February 2005Lehmann Audio Black Cube Linear Headphone Amplifier
by Vade Forrester
Why has headphone listening become such a serious part of the high-end-audio industry? I have a few theories. First, the immense popularity of iPods and similar portable music-playback devices has changed the way many people listen to music. But the cheapie earbuds included with these players are usually not the last word in audio quality, so upgrading to better headphones can make listening to music much more enjoyable. Also, the neighbors of apartment-dwelling audiophiles do not welcome loud music coming through their walls. Headphones can also produce quite decent sound at very reasonable prices, especially if driven by a good amplifier (not an iPod). Finally, headphones dont take up much room, and you can store them out of sight when youre not using them.
With all of this in mind, I eagerly accepted the offer to review the Lehmann Audio Black Cube Linear headphone amplifier, the latest product from the German company best known for its line of affordable, high-performance phono stages. Would I discover a few new supporting reasons for the popularity of headphone listening?
The $779 USD Black Cube Linear is a small, slender box with your choice of silver or black faceplace and knobs. It measures 11"L x 4 3/8"W x 1 3/4"H and weighs 3.3 pounds. These dimensions illustrate perhaps why Lehmann Audio named it the Black Cube Linear. The units small cross-section makes it easy to place on a shelf beside another component. Unlike many other headphone amps, this one will seldom require a separate shelf.
The Black Cube Linear's power supply is built in; there is an IEC connector in the rear, so you can pick a power cord that sounds good to you. Next to the IEC connector are the on/off switch and two pairs of gold-plated, Teflon-insulated RCA jacks: an input and an output pair. You can, therefore, feed the signal through the Black Cube Linear and use the front-mounted volume control to adjust the line-level-output volume as well as the headphone volume, making the Black Cube Linear a single-input line-stage preamp as well as a headphone amp. The volume control knob has a little dimple on the front, so you can tell how far its turned up, although on the black unit I auditioned, it was hard to see the dimple. Maybe a little white paint (or even an LED) in the dimple would make sense. The front panel holds two 1/4" Neutrik gold-plated headphone jacks.
The Black Cube Linear uses a zero-global-feedback class-A output stage. Output impedance is 5 ohms through the headphone jacks, and 60 ohms through the RCA output jacks. The Black Cube Linear should have no problems driving any known headphones or power amplifiers.
Two switches on the bottom of the unit allow you to set the gain at 0, 10 or 20dB. This allows you to make best use of the volume control. By setting the gain appropriately, you can assure that the normal setting of the volume control is around the center of its rotation. The gain setting also prevents blasting your ears with extremely loud music when the volume control is turned only a few degrees. With all the headphones I tried, the 20dB setting (maximum gain) worked best. The Black Cube Linear was totally silent through all the headphones I used it with, and all the amplifiers as well.
'Phones & stuff
A headphone amp has to drive a large variety of headphones, so I wanted to use 'phones that would accurately explore the capabilities of the Black Cube Linear. My old favorites, the Beyerdynamic DT 990 headphones, are a typical open-backed design, with sound reminiscent of the Sennheiser marque. They have a weighty low end, a somewhat recessed midrange, and slightly rolled-off highs. I would describe them as having row-T sound -- the way music would sound in the rear of a concert hall. They are also, by a fair margin, the most comfortable headphones I have worn.
However, I was concerned that the DT 990s would not do justice to the mids and highs of the Black Cube Linear, so my friend Roger Tiller at Blue Marble Audio offered me the use of some Ultrasone HFI-550 headphones. They made a good contrast to the DT 990s -- good midrange, extended bass with detail as well as weight, and a sparkling high end. I was unfamiliar with these headphones, and was impressed with how good they sounded. A closed-back design, the HFI-550 is comfortable to wear for long periods, which I came to appreciate during the review period.
