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January 2005
Enjoy the Music.com Review Magazine

Graham Slee Solo MC Versus The Aural Audition
Head To Head Headphone Amplifiers
Review By Phil Gold
Click here to e-mail reviewer


  This is a new story of David and Goliath....





David is represented here by the diminutive Graham Slee Solo MC Headphone Amplifier, a handcrafted plain silver anodized case weighing in at $950 US and 5 lbs.

PSU-1The Solo sports two sets of high quality gold plated RCA inputs, with a small front mounted toggle switch to choose between them. The central position on this switch is for muting, and Graham advises the unit be left on at all times with the mute on if you don't want sound from the headphones. No on/off switch is provided, since the longer the unit is left on, the better the sound. A single front mounted standard 0.25-inch headphone jack also graces the front panel, along with the specially selected Alps volume control. The single jack outlet can be used to drive two pairs of headphones having the same impedance using a headphone splitter lead. Power is provided by an outboard power supply, the PSU-1 as seen here.

Graham Slee designs and builds these units by hand in Yorkshire, England, alongside the much acclaimed Era Gold Phono Preamplifier.

Inside The Aural

Goliath, or the Aural Audition Headphone Amplifier, is simpler still. It sports just one box, one set of inputs, one headphone jack, and a large volume control with 24 positions. It comes in a very substantial and beautifully finished aluminum case and weighs in at $2,195 and 17 lbs.

The Audition is fed by an internal dual power supply rated at 200mA which features dedicated windings from fully potted mains transformer specifically designed for this amplifier. This proprietary power supply is a very high quality, wide band and low noise PS/regulator that also incorporates fast recovery diodes in the power supply and only FETs (JFETs and MOSFETs) in the regulators. Maximum input voltage is ±45V and maximum output voltage is ±40V. The headphone amplifier module is an ALL-FET line amplifier. Only FETs (JFETs and MOSFETs) are used as active elements in the amplifier. The resistors are all high quality Vishay-Dale and Caddock. All electrolytics are Nichicon FINE GOLD MUSE and the compensation caps are MICA or polystyrene. Two amplifiers are laid out on one board. The PCB is Teflon and the traces on the Teflon board are 70 microns in thickness. The amplifier module is biased at 80mA that facilitates "Class A" operation driving loads between 32 and 600 Ohms. All of the internal signal wiring is LEMO copper Litz that is also silver coated. The stepped attenuator is the DACT CT2-10k attenuator.

The Audition is designed in Germany, like all of Aural's product line, by Erno Borbely, and made in Jacksonville, Illinois. It takes a long time to get to grips with the sound of these two head amps, since they share more traits in common than the differences that divide them. Both impress with black backgrounds, high levels of detail and a complete absence of distortion. They share also a well-balanced frequency response and can dig strong bass response out of the Sennheiser 580s used in the testing.


Getting Heady

The strengths of the Solo are in the extra high frequency extension, a superb transient response and an unsurpassed dynamic range. Percussion, flutes and bells all have an extra sparkle that breathes life in to the music. The wide dynamic range and transient response can make some recordings uncomfortable, for no flaws will remain unhidden under this spotlight. The soundstage is wide, but there is little evidence of the hole-in-the-middle syndrome that headphone listening can sometimes provide.

The Audition brings more warmth into play, more color to the instruments, and a greater presence in the bass. It doesn't have the dynamic range or the transient snap of the Solo, and this makes for a more relaxed listening experience, like moving from transistors to tubes. I say that even though the Audition is also a transistor design, albeit class A all the way. The width of the soundstage is narrower than the Solo, and to my ears at least, more convincing on most of the material at my disposal.

