I first became aware of the Cayin brand some years ago as a neophyte to the Head-Fi hobby when a friend bought a C5 portable headphone amplifier.
Driving a pair of Beyerdynamic DT880 250 Ohm headphones, this svelte and well built powerhouse performed brilliantly, to my surprise it must be said.
It was the best of it's kind then, making comparable Fiio offerings seem anemic by comparison, and it remains a contender now.

Fast forward to present day, and Zhuhai Spark Electronic Equipment Company, better known as Cayin for the past 20 years, has rightly earned a serious rep in the community for their impressive tube amplifiers and DAPs. They do however also make a solid state, fully balanced headphone amplifier, the iHA-6, the subject of this brief review.

I felt compelled to air my impressions, not because Lumous Audio, in their wisdom, decided to loan me a unit for a while to "get my opinion" but because it is the finest sounding headphone amplifier I've heard south of twenty grand South African (about USD$1300).

More amazing is that it retails for a ZAR shy of half that (at the time of writing).
So I've gotten to the punchline upfront: it's a stellar performer, and it did not take long for me to come to this conclusion.

Out of the box one is struck by the solid mass of the thing, only 7 by 24 by 25 centimeters but weighing in at a dense 4 kilograms.
4mm thick aluminum panels are nicely finished in silver with nary a screw in sight.
Upfront one finds no less than 4 different headphone outputs covering low and high impedance 6.35mm and both 3 and 4 pin balanced XLR connectors by Neutrik.
The "low" has an output impedance of 10 Ohm whilst the "high" is 120 Ohm. More on this anon. The balanced outs both present a load of near as dammit Zero Ohm.

An illuminated power button resides next to three push-buttons for selecting RCA or XLR analogue inputs, high or low current delivery and low and high gain respectively.

The ability to increase the amp's quiescent current is a very neat trick too. Power output is decreased slightly while current delivery is increased.
This works very well indeed on low impedance headphones, and can be very noticeable or not so much, headphone dependent.
This should be the default mode for most Planars.

A large, onyx fronted silver knob controls the very smooth ALPS RK27 Blue fully balanced 4 way pot.
Since there are no pre-amp functions the rear panel is spartan, housing one pair each of RCA and 3 pin XLR connectors along with the usual IEC power socket.
I might mention that build quality and component choice are both excellent, and not just "at the price" - at any price.

Switching on one notices that the illuminated power button blinks three times and then settles into a constant white glow.
Protection against DC offset on power up. Nice. My headphones thank you. They don't need to be unplugged, ever.

Having given it 5 minutes to warm up I connect the trusty Sennheiser HD6XX to the 4 pin output on low gain and low current settings.
Source is a Windows 11 PC connected via AQ Pearl USB to Resonessence Concero DAC and thence single ended to the iHA-6 which coverts the signal to fully balanced internally using a bridge-tied load (BTL) circuit that converts single end input to balanced before feeding into the amplification circuit. Any signal loss via this process is said to be inaudible.

I almost always start listening with the HD6xx (and previously HD650) - since I know its sound so intimately, I can almost immediately hear where an amp is at based on this synergy. And synergy there certainly was! Santana's IV album in 24/48 FLAC sounded full, detailed and engaging at moderate volume - around 10 on the pot.
Absolutely no need to use high gain, the Cayin outputting a handy 1.2W into 300 Ohms...did I mention this thing is POWERFUL??

Andy Kong uses both the HiFiMan HE6 and Abyss 1266 to demo the iHA-6 at shows. Two of the most difficult to drive (properly) headphones on the planet, and this in front of a rabid and knowledgeable crowd...either a very brave man or very certain that his amp can do justice to these two behemoths.

Power is the last thing an owner needs to worry about, so let's put that to bed: 7W @ 32 Ohm balanced, 3W @ 120 Ohm and 600mW into 600 Ohm.
According to the Rolls Royce Scale, sufficient. Oh boy, is it ever sufficient..!

I have not needed to use the high gain setting (+12dB) at all, and that should speak volumes.

All that power is delivered with a velvet glove-like delicacy though, refined and smooth when required but capable of huge bursts of dynamic range and slam without any hint of compression or clipping. I simply loved the way it allowed the HD6xx to sound better than I've ever heard it, and I've heard it from MANY sources.

It is well-known that the venerable Sennheiser scales with better equipment and this absolutely holds true here.
There is no "veil', perceived or otherwise.
Midrange is rich and full-bodied as it should be and in fact all frequencies are evenly presented.

Similarly it gets the best out of all my headphones, from HE400SE to AKG K701 to Fostex TH-X00.
Three very different headphones, all of them fully and authoritatively driven.

