INTRODUCTION

Warning – completely subjective views below 
 
I enjoy the second order harmonic distortion from tubes. Very early on when I was getting back into audio, I had the chance to test and review a PortaTube tube based portable amp, and I loved it. It had plenty of power, very good resolution, and a rich tonality which was mesmerising - but without losing any detail. Anyone who knows me well will recognise that the one thing I don’t particularly like is warmth and darkness (i.e. bassy, overly smooth).  I need to be able to also hear detail – and that is what the PortaTube brought to the table. The drawback is that it was large, could get very warm, and was pretty expensive.
 
It wasn’t too long before I then added a Little Dot MkIV to my desktop set-up – and it is affordable, powerful, and depending on the tubes – quite linear and very detailed. In fact the more I’ve listened to some good tube set-ups (NZ local Head-Fi Meet), the more I’ve grown to appreciate their sonic ability.  I’m currently testing a full tube amp from Venture Electronics, and it is sublime.
 
But I’ve always wanted a portable tube amp with a small footprint which wouldn’t cost the earth but would give me that “flavour” of sound which I love. Then April 2015 I was told about a new Kickstarter project for a tube based portable hybrid amp – valves in the path for the signature, and solid state for power, size, portability and linearity. The surprising thing for me was that the developer was also from New Zealand. So I made contact, joined the KS (donation so I could access the comments), and made an introduction to Martin (the developer). This in turn led me to being able to try the prototypes, and conversing with Martin along the way regarding where the issues lay, what could be improved, and what needed to be rethought. The amps are being shipped right now to backers, and while it is still not perfect – we’ll cover that in the review – the end result is a combined high resolution DAC/amp with a really nice tonality.
 
ABOUT IMS 
Martin Young resides in Auckland, New Zealand – and has been working for the last few years on a portable valve amplifier design – which he completed and started selling in early 2015.  It met with some very good reviews, so in order to further expand production, and take the amp to the world he started a Kickstarter project in March/April 2015. I’ve known Martin since then, and we’ve conversed regularly by email and phone – and met last year at our local Head-Fi Meet. It was a good chance to sit down with him, discuss where some of the issues lay, and devise means of correcting them.
 
In the time I’ve known him, Martin has been very responsive to suggestions, meticulous in trying to develop the best product possible, and his only weak point so far has been saying “yes” to too many requested features – and thereby delaying the delivery of his product. I know that he is continuing to work on the current deliveries, and also working on some new ideas for the future.  He is someone who appreciates good music, and good design – which bodes well for his future.
 
DISCLAIMER
The IMS Hybrid Valve Headphone Amplifier was provided to me gratis as a review sample.  I have made it clear to Martin that I still regard any product he sends me as his sole property and available for return any time at his request. But I thank him for the ability to continue use of the HVA – both for follow up comparisons and also for my own personal use.  The IMS-HVA can be sourced from his website for approx. USD 179, with an add-on DAC a further USD 89.
 
PREAMBLE - 'ABOUT ME'.
(This is to give any readers a baseline for interpreting the review).
 
 
For the actual listening part of this review I used the IMS-HVA both with my X3ii, stand alone with PC and netbook (to test the DAC), and also paired with my iPhone 5S. 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.
 
FURTHER NOTES

1. The unit I have for test and review is the model with both DAC and amp combined.
2. Volume matching was done with a calibrated SPL meter and test tones (1 kHz) when required for comparison.
3. Frequency response and distortion measurements were taken using a relatively cheap Startech USB soundcard, which I know gives me a pretty good measure of objective data – but is somewhat limited by the card itself.
4. I measured the IMS-HVA DAC, but the readings seemed completely off (I really need a much better interface), so I haven’t included them.
5. From this point – I will refer to the IMS Hybrid Valve Headphone Amplifier as simply the IMS-HVA, or HVA.
6. I’ve spent about two weeks with this particular IMS-HVA unit, but previously have spent up to 100 hours testing prototypes of both the amp and the DAC.

 
WHAT I WOULD LOOK FOR IN A PORTABLE DAC/AMP
I thought I’d list (before I start with the review) what I would look for in a portable DAC/amp (YMMV). This is useful to remember when looking at my reasoning for scoring later in the review.

