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December 2012
Enjoy the Music.com Review Magazine
Exclusive Shootout!
High End Headphone Amp Group Test Shootout!
Bryston BHA-1, Graham Slee Solo SRGII, Graham Slee Solo Ultralinear Diamond Edition, Musical Fidelity M1HPA and Woo Audio WA22 headphone amplifiers.
Review By Phil Gold


Woo Audio WA22 Tubed Headphone Amplifier  More and more people are listening to music through headphones. Of course the iPod, iPhone and other MP3 devices have been a big factor, but today we're looking at the high end of the headphone market. Those who are looking for something better than MP3 quality, those looking for better sound than they can get from the headphone jack in their stereo or CD player, those looking for great sound from their computer or internet streaming device, those who in some cases are looking for the ultimate in HiFi – these are the potential buyers of the standalone headphone amplifier.

We have quite a mixed bag of headamps in this group test. The simplest is the Graham Slee Solo SRG-II, the latest in a long series of refinements of the original Solo design. A plain compact silver box with a volume control and source toggle, it should be placed at a distance from the dedicated PSU1 power supply. Over in the UK, Mr. Slee never sleeps. He has come up with an alternative design, the Solo Ultralinear (UL), which he now offers at a small premium. This version has been designed to offer the sound characteristics of a fine tube amp without actually using tubes to get there. He even offers a kit to upgrade earlier Solo amps to UL DE status. Both headamps offer a choice of two unbalanced inputs and are the only ones in this test with just a single headphone output, although you can drive two pairs comfortably with the aid of a splitter.

The least expensive component in this group is the Musical Fidelity M1HPA but it has quite a few tricks up its sleeve, including a Pure Class A design, a built in DAC and preamp outputs. It features a very large volume control which dominates the front panel and allows finer control of volume than the others in this test. It also offers two standard headphone jacks. It has a very healthy output level and a low output impedance that should allow it to mate well with a variety of headphones.

The Woo Audio WA22 is also a simple device, but it's built on a massive scale and offers two sets of balanced headphone outputs in addition to the regular ¼" jack socket. You can use them all at once if you like, although you cannot control the volume level separately for each output. It's a fully balanced tube powered Class A output transformer-coupled design. It features a 5AR4 rectifier tube, two 6SN7 driver tubes and two 6AS7 output tubes and weighs a massive 25 lbs. With the standard tubes it retails for $1900 but you can easily spend over $2500 if you specify upgraded tubes. You can select between balanced and unbalanced inputs on the rear panel, and between low and high impedance outputs on the front panel.

The newest component is the Bryston Audio BHA-1 and it offers the most flexibility of the bunch. It has the same three outputs on the front panel as the Woo Audio, but it also doubles as a preamp with balanced outputs on the rear panel paralleling the balanced headphone outputs up front. You get a front panel selector for balanced input, unbalanced input or mini-jack input and this is the only model here to offer a balance control. The BHA-1 has been a long time coming as Bryston tried out many different designs, soliciting user input along the way. As launched earlier this year and reviewed elsewhere, the BHA-1 did not have the preamp output, and I am very pleased to see this additional feature, since it works remarkably well as a fully balanced preamp.  Internally the analog output derives from six Class A fully discreet operation amplifiers of Bryston's own design.

Sadly none of these units has a remote control. Perhaps most users sit close by their preamp – I don't.

Let us take a closer look at the units in operation.


Graham Slee Solo SRGII and Ultralinear Diamond Edition
The Solo SRG-II is a very clean sounding wide-bandwidth unit. In the past I have much preferred it to the Solo Ultralinear when using sensitive headphone like my Ultimate Ears UE10Pro custom fit canal earphones. The SRG-II is silent as the grave but early Ultralinear models produced an uncomfortable level of hiss in this application. Mr. Slee never stops working to improve the Solos and recently shipped me a revised model from current production (the Diamond Edition) which not only puts all such noise issues to rest, it also improves clarity and dynamics. While the Solo SRG-II and Solo Ultralinear DE are clearly from the same family and look almost identical, the Ultralinear is warmer, more three dimensional and simply more musical to these ears. I think it now clearly eclipses the SRG-II and is well worth the price differential. With the Ultralinear DE you get a very immersive experience. While on paper its rated output is lower than the others here, it has enough power to drive the inefficient Sennheiser HD800 to high volumes while the volume control is calibrated well enough to cope well with efficient phones too.


