Graham Slee Solo SRG II and Solo Ultra-Linear Headphone Amplifiers Review

There’s a real English gentleman I’d like you all to meet. Assisted by two talented engineers that build the products, he designs solid state headphone amplifiers and phono-preamps in small innocuous looking anodized aluminum boxes, and he’s gathered a following all over the world without much fuss or a big advertising budget. I’ve been a fan of Graham Slee for at least seven years and I should tell you why. In my experience, his products are absolutely reliable, very well priced and upgradeable, but more than this, they sound wonderful, fully competitive with much more expensive units. In 2004 I bought a Graham Slee Solo headphone amp and a Graham Slee Era Gold V phono-preamp and they’ve been my references ever since. Reviewing the Solo MC in 2005 I wrote “The strengths of the Solo are in the extra high frequency extension, a superb transient response and an unsurpassed dynamic range.”

Today I’m going to look at two current Solo models to see if anything has changed. Thanks to the strength of the Canadian dollar the price has dropped from $1299 to $999 for the Solo SRG II, the current equivalent, and even the upscale Solo Ultra-Linear lists for just $1199. The casework is slightly changed, a millimeter or so taller, with nice looking grooves added on the top panel and a ground connector at the rear, but otherwise no big visible changes were made. You still get a simple ALPS volume control (no remote) and a three way toggle switch (input 1, input 2 or mute) on the front panel, with an indicator light – once white but now a more discreet green. The back panel has two sets of RCA inputs and a DC24V input socket. Graham Slee believes it best to keep the power supply outside the cabinet to minimize stray fields that might induce hum or otherwise affect the low level signals involved, and he includes a PSU1 fully regulated power supply with the Solos. This too is unchanged over the years apart from the mains connector, once captive, now socketed to meet European regulations.

So what’s changed inside the box? Let’s start with the Solo SRGII. There have been numerous revisions over the years, all aimed at improving the sound quality and extending the range of headphones that can be well driven by the Solo. One particular innovation is line driver load sensing that enables the Solo to adapt to the impedance of the headphones in use. One consequence of this change is that the volume control is calibrated quite differently than on a conventional amp, where you would expect a setting in the 9 to 12 o’clock range, and large amounts of distortion if you go too far clockwise. Here the whole range is usable to adjust to headphones of very different sensitivities. There were times with very inefficient headphones and a low output source that I was in the three or even four o’clock position, with no noticeable stress to the sound. Changes to the circuitry between the Solo MC and SRG II were particularly effective at improving the low frequency performance capabilities, while refinement of sound, separation and dynamic range have all steadily improved.

So today’s SRG II is a significantly better performer than the Solo MC, with the biggest improvements in the bass and the all-important midrange. Maximum sound pressure levels are higher while the clean treble and lightning reflexes have been maintained. Distortion and background noise are lower than before, making it easier to listen to the music, to enjoy the texture of the instruments and voices.

I have my own special way of testing headphones and head amps. I compare them directly with my reference Wilson Benesch Act 1 speakers, fed by a Parasound JC2 Preamp and the mighty Bryston 4B SST² power amp. By carefully adjusting the volume control on the headamp, I can pull the headphones on and off quickly to see how the headphone sound stacks up. With AKG K701s, I still hear a big difference. The AKG is much less complex musically, cooler, and more strident. They are still great phones, but they don’t stand up to this particular test. But when you substitute the far more expensive Sennheiser HD 800 with a Cardas Clear headphone cable, you’re getting into the same ballpark. The loudspeakers are still more relaxed and musical, sweeter and more three dimensional, but the differences are not great, while the balance across the frequency spectrum is quite similar.

Now let’s move on to the new top of the line Solo Ultra-Linear. The differences between this and the Solo SRGII are quite small, but also quite significant. The Ultra-Linear is the warmer, more natural sounding model, but the differences are less apparent on A/B switching than on extended listening. I found the more expensive Ultra-Linear allowed me to relax more into the music, and appreciate the texture of the instruments and voices. More significantly, the imaging felt more three dimensional and realistic. The frequency response seems pretty much the same, with a very wide extension at both ends, but the treble is sweeter and the bass end has more presence. That does not mean the Ultra-Linear is more accurate, indeed the bass performance of the SRGII may be the more controlled, but the Ultra-Linear sounded more musical. This is a feeling many people have about tube amps as compared to transistor amps, although there are no tubes involved here. Instead Mr. Slee has emulated a tube design while still using transistors. I don’t understand how he’s done this, and he’s none too keen on spilling the beans lest other manufacturers copy his lead. I think he has been successful in bringing the extra warmth and musicality of a good tube design into the Solo Ultra-Linear but we have none of the usual problems with tube failure, excess heat production, high voltages and so on. There is one drawback in the slightly increased level of background noise, but even if it measures noisier, I didn’t experience any issue with either the AKG or Sennheiser phones.