To complete things, I used a Blue Marble Audio power cord with the Black Cube Linear for most of the review period. A Purist Audio Design Venustas power cord worked very well also, but it strikes me as goofy to use a power cord that costs more ($1000) than the headphone amplifier with which it will be used. The source was a Meridian 508.24 CD player. For comparison I used a HeadRoom Total BitHead.
After the Lehmann amp had amassed lots of break-in time (about 300 hours worth), I plugged in my Beyerdynamic DT 990 headphones and was pleasantly surprised to hear how much better the Black Cube Linear sounded from the pre-break-in sound. Highs were now more extended, mids were clear and smooth, and the bass was still weighty and deep, although not as detailed as with the Ultrasones. The DT 990s were the harder headphones to drive of the two pairs I tried. Both the Lehmann and HeadRoom amps could produce good sound with them, although the Total BitHead couldnt drive them to very loud levels. The Ultrasone HFI-550 headphones are easier to drive, and the Total BitHead worked very well with them.
My first impression of the broken-in Black Cube Linear was that its creamy-smooth sound was very easy to listen to and enjoy. By this I dont mean that the Black Cube Linear glossed over detail or rounded off transients; it did not. It produced sound that was in no way edgy or rough. It did not try to simulate detail by accentuating the high frequencies. It sounded almost tube-like, without the euphonic coloration that some tube designs inflict. Its lows were very powerful and detailed, while the smooth highs were limited only by the headphones I was using.
My second impression of the Black Cube Linear's performance was of its utter effortlessness. It easily drove even the Beyerdynamic DT 990s to much louder levels than I wanted to listen to, and dynamics just happened naturally, without attracting attention. Crescendos showed no strain at all, and soft passages were delicate. Microdynamics, the minute variations in loudness that musicians use to express themselves, were also unfussy, but still distinct.
Harmonic accuracy was limited only by the headphones used. In a certain sense, I dont know how good the Black Cube Linear really sounds, due to the limits of the headphones with which I used it. Headphones tend to have more peaks and valleys in their response curves than speakers, and each listeners ear-canal structure creates a different response curve. Suffice it to say that the Black Cube Linear brought out the best in both sets of headphones it drove.
On the Eiji Oue and the Minnesota Orchestras Bolero! [Reference Recordings RR-92CD], Kabalevsky: "Colas Breugnon -- Overture", the DT 990s bass was almost overwhelming; highs were well defined and moderately extended; and the blatty trombones showed that the upper bass was realistically reproduced. The overall sound was smooth and relaxed. The complex percussion parts were clear and free from the shrill distortion that I sometimes hear. With the Ultrasone HFI-550s, the highs were more extended, sounding more like those I hear with loudspeakers. Although the bass did not plumb the depths to the same degree as it did with the DT 990s, there was still substantial low-frequency energy. String tone was lush but detailed; I could tell when each section of stringed instruments (celli, violas, violins) started to play.
On "I Cant Hide" from Jennifer Warnes The Hunter [Private Music 01005-82089-2], the DT 990s obscured some of the overtones of Warnes voice, but the bass-drum roll at the beginning was very deep and had enormous impact. Highs (percussion and the triangle in this case) were fairly extended, although not as much as they are through loudspeakers. The Black Cube Linear helped lots of detail pop out of the mix. Once again, the HFI-550s didnt go as low as the DT 990s, but the impact and detail of the drums in this piece were more detailed; I could hear each mallet striking the drum heads. The highs were more extended, too.
On Jordi Savall and associates La Folia 1490-1701 [AliaVox AV 9805], specifically "Folia: Rodrigo Martinez," the Black Cube Linear's portrayal of the opening cascabel was well defined tonally and spatially. Bass had great power and depth. Small dynamic and tempo changes in this complex, information-dense piece were easy to follow. Through the HFI-550s, there was much more high-frequency information, resulting in one of the best reproductions of this piece I have heard through any audio system. The guitar and harp parts were particularly easy to follow as they swirled around the central viola da gamba part. Exhilarating!
