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Slee Solo Mark III Headphone Amplifier

The Next Generation

[Italian version]

Product: Slee Solo Mark III Headphone Amplifier
Manufacturer: GSPAudio - Graham Slee Projects - UK
US Distributor: LPGear
Approx.cost: $795 USD - UK£399.95
Reviewer: Scott Faller - TNT USA
Published: September, 2003

[Solo Mark III]
Slee Solo Mark III

Trivial Bits

If you remember, a while back Nels and I did a co-review of the Maxed Out Headroom Amp and some other really cool headphone gear. Ever since then I've missed the Sennheiser HD600 cans, dreadfully. So much so that in desperation I went out and upgraded my Grado SR-60's to the SR-80's in an attempt to get closer to their sound. Granted, The Grado SR-80's aren't exactly the HD 600's, but it was a worthy upgrade. For the additional $30 or so over the price of the SR-60's, I got a better balanced and more listenable pair of cans that I can use around the house and on business trips. They are very enjoyable and I recommend them to anyone considering headphones for the first time.  

Over the years, I never really considered headphone use and audiophile gear to be in the same class. Mainly because, most of the cans I used in days gone by sounded pretty crappy. They always seemed harsh and nasty sounding. I usually had to EQ the hell out if them to get a decent sound. That didn't stop me from using them on a regular basis though. I loved headphones so much that I even bought one of those headphone extension cords so I could sit across the room in my comfy chair and just chill to my favorite music in total privacy.  

Some time ago I was turned onto the Grado SR-60's. They are a nice sounding pair of open backed headphones. I suddenly found I didn't have a need to EQ when I was listening to cans, very cool. Since that time I've found myself listening to cans almost as much as I listen to my big rig.  

I do a lot of typing and research for my regular job plus the writing I do here at TNT-Audio. In turn, I have a small stereo that flanks the computer monitor on my desk at home and at my real job. I use the Monsoon PM-9 plat panels with a separate sub and an in inexpensive CD player. I don't care for the sound that comes from a computer sound card. Not to mention, when you use the internal CD drive it eats system resources.  

At home, I'm lucky enough to have a fairly large family room. In turn I've taken over a small corner of it where command central resides. I used to have a separate room that I used but that meant I was separated from my lovely wife and kids. That's not overly conducive to being a good father and husband. So, I swiped a corner of our family room so I could still be around everyone.  

One problem, I don't watch much television. Sure, we've got the whole HT thing going on, but after a while I loose interest. Well, turning on my stereo at the computer is out because it interferes with my family watching the boob tube. So in turn, I listen to music on headphones.

[graham really knows how to bribe a reviewer]
I bet I know where the inputs are

I look at television like Groucho Marx did. "I find television very educating. Every time somebody turns on the set, I go into the other room and read a book."

All that as an aside, shortly after Nels and I published the Headroom review, TNT-Audio's old friend Graham Slee dropped me a note asking if I might be interested in giving a listen to his headphone amp, the Solo. Needless to say, I said sure!  

Not long after, I came home from work and found a box from Graham. Inside was a bunch of goodies. I found the Solo headphone amp, a few different power supplies to play with and the most excellent Jazz Club (which I purchased for my 78 collection, watch for the review this fall).  

How About a Little Detail

The Solo Mark III is Grahams third iteration (well duuuh) of his initial headphone amp design (also reviewed by TNT-Audio). Along it's development path, improvements have been made cosmetically and sonically. First I'll start with the cosmetics.  

The Solo comes in a very attractive, small, all aluminum enclosure. On the front is a ¼" TRS (tip ring sleeve) female socket (see pic above) for your headphone jack, a three position switch that controls two separate source inputs on the back of the unit (source 1, open, source 2) and a very smooth Alps volume pot. On the back you will find two source inputs are nice gold plated RCA jacks and finally the input for the power supply. The Solo is optimized for headphone impedances of 30 to 300 ohms.

[The guts of the Solo Mark III]
just what you wanted to see.....gory pictures


As you can (sort of) see from the pics, the Solo is an Opamp based headphone amp, much like it's contemporaries. The Solo uses the highly regarded AD 826. The Alps volume pot has a nice smooth feel to it and was appointed with a nice, machined aluminum knob. Over all, a seemingly nice, well thought out design.

