A headphone amplifier like Lehmann Audio's Black Cube Linear amplifier is a necessity if you're using high-end, demanding headphones like AKG's K701. However, this amplifier will still offer greatly improved sound quality with mid-range headphones.

We've only ever looked at one headphone amplifier before — the exemplary SXH2 from Perreaux. We applauded its low noise floor and metered treble — so how did the Black Cube Linear from Lehmann Audio fare? Both amplifiers have their own quirks and advantages that set them apart.

The Black Cube Linear is an imposing beast for a headphone amplifier. All the necessary power circuitry is built into its long, low body rather than being stored in an external power brick. This is great news for those who like to keep their entertainment setups tidy, but it also speaks volumes about the quality of internal wiring. If the power supply is electronically 'quiet' enough to be stored inside the body of the device, you can rest assured it won't introduce any noise into your audio.

The Black Cube Linear we tested had a set of unbalanced stereo RCA sockets as its primary input, though different versions also have USB and 3.5mm connectivity. In addition, the device has a second pair of RCA connectors, which are used as outputs. This means you can use the amplifier in-line between a source and an amplifier for a set of speakers — so you don't need to mess around with cable splitters or switches. Power is delivered through a standard IEC connector, so if you're dedicated (or foolhardy) enough to own an expensive power cable you can use that. The unit's power switch is also on the rear of the unit — instead of integrated into the volume control as is common, because that would degrade sound quality — as is as a removable fuse.

Moving to the front of the unit, the sense of exacting engineering is continued. The four millimetre thick aluminium faceplate is held on with hex screw, with the only components a volume control, two quarter-inch headphone jacks and a single blue power LED.

The volume control is of paramount quality. It's well weighted, requiring only a little force to start its movement. Moreover, the increase in volume is perfectly linear even at lower wattages. Whereas cheaper amplifiers bias to one channel of headphones at lower volumes, the Black Cube Linear is consistently even and balanced.

Obviously the amplifier is designed to drive headphones of premium quality and as such it doesn't bother with consumer-level 3.5mm headphone jacks. While the headphones that require or will benefit most from amplification usually have larger connectors, you will need the appropriate adapter if you're planning on using slightly cheaper headphones.

We tested the amplifier with two vastly different models of headphones from AKG. AKG's K701 are headphones that require an external amplifier to power them, which makes it hard to assess the amplifier's impact. We did find, however, that even with the amplifier's gain set to the lowest value (adjustable with two switches on the base of the unit), the Black Cube Linear was able to drive the headphones to painfully loud volumes. Another good test of the amplifier's capability was to hit maximum volume with no audio playing. There was no noticeable electronic noise or high-frequency hissing audible, which is testament to the fantastic internal components used. The amplifier didn't detract from the neutral, analytical nature of the headphones either — all frequencies were equally weighted, while immense detail was noticeable in complex orchestral music.

We also tested another facet of the Black Cube Linear's performance by connecting a cheaper pair of headphones, the K 272 HD — again from AKG. These headphones don't need an amplifier, being happy to run directly off a computer or MP3 player's 3.5mm headphone output. Connecting them to the Black Cube Linear with a 3.5mm to quarter inch adapter, however, exposed a difference as large as night and day.

Don't get us wrong — running straight from an MP3 player, the K272HD headphones still sounded great. When the amplifier was introduced into the mix, however, bass was punchier, tighter and more accurate, while treble became more pronounced and sweeter. The Black Cube Linear lent a slightly livelier tone to music, making it more involving. Music had a missing element added: that now-noticeable enhancement to bass and treble.

The Black Cube Linear headphone amplifier is a niche product. You probably won't consider it unless you have some equally expensive headphones to drive with it — in which case it will power them faithfully without altering sound quality. If you use it to drive more moderately priced headphones, though, you'll appreciate the extra boost it provides.