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Earthworks Sigma 6.2

Time-coherent Reference Monitors
By Hugh Robjohns

Earthworks are already world-renowned for the fidelity of their microphones and preamps, and now their studio monitors are setting a new benchmark for reference quality.

Earthworks Sigma 6.2Photo: Mike CameronThe American microphone manufacturer Earthworks was formed by David Blackmer, the original inventor of Dbx noise reduction, to further his ideals of a creating a technically and artistically perfect recording chain. His approach to the design of all kinds of audio equipment centres on his belief that extended frequency response and accurate time-domain performance are essential prerequisites to life-like audio fidelity. Sensibly enough, his company started to develop this philosophy by trying to design the perfect microphone — and with some considerable success, producing such revered studio mics as the QTC1 and SR78, as well as various measurement microphones, for example.

Once you have an accurate microphone, you need a very accurate preamplifier to look after the subtle nuances of audio captured in the first place, so Earthworks went on to design and manufacture a series of 'zero distortion' mic preamps in the form of the 1020 series. Of course, it's no good having all this perfectly captured and preserved signal information if you can't hear it, so the latest extension to Earthworks' signal chain is the new Sigma 6.2 reference monitor loudspeaker. It won't come as much of a surprise that this new monitor provides a level of performance which matches that of its sibling microphones and preamplifiers, but before any more detailed quality assessment, let's look at the technology.

Earthworks Sigma 6.2 £2350
  • Three-dimensional stereo imaging with rock-solid stability.
  • Superbly accurate transient detail.
  • Faithful rendition of acoustic instruments.
  • Unusual degree of bass extension without coloration.
  • None.
An unusual two-way passive monitor that achieves an exceptional level of performance, particularly in terms of imaging and transient detail.


Hardware & Technology

The Sigma 6.2 is a reasonably compact two-way passive design with a very unusual reflex cabinet construction, and it weighs a substantial 14kg. The two drive units are placed more or less on the centre line of the cabinet, although the positioning of the tweeter varies slightly for the left and right speakers. The 6.5-inch woofer is supported by a heavy die-cast aluminium frame which is mounted to the front of the cabinet in the usual way. A single slotted reflex port vents to the front at the top of the cabinet — its rectangular opening intentionally doubling as a useful hand hold for carrying or positioning the speaker. Internally, the cabinet is well damped with a generous amount of long-fibre wadding, and is braced effectively to control panel resonances.

I was intrigued by a reference in the specifications to the fact that the woofer is 'pre-conditioned' so that there is no requirement for a running-in period. In my experience most new loudspeakers benefit from a 'running-in' period, during which the sound quality and accuracy will change as the various materials 'loosen up' and take on their intended operating characteristics — although I know not everyone shares this view! However, in this case, not only was the woofer pre-conditioned to obviate the need for running in, but the review speakers were the distributor's well-used demo models, so I was able to set them up and start critical listening from the off, rather than having to spend a day or two running them in before starting to make my assessments.

Tweeter Baffle

The most unusual aspect of the cabinet design is that the tweeter is mounted on an isolated vertical open baffle set back slightly from the front of the woofer, on a flat step in the cabinet between the woofer baffle and the reflex port. A pair of tweeter terminals are mounted on the cabinet just behind this odd isolated baffle to convey the signal from the internal crossover to the HF driver. Every corner on the cabinet and tweeter baffle has been carefully radiused to minimise diffraction and secondary radiation, and the whole construction is extremely solid.

The unusual tweeter design and baffle make the Sigma 6.2 very directional, the advantages of which are precise stereo imaging and fewer problematic room reflections.The unusual tweeter design and baffle make the Sigma 6.2 very directional, the advantages of which are precise stereo imaging and fewer problematic room reflections.Both drive units are manufactured by Vifa, and represent the very latest in the Danish company's high-technology designs. They are both magnetically shielded to prevent colour casts on CRT monitors, and Earthworks claim the tweeter is the best they have ever measured, with a very uniform frequency response extending beyond 40kHz (matching the performance of the company's microphones). The tweeter construction is as unusual as the rest of this innovative loudspeaker, with a very distinctive phase cone extending from the centre of the driver surface, the latter comprising two concentric domed rings. As tweeters go, this one has a relatively large and flat surface, which will inherently have a narrower radiation pattern than, say, a domed design, and I'll come back to this point shortly. Although the Sigma monitor has no front grille (the nature of the cabinet precludes such a facility) the fact that the tweeter baffle is set back from the front baffle affords some level of protection to the exposed tweeter.

As a passive loudspeaker, the Sigma can be connected to an amplifier via either a Neutrik Speakon connector or a pair of 4mm binding posts, both interfaces being gold plated and both provided as standard. The internal crossover is deceptively simple with just nine components, but all are of the highest quality and selected specifically to create accurately matched stereo pairs of monitors. Chunky oxygen-free internal cabling links input terminals, crossover and drivers, with silver solder being used to form the joints. The speaker's nominal impedance is 8Ω, with peaks to 15Ω and a minimum of a safe 6Ω. Above about 100Hz the impedance settles to a stable 7Ω mean.

