What’s not to love about an audio company that are both based in the UK as well as make their products here. There is always a little bit of patriotism for a British company and at the same time it is nice that it is not just another amp getting shipped out of China (not that I have anything against Chinese products), it just seems a bit more personal. Graham Slee certainly knows his stuff and his résumé shows that and over the years of running Graham Slee, he has built quite a portfolio of products. He started making headphone amplifiers way back in 2001 with the original Solo and since then has continued to develop the design, leading us too the current flagship, the Diamond Edition Solo Ultra Linear (SUL). This amp has a price of £670 and this also come packed with the £185  PSU1 power supply. The SUL really is quite an understated looking amplifier and although I know this will never effect the performance and does allow a sound comes first approach, making an amplifier aesthetically pleasing is never a bad idea. What I can say about the design is that why it is not the most modern or even expensive looking piece, it does do a nice job of not being offensive, giving no reason not to like it but at the same time does not draw any attention to its self. In terms of what it has going on, it keeps itself simple, you have a single headphone output, two pairs of RCA input and a toggle to switch between them. It is a headphone amplifier and not much else and it makes this quite simple. Now a few things might seem like a glaring issue, a lack of a gain switch might seem like an obvious one but it actually bypasses this problem completely with the amplifier takes into account the impedance of the headphones and allows you to use the volume pot accordingly, which proved very handy, especially in occasions where I have switched from my HD580 too an IEM and forgot to reset the volume pot, my ears are saved from being blasted. There are some other features that this amplifier packs such as being able to be used with tape monitors but these are something I have no need for. 



The amp feels very industrial and strongly built. The all metal construction is what it is and does the most important job of looking after the circuitry inside. The volume pot is rather large and sits right in the center and while it does look and feel nice, it is very loose and a small knock can cause an accidental surge in volume either way, added resistance or having a stepped attenuator both seem like a much better idea. The size of the amp is fairly small. I say this in terms of how it is sized in terms of higher end competition because it is still clearly larger than the likes of the entry level VMV VA2 I have just reviewed or most transportable/portable amps. The main reason for this is that the SUL uses an external power supply, the PSU1, which is not much smaller than the amp its self. Sadly I cannot compare this to any other power supply but I do know that Graham Slee put a lot of effort into it and that it also has a few features itself. One of them is that it does not have an on/off switch but he has highlighted that this is not a problem. It is not something that will cause any negative aspects to sound quality nor will it ever cause a breakdown in the amp, everything is designed for it too be turned on 24/7 for the rest of its life. Now another thing I instantly thought is well this could end up being expensive, leaving an amp constantly on but it is efficient and Graham Slee have done some calculation showing that a year with it on will only set you back about £17 in electricity, not bad at all!





Now it is time to talk a little more about the versatility of this amp and a bit more about how it has been designed to work equally as well with IEMs as it is the hardest to drive headphones. Well it really is extraordinary how big a range of headphones/earphones this can cater for. I can listen to the SUL with the 8 Ohm Final Audio Design Pandora VI or my Lear LCM-5 with their sensitivity rating of 122db and yet have the most silent background you can imagine, it is almost eerie how silent the amp is, not even the slightest taint of hiss. This is something that I cannot always compliment dedicated portable amplifiers on and that should be their main focus. What is almost as important is that this does not forget its main purpose, driving full sized headphone as it is at the end of the day, a desktop amplifier and that should be its main duty. Plugging my 300 ohm Sennheiser HD580s in and I instantly know that it has all the power it should have, the job is hardly half hearted, the background is still silent and there is no worry of it showing any signs of strain, in getting the dynamics right or of it getting them loud enough. Now lastly I did have to give it a run in with my HiFiMAN HE-500s and it does not put out quite near the recommended 1 watt into 32 ohms with a significantly less 140mw into 32 ohms. Surprisingly though the numbers do not really tell the story with me being pretty satisfied with performance of the combination. It is not the best I have heard the HE-500 sound but is far from the worse and I have actually enjoyed many hours with this pairing. If you read any of my headphone amplifier reviews, versatility is one of the most important things too me behind sound quality because I have such a huge range of headphones and do use them, if someone in the house is moaning about the noise a pair of open backed headphones are making or there is a lot of noise in the house for some reasons (even if both these are not too rare occurrences), I do sometimes have to change to a closed back headphone or pair of IEMs and I want my desktop set up to be able to handle everything I throw at it with a combination of silent background and power and the SUL nails it completely, I have not plugged in anything and thought, hmmm, this is not cutting it power wise or hmmm, check out that hiss. Graham Slee has done an excellent job executing and I am very happy for this.



