the ps1000e offers, I'm sure you'll get it.
 
With the HE-6, I actually still kept it on low gain and controlled volume through an external preamp (with the pot maxed out). I had no issues with power and could get to good listening levels. Maybe not for head blasting volumes, but for average home listening it was more than enough.

 

[eugenius]

Come on, you make the best measuring amplifier out there and you stick a small analogue pot on it? Give this guy one of those new Muses chip volume controls.
 
http://www.njr.com/MUSES/series/MUSES72320.html

 

[sq225917]

Could be because the analogue pot gives a lower noise contribution.

 

[tomchr]

+1 on the unboxing videos and endless verbiage of how excited the "reviewer" was while waiting for the delivery truck to arrive. Humans are funny critters. Very emotional when it comes to it.
 
Quote:

  This amp looks really good to me. LME49600 is a good buffer imo. I like the xlr output, which is not just for ideal crosstalk but also less contact resistance iirc. I like that he put an emphasis on stability. The measurements look ridiculously good. I wish there was a 1x gain setting for more sensitive headphones. 

 
Thank you. Yeah. Stability is definitely important. There are a few LME49600 + OPA1611 amps out there. I do wonder if the designers have noted the phase wobble in the open loop response of the OPA1611 due to its internal compensation. That will cause the amp to oscillate at common cable load capacitances. You won't notice unless you have a high speed oscilloscope or RF spectrum analyzer as the oscillation is in the 100-200 MHz range. I had a lot of fun with my 400 MHz scope and 2.9 GHz spectrum analyzer during the design process.
 
2x is the lowest gain possible with the LME49600 + OPA1611 combo. 
 

  Come on, you make the best measuring amplifier out there and you stick a small analogue pot on it? Give this guy one of those new Muses chip volume controls.
 
http://www.njr.com/MUSES/series/MUSES72320.html

 
The MUSE chips are indeed quite nice, though in typical JRC fashion, the spec sheet does not contain very much information. The low noise floor seems impressive at first, however, as it is A-weighted it leaves me wondering how much 1/f noise this chip contributes. Also note that the THD, while good compared to many other volume control chips, is not that stellar. In particular the 0.001 % THD at 10 kHz (unknown measurement bandwidth) is not that impressive. Some of the solutions offered by Crystal Semiconductors (now part of Cirrus Logic) would be better candidates in my opinion.
 
The notion that a small pot is somehow inferior to a large pot is one of those "emotional human critters" things. The "gold standard" audio pot is the ALPS RK271 "blue velvet". There is much folklore surrounding that pot, and it is a good pot, but all it is is a good quality carbon track pot. Its main selling point is the good (±2 dB) tracking between channels. Its main drawbacks are size, cost, and availability. The RK097 uses a metal-ceramic track and offers ±3 dB tracking (slightly worse than the "blue velvet"). In exchange for the slightly worse tracking, it offers lower cost, better resistive material, better availability in low resistances (important for noise), and also better mechanical construction. The RK097 has leaf springs on the pins, thus, lock into the circuit board. This prevents the solder joints from failing over years of use.
 
I have measured both the RK271 and the RK097. They're both acoustically transparent. 
 
A note on pot tracking: An audio taper pot (logarithmic taper) is actually constructed from several linear segments. The tracking along each of the linear segments is usually stellar. I measure around ±0.1 dB or better on the RK097 and a little worse on the RK271 typically. The ±2 - ±3 dB tracking error on the spec sheet is only occurring at the transitions from one linear track to the other. So if you plot the tracking error versus the rotation of the pot, you'll find 2-3 bumps where the tracking error may reach ±2-3 dB within a narrow range of rotation and stellar tracking away from those bumps. The first bump down from the max volume setting is typically in the -40 to -60 dB attenuation range. There's another bump usually around -80 dB. You can deduce the locations of the bumps from the spec sheet (assuming it's a good spec sheet like the ALPS ones). If you turn the volume control a few degrees either way, you'll get away from the bumps. I'm not aware of anyone who's noticed this in actual use, except maybe in a high-gain amp.
 
Tom

 

[tomchr]

Could be because the analogue pot gives a lower noise contribution.

Bingo! Lower THD too.
 
Tom