1. Genuine portability
2. Good battery life
3. Clean, neutral signature
4. Easy to use
5. Low output impedance
6. Reasonable output power – should be able to drive IEMs and earphones up to 300 ohms
7. Good gain control
8. Hardware EQ if possible
9. Easy installation of DAC drivers
10. Value for money

 
PORTABLE AMP/DACs I HAVE EXPERIENCE WITH

1. Previous = FiiO E7, Beyerdynamic A200p
2. Current = FiiO E17K, Q1, K1, Cozoy Aegis, iFi Micro iDSD

 

THE REVIEW

PACKAGING AND ACCESSORIES
The package I have may not be the same as actual customers get. Mine arrived in a silver coloured tin box measuring 145 x 105 x 40 mm. Opening the lid reveals a top foam cover to protect the amp, and underneath this safely nestled in a thick foam form fitting inner compartment is the amp itself.
 
 
Also included with my unit were three short micro USB to USB cables (for charging / connecting to laptops etc), a 3.5mm to 3.5 mm interconnect, and an instruction / specification card. Martin is also looking at including an inter-connect with built in attenuation in future for those having issues with fixed line-outs. We didn’t worry about it for mine, as all my DAPs have the option to set the line-out to variable.
 

  
The entire package is practical, covering everything you might initially need for the IMS-HVA, but I do have a couple of suggestions for Martin. The first would be to think about including a simple protective carry pouch.  FiiO uses one for their E17K which would be perfect.  The second thought would be the inclusion of a shorter interconnect (see photos of FiiO’s latest which is included with their Q1 DAC/amp.  Lastly – and this may be included already in the final packages – a longer micro USB to USB charging / DAC cable. I have plenty already so I’m not worried, but ideally a 1m cable would be good for connecting to a PC.
 
 

TECHNICAL SPECIFICATIONS
The tables below list most of the known specifications for the IMS-HVA
 
 
BUILD / DESIGN
The IMS-HVA is rectangular shaped with rounded corners but quite “hard” front and back edges. It is essentially made up of two 59 x 104 x 2mm thick sheets of aircraft grade anodised aluminium (front and back), with a very solid silver one piece aluminium frame 3mm thick to hold the internals. In the front plate is a viewing window sealed by a 40 x 30mm piece of shatter proof plexi-glass, so that the tubes can be viewed. All of the print and graphics on the front and rear plates is laser engraved. The plates are attached by stainless steel hex mini fasteners. All of the internal components (front, sides, and pot) are electrically connected to the PCB’s ground for extra shielding from EMI and RFI.  This must be working brilliantly because there have been no issues at all working with my iPhone 5S.
 
   

On the left hand side is a single gain switch, and at the right hand side is the micro SUB charging port, which doubles as the digital input for the DAC if fitted. At the top left is the 3.5mm line-in, and top right the 3.5mm headphone out.  Between them is the potentiometer – which has a very smooth action.  This is topped by a CNC milled and knurled metal knob.
 
Internally, the IMS-HVA uses two military grade Raytheon 6418 glass vacuum tubes, and these essentially give the IMS-HVA its tonality. I asked martin about their origin, and he told me they are 1984 vintage, and were originally included destined to be included inside the Patriot missiles, so it would be fair to say that long life and top performance was essential with these valves. The valves actually don’t glow when active – which creates an issue for users expecting the amp to have that warm tube glow. So Martin arranged to include two very quiet orange LED back-lights behind the valves which glow when the amp is switched on. The result looks amazing – and I can assure you the tubes are actually working as they are designed. Without the filaments actually glowing, there are no issues with heat either – which makes the IMS-HVA perfect for pocketable use.
 
 

For the amp itself, Martin has used the Burr Brown OPA 2134 – mainly for its renowned low distortion figures. He also uses metal film resisters throughout, low ESR tantalum capacitors, and even the circuit boards have 2 oz. copper tracks to further lower circuit noise.
 
For the DAC section, Martin has used the Savi Audio SA9227 as a master DAC, with the Burr Brown PCM5102 as the slave. This gives a maximum resolution of 32 bit / 384 kHz PCM, and with the right drivers, should also be able to decode both DSD64 and 128.  Note – I have not tested this.
 
