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Eros: hybrid electrostatic speakers

Hybrid Electrostatic speakers are becoming popular, offering the ‘best’ of both worlds

Speakers come in all shapes and sizes these days. Some prefer the traditional cone-type while others are more interested in developing new ways of moving air.

The InnerSound Eros speakers use two types of driver; one electrostatic panel, and a cone-driven sub for all the lowest frequencies. This division of labour can reap great rewards but needs to be executed carefully for maximum benefit.

The difficulty traditionally lies in the different relative drive requirements for the panel and the cone. The panel’s drive unit is essentially a sheet of cling film stretched between two outer plates. It is essentially massless and is driven over its whole area evenly. It also requires a power supply to charge the plates for the panel to work. 

The cone woofer, in contrast, has a highly significant mass, radiates differently over its area, and needs a box of some sort in order to maximise its performance. Because of its mass it is relatively slow at stopping and starting, and in a bass application needs a significant amount of power to drive cleanly and adequately. Matching this to an electrostatic panel requires some very sophisticated electronics

The Eros package comes with inbuilt power supplies for the panels, built in woofers and an active cross-over and power amplifier for the bass units. As such, the system is effectively bi-amped: you use your own original amplifier to power the panels, and the Eros amp drives the woofers via its crossovers.

There have been many attempts to marry the two drive units by other manufacturers, but here the integration is utterly seamless and the aural presentation is as one driver.

The clue to the speaker’s success is in the way the active crossover works. The crossover frequency between the two drivers is unusually high, and would normally be very audible. However, because the crossover has been so well designed there’s barely any indication even when measured with accurate mics and monitors.

Some arguments have raged about the validity of combining an electrostatic with a conventional driver, but as InnerSound has said, the bass is a ‘slower’ frequency anyway. Although instantaneous stopping and starting with a massless driver would be an idea, it’s not a practical proposition.

There have been a number of ‘full range’ electrostatics on the past, the most notable being the two versions from Quad; the ELS (now no longer in production) and the current ESL63. However, in order for an electrostatic to produce true deep bass the diaphragm needs to be very large, and in many domestic environments, this is not an attractive proposition. Thus the smaller panel for the upper ftequinecies, and the cone woofer for the lower, has been a choice of many.

Again, though, Eros differs from traditional thinking, favouring a transmission line design over more ‘usual’ ported or reflex designs. This, InnerSound claim, leads to a more natural, deeper, cleaner and more articulate bass which more closely follows the output from an electrostatic panel.

One other aspect of InnerSound’s design is the bass ‘volume’ control on the bass amp. This can be used to vary the amount of bass output to suit the listening environment. The crossover point is unchanged, it is merely a bass volume control. However, it is particularly effective and obviated the need to change speakers if you move house. If you have the speakers tight into a corner a lesser bass volume is needed due to the corner reinforcement effects at the lower frequencies. In larger rooms, a higher bass volume setting will be more likely as the speakers are likely to be further into the room and less reliant on corner support. 

All in all, the thought and care which has gone into the design is commendable. The bottom line is, as always how they sound. Truly stunning.

In an ‘average’ listening room (15 by 12 feet) the panel can be driven by a little as a 10-watt amplifier, and imaging and presence is truly breathtaking. The upper reaches of the electrostatic’s performance are sparkling clear – an area where electrostatic has always scored over its cone based rivals. The midrange too is very, very transparent and gives a real ‘see-through’ aspect to every record.

Just to allay any fears all-tube lovers may have, the electronic crossover can be connected to provide two power amp feeds, with the ‘internal’ power amp unused. In this arrangement, any amplifier can be used for the bass amplification, so an all-tube system is easily possible. 

Even with a modest power amp the Eros will give a good account of dynamics. Again, most ELS speakers are hard to drive, providing the amplifier with a difficult load and making drive difficult having relatively low efficiency.

The Eros has, by careful research and design managed to achieve a 90db efficiency for the panel and the output from the sub has been matched to this. System efficiency is therefore very high and allows the ‘low power single-ended valve brigade’ a choice of real musical enjoyment with a combination hitherto difficult to get to work.

The bass end of the combination is also very articulate, and for me the ultimate test is with organ music. As a practising organist I am always interested in other performers’ interpretations of the great works, and in particular, the pedal combinations chosen. Many loudspeakers systems, whilst they will produce the lower notes, do so with a lack of clarity which masks the pedal ranks being used. They also make the following a pedal tune something of a hit-and-miss affair in some instances. 

However, here the Eros did a very commendable job of conveying all the weight and power of the bottom end, yet retained the clarity and dexterity of the player, and subtly of timbre so often lost in the mists of the weight. 

Another area so beloved of ‘aficionados’ is the reproduction of female vocals. Here was an area where the Eros excelled. There was such definition, such ease, such realism that at times it was difficult to accept that this was a loudspeaker in a hifi system at work. Everyone who visited while the speakers were installed commented on just how realistic and lifelike they were, and how they were so ‘un-hifi’, and how they made so much music. 

Despite their high sensitivity, they will also take a high power amplifier with no problem. In fact, the rated power handling is 250 watts, so we connected a big Krell.

And you could hear it was a Krell, with its distinctive sound, but the Eros happily took it all that they Krell could throw at it, and never sounded distressed at all. Getting close to the speakers at that volume was a bit difficult though sound pressure levels they are capable of the phenomenal!

The other virtue of the speakers is that they are completely happy with any program material as long as its good. Pop, rock, jazz, dance, classical, and many more all had their turn and none was disappointing. Put on good material and listening became a real joy and something which you didn’t want to stop once you got started. 

At its price, it has to be among the best, if not the best loudspeaker I’ve ever heard. 

Updated Sanders Systems version of the Eros loudspeaker