We now move onto the most important factor in my book and that is how this amp sounds. Now this is a solid state amp but it Graham Slee does mention that it does take on a warmth not unlike a tube amp and that is the first thing you spot with this amp, it is warm, smooth and resolving. Now when listening to this it does surprise me that this is meant to be the more neutral model in comparison too the likes of the slightly cheaper SRG II because it is very much a warm sounding amplifier, one that does certainly not strike me as neutral and will be influencing its signature onto a headphone. Now with it being a solid state and not a tube, this does come with its own perks, most noticeably the lack of distortion and also a very good amount of detail so it does offer some great traits of both types of amp. Now if this amp will suit you or not really depends on what your after, this is extremely musical, its easy going but it certainly is not neutral or source transparent. The tonal balance can be summed up with a warm, spongy bass, a very lush and forward midrange and then a rather relaxed treble.

The bass is very easy going, it is not the hardest hitting nor is the fastest you will come across but is not trying to be that. It is what is because it sounds very enjoyable, real and of course is warm. You do get a bit of extra decay and there does seem to be an extra bit of mid-bass added. Compared to some other amplifiers such as my CMA800R I do find the that deeper bass does seem a bit less prominent and does lack a bit of rumble, it is not rolled off but it is not digging out the deep notes with any extra bite. The midrange to me is the star of the show, it is the reason you want a tubey sound and this does it so excellently. It is forward, crystal clear and also is very natural sounding. The midrange is addicting, it just sounds so right, vocals are rendered in a way that strikes tingles down your spine and instruments sound rich. The only real nit picks are that sometimes you want the midrange to sit in the mix and not be the start of the show, this is more apparent sometimes on music without vocals. I also find that sometimes the higher midrange just feels a bit light, sometimes in a nice airy way but sometimes in a “could do with a bit more weight” way. The treble is probably why these types of amps do not suit me as much as your more simple neutral offerings. I find the treble to be laid back and smooth and I do like to hear a bit of flare and sparkle up top and this just does not seem to offer that. I do not like an uncontrolled treble but this does not give me that much at all. I guess it could be really nice along with some headphones that need some toning down or if you are someone is generally a bit insensitive to treble. No the treble is still nicely detailed, id super smooth and also extends well so you have an airy sound, it just does not make the sound have the super detailed feel to it, that crystal outline to the music that I am a big fan of. Now I know this is intentional, it is not a flaw and this is what Graham Slee is going for so you shouldn’t worry. 



The amp does a great job when it comes to having a spacious and layered soundstage, this is probably helped on by the resolving warm sound and it really is impressive to listen too. Height and depth are just fantastic and the width is also very well done with a great center image. Instrument separation on a silent background is always a great thing and this combines the two for very nice effect.



If you like a warm spacious sound that is easy going and will never fatigue then this does it with an abundance of details and no distortion and I have to say that is both no easy feat. They have the tonal balance just right for what they wanted to achieve, treble is still far from absence and the bass is far from being attention seeking. If you looking for a reference amplifier that has a dead flat frequency response then there are other options but if you want something that is made to enjoy music on then this is what Graham Slee have achieved in a very understated but versatile package. I can not stress how impressive it is that these can work with just any headphone or earphone because that is just something that not a lot can get right and the fact they have managed it without a variable gain to keep the sound quality consistent is just amazing. Graham Slee has certainly proven too me that he knows what he is doing when building an amplifier and they have made exactly what they say they have made with this product.


Sonny Trigg