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INTRODUCTION
When I first joined Head-Fi, I can remember that portable amps were all the rage – whether you needed one or not. And very popular amongst the amp market were CMOYs – which essentially were battery powered portable amps in an Altoids tin or similar. Strangely enough I never got to try one in the last 7 years, although I was always curious. Then earlier in 2017, Head-fier xrk971 from XRKAudio contacted me and asked me if I’d like to take his amp design for a spin. I said sure, and in late November the XRK-NHB (no holds barred) A Class portable amp arrived. Since then I’ve been putting it through its paces with a variety of different headphones. Those who follow me will know that I avoid using amplifiers unless they are truly justified (the headphones require the additional power), so what would I make of this portable Class A amp?
ABOUT XRKAudio
XRK Audio was a completely new contact for me – and one which came out of the blue. The company started as essentially a DIY project with its founder (xrk971) simply wanting to build himself a DIY portable amp as a fun project. After experimenting with the design, he allowed a few people to listen to it, and was pleasantly surprised with the overwhelmingly positive feedback. The common response was “you should sell this” – so he set up a storefront (XRK Audio) on DIYAudio and the company was born.
In talking to others, I’ve found that xrk971 is very responsive to feedback, and will also work individually with his customers to customise the amp to their needs – including changing components to meet needs for a specific use.
XRK Audio has a basic model A Class amp, an upgraded NHB model, and is currently also working on a desktop amp.
Further resources:
Head-Fi thread : here
XRK Audio Store : here
Discussion at DIY Audio : here
DISCLAIMER
The XRK NHB Portable A Class Amp that I’m reviewing today was provided to me gratis as a review sample. As I always do, I offered to return the amp on completion of the review, but I've been asked to hang onto it for follow uo questions or comparisons. So I acknowledge now that the XRK NHB I have is supplied and gifted completely free of any charge or obligation. I have now had the amp for approximately 5 weeks. The retail price at time of review is ~USD 199 for the standard model and ~277 for the upgraded model which I’m reviewing today.
PREAMBLE - 'ABOUT ME'. (or a base-line for interpreting my thoughts and bias)
I'm a 50 year old music lover. I don't say audiophile – I just love my music. Over the last couple of years, I have slowly changed from cheaper listening set-ups to my current set-up. I vary my listening from portables (mostly now from the FiiO X5iii, X7ii and iPhone SE) to my desk-top's set-up (PC > USB > iFi iDSD). My main full sized headphones at the time of writing are the Sennheiser HD800S, Sennheiser HD600 & HD630VB, MS Pro and AKG K553. Most of my portable listening is done with IEMs, and lately it has mainly been with the Jays q-Jays, Alclair Curve2, and LZ Big Dipper. A full list of the gear I have owned (past and present – although needs updating) is listed in my Head-Fi profile.
I have very eclectic music tastes listening to a variety from classical/opera and jazz, to grunge and general rock. I listen to a lot of blues, jazz, folk music, classic rock, indie and alternative rock. I am particularly fond of female vocals. I generally tend toward cans that are relatively neutral/balanced, but I do have a fondness for clarity, and suspect I might have slight ‘treble-head’ preferences. I am not treble sensitive (at all), and in the past have really enjoyed headphones like the K701, SR325i, and of course the T1 and DT880. I have a specific sensitivity to the 2-3 kHz frequency area (most humans do) but my sensitivity is particularly strong, and I tend to like a relatively flat mid-range with slight elevation in the upper-mids around this area.
I have extensively tested myself (ABX) and I find aac256 or higher to be completely transparent. I do use exclusively red-book 16/44.1 if space is not an issue. All of my music is legally purchased (mostly CD – the rest FLAC purchased on-line). I tend to be sceptical about audiophile ‘claims’, don’t generally believe in burn-in, have never heard a difference with different cables (unless impedance related etc), and would rather test myself blind on perceived differences. I am not a ‘golden eared listener’. I suffer from mild tinnitus, and at 50, my hearing is less than perfect (it only extends to around 14 kHz nowadays). My usual listening level is around 65-75 dB.
I have very eclectic music tastes listening to a variety from classical/opera and jazz, to grunge and general rock. I listen to a lot of blues, jazz, folk music, classic rock, indie and alternative rock. I am particularly fond of female vocals. I generally tend toward cans that are relatively neutral/balanced, but I do have a fondness for clarity, and suspect I might have slight ‘treble-head’ preferences. I am not treble sensitive (at all), and in the past have really enjoyed headphones like the K701, SR325i, and of course the T1 and DT880. I have a specific sensitivity to the 2-3 kHz frequency area (most humans do) but my sensitivity is particularly strong, and I tend to like a relatively flat mid-range with slight elevation in the upper-mids around this area.