Musical Fidelity M1HPA
In terms of sound quality, the Musical Fidelity is a close match to the Solo SRG-II. It is very quick like the Solo but has a somewhat leaner balance. It is capable of driving the most inefficient phones with ease and it has a very sweet treble. I didn't try it as a DAC – I understand this is a convenience feature and not designed to be a high performance option like you see on Musical Fidelity DACs. But I did use it extensively as a preamplifier as I will discuss later.


Bryston BHA1
If the Musical Fidelity is a close sonic match to the Solo SRG-II, it will take some kind of sonic microscope to separate the Solo Ultralinear from the sound of the Bryston BHA-1 through its unbalanced output jack. Both of them reach very high levels of performance with every phone I tried, and I strongly suspect they sound so alike because they are converging on some kind of acoustic truth here. Both have superb imaging, especially with the canal type phones, and I had no difficulty listening for long periods. Both offer a very evenly balanced frequency response, vanishingly low levels of distortion and a high level of resolution coupled with powerful dynamics, where perhaps the Solo Ultralinear does pull slightly ahead.

The Woo Audio WA22 is the final amplifier in this grouping. It is hand built using point to point wiring throughout, which is much more expensive than printed circuit boards assembled by machine. It is based on a true balanced topology and no semiconductors are used in the design.

Sadly it had to go back before the Bryston arrived, so I was not able to compare them side by side. I did listen to it next to all the other amps and it certainly is a contender in terms of sound quality, especially when used in balanced mode. As a tube amp you'd expect it to be on the warm side of neutral, but if this was the case it was only to a very slight degree. It lacks preamp outputs but otherwise it has similar features to the Bryston, and rates even higher in terms of maximum output, making it the best placed to fully drive the AKG K1000s.

But like the first version of the Solo Ultralinear, it is not a good match for highly efficient phones like the Ultimate Ears. The problem is not hiss this time, but hum. In fact hum was present with all phones to a certain degree, more on the left channel than the right. Despite all efforts to avoid grounding problems, I could never completely eliminate it. It seemed to be independent of volume level. You could only just hear it on inefficient phones like the HD800s where it was inaudible when music was playing, but it intruded with the Ultimate Ears so that even though the sound quality was clearly excellent, the hum for me was a distraction. Now your mileage may vary and hum may never be a problem for you but I strongly recommend you try the Woo Audio in your home system before committing. You can try different tubes to suit your taste and your wallet and the good guys at Woo Audio offer a great deal of advice on their website in this regard. This is the only amp here which can be tweaked in this way. I did some tube rolling and could easily hear the benefits of the upgrades which added a greater sense of ease and intimacy to the already fine sound.

The Sennheiser HD800s are the only phones that I can drive both balanced and unbalanced, and I am very grateful to George Cardas for supplying his superb Clear headphone cables in both balanced and unbalanced formats for use in these tests. Now that I've done the comparison I am convinced that the balanced option is not just of theoretical benefit, even if it academic for most listeners whose headphones may not offer a balanced cable option.

While most of my listening comparisons are centered around the HD800, I also used Ultimate Ears UE10Pro and AKG K701 phones on all the machines in this group test. The results are consistent all the way through, so I will not detail them separately. Listening tests involved recordings of the highest quality including Schubert's Death & The Maiden Quartet by the Lindsays [CD RSB 403], "Alfie's Theme" by Sonny Rollins [Impulse IMPD 224], "While My Guitar Gently Weeps" from the Beatles Love Album [Capitol 094637981023], MA on SA [Crystal Cables Sampler] and Bartok Concerto for Orchestra conducted by ZoltanKoscis [HungarotonHSACD 32187].


Woo Audio WA22
The WA22 has the highest output level of any headamp here, taking all the inefficient headphones in its stride. As a tube amp it has settings for low and high impedance phones and I found it best just to try each setting and settle for the one that sounded best. The imaging is stronger than all others too, which helps to alleviate that in-the-head sound some find objectionable with headphones. I don't suffer from that symptom, perhaps due to my choice of open headphones with drivers positioned forward and away from the ears, such as the AKG K1000 and Sennheiser HD800. The one issue I had with the Woo Audio was also an issue with a number of other tube-based components – a low level of tube hum, especially in the left channel. This is not an issue with most headphones but for high efficiency phones such as my Ultimate Ears UE10Pro canal phones, this hum is problematic. Using the Sennheiser HD800 this was a non-issue and I'd have to say the sound is a small notch above all the others, exceptionally present and colorful while at the same time high in resolution and well balanced.

