EROS LOUDSPEAKER Ivor Humphreys, Reviewer

Gramophone, May 1999, Ivor Humphreys, Reviewer

For all the sophistication in acoustics and electronics design in the world today, one problem  the loudspeaker/room interface  resolutely refuses to go away.  Not only will my loudspeakers sound very different in your room, they will also behave very differently if I move them to another position within my own room.  This is a fundamentally worrying problem for all loudspeaker designers since that idealized test bench performance can so easily be compromised in the home.  It is also the loudspeaker reviewer's ultimate let-out of course; I can maintain that the Eros is a very special loudspeaker, but I can't guarantee that it will perform quite as well for you.  Probably it will.
Not that I was able to hear it at home myself.  The Eros is too tall for the listening room in our converted loft, whose ceiling follows the contours of the pitched roof; the side walls are only a little higher than the Quad ESL-63.  Fortunately, with their figure-of-eight radiation pattern, dipole radiators can go close to a side wall  and in my case, sloping ceiling  to little or no deleterious effect but in my room the taller they are, the closer together the pair has to be and at the Eros' exceptional 1,700mm (68 in) the gap would be difficult to squeeze through!  All my testing was done, therefore, in the listening room at the Gramophone offices in Harrow, a good sized but rather lively acoustic in which, after some considerable experimentation, Eros made some very splendid sounds.

InnerSound is a new company formed last year by a passionate enthusiast for electrostatic loudspeakers, Raj Varma, whose digital electronics company, Omni Digital, has been supplying computer systems to professional users in the UK for some nine years.  Varma commissioned American electrostatic designer Roger Sanders, well known for his articles in Speaker Builder magazine and for his book The Electrostatic Loudspeaker Design Cookbook, to design the best loudspeaker he could, the only constriction imposed being a limit to its physical size.  The project was developed on both sides of the Atlantic and Eros is the result.

Apart from its height, probably the most immediately striking thing about this loudspeaker is its marrying of an electrostatic panel with a transmission line loaded, moving-coil, low frequency driver.  Electrostatic panels have much to offer in terms of transparency and transient response, but are thwarted at low frequencies by the limited excursion of the diaphragm and its dipole operation.  The lower the frequency, the greater the wavelength and the greater the volume of air that has to be moved in order to maintain a level response.  Dipole construction (where the front and rear of the diaphragm are open to the room and of course radiate in opposite phase) means that as the wavelength increases, so does the cancellation effect due to the path length around the loudspeaker.  The only way to achieve real bass with a panel loudspeaker is to make it very large, thus increasing both the air displacement and the distance from centre front to centre rear.

Good low frequency performance is easier to obtain (and certainly domestically far more acceptable) with moving-coil drivers, where the diaphragm excursion can be much greater.  Here the rear-going radiation can either be contained (in a closed box, a so-called infinite baffle) or converted (via a reflex port) to complement that from the front.  An appropriately designed moving-coil based, low frequency system can satisfy the most demanding loudness requirements and, given the right crossover point, cover for the electrostatic driver's limited displacement.  The result can be a loudspeaker which has both substance and subtlety but the problem is this transition between disparate drivers.  Too often the joint is blatant.

Given this it is perhaps surprising to find the Eros crossover point set at 450Hz, right in the middle of the piano compass, where any anomalies in response or phase will be ruthlessly exposed.  Sanders has chosen a lightweight coned, 250mm diameter bass driver and damped transmission line loading as offering the best potential match to the ESL driver.  The spring effect of closed box operation and the non-linear, essentially resonant operation of reflex loading were dismissed as inherently incompatible.  Despite its compact cabinet, the folded line is some 2.4m (8ft) long and is partially filled with absorbent fibre.  Only the lowest frequencies find their way to the port where they complement the radiation from the front of the driver.  This combination of a low mass, large excursion driver and transmission line loading, produces a very clean, tight response, providing a good match in this respect for the ESL panel.  The principal residual difference is their dissimilar radiation patterns, although here again, the room and the loudspeaker's position within it are probably dominant factors.