Mahler's First Symphony, played by the Concertgebouw Orchestra under Leonard Bernstein [DG 427303-2] is a challenge for the largest speakers, never mind the little transducers inside a pair of headphones. How would these head amplifiers fare? The Solo is impressive here, offering excellent clarity of line and tremendous dynamics. As the first movement drew to a close I found myself out of my seat and wind conducting up a storm. The cuckoo's repeated calls jump out, the bells peel true, and there is a tremendous sense of pace and rhythm. The Audition shows its strengths in the wonderful color of the various instruments, especially the string tones, and sustained power in the bass. This is a more organic sound, revealing extra subtleties in the playing, especially in the low level detail. The Audition excels at revealing the clarity of a multitude of simultaneous musical lines, even when some are much louder than others. Without wishing to detract from the Solo's achievements in this area, the Audition takes this accomplishment to a higher level and in this music, which gives it the edge.

I love the sound of the Beatles Yellow Submarine Songbook [Capitol 7243 5 21481 27]. Here are the tracks we all know so well, portrayed in all their glory for the first time. "Eleanor Rigby" quickly reveals the differences between the two contenders. The Audition is magnificent, with strong biting strings flanking Paul's vocal lead. Even at elevated volumes, there is no trace of distortion. The Solo gives a wider spread and an over the top dynamic, losing some of the coherence of the image and also some of the warmth and ambiance. "When I'm 64" reinforces the impressions from the Mahler and "Eleanor Rigby".

On highly dynamic recordings, I would give the edge to the Audition, and it would get my vote also for any thin or edgy digital recordings where the extra warmth and a slightly reticent treble work to its advantage. But there are plenty of recordings that call out for the dynamic strengths of the Solo. If the stereo mix offers a narrow soundstage, the Solo will get the most out of it. If the recording does not have a lot of dynamic range, the Solo will preserve whatever is in the grooves, and if the recording lacks crispness already, you may prefer the Solo's way with it to the smooth Audition. "Tears In Heaven" from Eric Clapton's Unplugged [Reprise 9362 45024 2] sounded more realistic through the Solo than the Audition. You can hear the leading edges that separate the notes, while the Audition blurs the notes together. "You can't always get what you want" on the Rolling Stones magnificent sixties album Let it Bleed [abkco 90042 - this record should be played loud] also thrives on the Solo, with greater articulation in the opening choir and a well balanced sound throughout. The Audition gives us a flat sound to the choir and never provides this track the edge it deserves. Hank Williams' classic "Cold Cold Heart" appears on the Nora Jones album Come Away With Me [Blue Note 7243 5 41747 2 8], and again the Solo shines with a more intimate and realistic voice and a crisper piano sound. On this and others tracks on this album, the Slee has the fuller bass, while the Audition's bass is more agile and the voice warmer.

The Quatuor Mosaïques playing Haydn's String Quartet Opus 20 No 2 [Astrée E8786] leaves me torn. I like the Solo's strong separation between instruments and openness at the high frequencies, but the Audition counters with increased presence, greater realism and fills in the hole in the middle. Brendel's Beethoven [Philips 446 701-2] creates another dilemma. The Solo brings a very good piano singing tone and the size and scale of the instrument is as I remember it from live performance, but the Audition has the more beautiful sound, but the bass runs are less clear. I know my Brendel, and the clarity of his left hand should certainly be a standout feature, as it is with the Solo.

I'm not normally into headphone amplifiers, since I usually drive my AKG K1000 ear-speakers directly from the binding posts of my Perreaux Radiance R200i 200wpc channel amplifier. The Perreaux has two sets of binding posts, and the second set is switchable. I have my loudspeakers connected to the second set, and I can toggle them on or off from the remote control. The AKGs are always in circuit, except when I unplug them.

The Aural Audition Headphone Amp is man enough to drive the AKGs, although the volume control runs out of notches before you reach head-banging levels. This achievement puts it in a select group of head amps, since the AKGs are perhaps the least efficient dynamic headphones ever made. I spent a lot of time comparing the performance of the Audition to the Perreaux through the AKGs, and I'm impressed with both, but for different reasons. While both major in low distortion and high levels of detail, the Perreaux exerts an iron grip on the sound, with outstanding dynamics, fast transients and an ultra wide band frequency response. The Audition offers a warmer and more intimate sound, with a narrower image width and a smoother high frequency sound. I preferred the Perreaux for well-recorded source material, while on many discs with brittle digital sound, the more forgiving nature of the Audition gave it the edge.