 

If I was to describe the build and design in a couple of words I’d say clean and industrial. The finish might not be 100% perfect (the gain switch is slightly low on my unit), but it exudes a feel of robustness / sturdiness which I find oddly endearing.  And knowing how much time went into selecting and testing components (from my various discussions with Martin), I know that although some tough design decisions had to be made, corners were not actually cut during the journey.
 
HEAT AND POWER
As discussed earlier, the Raytheon valves actually run correctly at a very low temperature, so heat is not an issue (at all) with IMS-HVA.  You can happily walk around with this in your pocket all day.
 
Power is something Martin and I have talked about a lot. The battery will provide 4.2 volts, but included in the circuitry is an on-board booster which takes this to 12.5 volts, which is then applied to the anodes to power the valves. And although I listed above the actual power output volumes at full power – they aren’t quite the full story. The IMS-HVA has one big issue at the heart of its design – and that is when you put a full line-out voltage into the amp, it can overdrive the Raytheon tubes into distortion. A solution would be to lower the amount reaching the tubes (via use of resistors), but when we tested that, we then needed to increase the output gain which led to unwanted noise after the tube stage, and also at the low point of the pot. The only other solution would have been to change the tubes, but this would have compromised the entire design – so this is where compromise had to be reached.  Martin did put resistances in before the tubes – but compromised between that and the output stage. For most sources – including PC’s I would recommend keeping the output volume of your source at no higher than 75% - otherwise there is a high chance that the valves will over drive and you will hear distortion (particular in the lower frequencies).  I know some people are going to cry “foul” at this – but it is what it is, and I personally have no issues with it on any of my devices.  If it something that will bother you – look elsewhere.
 

Testing with the 320 ohm VE Zen1 and 14 ohm Campfire Audio Orion - I also used the HD600 (not shown)
 
So how to give you a usable power metric? The easiest way was to measure the IMS-HVA under a demanding load and see how loud it could get.  So I used test tones, set the PC to 75% into the DAC, and measured using a SPL meter.  My readings were 72.7 dB at low gain and 85.4 dB at high gain.  I then measured FiiO’s Q1 using the same metrics, but this time at 100% power from the PC, and it gave readings at 78.9 dB and 90.2 dB respectively.  This was test tones though – not actual music. 
 

Screenshot of the channel matching on my amp - provided by Martin.
 
Martin actually states on the documentation that his target headphone impedance is 4-32 ohms, and I can understand exactly why he’s done this. I can tell you that it drives the 32 ohm 114 dB SPL/V AKG K553 Pro beautifully on low gain at very low on the pot and 75% on the iPhone (digital out).  But how about throwing a much bigger load – the 300 ohm HD600?  Again with the iPhone at 75% the HVA manages to output a very healthy 75 dB ave with peaks at 80-85 dB but on high gain and the pot really doesn’t have anything left to give. So for my listening preferences (low volume listener – low to mid 70dB) the HD600 sounds wonderful. For those wanting more volume the amp will struggle. Based on this I’d suggest the HVA should be OK with most headphones up to around 150 ohms (as long as they have reasonable sensitivity).
 
On the reverse side, with sensitive IEMs with the 14 ohm 113 dB SPL/mW Campfire Audio Orion, I found it necessary to switch to low gain, and actually drop the input volume a little bit more.  The audio is crystal clear – but it just shows the complexity of trying to produce an amp for many different inputs and outputs – especially if you need to take the valves into account as well.
 
FEATURES / USEABILITY / SONICS
The IMS-HVA is a very simple amplifier to use. Simply plug your source into the input socket, headphones into the headphone out, press play on the source, and adjust the volume on the HVA to suit. The only complexity in this equation may be that you’ll have to adjust your source volume as well to find a sweet spot.
 
The HVA has a very low output impedance (<0.3 ohm) so it should suit even the most sensitive earphones without running into damping issues. As usual, I tested for hiss – this time with the 14 ohm Orions. I couldn’t detect any with the Orions but I’m a poor subject because my high freq hearing is masked by my permanent tinnitus. My wife has super hearing though (she can hear our small cat on carpet from about 5-10 meters away).  So she ran the tests for me, and at low gain the amp was very, very silent.  At high gain though, the hissing started at just under the 50% mark on the pot. Given that it would be unlikely anyone is listening to sensitive IEMs on high gain at 50% volume – this should not be an issue.
 