Since this is the Mark III design, I asked Graham what improvements he had made to the Solo over it's previous design. Here's just a few of what he mentioned - Improvement to infra-bass stability (improved bass harmonic structure and phase integrity), Improved earthing arrangement brings out more spatial information, Tighter selection on Alps volume control channel tracking, Improved build quality, Improved dielectric performance selection to key capacitive components,

Rather than me trying to explain the intricacies of Grahams design and the improvements he made, I'd like to share some of the email conversations Graham and I exchanged during the course of this review.

SF - As I look inside, I see the little yellow caps, are they Silver Micas? Also, are those Wimas I see?  

GS - Fraid not, The (yellow) beads are tants (4x), the axials are multilayer ceramics of the X7R variety (2x). All these are in the power supply section and were selected by listening. The X7Rs were interesting. The radial leaded versions wouldn't cut the mustard but the axial version really rock. Oh, and I must correct you, the Wima's.... aren't. They're Evox, which are sonically superior to the cold hard sounding Wima's.  

OK, I know I'm out of step - Wima's are the best????? aren't they????  

The 2 big yellows that are side by side are my big secret. They're good, in fact they're fantastic, but still need bypassing, which I do on every Solo beneath the board with an Evox calculated to be the exact value to take over in the upper midrange.  

Lastly, the single tant on the right of the board is bypassed (underside) by a general purpose electrolytic, whereas an exotic equivalent made no difference.

SF - As part of the improvements you noted, you made mention of "Improvement to infra-bass stability (improved bass harmonic structure)" and "phase integrity". Can you give me some details of what you mean without giving away the farm?  

GS - I know someone who says "all capacitors are bad". I compare such statements with "I like breathing fresh air but don't see the need for the Amazon rain forest". You see, there is a place for everything in the universe, and if capacitors are bad, why did we ever invent them?  

If you look into state of the art video electronics you will see AC coupling is used. AC coupling means placing capacitors in the signal path. I am not ashamed to admit to doing this in my products as it removes the very awkward stresses DC coupling presents (Servo'ing out DC error invariably means things are forced into equilibrium, rather than being "relaxed" into equilibrium). So, therefore my products are very similar to tube designs of yesteryear but use solid state semi's.  

However, tube designs always suffered a phenomenon called "motor-boating", which is low frequency instability. You could hardly accuse my products of such instability, having the honest noise performance that they do. But in theory, if it can happen, it does happen, even if it's miles below the noise floor - it is there and it could just have an effect on the sound.  

By taking the margin for guarding against the possibility of low frequency instability a further order of magnitude greater (far greater than any remotely similar circuit I've seen in my 30 years at it), should therefore improve things a tad :-) In fact, it improves the mid bass and lower midrange. But how? Phase integrity. You see, if all the fundamentals and harmonics set off together, then the sound is bound to be realistic. You can tackle the highs all day, but here's where I tackled the lows as well, and it made a difference.

SF - Can you give our readers and idea what "Improved dielectric performance selection to key capacitive components" is.

GS - The last bit means I have paid great attention to a thing called dielectric absorption which is a property of capacitors. In a power supply you want stability and minimal effect due to change caused by power demands of the signal. Therefore you want a dielectric that doesn't like changing state all that quickly, and the best match of this statement is a tantalum. However you cannot get big tantalums but you can amplify capacitance! The Solo does this. In the signal path you may or may not want a dielectric that likes changing state faster. If it's across the signal it needs to be faster. If it's in series with the signal it wants to be slower. This is the starting point, and then you start listening for tonal balance, timing, spatial information, recording venue atmosphere (how live the studio/hall is - doesn't matter if it's real or artificially generated). Then you start tweaking. You may find that a power supply needs to be less tight in places, after all everything is in the signal path when you really think about it. This is why some amps sound better with smaller psu caps and others with bigger psu caps. It's all a question of balance. And so the story goes....

Meanwhile Back at the Farm.....

A fair chunk of the time I spent with the Solo, I had it on my desk at home plugged into the back of my dual deck Philips CDR. Not the most refined CD player in the world but it worked well to break in the Solo and it provided me with some much needed tunes when I'd get bored with TV (which is pretty often).  