I also briefly used the Black Cube Linear as a line-stage preamp, in which application it was very quiet and had plenty of drive for my notably insensitive Wright Sound Company WPA3.5 mono amplifiers. However, the Black Cube Linear's sound was different from what I heard with headphones. Through amp and speakers, the highs were more pronounced -- for my tastes, they were a bit over-emphasized. A favorite song from Eva Cassidys Live at Blues Alley [Blix Street Records G2-10046] is "Autumn Leaves." Through both headphones, it sounded smooth, detailed, and exquisitely nuanced. Through the speakers, however, Cassidys voice became rather aggressive during some of the louder passages. I speculate that the Black Cube Linear might be a good match for a SET amp with rolled-off highs (no, Im not saying that all SET amps have rolled-off highs, but some do). In my system, however, the Black Cube Linear never reached the sonic heights as a line-stage preamp that it did as a headphone amp.
Doug Paratore wrote about the tiny $269 HeadRoom Total BitHead headphone amplifier on SoundStage! last year, so Ill let you read his excellent evaluation for a complete description of the Total BitHead. Its a portable unit powered by either four AAA batteries or, if its used with a computer, by the universal serial bus (USB).
The Total BitHead produced very deep bass and a well-fleshed-out tonal palette, with highs as extended as those of the Black Cube Linear. I heard lots of detail, but no edginess. This diminutive headphone amp surprised me with its robust sound, but it couldnt drive the Beyerdynamic DT 990s to my preferred listening level without clipping. Of course, I inserted a set of fresh batteries before any serious listening, so I had the maximum amount of power available. The dynamic "Colas Breugnon -- Overture" quickly illustrated the limited levels to which the Total BitHead could drive the DT 990s. It had even deeper bass than the Black Cube Linear, but did not seem as detailed and refined in the midrange. The opening drum roll on Jennifer Warnes "I Cant Hide" had cavernous deep bass through the DT 990s, although it was limited in volume.
You need to pair the Total BitHead with headphones it can comfortably drive, just as you would match an amplifier to appropriate speakers. Fortunately, the Ultrasone HFI-550s were just the headphones for the Total BitHead, and they even had a mini-plug at the end of their long, flexible cable, so I could dispense with the Radio Shack adapter I had to use with the DT 990s. With the HeadRoom amp, the HFI-550s had more distinct bass-drum impacts, and lots of power, too. Their abundant high-frequency energy restored the missing overtones to Warnes voice. "Folia: Rodrigo Martinez" sounded very dynamic. The HFI-550s were (again) more energetic in the highs, revealing much more information about percussion instruments.
When I finished auditioning the Total BitHead as a standard headphone amp, I had to try it with a computer. Connecting the amp to one of the USB ports on my Dell 4600 computer, I played "Colas Breugnon -- Overture" through the DT 990 headphones. This fed the signal, in digital form, into the Total BitHead, where its internal DAC decoded it and fed it to the internal amplifier section. The overall sound was very musical and enjoyable, although some of the highs and mids seemed more evident through the computer. I could get a little more volume when I let the AAA batteries power the output amp, while the USB powered the Total BitHeads DAC. The Total BitHead turned out to be a great way to enhance the computing experience.
The Lehmann Audio Black Cube Linear was a pleasure to use. It was never fussy -- it just went about its business of producing smooth sound through every set of headphones I plugged into it, even the somewhat-difficult-to-drive Beyerdynamic headphones I use. Moreover, it always extracted the best performance from these headphones, and its transparency clearly revealed the sonic characteristics of each pair. Music though the Black Cube Linear was warm, relaxing, and always involving. With comfortable headphones, the Black Cube Linear produced sound I could (and several times did) listen to for prolonged periods. Given all of this, its $779 asking price seems very fair.
My time with the Black Cube Linear left me enlightened. I now understand what the headphone craze is about -- and how good headphones can sound. Im ready for more!
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