Default Signature
I normally dislike talking about an amp’s signature – and especially trying to break things into bass / mids / treble, and especially about soundstage (which is really a by-product of the recording and transducers rather than the amp).  But in this case it wouldn’t be right without saying something about what you can expect.  This is very subjective. The first thing I’ve noticed using the HVA is that it does have a warm signature – but it’s not warm/dark, and instead more warm/sweet or warm/rich, or warm/vibrant.  It’s really hard to describe, but people who have a tube set-up already will know exactly what I’m talking about.  There is no lack of detail at all, and using the HD600s tonight with the HVA as DAC from my main PC has been thoroughly enjoyable while I’ve been writing the final version and bringing things together. If I had to make an analogy (and probably a  poor one) – it’s a little like the different colouring you can get with lightbulbs. With some straight digital amps, you can get illumination from a cold blue light source – very revealing, hiding nothing, very clinical.  I like this type of presentation too. With the HVA it’s like switching from the cold blue light to a warmer bulb.  You get the same illumination, but it isn’t as stark, or as readily apparent. There is a slightly warmer more natural tinge – and although I know it is a colouration, it is a colouration I really like.  It is relaxing, enveloping, and for my tastes utterly enjoyable. I’m a tube fan – and the HVA just sounds “right”.
 
Gain & Frequency
I mentioned the gain earlier, and so I duly measured the gain from both the amp section (used as amp only) and also as a DAC.  The gain measured on both was pretty close to +13 dB. For martin’s next portable project though, I think a gain solution which allowed for negative low gain (for IEMs), and then the much higher gain for full sized headphones, would be a fantastic solution.
 

 
Measuring the frequency response showed a relatively flat plot from 20 Hz to 20 kHz with a very slight rise in the extreme upper frequencies, and a very slight drop from about 40 Hz down.  Given that these deviations are less than 1 dB, they will not be noticeable – especially at the extremes of frequency.
 
Use as a DAC (PC/Laptop)
The IMS-HVA can be used as a DAC for your PC, Linux box or MAC. I was unable to test this on the MAC but was able to on my EEEpc running Linux.  The DAC was recognised (SA9227 USB Audio) without any need for further drivers, and within a couple of minutes I was up and running. I didn’t try to manipulate it too much within Alsa (due to time constraints), but I have no doubts that the full resolution would have been available. I should also mention that this is a very old, very low powered laptop, and with the HVA combined with the HD600’s and Clementine audio player, I was an extremely happy camper.  The resolution was fantastic, and I really didn’t feel I was missing anything – even compared to my main system.
 
Moving to Windows, and this time you need to have the DAC driver installed.  Interestingly with my netbook, a previous driver I’d used for another DAC utilising the Savi chipset was installed and so it utilised that driver and I immediately had sound and resolution up to 24/192 (if I’d installed the correct driver this would have been at 32/384).  On both Linux and Win 7 starter on the Netbook, the DAC ran with no hitches (both times I had the output set at 75%), and the difference to the on-board  was stark.  The on-board Intel has always been slightly noisy, and like the Q1 I tested recently, the improvement sonically was wonderful.  The difference between the Q1 and the HVA though is in the resolution of the DAC, and the HVA really does sound rather wonderful – even on this very old piece netbook.
 
Lastly I tried the HVA with my desktop, and again after getting the output from PC to DAC correct – PC at 50% output this time (once the drivers were installed), the DAC ran without a hitch (this time with 32/384 full resolution available, and again the actual resolution of the DAC is really very good. I also tried the HVA with a YouTube video and there was no latency or lag. While for me personally I won’t be using the HVA as part of my main system, as I have access to any number of DACs, and it simply does not have the power of my iDSD, its performance with my netbook was so good that it would definitely be an option for me next time I’m heading away.
 