I tried the AKGs with the Solo but it simply did not have the power output to drive them to a reasonable volume on dynamic recordings. No problem, since The AKGs will defeat most head amplifiers.

I could be very happy with either of these superb headphone amplifiers. On balance I prefer the Audition, but it's a win on points, not a knockout. It may not be fair to match a welterweight and a heavyweight in the same ring. Taken pound for pound or dollar for dollar, the Solo comes out ahead.


You pays your money and you takes your choice.


Interview With Graham Slee

Phil: Your products are very compact. Is there a sonic advantage to the small size?

Graham: The enclosure is compact because the PCB is compact. As well as audio I designed high-speed logic boards for a number of years. Many of the glitches that cause this technology to fail are directly attributable to misunderstanding of PCB layout. By using similar techniques in laying out audio PCBs many of the glitches that afflict audio can also be rectified. It just happens that in some cases the layout turns out to be small.

Phil: In the Solo you are using a potentiometer volume control. Have you experimented with other types of volume control, and why did you choose this one?

Graham: The one used in the Solo (made by Alps) was found to be the most reliable and to yield excellent matching results. It is a tried and tested design having a long history of use in audio. Other trendy controls could be used and send the price into orbit. Some say I should use an Alps "Blue Velvet", but it's just made like any other pot - a carbon track deposited on paper-phenolic board - you'd expect fiberglass for the price! Switched attenuators are also quite trendy, but I can't get over the fact that at each setting there is a contact exhibiting contact resistance as well as a soldered joint...

Phil: There are not a lot of components inside the Solo. Is a low component count a design objective?

Graham: The design objective is to make a product that people will enjoy listening to music with. Some people like to see a minimalist design, some want to see a massive number of components, some just want to listen to music. The Solo design represents the maxima on the design curve.

Phil: The product has evolved considerably over the years. What improvements have you made? Are there any new developments around the corner or have you taken this design as far as it will go?

Graham: The product has evolved through listening to the consumer. I wanted to make the best sounding audio in cost effective black plastic boxes that everybody could afford (Mk I). But it seems I was in a minority of one. So the first improvement was a metal enclosure (Mk II).  The sound I started with was one which captured the emotion of the music, but as some source equipment and headphones couldn't do this, I had to make it sound more in keeping with what was expected, while retaining the emotional characteristic for those who could appreciate it (Mk III). The Solo began to be accepted by the famed Head-Fi forum and I took on board many of the comments like "please reduce the gain". The "chip-rollers" made a number of suggestions and I tried a number of different op-amps in the voltage amp stage, and in early 2004 a different op-amp and recalculated circuit was introduced. As part of the recalculation the simulated electronic capacitor in the op-amp supply was uprated to 0.1 farad and with the introduction of the new case the output caps were made bigger to improve low frequency phase accuracy and current drive was improved to cater for a wider group of "difficult" headphones.

Phil: Your products are engineered for extremely wide bandwidths. Why do you feel it necessary to go so far beyond the range of human hearing?

Graham: Phase accuracy! Bandwidth limiting introduces phase shift inside the band, which alters the position of the many frequencies that determine how the music sounds. By careful consideration the bandwidth is extended to give a closer approximation to the original positions while ensuring the stability of the circuit.

Phil: Many designers place the utmost importance on the power supply. What are the special features of the PSU-1 power supply you use for the Solo and the Era Gold phono preamp, and why have you chosen to use an outboard power supply?

Graham: The last part of your question is easy. Mains transformers hum. Toroids enclose the magnetic field but not the electromechanical vibration, which is induced even into non-ferrous parts. Then there's the little understood thing called charging currents. By comparison with exotic power supply circuitry, the PSU-1 is rather basic, but has addressed the charging current phenomenon, which makes a big difference with audio circuits. An amp stage and power supply should work hand-in-hand.