Woo Audio offers a large range of tube based headphone amps. This one sits around the middle and is optimized for full sized relatively inefficient headphones. If you are driving more efficient headphones you might consider the WA6.


Balanced Headphones
The best sound, without question, comes when I replace the unbalanced Cardas Clear headphone cable on the Sennheiser HD800 (itself a major upgrade from the stock cable) with a fully balanced Cardas Clear headphone cable terminated in a pair of three pin DIN plugs, one for each channel. Connecting these to the Bryston gives a subtle but significant jump in the size of the image, the level of detail and hence the musical realism. When played in this fashion, balanced in, balanced out, the Bryston has a clear advantage over the Solo Ultralinear, which like most headphone amps lacks the balanced output option.

The famous old AKG K1000 ear speakers have seen little use in recent years in my house, because they need the direct output of a power amp, and I haven't had a power amp with speaker switching since my old Perreaux Radiance died. The K1000 has a two piece cable, with a DIN connection mid length. This DIN connector allowed me to try it with the Bryston, as long as I sat within a short distance from the amp. The K1000 is very inefficient indeed and even the powerful Bryston could not drive it to high levels but within a civilized volume limit it performed very well indeed. The sound is definitely tipped up towards the high frequencies, but this fault cannot be laid at the foot of the Bryston. The combination is stunning in its clarity and may be ideal on those recordings that possess something of a dark balance. Finally James Tanner of Bryston lent me his personal pair of Grado PS1000 phones, with a single balanced connector like the AKG K1000. Once again the Bryston drove these phones very easily and the sound was enormously engaging. I felt these phones were editorializing the music to a certain extent, making it more colorful than through the Sennheisers, and this was confirmed by switching to my reference speakers, the YGCarmels. The Sennheisers sound quite similar to the Carmels, but the AKG have quite a different balance. In any event I could only try the Grados through the Bryston, so I can only use the experience to indicate whether the Bryston would make a good partner for them rather than to compare the Bryston to the other amps.

The Woo Audio also sounded best through its balanced outputs but showed less of an improvement when moving from unbalanced to balanced headphones than the Bryston. It still retains the smallest of leads over the Bryston with balanced phones due to superior instrumental color that enriches vocals, strings and wind instruments alike.


The Musical Fidelity And Bryston As Preamps
I used the Musical Fidelity to drive a Bryston 4BSST² and a ModWright KWA150SE through their unbalanced inputs. The sound was entirely consistent with what I was hearing through headphones. Dynamic, quick, high resolution but lacking somewhat in presence and a little bass shy. By comparison the Bryston was more three dimensional, warmer, richer and altogether more competitive with dedicated preamps. However the Bryston's preamp output level seemed uncomfortably high in comparison with other preamps, so all adjustments were made in the first 15 or 20 degrees of rotation with balanced inputs, even on the low output toggle. If you use an unbalanced input this situation improves.

For sound quality, the Bryston does exceptionally well. This is especially truewith a balanced input. It may in fact sound better than a Bryston standalone stereo preamp since the discrete operational amplifiers used here are of a new iteration developed for the new Bryston Surround processor. Bryston has found a way to reduce the parts count and this is said to be responsible for greater presence and lower noise. Of course the calibration of the volume control may be a better match for power amps with lower or adjustable gain (Bryston power amps have such a switch) and remote control would be very valuable here. Substituting the costly EMM Labs Pre2 preamp quickly shows the sonic limitations of both Musical Fidelity and Bryston headamps by opening up a wider and deeper soundstage with increased density of musical information and openness at frequency extremes. Nevertheless, the Bryston's quality is fully competitive with stereo transistor preamps in the $1000 to $3000 range.  


A Few Words About Build Quality
Top marks here go to Musical Fidelity, with a silk touch to the controls and a very high level of fit and finish. The Bryston comes next, and remember that the BHA-1 comes with a 20 year warranty so you know it is very well screwed together. The Woo Audio is big and heavy and well-built but perhaps a little old fashioned in its physical presentation. The two Solo models are in fact very well built, but they don't look the part. Being so small and with the compact external power supply you might not at first think that these are high quality components. In fact the small dimensions are deliberate. It is much easier to control resonances in a small box than a large one, and keeping the power supply at arm's length is the best way to keep the noise level at a minimum. All Graham Slee components are small, some much smaller than these. I've had many Graham Slee components through my house over the years and never had any issues at all, for what it's worth.


