Audio engineer Norbert Lehmann founded the firm Lehmannaudio in 1988 in Cologne, Germany (or Köln, as the Deutsche spell the city’s name). The firm’s first product, the Black Cube phono stage, drew considerable positive attention for Lehmann and his firm, such that the Black Cube -- and its ever-evolving line of descendants -- became an analog classic with a nearly cult-like following. Over time, though, Lehmann’s attentions expanded to include a family of headphone amplifiers collectively known as the Linear models. This review focuses on Lehmann’s latest, the Linear USB II ($2,199).
The Lehmannaudio Linear USB II is a minimalist yet also very high-performance DAC/headphone amp/preamp that is targeted toward people who care more about sound quality than about long lists of features. Similarly, it is a product for those whose preferences in design aesthetics strictly follow the “simpler is better” creed.
Accordingly, the Linear USB II arrives with a compact chassis that is considerably deeper than it is wide. The front aluminum faceplate, available in silver, black, or chrome finishes, is the epitome of visual simplicity. It sports a blue power indicator light, two 6.35mm headphone jacks, and a polished volume control knob. Out back, the rear panel features a USB port, a stereo pair of RCA analog input jacks, a pair of RCA analog preamp output jacks, a power switch, and an IEC inlet socket for the included power cord. And that’s pretty much it. There are no input selector switches, no digital filter switches, no additional digital inputs ports or sockets apart from USB, and absolutely zero gongs and whistles. The sole exceptions are two small banks of DIP switches (one each for the left and right channels) located on the bottom of the unit, which are used to set one of three available gain options: 1, 10, and 20 dB. Somewhat confusingly, Lehmannaudio’s online documentation suggests there are four gain settings available, but that is not the case.
Input switching follows an elegantly minimalist convention: if there’s a USB audio signal present, the Linear USB II automatically decodes and plays the digital audio file, but where no USB signal is present, the unit reverts back to its analog input. Output switching is equally elegant: if there are no headphones plugged in, the rear panel preamplifier outputs are enabled, but if a headphone is plugged into the left-hand output jack, the rear panel outputs are muted and the unit operates in headphone amp mode.
The analog section of the Linear USB II is basically an evolved and improved version of the analog section used in Lehmann’s original Linear headphone amplifier, or as Lehmann puts it, “…a fully revised analog board using the high-grade Low-Z Copper technology.” High-quality parts are used throughout, including a stout, well shielded, toroidal power supply transformer and Mundorf capacitors in key power-supply locations. Headphone jacks with gold-plated contacts are sourced from Neutrik, while all RCA sockets are likewise gold-plated and use Teflon insulation. Finally, the unit is suspended on three vibration-absorbing feet from SSC Audio.
Power output is an adequate 400 mW into 60 ohms or 200 mW into 300ohms, while distortion is very low: <0.001 percent at 6mW into 300 ohms. Specified frequency response bandwidth is 10 Hz (-0.3 dB) to 35 kHz (-1 dB). The signal to noise ration is excellent: >100 dB at 0 DB gain (the best of any of Lehmann’s headphone amplifier models).
In turn, the digital section uses the latest ESS Sabre K2M DAC chipset, an XMOS USB interface, and an analog filter featuring Silver Mica capacitors. The DAC can handle PCM at resolution levels up to 384 kHz/32-bits, and DSD files (via DOP) up to DSD 128. The DAC also accepts and decodes FLAC, MP3, and WAV files.
For my listening tests I connected the Linear USB II to an AURALiC ARIES wireless streaming bridge, which in turn was attached to a large USB hard drive containing my digital music library. (The library contains a mix of standard and high-res PCM and DSD audio files with various encoding formats; all files are CD-quality or better.) On the output side, I connected the Linear USB II to a speaker-based system including a Rega Isis Reference CD player/DAC, a Rega Osiris Reference integrated amplifier, and a pair of Magnepan LRS loudspeakers. Power was fed through a Furutech Daytona 303 power conditioner/distribution box, while power and signal cables were also from Furutech. USB audio signals from the ARIES were carried to the Linear USB II via an AudioQuest Diamond USB cable (I use two of these, one in my headphone reference system and the other in my speaker-based system.) Room treatments included a mix of Auralex, RPG, and Vicoustic absorption and/or diffusion panels.
Headphones on hand for my listening tests including the excellent Dan Clark Audio ETHER 2, the Final D8000, the Meze Empyrean, and the Quad ERA-1. All four offer very high resolution, reasonably neutral voicing, and are at least moderately easy to drive.