I have extensively tested myself (ABX) and I find aac256 or higher to be completely transparent. I do use exclusively red-book 16/44.1 if space is not an issue. All of my music is legally purchased (mostly CD – the rest FLAC purchased on-line). I tend to be sceptical about audiophile ‘claims’, don’t generally believe in burn-in, have never heard a difference with different cables (unless impedance related etc), and would rather test myself blind on perceived differences. I am not a ‘golden eared listener’. I suffer from mild tinnitus, and at 50, my hearing is less than perfect (it only extends to around 14 kHz nowadays). My usual listening level is around 65-75 dB.
This is a purely subjective review - my gear, my ears, and my experience. Please take it all with a grain of salt - especially if it does not match your own experience.
WHAT I LOOK FOR IN A PORTABLE AMP
I usually list (before I start with the review) what I would look for in a portable amp. This is useful to remember when looking at my scoring later in the review.
- Genuine portability
- Good battery life
- Clean, neutral signature
- Easy to use
- Low output impedance
- Reasonable output power – should be able to drive IEMs and earphones up to 300 ohms
- Good gain control
- Hardware EQ if possible
- Value for money
- Free of EMI (use with smartphones)
PACKAGING AND ACCESSORIES
The XRK-NHB arrived in a standard but well padded courier pack. Included was the amplifier in a plan thin steel “mint” tin, and accompanied by a good quality 1m 3.5-3.5mm interconnect.
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| The XRK-NHB | XRK-NHB + cable |
TECHNICAL SPECIFICATIONS
These were graciously supplied by xrk971 on request.
| SPECIFICATION | XRK-NHB Class A Amplifier |
| Approx price USD | ~ USD 199 STD or 277 for the NHB |
| Amplification | BF862 JFET (input), ZVN4306 MOSFET |
| Output Impedance H/O | ~ 3.9 ohm |
| Max Output Power @ 32 ohm | 34 mW RMS, 3.0Vpp at 0.46% THD |
| Max Output Power @ 50 ohm | 50 mW RMS, at < 0.5% THD |
| Max Output Power @ 250 ohm | 50 mW RMS, at < 0.5% THD |
| Max Output Power @ 300 ohm | 54 mW RMS, 11.4Vpp at 0.47% THD |
| SNR | ~ 96 dB with THD at 0.029% (@ 1 kHz with 270 ohm load) |
| Stereo Crosstalk | Better than -72dB |
| Frequency Response | Flat response from 15Hz to 40kHz (-0.5dB) |
| Dimensions | 60 x 95 x 20mm (just the tin) |
| Battery Capacity / Life | 600 mAh 9v Li-ion/ ~ 4-5 hours |
The XRK-NHB is essentially similar size and exterior appearance as the old classic Cmoy type DIY amp. The steel tin is just your basic low cost container (I've seen some as Altoids tins – mine is just plain thin plate steel), and it belies the actual components within. The tin has a hinged lid rounded corners, and is a it bulky compared to many of the slim-line portable amplifiers now on the market.
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| The XRK-NHB in profile | Input, pot, LED and output |
And thats pretty much it.
Internally is where the magic happens though. The standard amp features a custom designed double sided SMT/through-hole PCB with top-quality components including:
- Panasonic OSCON solid organic polymer ultra-low ESR caps
- Nichicon AK audio grade caps
- WIMA audio grade film caps
- Switchcraft jacks
- Alps RK09 potentiometer
- Zetex power MOSFETs
- NXP JFETs
- 1% SMT resistors
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| Inside the tin | Battery connectors |
- Upgraded input caps increased to 10uF + 100nF 100v MKT for deeper bass extension
- Upgraded output caps increased to Nichicon AK series 470uF 16v + 2.2uF Wima MKS for deeper bass extension
- FETS hand-matched to 4 significant figures for superb channel balance and stereo imaging and improved sound stage
- Hand-tuned harmonic profile and FET bias set points to achieve -66dB H2 and -96dB H3
- Frequency response per LTSpice simulation is now 5.8Hz to 20MHz (-0.2dB point)
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| Circuit board | Components |
HEAT AND POWER
The XRK-NHB is class A by design, and as a result it can get mildly warm when in use. After half an hour's use with the X7ii as source, and the HD800S as transducer, the temperature registered at just under 39° celcius on the hottest part of the amp. Definitely on the physically warm side, but nothing that is going to cause discomfort.