When I started to get serious about writing this review, I moved the Solo and the two power supplies down into my listening room. Here it resided on my coffee table right in front of my sofa. The interconnect I used is one of my own designs which uses shielded, silver plated copper wire with a thick Teflon insulation. The interconnects are about 15 feet long.  

Well, as I started to listen to the Solo behind the Korato KVP 10, something didn't quite sound right. I wasn't getting as much bass as I was when I had it plugged into the back of my Philips CDR. I was getting an OK amount of bass but it just didn't seem to sound right. Then it dawned on me, hey, the Korato is a phase reversing design. In other words, when you use the Korato pre's, you have to hook up the speakers out of phase (negative to positive, positive to negative) in order to get the proper bass. So I reached around to the back of my rack, unplugged the interconnect from the Korato and plugged in the Solo directly into the second pair of outputs on my Arcam 8se. Bingo! Bass again.  

Now that I've gotten past that moment of stupidity, I could do some serious listening.   

A Tale of Two (power) Supplies.

The Solo has an option of two different power supplies. The Intro model is your pretty standard wall wart. Nothing special, just your typical inexpensive power supply. The Monitor Class is Grahams (recommended) PSU1. This is a serious power supply. No longer in the wall wart fashion, this power supply is cased in a much larger ULV094 housing that sits on the floor and has about a four foot lead that plugs into the wall.  

In listening to these power supplies, there is a distinct difference in the sound between the two. I tried to pick some acoustic music with vocals to use for comparison of the two power supplies. I settled on Allison Krauss and Union Stations release, New Favorite. This is a stunning recording loaded with details.  

For the test I picked track 2, The Boy Who Wouldn't Hoe Corn. The track starts off with Jerry Douglas lightly picking on his Signature Dobro then leads into some fantastic "old tymie" vocals by Dan Tyminski. Playing the intro of this track over and over and switching power supplies gave me a really good sampling of how they both sounded.  

Listening to the Intro power supply, I found the soundstage to be very nice. On the proper source material, the soundstage would reach outside the boundaries of the headphones. All of the instruments were in a sharp focus and tonally well balanced. Bass reproduction was very good.  

Next up was the PSU1. This is about a $179 (US) (UK£119) upgrade to the Solo. Now the soundstage has opened even further extending well beyond the Grado's boundaries. I started getting the impressions of a recording venue rather than just a recording. I'm getting even more of the harmonic structures of the vocals and instruments. Any remaining ragged edges of the music have disappeared. This power supply provides a far smoother and more accurate presentation to the music.  

Do yourself a favor, if you are considering the Solo and want those last few, precious ounces of refinement, invest in the PSU-1. It's well worth the money.  

Needless to say, for the rest of the review I used the PSU1 as the favored power supply.

More Listening Impressions

Here I moved onto some Pink Floyd, Momentary Lapse of Reason. The opening song track has some great recorded effects of a boat dock and water lapping against the shoreline. As I listened, the sound of the water lapped well beyond the outside of the Grados while the dock creaked in a well focused image at the top of my head. Later in the intro, you hear several single drops of water on a still body of water mixed in just after the synth starts. The drops were located well to the rear of my head. For those of you that don't listen to cans (much, if at all) this may sound a bit strange but it's actually an extremely cool phenomenon. It's almost like having a Dolby 5.1 rig strapped to your head. That's what a good pair of cans and amp can do. Psycho-acoustics, ya gotta love 'em.

A little more from Graham

BTW, the Pink Floyd track is digitally mastered I believe (I have the LP version), so to get the purity of sound required to reproduce water sounds properly (instead of sounding like frying oil) takes a bit of doing. The great thing about the AD 826 opamp is the speed, and it is this which allows the purity of sound through (my phase integrity philosophy). But it would not reach the cans if the output stage interfered with this speed, and therefore I designed the Solo output stage to be as fast/exceed the speed of the AD826.

Next up is Mouth Music, MoDi. I realize most (if not all) of you have never heard of this group. Unfortunately, this CD is long out of print. It has some of the coolest music ever recorded. The music is really hard to describe. It has (almost) a world-trance-acid-trip feel to it. Bizarre as that sounds, it's a really cool listen. This recording has unbelievable bass lines and a huge soundstage. Listening primarily to the deepest of bass lines, the Solo faired extremely well even at high listening volumes. The synthed bass was tight, fast and well defined.  

