Green Mountain Audio Eos (£4400)

Colorado springs a speaker whose technology and form strike more than one chord with our reviewer


The famous Bauhaus diktat ‘form follows function’ was an aesthetic imperative rather than an engineering philosophy, and a good job too, because for structural engineers in particular the concept was already old hat. All those Martello towers littering England’s south coast, for instance, are not round in plan view on a whim: it’s because castle builders, centuries earlier, had discovered that round towers better resisted artillery bombardment than those with corners.

It’s natural to suppose that modern engineers would never do anything so crass as to make something fundamentally the wrong shape, but don’t be so sure. Most loudspeaker manufacturers have done it, continue to do it, and give every sign of proposing to do it in perpetuity. I’m talking of the rectangular box cabinet, which despite its ubiquity is the worst possible form for a loudspeaker enclosure, both structurally and acoustically.

I’ve been a voice in the wilderness on this subject for almost 30 years, so you can appreciate what a heartening change it is to have in front of me a speaker that eschews not only the conventional box cabinet but also the materials from which it is traditionally constructed, and the predictable – and oh so dull – wood veneer finish. Until the day before the pair of Green Mountain Audio Eos arrived at my door, though, I’d never heard of the US company that makes them – even though it has been in business for 22 years.

Based in Colorado Springs and founded by designer Roy Johnson, whose CV includes a spell as recording engineer for the Colorado Springs Symphony Orchestra, Green Mountain Audio launched the Eos and Eos HD in 2008, the HD being a ‘breathed on’ version with enhanced crossover and Marigo Labs’ CopperMatrix internal wiring. What both models share is a moulded Q-Stone cabinet of marble-loaded resin, a first-order and hence linear-phase crossover [see box-out] with Zobel network compensation of driver inductance, and a unique tweeter arrangement whereby the entire tweeter module atop the cabinet can be slid back and forth in a groove in order to time-align it accurately with the 170mm Aura Sound bass-mid driver. The tweeter is a soft dome unit from SEAS that lacks the ultrasonic resonance of metal dome designs but provides useful output to beyond 30kHz.



It’s this novel tweeter provision that gives rise to one of the stranger audio accessories I’ve encountered, called the EarStick. The idea is that you mount this on a camera tripod and align the tip with the entrance to each ear canal in turn. With a helper you then measure the distance to the centre of the thin foam cover of the nearer speaker’s bass-mid unit, add three inches, and adjust the position of the tweeter – it’s locked in place via a thumbwheel – so that the centre of its foam cover is that distance from the end of the EarStick. Voila! The two drivers are now time-aligned.

Take my word for it: this adjustment is well worth making. I hate to use the awful old cliché ‘snaps into focus’ but that’s exactly what happens to the Eos’s sound when the tweeter position is optimised. It’s quite remarkable, and the most convincing demonstration of the importance of accurate driver time alignment that I’ve ever experienced.

What the moulded cabinet does, apart from making the Eos surprisingly heavy for such a compact design, at a little over 20kg apiece, is afford the opportunity to shape the cabinet for better acoustic and structural performance. The upper section of the enclosure resembles a cylinder, which Don Barlow demonstrated years ago is one of the favoured structural forms for a speaker cabinet because of its inherent stiffness. And the transition from the edge of the bass-mid driver to this cylindrical portion of the cabinet is curved so as to avoid the sudden change in acoustic impedance inherent in sharp baffle edges, which give rise to secondary radiation and consequent smearing of time domain performance.



The traditional knuckle-rap test confirms the inertness of the Eos cabinet, and it’s there to be valued in this speaker’s unusual lack of the false lower-midrange bloom that we’re habituated to hearing from conventional slab-faced box cabinets. Once it’s removed, you wonder how you ever lived with such a colouration.

This is one contributor to the Eos’s outstandingly open, clean sound. Another, I’m sure, is the first-order crossover. There’s a widespread orthodoxy in audio’s academic circles that the phase distortion caused by typical passive crossovers (ie, up to fourth order) is inaudible. Well, I demur, and not just because I tend to like the sound of speakers with first-order crossovers. I’ve experimented in this area over a number of years and have no doubt that crossover phase distortion has clear audible consequences. A significant part of the Eos’s fine, spacious imaging unquestionably owes itself to that simple crossover.