Use as a DAC (iPhone 5S)
I spoke to Martin a couple of weeks ago with the outstanding news that I could actually run the HVA with my iPhone 5S, but with the iP5S running digital out to the HVA DAC! Unfortunately since then I’ve had an update on the phone, and that functionality is no longer there again.  The 5S now gives me the message that the add-on device draws too much power.  Damn!
 
However very recently I’ve had the chance to test a new cable (I can’t say too much about it right now – except to say that it works!), and the functionality is back again.  The downside is that it chews through the battery of the iPhone very quickly (I’d estimate I might get 4-6 hours max).  The solution would for Martin’s next project to have a simple switch to turn USB charging off.  This would then mean that most cables would work – and the next HVA would become a smartphone’s best friend.
 
 
And like I recently wrote with FiiO’s Q1, the ability to actually use a different DAC with the iPhone as transport, has shown me what is available.  The iPhone5S still has a very good DAC – and it does truly sound great. But the HVA adds another dimension – subjectively a richer tonality and resolution that is a little more vivid and alive – and it is mesmerising. I also tested the HVA just by using the iPhones headphone out into the HVA line-in (so as pure amp), and this was also a sonic improvement, adding some warmth – but vibrant warmth rather than simply addition in the low end. If I was to rank the sound from top down, it would be HVA (DAC/amp) > HVA (amp only) > iP5S.
 
So what would make this pairing even better besides the ability to turn the USB charging off – the position of the DAC port.  As you can see from the photos, it isn’t ideal for the iPhones, and the ideal for me would be to have the port on the bottom. I wouldn’t have been worried about this before, but now that I know the DAC can be used quite easily (with the right cable), I’d be keen to find another solution which fits the bill while retaining the excellent sonic signature of the HVA.
 
Other general notes
I also measured distortion with my equipment and was pretty amazed to see that the main body of noise (low gain, but full volume from the amp, and about 75% from the PC) was measuring well under 100 dB down.  Then of course I noticed the signal at 1 kHz was correspondingly starting at about -20 dB, so the SNR was probably around 85dB.  Not stellar, but good enough to be unnoticeable with music playing and to be fair, on low gain, the amp has a pretty quiet background. The second very noticeable thing is the harmonics from the tubes. THD and THD+N both spike to around 3% - but again this will be in ratio to the lower SNR, and exist because of the tubes. The reason many of us use tubes is for the pleasant 2^nd order harmonics.  If this bothers you (the higher THD) then for sure stick to solid state.
 

 
Comparisons
Unfortunately I’m probably going to disappoint with this section. The problem is that I really have nothing even remotely close to this price range or feature set – other than the Cozoy Aegis – and again these two really would be chalk and cheese. To make a valid comparison, I’d really need to be comparing another portable valve amp – and I don’t really have one for comparison. So what I will attempt is a quick comparison with the $140 FiiO E17K.  It may not be useful comparisons – but hopefully you’ll get a feel for overall performance vs price.  At a later stage if I have another suitable am to add for comparison, I’ll revisit the review.
 
IMS-HVA vs FiiO E17K
For this test I was able to use my iPhone or PC – and both work faultlessly. Size and weight are somewhat similar with the E17K being almost the same dimensions but about 40g lighter. The E17K has a driverless solution for the DAC and is pretty much plug and play with any PC, but the cost of this is reduced resolution at the DAC.
 
In the amp section, both have similar total gain settings, and both have similar output power on paper – although the HVA is hampered by the issue with overdriving valves (so effective output on the FiiO is definitely higher by quite a margin).  The FiiO measures better – in linearity, distortion and noise floor (SNR).  It also has a lot more EQ features with the tone controls. Both can be used with the iPhone 5S with the correct cable – but it easier with the E17K due to being able to turn charging off.
 
Sonically, both are wonderful sounding units – but in direct comparison, the E17K is like sitting at the desk with my work light on, everything revealed – sonically wonderful, and this is why it remains my workforce for reviewing with my X3ii.  The HVA with the same music is sitting in the same room, but with the softer main lights on, and in my comfy reading chair rather than at the desk. I love both devices – they are just for different purposes.
 