The products being powered by the PSU-1 are all SELV (safety extra low voltage) so the only safety concern is the power supply. The PSU-1 has been awarded safety approvals by the IEC and CSA that satisfy European and North American safety requirements including UL.


Phil: I didn't find the Solo a good match for the AKG K1000s.

Graham: The K1000 is not what would be classed as a sensitive headphone; in fact it is quite inefficient at 74dB for 1mW, whereas most headphones are in the region of 84dB to 96dB for 1mW. Normally the K1000 would expect the output of a power amp and is not supplied with the usual headphone jack, but the production mod on the new cased Solo has already been attested by some users to drive the K1000 to a reasonable, if not ear-shattering level. Calculations show it should drive it to 91dB peak, and about 83dB average.

Phil: Maybe I listen at higher levels than some people. Do you have plans for any new headphone amplifier products?

Graham: On the drawing board at present is a portable, shirt pocket sized, rechargeable headphone amp based around the Solo design.

Phil: Is it true you make every unit yourself? Wouldn't it be cheaper to outsource?

Graham: Unless you have the clout of the big boys to make dealers stock your products up to the rafters (which gives them the incentive to sell such products as being better than anything else), the quantities manufactured will always be relatively small making outsourcing uneconomical. I have to balance things up very carefully so that I and a colleague (both ex-broadcast audio) can hand-build quality units, give them a full professional specification test, and provide my few dealers a monetary incentive, while providing the customer with the excellent value for money and the sound I know they've been waiting for. We won't get rich quick, but using hard earned business experience, we intend to stay around.

Phil: Thanks for your time today, Graham.


Interview With Larry Black

Phil: Can you give me a little history of the company and your past experience?

Larry: Aural has been operating for many years under a sole proprietorship, specifically building custom audio systems. Aural began building home theater systems as well as 2 channel audio equipment in 1995. All of Aural's equipment is designed by Erno Borbely of Germany. Erno has had a tremendous audio career working as lead designer for companies such as David Hafler and National Semiconductor. In 2001, Erno Borbely and Aural reached an agreement for proprietary designs in Aural products. Aural specializes in Class A products. Since graduating from Texas University in 1985, I've held many positions with companies that launched new products and startup companies. In 1994, I was instrumental in the successful launch of a company called DIRECTV. Since 2001, I've dedicated my entire efforts to Aural.

Phil: This is quite a large unit for a headphone amplifier. Why is that?

Larry: The size of the Audition is to accommodate the Toroid transformer as well as the amplifier board and power supply. The amplifier board dimensions are 5.5" x 4.25". The power supply board dimensions are 6.5" x 5.75". Remember that the JFETs and MOSFETs are discrete devices, so the PC board is larger to some degree. The output MOSFETs are biased for Class A operation. The heat-sinks for each of the MOSFETs are mechanically connected to the sides of the chassis. This allows greater dissipation of heat, and allows me to bias the transistors in "Class A" as far as I can.

Phil: Why is there no volume control marking or readout, and how far apart are the 24 steps?

Larry: Volume control markings will be added to the next inventory run of chassis slated in December 2004. Plans are to include an area around the volume knob that is backlit with markings in dB. As it is now, using the CT2-10K control, each click of the control is 2dB.

Phil: Did you consider offering balanced inputs?

Larry: I've pondered the use of balanced inputs and after receiving emails from customers asking for them, we'll be releasing a Signature Series in spring 2005 that will have balanced inputs, two headphone jacks (on the front), and a selector switch for two inputs. One input will utilize the balanced inputs and the remaining will be used for single-ended. The PC board has these capabilities already, so we'll offer an upgrade to the existing customer base for a nominal fee. I guess I never realized how many are using headphone amplifiers as preamps as well. I knew that many companies were advertising their units as preamps, but in my estimation they are a far cry from a good preamp. We'll keep the Audition as a headphone amp only. Its design is specifically used for that purpose. I get so many emails from existing and potential customers asking if we can incorporate a wireless remote for the volume control. We can, and have used the DACT control with remote capabilities, but that's being reserved for the Cortege, our preamp to be released.