My final test was with the far more sensitive Ultimate Ears UE10Pro custom fit canal earphones. The Solo MC does a good job with these superb phones, but the SRGII is a much better match, since I’m not left struggling with the first 15 degrees of turn in the volume control anymore, and the frequency range seems more extended than before, while the level of detail is significantly higher. But switch to the Solo Ultra-Linear and there is a bigger jump in performance than I noticed with the full size headphones. The music is much warmer and more realistic, without losing any of the detail or extension. It just felt more full bodied and spacious in every direction – a marriage made in heaven. Not as detailed or as open topped as the Sennheiser, but fuller in the bass and with a strong dose of magic, this is the first head amp to draw so much out of these miniature marvels. There is one small caveat. Regardless of the volume setting, there is a constant low level of hiss with the Ultra-linear that is entirely absent with the SRGII. You may not notice it when the music is playing but it is apparent between tracks.

I pulled out all the phones I could find. The Sennheiser HD580 and Sony MDR-V150 showed no hiss at all, the Klipsch Image One had barely detectable hiss, while the Sleek Audio CT6 and Sony MDR W25 showed a slightly higher level than the UE10Pros. Phones that are designed for MP3 players and iPhones may be problematic, while full size phones designed for serious listening will show no symptoms. If you are using exceptionally efficient phones you should try before you buy the Ultra-Linear or stick to the SRGII.
What if, like me, you own an older Graham Slee Solo? Should you pull your hair out over the major refinements available in the SRGII, or the additional but smaller benefits of the Ultra-Linear circuitry? This is where the story gets interesting. You can buy a do-it-yourself upgrade kit to bring your Solo right up to the Ultra-Linear specifications. No need to change anything in the chassis or power supply. Your job will be to remove the old circuit board and replace it with a new one. The upgrade is available for any Solo made since early 2004.

The upgrade kit will set you back just £175 (plus shipping and taxes if applicable), and the skill level required is not very demanding. There is no soldering required, but you will need:

•   A small adjustable wrench
•   2mm AF Allen Key
•   Small flat blade screwdriver
•   M4/7mm AF open ended spanner or nut
spinner
•   No 1 Philips screwdriver
•   No 2 pozi-driver or cranked pozi-driver
See: www.diy-audio-kits.com/kits/diysoloUltra-Linearheadphoneampupgrade.htm.

If this task looks too daunting for you, a factory upgrade is also available. This will involve packing just the main chassis and shipping it over to the UK and forking out £219 (plus shipping and taxes if applicable). The fact that Graham Slee is constantly working to improve his products is of course a good thing. The fact that he offers you two different ways to upgrade your old model to current specs is particularly commendable. How many companies offer that level of service?

If the Solo SRGII ($999) and Solo Ultra-Linear ($1199) are beyond your budget, Graham Slee has three less expensive models that might interest you. An entry level Solo SRGII package is offered with a switched-mode power supply. The Novo Discrete headamp is available assembled ($499) or in kit form. A portable headamp, the Voyager ($349) rounds out the range.

If you can afford the best, remember you can spend a whole lot more and not find any significant improvement over these two Solo models. And yes, I think the extra $200 is well spent on the Solo Ultra-Linear, which is why Graham Slee no longer makes a kit to bring your old Solo up to SRGII specs.

Whatever your taste in music, this formula holds: Sennhesier HD800 + Cardas headphone cable + Solo Ultra-Linear = A high level of musical enjoyment.

Graham Slee
www.gspaudio.co.uk
+44 (0) 1909 568739

Distributed in Canada by Planet of Sound HiFi Inc.
www.planetofsounddistribution.com
(416) 461-3939

Graham Slee Solo Ultra-Linear
Price: $1,199

Graham Slee Solo SRGII
Price: $999

SIDE BAR: Why would you even want a headphone amp?

If you’re into high end audio already, you probably won’t be asking this question. Your main amplifier or preamp most likely does not have a headphone output. You may connect a high quality headphone amp directly to the unbalanced output of your CD player, while the balanced output goes to feed your preamplifier or integrated amplifier. Or you may connect the fixed line level output from your preamp to the headphone amp.

But if your CD player or amplifier already boasts a headphone output jack, why complicate the picture with more boxes and cables? Is the quality so much better? That depends on how good the headphone circuitry is in your current component. Often it’s an inexpensive chip that’s driving the headphone socket, and the limitations are not just in terms of quality. It may do a reasonable job with an easy to drive pair of headphones, not as well as a component like the Solo, but good enough. But if you’ve invested in a high end set of phones like the Sennheiser HD650 or HD800, or a top model from one of the other competing manufacturers, you’ll find these can be very demanding. They will reveal every fault in the headphone circuit of your amp or CD player, and they may very well prove an electrical mismatch in terms of impedance or sensitivity for the headphones you’ve saved up for. With that mismatch you’ll find an uneven frequency response, a lack of dynamics or even a plain inability to drive the phones to realistic levels. You’ll also hear a very flat soundstage, with sounds at each ear rather than appearing solid in space as the best equipment can manage. You certainly won’t get the superb definition and tonal accuracy that today’s best phones can render.

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