The Verdict
There are five winners here. Each unit was selected for inclusion on the basis of a very strong reputation from people I trust, and each has its particular strengths.

Graham Slee Solo SRGI
This is the unit I've lived with the longest and it's a strong performer if you're looking for a compact reliable unit. Although I rated its sound below the Solo Ultralinear, you should know that there are some that prefer it for use with particular headphones and it does save you a couple of hundred dollars. There is never a hint of noise with this unit, resolution is high, distortion non-existent and it's thoroughly musical.

Graham Slee Ultralinear Diamond Edition
While early Ultralinear samples were noisy when paired with high efficiency phones, this problem has now been licked making this model much more widely applicable. The superior imaging and greater musicality on all the single ended phones in my collection make this component worth the extra money. You can't beat the sound at this price and this is the one I'm keeping.

Musical Fidelity M1HPA
This versatile and good looking box comes in at an unbeatable price and has the best ergonomics of the pack together with useful extras such as a preamp output and USB DAC. While it doesn't match the sound quality of the best models it isn't far off at all, and I recommend it strongly.

Bryston BHA-1
If I had to pick a winner here it would have to be the Bryston. It is a close match to the best in performance while offering superior versatility and value. It is the least expensive of the headamps here that support balanced operation, which I have found invaluable. Frankly I think it is underpriced and with the recent addition of balanced preamp outputs I doubt it will stay at the original $1295 for long. I predict a smash hit for Bryston. I'd like to see Bryston introduce another model sitting above the BHA-1 with multiple balanced inputs, higher maximum output, a remote control and a volume control covering a wider range of output levels.

Woo Audio WA22
This is a superb sounding full-blooded headphone amp which users can tweak to their heart's content through tube rolling. The Woo Audio website gives extensive advice in this regard and you can order various upgrades directly from them. In my system it was let down by a small hum issue which precludes its use with high efficiency phones, but on the other hand it is the only device here which can drive the AKG K1000s fully. It has a different balance to the Bryston, a little warmer and even more dynamic, while not quite as clean and resolute in the bass. It is the most expensive model here but a very strong candidate if you are driving balanced headphones, in which case you should audition it up against the Bryston BHA-1.


                      Solo SRGII       Solo UL        BHA-1               M1HPA         WA22  Weighting

Versatility:         5                     5                   10                       8                 9          15%   

Max output:       5                     6                     8                      8                 10         10%

Noise level:     10                    10                    10                     10                 3          15%

Sound Quality:  8                    10                    10                       7                10         35%

Imaging:           8                     9                      9                      8                  10         15%

Build:                8                     8                      9                    10                   9          10%


Overall:          75.5%               85.0%               95.5%               81.5%               87.0%

Price:             $999                 $1195               $1295               $799                 $1900

Value:             3.5                    4.0                    5.0                    5.0                    3.0


Specifications /  Company Information
Graham Slee Solo SRGII
Phone impedance:        16 to 600 Ohms preferred; 8 to 2,000 Ohms acceptable
Headphone Jack:          1/4 inch (6.35mm) A gauge (BPO B gauge to special order)
Power output:               32 Ohms: 27mW / channel; 600 Ohms: 23mW / channel
Distortion:                     0.02% (THD plus noise, ref 1kHz)
Frequency response:     32 Ohms load: 27Hz - 35kHz; 600 Ohms load: 10Hz - 39kHz (-3dB)
Output noise:                -84dB (20Hz-20kHz, CCIR quasi-peak)
Input sensitivity:            32 Ohms: 250mV rms; 600 Ohms: 775mV rms
Channel balance:           Better than 1dB, "9 to 3 o'clock" positions
Crosstalk:                     Left to Right -57dB; Input to Input -68dB
Mute:                            Signal off mute, non-shorting
Output Stage:               Bipolar class AB
Supply voltage:                         24V DC unipolar
Size:                             4.2" x 2" x 7.3"(WHD)
Website:                       www.gspaudio.co.uk