I heard detail from the HiFiMan I didn't know it could resolve.
Bass on the power-hungry AKG filled out and became...more...and better defined, with seemingly much less roll-off.
The 25 Ohm Fostex is sensitive and easy to drive and the iHA-6 allowed me to experience what it could really do when subjected to massive power, current and control.

This headphone just thrives on current and with the amp in high current mode it's the best I have heard them.
They deliver legendary bass but now it's even better defined and there seems no end to its extension.
Highs in particular have even better extension with nary a hint of sibilance.
Considering that the Mahoganies have a tendency to sound a bit "zingy" with the wrong amplification, this is impressive control.

Whilst I would not call the iHA-6 a reference amp (it is ever so slightly warm of neutral), compared to the Topping A90 I found it to be more "musical". Not easy to quantify but noticeable once one has had a decent listen - for "musical" one might substitute "organic" or 'natural". Listen to Amber Rubarth's "Sessions from the 17th Ward" on DSD and you'll quickly understand the difference.

Perhaps the only negative about this amp might be the high output impedance of the SE headphone jacks, but truthfully its a non-issue for me. It is designed to deliver as a fully differential device and should be used that way.
In short, it sounds great running SE, but it is exceptional when used balanced.
'Nuff said!

I could ramble on, but I've got too much music to listen to...the honeymoon is not yet over.
It is all the headphone amp most (normal) people should ever need and redifines the cost to performance ratio in my book.

I love switching it on every day and can highly recommend it.

UPDATE 08/04:
Having now had the opportunity to hear my HD6xx using the 120Ohm SE output I will need to change my initial opinion.
This is in no way a con now that I understand why it's there in the first place, rather a stroke of genius from the designer(s).

As per my thread post:

There's definitely a heftier, rounder bottom end but no bleed into mids and no loss of detail (not that the HD6xx is the most resolving headphone to begin with).
Yes, it's a tad warmer but I like it!
Yes, I would say slightly more euphonic (ok, tube-like) which was precisely the result I was hoping for.
HF seem to be largely untouched, but that's perhaps due to me not hearing much above 13kHz...
I will need a bit more time to hear if I can pick up on any LF nasties, but so far, so good.

It’s a wonderful thing to have the fact that one is never too old to learn something new emphatically proven, and even better when it is totally unexpected. My new Solo Ultra Linear was both unexpected and has proven to be quite an education.

I’ve a fair amount of experience with high fidelity gear of all kinds, having been an enthusiastic audiophile since my late ‘teens. The spark was rekindled after a long hiatus about 3 years ago, when I discovered this thing now commonly dubbed “Head-Fi”.

This rabbit hole has been slippery and deep, and the journey of discovering a new way of listening to my favourite music has been exhilarating and incredibly satisfying on many levels.

I wasn’t planning on another headphone amplifier. I already had five! Each carefully chosen for it’s particular attributes, I felt I had achieved the synergy between gears that I was looking for, and was as satisfied as any gearhead could be, I suppose.

Then I came across an ad for a pre-owned Solo with upgrade UL board fitted and PSU1…and ignored it for about a month. Of course, I had come across the work done by Mr. Slee and Company during my ramblings on the interwebs, but had never seen one in the wild, nor considered it as something I needed to have. After about 6 weeks, with the amp seemingly still unsold and having hoovered up as much information as I could about it, I had to enquire after it. The rest, as they say, is history…

It took a while for the package to arrive, and I’m not the most patient, so when I finally unboxed the amp and PSU it was in a state of extreme anticipation. First thought: damn, it’s small! But beautiful. I love the utilitarian look. Classic. It won’t date and I’ll always enjoy looking at it.



Here’s the rig used for initial listening:

PC running Windows 7 Pro/JRiver MC24 and little else, optimised for media -- Audioquest Pearl USB -- Resonessence Labs Concero DAC used as DDC (SPDIF bridge) -- 75 Ohm Lindy Gold coaxial cable -- Benchmark DAC1 USB -- Sommer Stage 22/Amphenol interconnects -- Solo SRGII with Ultra Linear board upgrade and PSU1 -- HD600/LCD2F/K612/DT990

It’s a tricky thing, trying to describe what something sounds like. Or rather, in the case of an amplifier, how it makes other things sound really: the DAC and headphones connected to it. My first impressions after letting the Solo warm up for a few hours were quite honestly underwhelming…it sounded good, but a tad thin and dry. Bass was present but not very extended or detailed. Plenty of promise though…that soundstage was already very apparent. I expected this, since the amp had not been used for at least 2 months, and left it powered on until I could again do some listening the following night.