To finish up at the complete, opposite end of the musical spectrum, I flipped on some YoYo Ma and Emmanuelle Axe playing Braham's. I can't say enough about how fine this CD is. I'm not a big Classical Music fan but this CD has so much emotion, it's hard not to keep using it as a reference. Closing my eyes and listening, my head is filled with a deeply, rich sound. A sound much like I would expect from a piece of tubed gear. The lushness of YoYo's quarter century old cellos came through without a hint of being cold, sterile or being stripped of life. The only way I can describe it is, simply lovely.  

Useless Drivel

These thoughts keep begging an answer. The more I listen to the Solo, I feel it is as far from cold and lifeless as you can get. With a statement like that you would expect a piece to sound slow, overly warm and lacking in detail. The Solo is anything but that. The detail is ample. The bass is solid, in fact it slams with the proper source material. The highs are well defined and extended without a hint of being brittle.  

Now, when you take a look inside at the design and read Grahams choice of components, you think (or at least I do) "Where are all of the ultra-fi components ?". I don't see any Auracaps or big fat Solens in there. Where are the Black Gates (not that Elnas are slouches)? Where are the precision tantalum resistors or mil-spec Dales? The three parts closest to the ultra-fi world are the AD 826 opamp, Elna Stargets and the Alps pot. So just how can this headphone amp sound so damned good and be so musical without all of those audiovile parts?  

Well, I guess it all boils down to proper design and extensive listening tests. Graham spent loads of time changing parts until he got it to sound right. It's more than obvious to me that Graham has done his homework in the building the next generation of this headphone amp. More living proof that .... all that glitters isn't gold (plated). If you have enough experience in design, the way parts sound in particular, you can make a damned fine sounding piece of gear without going "over the top" using all of the cottage industry parts. It's kind of like defying (the industry) gravity.

[i'm not sending the girl back]


As you would expect from a quality piece of gear, the Solo performs extremely well. The bass is deep, controlled and well defined. The treble is crisp and extended without being harsh or splashy. The midrange is detailed and fluid. The combination of the Solo and the PSU1 or as Graham calls it, the Monitor Class, provides your head with a very open and spacious musical presentation. All in all, the Solo Mark III is very smooth and coherent top to bottom.  

As my music memory has it, comparing the Solo to the Headroom, I'd say the Solo Mark III is probably the more musical of the two amps. That in itself is pretty surprising to me. Here we have a competing design that is less than ½ of the price of what many consider the do all - end all of headphone amps. Granted the Solo doesn't have all the bells and whistles (all of the EQ settings and channel summing options) the Headroom Max has, but on a straight up sonic comparison of the two, I'd bet you'd find the Solo easier to live with.  

The other major difference is the build. The Headroom Max is this big, heavy, machined, beast of a unit. Kind of over kill for a headphone amp. Amps, pre's, turntables and CD players need to have mass to help deal with vibrations created by the speakers. Headphone amps don't. Cool as all that mass and machining may be, headphone amps only need to be heavy enough not to slide off your table beside you, that's it.  

The Solo measures about 6" deep, 4" wide and stands about 1 ½" tall. It's easy to place near your listening seat and doesn't take up near as much room as other amps. It's all machined aluminum with a nice machined volume knob. What more do you need (besides a good source to mate with the Solo)?  

So, if you're in the market for a great sounding headphone amp and don't want to drop the kilo-bucks needed for "the other guys" amp, I'd seriously consider the Graham Slee's Solo. After listening to both, I'm not sure I'd the go extra grand (or so) for the "other" amp. That's not a bash against the competition because it's a great sounding amp too. It's just simple economics.  

I'd personally like to thank Graham Slee of GSP Audio and Rome from LP Gear for the use of the products during the course of this review.  

Manufacturers Comments

I again thank TNT-Audio for their honest and unbiased reviews which help all those seriously interested in listening to music obtain the fun and enjoyment they rightly deserve when spending their hard earned cash.
Graham Slee


© Copyright 2003 Scott Faller - www.tnt-audio.com

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