In terms of power output, the specs say that it'll put the following output into these loads:
- Into 32 ohms = 34 mW RMS
- Into 300 ohms = 54 mW RMS
IEMs / Earbuds
First up was the Alclair Curve (22 ohm impedance, 111 dB/mW sensitivity), and just shy of 3pm on the pot (or about 10% of the total pot) was able to easily get me my preferred ave 70-75 dB volume level. The sound was very clean and clear, with a beautifully subtle warmth to the overall tonality. Even thought the pot was quite low, I could detect no sign of channel imbalance.
The second test was with VE's brilliant Zen2 open ear-buds (320ohm impedance, 108 dB/mW sensitivity), and this time I needed just over 3 pm on the pot (again not much more than 10-15%) to achieve the 70-75dB (this was quite hard to measure with the SPL meter – so take with a grain of salt). Again the Zen2 sounded beautifully driven with a rich smooth tonality which was also effortless in its overall presentation.
So what about headphones?
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| Testing the Curve and Zen2 | Testing the MS Pro and HD800S |
First up this time was my Alessandro's MS Pro (32ohm impedance, 98 dB/mW sensitivity), and this required slightly less than 3pm on the pot (or roughly 10% of the total pot) to get to my preferred listening level. Again the tonality is simply sublime with the MS Pros, and the XRK-NHB seems to have this ability to reach into the music and simply carry me away.
Time to really push the amp, and this time my 300 ohm, 102 dB/mW sensitivity HD800S. The power output specs tell me that this should be an easy job for the XRK-NHB, and that proved to be eminently true. At about 3.30 on the pot I was ionce again at my personal ideal listening level (ave 75 dB) listening to Sarah Jarosz's album “Build Me Up From Bones”. Even if I switched to Amber Rubarth's “Tundra” (which is an excellent recording, but needs a volume boost), 5pm on the pot (or about 30% of the total pot) was hitting a 75 dB average. Time to see what maxing the pot could do with the HD800S. Even with this quietly recorded track, the HD800S was measuring peaks into the mid to high 90 dB range, and with more modern music well over 100 dB. The XRK-NHB is certainly a power house!
And how did the HD800S sound with XRK-NHB? Put it this way, I've just ordered some more rechargable batteries for it. I'm not one who usually uses an amp when I can afford it, but the overall tonality of the XRK-NHB – especially with full sized cans lioke the HD800S and HD600 is simply stunning.
BATTERY LIFE
Being class A output, the XRK-NHB is a bit of a battery drainer. Its powered by 2 9V batteries, and performance will depend on the batteries you use. Unfortunately NZ is a bit of a backwater, and I was unable to find any Li-ion rechargable batteries, and its even harder in the mmodern age to get them shipped here. So I've been living with a pair of Powertech 8.4v 2000mAh NiMH rechargables. Unfortunately they only give me a little over 2 hours perfect output, and as they start running out of power, the amp starts distorting. I've ordered a pack of 4 x EBL 6000 mAh Li-ion batteries (which should arrive in 7-10 days), and they should give me close to 5 hours per pair.
Personally I'd love the life to be much longer, but it is what it is, and the sonic performance is worth the outlay and the hassle IMO.
SONIC PERFORMANCE
Preface
I’m going to preface this section with a little critique I received a while ago (by PM), and my answer to it – so that you can understand why I don’t comment on some things, and why I do comment on others. I was told my review on another amp was poor because I didn’t include sections on bass, mid-range, treble, sound-stage, imaging etc – yet referred to an amp as warm, full, or lean.
Now I can understand the reference to warm / full / lean – as they are very subjective terms, and whilst I’d like to avoid their use, they are invaluable to convey true meaning. Comparing my NFB-12 to the Aune X1S for example – the Audio-gd does sound richer and warmer. It’s the nature of the DAC and amp circuitry which is used.
But I choose not to comment on bass, mids, treble, and most definitely not sound-stage – simply because when we are talking about an amp – IMO they shouldn’t be discussed. An amp’s job is to amplify the signal with as low distortion as possible, and output as linear signal as possible. If it is doing its job properly, there is no effect on bass, mids, or treble – except if hardware boost is concerned. And IME an amp does not affect soundstage (unless there is DSP or cross-feed in play) – that is solely the realm of the transducers and the actual recording.
So we have that out of the way how does the XRK-NHB perform sonically, and can it match some of my other portables?