I have a selection of favourite test tracks that I know note for note and chose “Yambu” off the stellar Santana IV album to kick off with, a track that has everything AND the kitchen sink: complex, multi-layered instruments, two percussionists, multiple vocalists and of course THAT Santana guitar tone. Well, what a revelation! Now I was hearing something rather special, but it would take a bit more time to realise exactly what that was. Detail and instrument separation was simply in another league compared to my Kingrex HQ-1 and Audio-gd NFB11.32 and they are both highly resolving amps. Bass now extended much deeper and punched harder. Wide soundstage with everything in its place, front to back and side to side. The mark of a well-sorted amp in my book. I was immediately sucked into the music.

Dave Weckl’s “Heads Up” (1992, GRP) can sound dry, as if it were recorded in a bare studio. Not so with the UL. I’ve never heard so much air in this recording. Snare drum decay is amazing, hi-hats actually sound suspended exactly where they should be. Precise imaging as good as anything I’ve heard on the MANY (expensive) amps I’ve had through here over the past 36 months or so. Drums are not easy to reproduce accurately, and the UL rendered them stunningly well and with seemingly endless headroom. This is a high DR recording that requires an amp with decent grunt and the volume knob never climbed above 11. This well and truly laid to rest any lingering misgivings about sufficient power. This amp is highly dynamic and I’ve not yet managed to get close to its limits – it just sounds effortless on any and all genres.

Jeff Golub’s ‘The Vault” (2015, eOne) is a fitting tribute to the late, great guitarist and again I was delighted by the smooth yet punchy delivery of the UL/HD600 combo with Jazz/Rock

So far so good, but my tastes are as wide-ranging as a Bedouin with itchy feet – how would the Slee deal with some Death Metal?? Omnium Gatherum’s The Burning Cold is that rare beast, an extremely well mastered Metal album, and the UL didn’t break a sweat in rendering double bass drums and guitars with power and precision – hugely impressive considering the specifications give little clue as to the ability of the UL to deal with such demanding music.

Thankfully winter is well and truly with us here at the Southern Tip of Africa, and I need no further excuse to spend as much time indoors as possible bonding with the Ultra Linear, as I troll through my collection at random. It takes a bit of time to fully appreciate the effect of the Ultra Linear circuit. This amp DOES sound different to most other solid-state amps and the secret lies in its musicality – the ability to render a recording organically. No hint of the dreaded digititis, unless your DAC is that way inclined…which it should not be! Plenty of great DACs out there these days for beer money.

Paired with my Resonessence Labs Concero the Ultra Linear seems to revel in having to work a bit harder to translate the 1.2V line output of the Canadian mini-marvel to ear-splitting levels on Marillion’s magnificent “All One Tonight (Live at the Royal Albert Hall)” in 24 bit 96kHz FLAC. The Solo just makes me want to turn the pot up a LITTLE bit more…Now sometimes that can be due to a lack of punch or energy in the music, or artificial detail, neither of which is the case here. It just sounds so damn excellent with the Audeze LCD2F at volumes higher than I might normally listen. It’s all about control, which allows for a neat segue…

What sets great amplifiers apart from the herd is control. Regardless of source or material a great amp just gets out of the way and lets the music arrive, unencumbered by distortion or artefact. You realise this after you look at your watch and you’ve been listening for 3 hours and could continue another three. At least. The Solo Ultra Linear facilitates this happy process unfailingly. It makes me want to get home and listen to music rather than watch TV – Cricket World Cup? Don’t even go there. Definitely rather find out what the Solo can do with some ultra-revealing DSD recordings.

Amber Rubarth is as good a test of vocal rendering ability as any, and I’ve never heard “Sessions in the 17th Ward” (2012, Chesky Records DSD128) sound quite this binaurally huge. Her voice can veer towards shrill to my ears on certain gear – not so with the Solo and Audeze planars. Also excellent with my AKG K612, soundstage for Africa (!) and a steely grip on the power-hungry Austrians.

Beyerdynamic DT990 Edition 600 Ohm? Spectacular headphone with the Slee, which tames the infamous chainsaw treble just a tad. Although that said, my battle-scarred aural appendages don’t hear much above 15kHz anyway and I LOVE the DT990 for that very reason. Few other headphones at any price deliver the visceral punch that the Beyers can with the right amplification. Budget they may be, but don’t let anyone tell you they are entry level. Technically excellent headphones, start and stop on a dime. A Metalhead’s dream.

Much more listening to be done, apologies for the lengthy ramble but I got inspired to spew forth this stream of consciousness babble and just had to share. Great gear can do that...

A final word: although there is some positive feedback on the Slee amps on Head-Fi (if one looks hard enough), it is obvious that they don't get very much copy here. This is a shame, but understandable to the extent that the Solo is not a mass market product and won't attract your typical headfi punter. It isn't big and complicated, neither does it come with a particularly impressive set of specifications at first glance. Let me just say that this amp is not about specs or appearance, it's about world class performance delivered to those prepared to look out of the box and whose audiophile journey has brought them to a point where they are experienced and mature enough to appreciate it.

A very special piece of equipment this one.