The rear panel of the monitor offers both Speakon connector and binding posts.The rear panel of the monitor offers both Speakon connector and binding posts.The specifications for the Sigma 6.2 neglect to provide the crossover frequency, but I would estimate it to be fairly low, probably around 1.5kHz or so. The overall frequency response is given as 40Hz to beyond 40kHz ±2dB and the published frequency response chart shows it to be very well balanced and substantially flat across the entire range. The speaker is continuously rated at 150W, with peak power handling of 400W, and the sensitivity of 87dB/W/m means a chunky amplifier is required to enable the Sigma to give of its best. I partnered it with my trusty Bryston 4B amplifier, which seemed a perfect match.

Time-coherent Design

While most loudspeaker manufacturers are happy to boast about the smooth frequency response of their designs, few will discuss the time-domain response of their speakers. Moving-coil drive units are notoriously bad when it comes to recreating accurate impulse responses — all that inertia of diaphragm and coil, the mechanical and acoustic resonances, and so forth. That's not to say moving-coil speakers can't have a good impulse response, just that it is hard to achieve. Earthworks claim to have done just that, though, and the responses printed in the specifications would appear to bear witness to their claims, with very tidy impulse and step responses. What this means in practice is that the Sigma is able to recreate the all-important transient elements of audio signals with better fidelity than usual, resulting in more accurate, clean and fatigue-free monitoring.

Another very important aspect of the design of the Sigma is its polar response, or how it puts energy into the room in different directions. Most speakers produce a kind of cardioid response, with maximum energy being sent forward on the listening axis, and gradually decreasing levels of energy as you move off axis. This off-axis sound obviously reflects around the room, which is why most control rooms are well damped near the speakers, to soak up this unwanted radiation. Ideally, the off-axis signals should also remain spectrally balanced, with equal amounts of treble and bass, so that reflected sound has the same sonic characteristic as the direct sound, although this is difficult to achieve because of the inherent directional characteristics of the drive units, amongst other factors.

In contrast to this 'standard' approach, the Sigma monitors appear to have a very directional characteristic indeed, with remarkably little energy being emitted to the sides and rear of the cabinet. The drive units provide a wide enough spread of sound to ensure a usefully wide 'sweet spot', but move to 90 degrees off a speaker and it gets very quiet, very quickly!


Presumably partly as a result of less energy being splashed around the room from reflections of the (absent) side and rear output of the Sigma speakers, I discovered that they provide exceptionally detailed stereo imaging. The isolation of the tweeter on its own vertical baffle, time-aligned with the woofer, and with its edges well rounded is also likely to be a significant contributor to this imaging excellence. This attribute was immediately noticeable in an acoustically treated environment, and even more so in a highly reflective and untreated room. In fact, I found the precision of stereo imaging to be quite exceptional and amongst the best I have heard almost regardless of price. Not only was the image wide and stable, but it also had a tangible and realistic depth with appropriate sources, and some test recordings I have made with a Soundfield microphone were recreated in stunningly precise three-dimensional realism.

So, a big tick for spatial imaging, and I'll give it another for transient reproduction. Acoustic instruments and percussion are portrayed with a natural, uncoloured realism — that word again — which is more usually associated with electrostatic loudspeakers than reflex-ported moving-coil designs. These speakers are extremely fast and dynamic, and with a very clean, transparent and neutral quality. I think I can honestly say that these Sigma speakers are the closest I have yet come to listening to the source — rather than a loudspeaker recreating the source — in a compact two-way passive design.

The overall spectral balance is just as good too. There's a smooth, even response, with little variation caused by placement in the room (within reason), and a naturally balanced bass response which has an unusually deep bass extension. Given that the Sigma is a similar overall size to my trusty transmission-line TB1 monitors, I was surprised to find that the bass extension was very similar. I know of no other similarly sized reflex cabinets that provide as much bass extension with as little coloration — although, having thought about it a little more, the manner in which the Sigma cabinet is constructed (with its stepped arrangement) tends towards a simple transmission line in some regards, and may account for some of the unusual degree of bass extension.

The Sigma 6.2 is a very fine monitor, of that there can be no doubt. Partnered with a wide-bandwidth, clean and powerful amplifier — such as the Bryston — and appropriate source electronics, this monitor provides a reference audio quality which is beyond reproach in every regard: imaging, transient accuracy, tonal fidelity, and more. It is possible to achieve a higher level of performance, but only with substantially more expensive (and much larger) three-way passive designs, and arguably with a very few of the most sophisticated two-way active models.

These Earthworks monitors may initially seem expensive in the UK for a compact two-way passive monitor speaker, and by the time a decent amplifier has been added to the budget we are talking serious money here. Nevertheless, the performance is nothing short of startling, and compares very favourably with equivalently priced designs. I found the Sigma 6.2 made a perfect mastering grade monitor and I was very sad to see the review models go. I may very well have to have them back, bank manager permitting!

infop.gif £2350 per pair including VAT.
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Published April 2003