Tonality
The XRK-NHB is what I would describe as smooth, on the warm side of neutral, and probably as close as I've heard (from an SS portable amp) to a good tube amp. And in fact that is what xrk971 was aiming for. He wanted an amp that measured extremely well (and the SNR and noise floor measurements bear this out), was very linear in it's frequency response (check the graph below), and finally an amp which had a musical tonality – based on 2nd order harmonics (the same harmonics which give tube amps their distinct sound). This familiar with good tube amps will recognise what I'm trying to describe – an amp that is clear and clean, yet somehow retains a smooth and utterly cativating overall presentation. The thing I really like with this amp is that while its warm, its not thick and dark and over-bearing.
If you do follow the links (further reading at bottom of the review), you'll get to many measurements – including proof of the linearity and very good THD. I don't generally measure distortion because the sound card I have is limited (too noisy to accurately measure noise floor – ie you'd be limited by the measurement device). I can however measure frequency response, and the XRK-NHB is essentially flat from 20Hz to 20 kHz. This is a very linear amp.
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| Beautifully linear output | Tonal test against the E17K (my neutral reference) |
COMPARISON WITH SIMILAR DEVICES
I thought at this stage it would be a good idea to try and compare the XRK-NHB with some alternatives. My prerequisite was that the comparable units should all be portable amps, and ultimately be capable of driving full sized headphones (such as the HD600 and HD800S). For testing I used the HD800S for subjective evaluation with the FiiO X7ii as source. I volume matched output for each of the amps using an SPL meter and 1 kHz tone, and then used a splitter from the X7ii's line-out so that I could rapidly switch the HD800S between the amps I was testing.
The amps I've chosen to test are:
- VE Runabout (V1)
- IMS Hybrid Valve Amp
- FiiO A5
- ifi iDSD
XRK-NHB (~USD 277) vs VE Runabout original (~USD 99)
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| XRK-NHB vs VE Runabout | Size comparison |
Sonically the two both sound pretty good – very clean backgrounds, and good definition and clarity. With both of them I'm only at around a qurter of the pot too. The big difference is in the smoothness of the presentation, and the XRK-NHB just sounds effortless with the HD800S in this direct comparison. The Runabout still sounds pretty good with the HD800S, but as soon as I switch there is that instant “connection” with the music – its difficult to desribe – it just sounds right (and it is very addictive).
XRK-NHB (~USD 277) vs IMS Hybrid Valve Amp (~USD 179-270)
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| XRK-NHB vs IMS HVA | Size comparison |
Tonally both sound very similar, and this is definitely the second order harmonics of the tubes on the HVA giving a comparable output to the created harmonics of the XRK-NHB. Both have a very good SNR and appreciably black noise floor. The difference for me is that the HVA has slightly more thickness to notes, where the XRK-NHB is just a little clearer. Both appear to drive the HD800S rather well although the bass from the XRK-NHB is either delivered a little cleaner or in slightly more quantity. I really like the HVA's tonality, but in this comparison, the XRK-NHB just (again) manages to connect with me a little more.
XRK-NHB (~USD 277) vs FiiO A5 (~USD 130)
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| XRK-NHB vs FiiO A5 | Size comparison |
When I started the comparison, I first listened to the A5, and was once again surprised by how good it sounds with the HD800S. I had Norah playing, and could have quite easily just stopped for half an hour and enjoyed the music. But I had to switch, and I was expecting this comparison to be subjectively close. It wasn't. The A5 is excellent – clean, clear, engaging – everything you want an amp to be. The XRK-NHB is just in another realm – smooth and enveloping, and the bass once again seems deeper. I could live with either amp quite happily – but based purely on sounds, the XRK-NHB takes me deep into the music, and into places I simply didn't know existed with the A5.
XRK-NHB (~USD 277) vs ifi Micro iDSD (~USD 499)
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| XRK-NHB vs ifi iDSD | Size comparison |
The iDSD is at least twice the size of the XRK-NHB, so I'd personally consider it transportable rather than portable. It's best features are the switchable gain settings which allow you to drive many demanding headphones, or the most sensitive of IEMs. It also comes with a very good Burr Brown DAC and has a subtle bass boost and ghardware DSP for speaker emulation.
It is of course wonderful with the HD800S (although it was bested when I used it as a DAC and also VE's Enterprise statement tube amp). The XRK-NHB doesn't get to quite these lofty heights, but it does easily keep up with iDSD subjectively. Both have an easy armth while retaining clarity and depth. The XRK-NHB is the warmer of the two, and appears to have a depth of tonality which I would have to say I actually enjoy more than the tonality of the already excellent iDSD.
Listening to this now, I can't but help wondering how good a desktop amp from xrk971 would be, especially if it was an all in one with a nice chassis, decent DAC etc.