BATTERY LIFE
The internal battery is either a NiMH (nickel metal hydride) or LiPo (lithium polymer) 1800 mAh long life battery.  I’m not sure why Martin has the two different options, but I suspect it may be to do with different postal regulations around the world.  Mine is the LiPo.  Martin recommends an 8-10 hour first charge, and after that around 4 hours seem to charge from empty to full.  The amp runs at roughly 140mA per hour which should give around 12 hours use – depending on the load being driven.
 
 
When you plug the amp into a power source, there is a little red LED on the far lower right of the plexi-glass window which lights up.  When the amp is fully charged, a green LED is visible.
 
Martin’s estimate for both battery life and recharge time are pretty much spot on. Recharging has generally been about 10 minutes either side of 4 hours for me, and testing under load netted a little over 10 hours for the HD600, and slightly over 12 hours with IEMs.

VALUE & CONCLUSION

So how do I draw a conclusion on the IMS-HVA, and did it meet my expectations of what constitutes a good portable DAC/amp?
 
The HVA DAC/amp has an extremely good build, and I know personally how good the components are which Martin has included – mainly because we’ve discussed many of them. Aesthetically it is a fantastic looking unit, and for lovers of tube visual warmth, you’ll be impressed with industrial looks combined with the warm glow behind the very clear glass. The amp has definitely been a talking point whenever I’ve had it out and about.
 
The accessory package is but a couple of things I’d love to see included would be some sort of soft protective case, shorter inter-connect, and maybe protection for when stacking (a rear pad perhaps).  Including these items would not add a lot of cost but would increase the perception of value.
 
Battery life (10-12 hours) is probably around average, and output power is ideal for most IEMs or portable headphones, and depending on your listening level, is also capable of handling headphones up to 300 ohms (my HD600 and Zen/Zen2 earbuds both were driven reasonably well for my listening levels by the HVA). I think the ideal range is probably going to be more in the 10-150 ohm range though (depending on sensitivity). Gain at +13 dB is practical and useful, but volume output at times may be too much for really sensitive IEMs.
 
Separate Windows drivers are required for the DAC section – and will give resolution up to 32/384. The DAC decoder is capable of handling DSD (64/128) but this was not tested by me. The DAC works natively with Linux, and this should be true of Macs as well (driver support included in the kernel).
 
With the right cable – I was able to get the HVA working natively as DAC/amp with my iPhone 5S, and the sonic improvement (subjective) to me was definitely worth it – but it came at a cost of battery life on the iPhone. And the location of the DAC micro USB port is not exactly friendly for stacking my iPhone.
 
Sonically the IMS-HVA brings together very good resolution with typical rich, warm, and subjectively enjoyable tube warmth (second order harmonics). How good does it sound? Well I’ve been doing the final edit tonight, and I used the E17K for around ½ an hour for the comparison piece (with my HD600s), but as soon as I could, I switched back to the HVA again – and I genuinely like the E17K. If you appreciate the natural tonality a linear tube set-up can bring, you’ll probably like the IMS-HVA.
 
But let’s not forget some of the issues:

1. Due to the tubes having limitations for input power, they can be driven into distortion from a full strength line-out. My recommendation is to run most sources into the HVA at around 75% if using a PC, laptop, or DAP with a variable line-out.  If you only have a fixed line-out, you may be better to avoid the HVA, or be prepared to amp via the headphone out.
2. There is some hiss with sensitive IEMs if you intend using high gain, and the noise floor is also audibly higher at close to high volume on high gain.  This of course will depend on sensitivity of your headphones and hearing (I couldn’t detect it, but my wife could)

 
So we come to decision time, and I’m torn. USD 280 is a reasonably high price to pay for a DAC/amp compared to some of the very good solid state options – and there is the issue of lowering the input (which for some will be unsurmountable). But the build and overall quality of components, the sheer joy of the sonic signature, and the fact that with my gear I can have some of the beauty of a good tube set-up to take with me anywhere – 4 stars from me. And yes – if you like a tube set-up and can live with the caveats, I’d recommend trying the IMS-HVA
 
Finally to Martin, thank you for including me on the journey. It has been inspirational watching you solve the problems as they’ve cropped up.  What you have at the moment isn’t perfect – but at this price point it is going to bring a lot of people pleasure, and it leaves you ground to build upon in the future.