From the outset the Linear USB II exhibited four compelling and very positive sonic characteristics.
First, it offered pretty much dead-neutral voicing. On track after track and album after album, it just quietly reveals to you what the material at hand has (or doesn’t have) to offer, whether good, bad, ugly, or just plain “meh.” When you listen through the Linear USB II, you quickly realize a lot of audio electronics components have a subtle tendency to homogenize the sound (in other words, to make the sonic qualities of various record sound similar to one another). In contrast, the Linear USB II is a powerful differentiator, teasing out subtle and not-so-subtle distinctions between recordings without imposing colorations or adding editorial commentary on the music.
Second, the Linear USB II helps good headphones deliver uncommonly high levels of three-dimensionality, which means it does an extremely good job of retrieving low-level spatial cues in many recordings. To highlight this point, I tried a comparison between the “Appalachia Waltz” performed by Mark O’Connor’s Appalachia Waltz Trio on Crossing Bridges (OMAC Records, 44.1/16) and the “Misty Moonlight Waltz” performed by Yo-Yo Ma, Edgar Meyer, and Mark O’Connor on Appalachian Journey (Sony Classical, DSD 64). Granted, the two songs and ensembles are different, but they are quite similar in style, genre, and overall vibe, and also feature the same primary violinist (O’Connor).
The Linear USB II quickly reveals that there are substantial differences in sound quality between the two recordings -- recordings that many amp/DACs make to sound similar. The Linear USB II shows that while Crossing Bridges is a good (even a very good) live recording, it offers a comparatively flat, two-dimensional presentation, almost like that of a closely mic’d studio recording. By contrast, Appalachian Journey delivers a strikingly and intensely three-dimensional sound, where each instrument (violin, cello, and bass) has palpable depth, shape, and body and where there is readily apparent air and space around and between the three instruments in the ensemble. The experience was a bit like turning the page on a well-designed 3D pop-up book. When it happens, you can’t (or at least I can’t) help but feel a certain sense of surprise and wonder. Let me stress that this isn’t a case where other electronics fail to show differences in three-dimensionality at all, but rather that the Linear USB II makes them so much more apparent, convincing, and easy to hear and appreciate.
Third, the Linear USB II offers excellent resolution of delicate, low-level sonic details, provided your recordings and headphones are up to the task. The Final D8000 headphone, which is highly revealing and uses a distinctive AFDS (Air Film Damping System) design similar to systems used on some high-end Sony recording microphones, proved a useful tool for hearing all the delicate details the Linear USB II could deliver. When I listened the Final/Lehmannaudio combo on familiar recordings that are rich in detail, the Liner USB II often did something quite unexpected and sonically beneficial. Namely, it took me down below what I had thought was the noise floor of the recordings to reveal additional layers of musical information hidden under the floor. (The experience is something like living in a house with a concrete slab foundation, only to discover after-the-fact that the home actually has a beautifully appointed basement space you never knew about.)
A track that dramatically highlights the Linear USB II’s excellent handling of low-level details is the brilliant cover of the Cyndi Lauper favorite “Time After Time” found on Miles Davis’ Live Around the World (Warner Bros., 44.1/16]. The track was captured in what I take to be a large, open-air venue and presents the song with a very wide range of dynamics, from “barely above a whisper level” to powerful swells -- especially on the song’s unforgettable chorus. What’s interesting is that the Linear USB II lets you hear subtle (indeed, almost inaudible) crowd noises and stage sounds in a highly three-dimensional way that conveys the vivid impression of being in the audience and present at the original live performance.
One of my favorite moments comes during a quiet passage where you can hear a hush fall over the crowd as the band plays very softly and delicate percussion sounds can be heard emanating from various point onstage. Once again, it is not the case that other amp/DACs completely miss out on these beautiful low-level details, but rather that the Linear USB II makes them sound so much more whole, three-dimensional, and complete.
Fourth, the Linear USB II offers surprisingly hearty dynamics in general and unexpectedly rich and appropriately weighted bass. My past experience with such high-purity devices is that the sonic price to be paid for their perceived purity is often manifest as a quality of sonic leanness and/or a lack of appropriate bass weight, warmth, and grunt. Thankfully, the Linear USB II has no such problems; it successfully combines purity and precision with bass that exhibits excellent, natural organic warmth and punch. A fine example would be “Bass Suite #1” from Avishai Cohen’s Adama (Concord Records, 44.1/16), where the Linear USB II beautifully renders the dark, woody growl of Cohen’s bass -- always with appropriate depth, gravitas, and a jaunty rhythmic bounce.