Part and parcel of Eros is an electronics unit which comprises an active crossover and a 200 watt per channel (into 4 ohms) solid-state stereo power amplifier.  Active line-level crossovers are usually superior to their passive loudspeaker-level equivalents, allowing a more precisely tailored response and complete freedom from saturation effects at high working levels.  InnerSound has used steep 24dB/octave slopes set, as noted above, at 450Hz.  The low-pass output is fed to the 200W amplifier and thence to the bass drivers.  (This signal is also available direct from the crossover for those who prefer to use a different power amplifier for the bass drivers; two switches allow the crossover to be powered with or without the on-board amplifier.)  The high pass (treble) signal is fed to phono output sockets for connection to a stereo amplifier (or two monoblocks) of the user's choosing, this amplifier powering only the electrostatic elements.  The crossover point is fixed but the level (gain) of the bass output is adjustable to accommodate differing amplifier sensitivities and loudspeaker positioning.

Build quality is excellent with subtle radiusing of exposed cabinet rear edges and a good aesthetic balance overall.  The bass enclosure provides obvious physical and visual stability and although the loudspeaker is undeniable large, its relatively narrow front and shallow, semi-transparent top two-thirds make it appear reasonably-sized.  Adjustable feet are supplied for use on hard floors and spikes for carpets.  

The electronics unit has a finish to challenge the American heavyweights, and a glance inside confirms that high quality components have been used throughout with much care over the layout, a massive toroidal mains transformer, an Alps pot for the level control located on the crossover board adjacent to the rear panel socketry but controlled via a rod reaching to the fascia.


I should say straight away that this is a formidably good loudspeaker system, provided it is set up with care and provided one accepts that the optimum image definition and tonal balance are available only to one person sitting in the stereo hot seat.  Although perfectly listenable results are heard off-axis, the image only clicks into place in a small area defined by simple, self-evident geometry.  It is crucial that the path lengths from each loudspeaker to the listener are equal and crucial too that each loudspeaker is at right angles to the listener.  Because the plastic diaphragms are visible and light-reflective, the simple expedient of shining a torch (flashlight) at them from above one's head confirms that the angle is correct!

Eros is intended for near-wall placement but some experimentation is necessary to secure the best tonal balance.  This is intimately tied in with the bass level control, which as implied earlier, is basically a preset which once set should be left untouched.  This control can only be set by ear and is extremely, I would say overly, sensitive.  I would like to see a secondary, fine-tune control and also provision to lock the preferred setting.  Since it hinges at 450Hz, this control has a profound effect; set it too high and the sound will be bloated with a markedly cupped hands quality to middle range voices; set it too low and the result is emaciated.  That is the main caveat.

The other concerns the necessity to ensure that the drive to the electrostatics is in-phase with the bass.  Most power amplifiers are non-inverting, but if, for example, one of the older Quad amplifiers (e.g. the 405) is pressed into service, it will be necessary to reverse the cable connections (red to black, black to red) at either the amplifier or loudspeaker (but not both!) to correct the phase.  I used my own Exposure XVIII Mono Regulated power amplifiers.  The need also to get the phase correct between channels is obvious enough, though I have to confess that I listened puzzled for some time with one electrostatic panel out of phase while the other and the two bass drivers were correct before twigging why the stereo image was somewhat vague. (By the way, in addition to two signal input cables, each loudspeaker requires a mains connection for the electrostatic polarizing supply.)

Get all this right, though, and the result is terrific, with a virtually seamless transition at the crossover point and a definition of image rivalled by very few loudspeakers at any price.  True pin-point stereo is a joy in well-recorded classical music, definition such that one can hear right into a sound stage, feel the hall ambience, and almost reach out to touch the performers.  I don't think I've heard this bettered.  With the crossover point set this high, the electrostatic panel is well within its drive capabilities, even at very loud levels, so that with its correspondingly crisp bass response, Eros has a slam that puts it on a par with the big hitters; quite disconcerting this, for one who lives with the normal limitations of a full range electrostatic.  Where the ESL-63 will rather abruptly decline if pushed too hard (its sniffer detecting the onset of ionization and causing an electronic crowbar to short the input  enough to wreck some amplifiers), Eros is game for very much more, enabling it, for example, to invest close-miked percussion with a highly tangible, indeed physical impact.

The joy with all electrostatics, though, is that ability to render subtle detail with such clarity.  Combine that with remarkably even-handed and complete coverage of the entire audio band and you have a loudspeaker worthy of being called a monitor.  For a first product, this is a remarkable achievement, and with it InnerSound has not only made an appearance, it has arrived.

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