It has to be said, though, that the Eos’s rare clarity is also in part due to a less than scrupulously neutral tonal balance. There’s a lack of midrange warmth here, although curiously it’s more apparent on some programme material than others. For instance, I didn’t feel that Ella Fitzgerald’s masterly ‘Reach For Tomorrow’ [Verve 839 838-2] – a track that’s very sensitive to a speaker’s ability to reveal vocal nuance – was tonally cool or upper-mid forward, but other items clearly were. One such was Diana Krall’s ‘Narrow Daylight’ (24/96 version from Dual Disc, Verve 0602498648247), where both the vocal and acoustic guitar break had a little more edge to them than they should.

This is part of the Eos’s makeup, and you either accept it or you don’t. My tolerance of it was due in no small part, I think, to my partnering the Eos with the utterly grain-free Pass Labs XA60.5 power amplifiers. Certainly this is not a speaker you’d want to hitch up to an amplifier or signal source that’s prone to harshness.

What you gain from this acceptance of the Eos’s sometimes forward nature is a really quite special blend of insight, spaciousness and sheer persuasiveness on material to which it’s well suited. To label this as a speaker best adapted to small musical forces would be to do it an injustice, but there’s no question that it excels on simply accompanied vocals, small jazz ensembles, chamber music, etc. Particularly vocals.



As I’d just completed this month’s review of the Pass Labs amps [see p52], conducted using Thiel CS1.6s – a speaker I’ve praised in the past for its midrange lucidity, and which is a good review tool because of it – it was a no-brainer to play again two of the vocal tracks I’d just used: Sara K’s ‘Vincent’ [Chesky JD 133] and Kurt Elling’s ‘Goin’ Back to Joe’s’ [Naim CD080]. So impressed was I – particularly with the Eos’s ability to cut through the slight thickening that can afflict the piano accompaniment of the latter – that it was these tracks I used to verify the importance of correctly time-aligning the tweeter.

With the tweeter optimally positioned, the Eos’s ability to transport the singer to the listening room was simply uncanny. The sound was pretty good even with the tweeter off its sweet spot, but with correct tweeter alignment the sheer resolution of vocal nuance and inflection came as something of a shock. Kurt Elling’s voice in particular I’ve never heard more realistically rendered, and this wasn’t a mere matter of hi-fi nerdiness. With the enhanced sound came an even deeper appreciation of Elling’s skill at moulding and delivering this prosaic paean to lost love. I kept repeating the track with a sense of wonderment.



Another benefit of the Eos’s inherent transparency is its ability to refresh and reinvigorate recordings which you’ve classified as being on the dull and turgid side. An example from my collection was the Technics-sponsored recording of Andrzej Panufnik’s Violin Concerto [Conifer – alas deleted]. In place of the usual generalised bloom there was now a specific acoustic.

What the Eos doesn’t do – unsurprisingly given its high sensitivity and small internal volume – is bass extension and weight. It has sufficient bass, helped by room boundary boost, to get by with most orchestral material but a lot of rock programme will show up its deficiency. The version of ‘Come Together’ from The Beatles’ Love remix [Parlophone 094638078920], for example, just didn’t have the necessary low frequency heft to propel the track along as it should. 

What bass there is, mind you, is always tuneful, without any sense that upper bass output has been deliberately fattened in a misguided attempt to disguise the shortfall in deep bass fundamentals. Of course, moving the speakers closer to the back wall will help prop-up the bass, but this has to be done with care.

Unfortunately I didn’t have a powered subwoofer on hand to try with the Eos but I’d expect a good example to plug the LF gap effectively, and quite cheaply too if carefully chosen. The Tannoy TS 1201 I reviewed a few months ago, for example, would in all likelihood be a good partner, adding weight without boom. And, sensitively set up, it should only enhance the Eos’s already outstanding spatial qualities.

To finish on what may seem a downbeat note, but one that only adds to the attractions of Eos ownership, Green Mountain Audio offers a ‘Happy Ears for Life’ undertaking to original owners, which means that the company warrants its workmanship and the sonic performance of the speakers for life. A comforting thought…  


Not everyone will like the Eos. Its lack of bass clout will put off some, and its upper-midrange forwardness others. But if your signal sources and amplifiers are free of hardness, and particularly if your preference is for smaller musical forces and especially the human voice, this novel, thoughtful speaker has a lot to commend it. As an alternative to the usual veneered wooden box, it comes as a breath of fresh air.

Sound Quality: 86%


Originally published in the April 2010 issue