Phil: What can you tell me about the fully balanced preamp?

Larry: Our fully balanced preamp will be released first quarter 2005. Our preamp uses the same topology as our Audition headphone amplifier; however its design is that of a true preamp. The Cortege will have three inputs including 1 balanced input as well as two single-ended inputs. The Cortege will offer single-ended outputs and 1 balanced output. There will be a mechanical selector switch on the front of the unit as well as a volume control using the DACT CT2-10K stepped attenuator with remote control capabilities. The look and feel of the Cortege will be very similar to the Audition. I like the heaviness of the Audition. It doesn't slide around when listening and it has a remarkable "deadness" to unwanted ambient noises. The Cortege will be sold in two chassis; one for the amplifier itself and the power supply will be in its own chassis. They will be connected with an umbilical cord.  I'll send you pictures when units go into production.

Phil: Will there be other Aural products coming?

Larry: Our newest amplifier, the Telo, a 40W Class A power amplifier will be released at the CES show in January. This newest amp was auditioned by our dealers at the Rocky Mountain Audio Fest last month in Denver. We currently have a waiting list for this product. Some of the other products in prototype are:

 Aleron – 75W "Class A" stereo power amplifier.

 Fahren – MM/MC "Class A" phono preamp.

Phil: How do you position Aural in the crowded HiFi marketplace?

Larry: I guess the last thing I could say about our products is our desire for exceptional quality. We want to keep our products easy to use without the frills of some comfort features that degrade the signal and provide for the customer true value in their purchase. Our listening tests tell us that changing volume controls (for example) to a less expensive wiper type control would save us money in production, but weighing the effects of signal quality isn't worth it. We purchase in quantities that provide us the lowest cost, yet we won't skimp on quality. Using Caddock resistors, Nichicon Fine Gold Muse capacitors and Teflon PC boards gives us the sound we want without reservations.

Phil: A pleasure talking to you, Larry.




Graham Slee Solo

Aural Audition




Sub-bass (10Hz - 60Hz)



Mid-bass (80Hz - 200Hz)



Midrange (200Hz - 3,000Hz)



High-frequencies (3,000Hz on up)









Inner Resolution



Soundscape width front



Soundscape width rear






Fit and Finish



Self Noise



Value for the Money






Type: stereo headphone amplifier


Graham Slee Solo MC

SPL gain into 32Ω: +15dB       

SPL gain into 300Ω: +16dB

SPL gain into 600Ω: +14dB       

Input Sensitivity: 125mV into 30kOhms

Frequency Response: 12Hz (-.5dB) to 100 kHz (-3dB) at half power

Distortion: 0.01% at half power

Signal to noise: 89dB

Headphone Impedance: Optimized for 30 to 600 Ω

Dimensions: 4.1 x 2 x 7.1 (WxHxD in inches)

Weight: 5 lbs.

Finish: Silver

Price: $950 US, $1,299 CDN, £460

Warranty: 2 years parts and labor


Aural Audition

Gain: +20dB

Frequency Response: 1MHz

Distortion: < 0.05% at 1kHz / 3V rms

Rise Time: < .2uS

Output Impedance: < 2Ω

Headphone Impedance: Optimized for 32 to 600 Ω

Power Consumption: 18 watts

Dimensions: 8 x 4 x 12.75 (WxHxD in inches)

Weight: 17 lbs.

Finish: Silver

Price: $2,195

Warranty: 3 years parts and labor


Company Information

Graham Slee Projects
1 Monks Way
Monk Bretton
South Yorkshire S71 2JD
Voice/Fax: +44 (0)1226 244908
Website: www.gspaudio.co.uk


Aural Audio
36 Applebee Farms Drive
Jacksonville, IL 62650

Voice: (888) 863-3482
Website: www.auralaudio.com













































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