Graham Slee Solo Ultra Linear Headphone Amplifier

General Information

The Solo Ultra-Linear uses advanced linearization techniques that mimic valve behaviour but in a solid-state implementation

Latest reviews

Pros: Stellar sound quality with all genres and most headphones. Gets out the way and allows the music to shine.
Cons: None that really matter, but better suited to higher impedance headphones (above 50 Ohm)
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.

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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.
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Great to see a Graham Slee amp getting a review in 2019! I still own and love my little Novo, always a great listen!
Pros: Sounds exquisitely good. Eeks out every last drop of performance form your headphones.
Cons: Your wallet may not be best pleased.
Graham Slee Solo Ultra Linear Quick Review
Thanks to Graham Slee for the loan.
Full review here
Brief:  Teeny amp, giant name.
Price:  £670 or circa US$1020
Specification:  Lots, see here
Accessories:  Err none really, power supply if you count that.
Build Quality:  Good.  It’s a big block of aluminium which you can see the screws holding it together.  It’s sturdy and practical.
Aesthetics:  Pleasant, plain, perhaps a little utilitarian.
Sound:  Open, detailed, transparent and I could see some thinking it contradictorily either bright or valvey.  Bright because it’s so very detailed and this heightened clarity in the uppers makes the highs more articulated and readily distinct.  Rather than the smearing you get when there is a lot going on up top, the clarity retains all the detail making it more noticeable and hence could be interpreted as bright.  It’s valvey in the sense that the initial impact of metallic edges are a might soft.  The highly fluid smooth detail you get pervades everything and offers a hyper liquid presentation.  There isn’t any abrasiveness or harshness despite its very open and clear nature.  Normally you see, that “clarity” is enhanced by cranking up the abrasiveness so it “feels” more clear.  The Solo Ultra doesn’t do that, it does it the hard way by actual giving you more detail rather than just trying to trick your ears in to then thinking there is.  It’s pretty awesome.  The enhanced clarity is everywhere, from adding a certain vigour to the lows, they do reduce the tendency of headphones to bloom a little so you get a more well sculpted bottom.  It firm and luscious.  Mids share the same nudging, highly detailed, highly detailed pushing towards detail and nuance.  It does a little over do vocals which suit a more creamy presentation.  I can’t fault it for being too detailed but Nora likes a more rich and creamy experience, yes her vocals are super fluid but she isn’t about extracting every last detail she’s about melting away.  The Solo Ultra enjoys being desperately detailed and nuanced, not so much melty.  Highs are super good too.  Supremely detailed so feed it good quality stuff, while it does do a little valve esq softening of the metallic edge of a clatter but only a hint of it, feed it crap and it’ll try not to cheese grater your ears but if you buy this amp and feed it rubbish, it will assume you know what your doing and dish it up.
Value:  Eek.  It’s the best part of £700 here for a headphone amp.  So we are well into diminishing returns but….. if you’ve got some first class, kick ass headphones already and you want to see just how well they can shine, you may well want to get your wallet out and give them a bash.  It’s pretty damn fine.
Pro’s:   Sounds exquisitely good.  Eeks out every last drop of performance form your headphones.
Con’s:  Your wallet may not be best pleased.

Pros: Compact, remarkably clear sound, easy to listen for ages
Cons: several weeks of burn-in (but worth waiting for)
I had built and enjoyed the Novo amplifier kit from GSP. I wanted to upgrade to get one of the best headphone amps available.
The Solo Ultralinear is remarkably musical, it opens up the sounds within a mix and brings details out without being harsh. It does a fine job on guitars, acoustic and amplified. The detail and stereo soundstage make individual vocal tracks clear, I could hear vocal backing parts that were previously lost in the mix.
The individual drums and percussion were clearer than before on tracks I knew well, but the punch and resonance of toms surprised me. Basslines have body and clarity, with the track being clearly audible in the mix.
I was impressed with the intial sound which was pretty clear, if a slightly bright new electronics sound. It quickly mellowed with some detail and sounded full with good bass. Over the next couple of weeks the low level details emerged and the positioning of parts in the stereo mix became better and better. The burn-in is 3 or 4 weeks of being left powered up, some users say 500 hours. It is well worth waiting for the end results.
There is a light soft hiss but it is unobtrusive at normal listening levels.
The two inputs and switching arrangement is ideal for my use for phono and CD/DAC inputs.
Ultra-Linear Diamond Edition is hiss free!
@treebug ... I sure hope so as I just ordered one :) I'll post a review after burn in with my impressions.


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