VALUE
How do you make a call on this? You can buy a standard kit with everything you need for just $99 and build it yourself. For approx $140 you can get a prepopulated standard kit, or $160 for a prepopulated NHB kit. Standard matched FETs (4) will set you back $19 or $32 for the NHB version. But if you're like me, and just want the amp – then you can get a compeltely prebuilt and tuned amp for $199 for the Standard version, or $277 for the NHB version. The best part about doing things this way is you get personalised tuning – ie suited to your individual needs. Xrk971 hand-tunes the resistor network for best sound and harmonic profile, and does this with your own cans in mind. With this amp, I told him I had the HD800S and HD600 and I've got something which matches those cans beautifully. For others he's even tuned specifically for sensitive IEMs like the Andromeda. Other popular tunings include bnoth the HD650 and LCD2.
If you look at the chassis, the lack of features, the short battery life – it would be very easy to scoff at the price tag. But once you hear it, and spend time with it, the perception of value rappidly changes. I've felt this a few times – one was when I actually bought the HD800S. When you hear something which sounds so right you just have to have it, no matter the cost. A telling point for me was when I spent 3-4 hours scouring the net simply so I could get some decent rechargeable Li-ion batteries sent to NZ. Hooked = Yes, and loving it. The sad part now is that I'll have to buy this off xrk971 (I WANT to own this personally). That should indicate its value. Now how to explain this to the wife ......
XRK-NHB – SUMMARY
If you showed me a picture of the XRK-NHB, quoted its price, and told me that it has poor battery life, very basic operation, and no real feature set common to a lot of portable amps, I'd probably tell you to peddle your wares elsewhere. I am so glad that xrk971 approached me to try this amp. It has quite simply been a revelation for me.
At a glance, the XRK-NHB is an A-class amp in a C-moy type tin. It runs on a couple of 9V batteries, and depending on the batteries you use, you'll get anywhere from around 2-5 or 6 hours life. It'll easily drive anything up to about 300 ohms (although xrk971 says it doesn't do quite as well with low impedance low sensitivity headphones). It has very good specs including an impressive SNR and noise floor. Distortion numbers are pretty good too, and xrk971 has pruposely designed the amp with 2nd order harmonics in mind (similar to very good tube amps).
The magic starts when you pair it with a decent headphone. I've compared the XRK-NHB to a liot of my amps over the last month and consistently come away impressed. For someone who only uses a portable amp when he absolutely has to, I'm becoming someone setting up a permanent portable stack with the XRK-NHB as center piece.
Is the amp perfect? No – its still a pretty basic looking amp, and the battery life is poor by today's standards. But it sounds better than most of my portable set-ups. The clincher for me was when I used it with the L3 and compared it to L&P's LP5 Gold. Yep – the ~ $600 combo sounded on par with a DAC/amp more than twice its price. I would recommend this amp without hesitation – and especially if you are a tube lover.
I've used my objective scoring chart (for consistency), but the score for me means nothing at the end of the day. This one is a keeper. I just want to close with thanking xrk791 for arranging the review sample.
Scoring Chart
| Amplifier | XRK-NHB | (out of 10) | ||
| My Score | Out Of | Weighting | Weighted Score | |
| Accessories | 7 | 10 | 2.50% | 0.175 |
| Build/Design | 7 | 10 | 7.50% | 0.525 |
| Portability | 7.0 | 10 | 15.00% | 0.700 |
| Battery Life | 5 | 10 | 10.00% | 0.500 |
| Output Impedance | 6 | 10 | 5.00% | 0.300 |
| Output Power | 8 | 10 | 15.00% | 1.200 |
| Linearity | 9 | 10 | 15.00% | 1.350 |
| Overall SQ | 10 | 10 | 15.00% | 1.500 |
| Gain Control | 0 | 10 | 2.50% | 0.000 |
| Hardware EQ | 0 | 10 | 2.50% | 0.000 |
| EMI Free | 10 | 10 | 5.00% | 0.500 |
| Value | 7 | 10 | 10.00% | 0.700 |
| TOTALS | 76 | 120 | 100.00% | 7.45 |
If I could request anything as far as improvements go, the list would be relatively short, but will indicate how much I value the XRK-NHB, both in it's current form and also its potential. Anyway - here is some food for thought ...
- a smarter and more solid chassis – preferably one easy to stack
- a rechargable (in unit) battery, and something which could provide 7 to 8 hours life preferably
- a gain switch (maybe something which also changes the tuning slightly) – so that you could have the option of more headroom for IEMs, but still use full sized headphones with a flick of the switch
- a desktop version – powered, with rear RCA's for speakers, and if possible a version with DAC (and bluetooth – OK I'm reaching!)
Basically – I'd pay reasonable money for something with a few more features. Something to think about anyway.