Graham Slee Solo Ultralinear Diamond
Phone impedance:        16 to 600 Ohms preferred; 8 to 2,000 Ohms acceptable
Headphone Jack:          1/4 inch (6.35mm) A gauge (BPO B gauge to special order)
Power output:               32 Ohms: 140mW / channel; 600 Ohms: 30mW / channel
Distortion:                     < 0.04% (THD plus noise, 10Hz – 20kHz)
Frequency response:     10Hz – 35 kHz (+0, -3dB)
Output noise:                -95dB (22Hz-22kHz, unweighted quasi-peak)
Input sensitivity:            511mV rms
Input impedance:           37 kOhms at max volume; 50 kOhms at min volume
Channel balance:           Better than 1dB
Crosstalk:                     Left to Right -56dB; Input to Input -68dB
Mute:                            Signal off mute, non-shorting
Output Stage:               Bipolar class AB
Supply voltage:                         24V DC unipolar
Size:                             4.2" x 2" x 7.3"(WHD)
Website:                       www.gspaudio.co.uk


Musical Fidelity M1HPA
Output Power:               1.1W into 32 Ohm
Output impedance:        < 1 Ohm
Output level:                 5.2V rms max, 1.1W into 32 ohms
Output impedance:        < 1 Ohm
Frequency response:     15Hz to 75kHz (+0, -3dB)
Signal to noise:             >109dB "A"- wtd
Distortion:                     < 0.008% 10Hz to 20kHz
Inputs:                          Line level input 1 pair RCA in
Outputs:                       Line level outputs 1 pair RCA out, 1 pair RCA pre-out, 2 headphone outs
Digital input:                 1 USB type ‘B' connector for computer/PDA
Power:                          90 - 250 V 50/60Hz, 25 watts
Weight:                        3.4 kg (7½ lbs), 4.1 kg (9 lbs) in shipping carton
Dimensions:                  8⅔" x 4" x 12" (WHD)
Standard accessories:   IEC type mains lead (10-Amp type)
Website:                       www.musicalfidelity.com


Bryston BHA-1
Inputs:                          Balanced XLR Pair, Single Ended Pair (RCA), 3.5mm Stereo Mini Jack 
Outputs:                       Balanced XLR (4 Pin), Balanced XLR Pair (3 Pin), 0.25" Headphone Jack
Preamp Outputs:           Balanced XLR Pair
Control:                        Remote Trigger Input (5 - 12V AC/DC @10mA)
Power:                          Rated at 0.5 wpc into 32 Ohms or 0.66 wpc into 600 Ohms
IM distortion:                    <.001%
Harmonic Distortion:      <.003% at 32 Ohms at 100 mW (20Hz -20kHz)
S/N:                              >105dB
Frequency response:     20Hz to 20kHz ± .1dB
Balance control:            Included
Analog Output:              Six fully discrete Class A Bryston operational amplifiers
Gain:                            14dB or 20dB switchable
Input impedance:           10 kOhms
Volume control:High quality noble stereo volume control (laser trimmed)
Connectors:                  All gold plated
Finish:                          Available with Silver or Black Faceplate (2U)
Faceplate:                     17" or 19" Faceplate available
Dimesions:                   3⅛" x 17" x 11¼" (HWD) or 3⅛" x 19" x 11¼" (HWD)
Weight:                         8 lbs
Rack Mounting:             Available only in 19" Black
XLR Output connectors:   Locking mechanisms, Female or Male Balanced XLR available
Website:                       www.bryston.com


Woo Audio WA-22
Phone impedance:        8-600 Ohms
Input impedance:           100 kOhms
Frequency response:     8 Hz - 50 KHz, -3dB
Output:                         1.5 wpc @32 ohms, .7 wpc @600 ohms
Signal/Noise:                92 dB
THD:                             <= 0.3%
Voltage:                        AC 110/220V, 50/60 Hz
Rectifier tube:               1 5AR4 rectifier tube. Direct substitute: 5U4G
Driver tubes:                 2 6SN7 drive tubes
Power tubes:                 2 6AS7 / 6080 power tubes
Volume:                        DACT CT2 balanced stereo stepped attenuator
Inputs:                          One balanced XLR input, one single-ended RCA input
Headphone outputs:      Two 3-pin balanced, one 4-pin balanced output, single-ended
Impedance switch:         High and low impedance settings
Finish:                          Black or silver
External dimension:       12" x 7" x 10.5" (WHD)
Weight:                         25 pounds
Website:                       www.wooaudio.com  













































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