Taken strictly on its own terms, the Linear USB II has few downsides, apart from the fact that some might wish for the unit’s gain switches to be on the front and not the bottom of the chassis. Viewed in a broader context, however, I think discerning listeners could want three characteristics that some, though not all, competing amp/DACs provide: 1) more output power, 2) multiple user selectable digital audio filters, and 3) balanced as well as single-ended headphone and preamp outputs.
Three competing amp/DACs that fall roughly in the same market space occupied by the Linear USB II are the Chord Electronics Hugo 2 ($2495), the Questyle Audio Engineering CMA Twelve ($1499), and the Woo Audio WA11 Topaz ($1399 to $1799).
All three competitors offer higher output power than the Linear USB II, meaning they can drive a broader range of power-hungry/low-sensitivity headphones than can the Linear USB II.
Two of the competitors -- the Questyle and Woo Audio -- offer both single-ended and balanced outputs, while the third -- the Chord -- offers such an incredibly high signal-to-noise ratio (126 dB, “A” weighted) that its manufacturer argues there would be no practical sonic benefit to adding balanced outputs. The Woo Audio and the Chord also have the added benefit of being portable, or at least readily transportable.
Of the three competitors, the Questyle arguably offers the most sophisticated amplifier section, as it employs the firm’s distinctive and patented Current Mode Amplification circuit topology, complete with a user adjustable Pure Class A bias control. What is more, Questyle’s CMA Twelve is offered in two versions: the standard CMA Twelve (which uses very high-quality parts, such as WIMA and Nichicon FW capacitors and Dale military-grade resistors) and the CMA Twelve Master (which uses even higher-quality parts, such as a Rogers ceramic PCB).
The Chord Hugo 2 arguably offers the most sophisticated DAC section, with a Xilinx Artix 7 FPGA-based DAC with a claimed tap-length of 49.152 taps in a 10-element pulse-array configuration. Moreover, the Hugo 2 offers four user-selectable digital filters (Incisive Neutral, Incisive Neutral with HF roll-off, Warm, and Warmer with HF roll-off).
Sonically, the Chord Hugo 2 offers the warmest and most three-dimensional sound, the Woo Audio WA11 Topaz offer the expressive dynamics and vibrant tonal colors for which Woo components are known, and the Questyle Audio Engineering CMA Twelve offers extreme purity, focus, and definition. I would place the sound of the Linear USB II somewhere between that of the Woo Audio and the Questyle amp/DACs, but with strong hints of Chord-like three-dimensionality.
Lehmannaudio’s Linear USB II headphone amp/DAC/preamp is a minimalist gem. While it offers fewer features and functions and also less power output than some competing designs, it’s incredibly easy and straightforward to use and delivers a high-purity sound that combines -- in roughly equal proportions -- admirably neutral voicing, excellent resolution of low-level sonic details, superb three-dimensionality, and strong dynamics (especially bass dynamics).
| Analog section | |
| Input impedance | 47 kohms |
| Maximum gain | 0 dB, 10 dB, 18 dB, 20 dB selectable via DIP switches |
| Frequency response | 10 Hz (-0.3 dB) to 35 kHz (-1 dB) |
| Signal to noise ratio | > 100 dB at gain 0 dB |
| THD | < 0.001 % at 6 mW/300 ohms |
| Channel separation | > 80 dB at 10 kHz |
| Output power | 400 mW/60 ohms 200 mW/300 ohms |
| Output impedance | pre-out 50 ohms phones out 5 ohms |
| Connectors audio | Neutrik headphone jacks with gold plated contacts 1 x pre-out 2 x phones out gold plated RCA sockets with teflon insulation |
| Power consumption | 15 W through internal, regulated power supply |
| Special feature | PC board in Low-Z Copper technology vibration-absorbing 3S Device Feet |
| Outer dimensions W x D x H | 110 mm x 280 mm x 44 mm (4.3 x 11 x 2 in.) |
| Weight | 1.5 kg (3.3 lbs.) net |
Digital-Analog Converter | |
| Digital inputs | 1 x USB B |
| Sampling rate | up to 384 Khz |
| Resolution | up to 32 bits |
| Formats | PCM, DOP 128, FLAC, MP3, WAV |
| Chipset | ESS Sabre K2M |
| Analogue filter | Silver Mica capacitators |
| Max. output level DAC | 1.8 V eff. |
| Special feature | The analogue input is automatically activated, when no digital signal is recognised |
