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HIFICRITIC
Volume 5 No. 4
The Supravox Field-Coil
Chris Bryan gets back to basics with a simple full-range drive unit driven from an electromagnet.
Review By Chris Bryant

 

  Supravox emerged from a company that was founded in France in the 1930s, and proved successful in the heyday of field-coil driver production. Since 1945 the company has successfully developed in house technologies and continued producing loudspeakers, almost uninterrupted, to the present day. This Supravox 215 EXC is one of very few survivors of a once dominant species. This raw drive unit comes in several forms, depending on the magnet type used in its motor. This EXC version uses a large ‘field-coil' electromagnet and costs around £2,000/pair; normal permanent ferrite and alnico magnet versions are also available.

 

Background
The fact that field-coil (electromagnet) energized loudspeaker drive units still exist is a triumph for the enthusiasts who've managed to conserve what they consider to be a worthwhile engineering approach, in spite of the fact that it has subsequently been almost swamped by advances in technology and mass production.

Back in the earliest 1930s days of hi-fi, powerful permanent magnets were exceptionally heavy. The only way to create the required magnetic field strength in a manageable packaged loudspeaker was to use a field-coil magnet. Back then the coils were designed to work at a high voltage (100s of volts DC), and were often also used as a filter choke in a valve amp's high tension power supply. Hum could frequently be a problem.

The ubiquitous modern direct radiating moving coil loudspeaker that dominates audio today was made possible by subsequent advances in magnetic materials. When relatively lightweight permanent magnets with high field strength became available (in practical terms after the Second World War), and proved capable of delivering an acceptable result in both sensitivity and sound quality, field-coil drive units were rendered all but obsolete and extinct. However, some devotees still believed field-coil drivers were intrinsically better, so limited development continued for these extreme enthusiasts.

Although design diversity was the rule rather than the exception, loudspeaker companies all searched for wide bandwidth, good efficiency and domestically acceptable products, especially after two loudspeakers were needed once stereophony arrived in the late 1950s. Cheap permanent magnet moving-coil cone loudspeakers that could fit into small cabinets and were relatively easy to make proliferated. In search of better performance and advanced technical specification, the industry developed complex multi-driver systems in similar compact packages.

 

Voigt Inspiration
The Supravox Field-CoilWhen visiting PM a couple or years ago, I heard some vintage Voigt Domestic Corner Horns using single field-coil drivers. They sounded so interesting and musically natural that I began to wonder why even the high end had virtually abandoned such technology. Looking back, had the industry taken the best direction for musical satisfaction?

I believe it's easy to understand why the industry followed the course it did. Once the systems of mass production are set in motion, it's far easier to make advances in those technologies familiar to engineers. Most of the industry and the market accepted that the development and refinement of permanent magnet driver technologies was capable of producing ever better loudspeakers. Field-coil technology had been usurped; long live the simpler and cheaper permanent magnet solution.

However, like so many activities that captivate people emotionally, hi-fi is divided into factions. Although such factions may remain aware of mainstream trends, they independently adopt pertinent techniques to develop extremes of performance in their own desired directions. Most of us (including hi-fi reviewers) necessarily receive mainstream products with widespread availability. While we may become aware of the rarer and more exotic components around the periphery, those interesting technologies are constrained by restricted access and tend to remain forever out of sight for all but the inquisitive few.

We seem to expect the best to rise to the top, but these days I consider that the companies with the better marketing muscle and expertise tend to dominate the scene. Although the internet has made it far easier, significant time and energy is needed to discover that enthusiasts around the world are forging alternate paths to excellence.

One unfortunate down side is that the cost of small scale production often means that many of these exotic products have price tags that will forever ensure their exclusivity. With the contraction of interest in performance driven hi-fi and the decrease of interest in quality audio by the major electronics manufactures, we may yet see a greater acceptance of and interest in the smaller firms creating their uniquely exotic solutions.

 

Supravox
Supravox emerged from a company called Super Electro Mecanique (SEM) that was founded in France in the 1930s. Since 1945 the company successfully developed in-house technology and continued production, almost uninterrupted, to the present day, establishing the Supravox name in 1956.

The 215 EXC reviewed here is a full-range raw drive unit with a low mass hyperbolic paper cone. It has a stationary phase plug in the centre, no extra whizzer cone to lift the treble, and a concertina form coated paper surround that gives a maximum linear displacement of +/-10mm. A massive field-coil electromagnet replaces the normal ferrite or alnico permanent magnets. In many ways this old fashioned looking unit seems an unlikely candidate to produce a high end sound. It may be said to be capable of full-range operation, but looks more like a powerful midrange driver.

On opposite sides of the magnet assembly, mounted on the thick aluminium frame, two pairs of gold plated 4mm socket/binding posts accommodate the signal and power supply connections. An external power supply is required to energise the field-coil (not supplied), and Supravox recommends a 10-14V DC output with a 5A capability. Changing the voltage to the field-coil alters the field strength which changes various driver parameters, especially bass damping and sensitivity. For the technically minded, Supravox provides a table of how these vary with voltage. Assuming no need for a crossover and feeding the electromagnet with 8-13V, sensitivity ranges from 98-100dB/W/m. Even though it has a 4ohm voice coil, this level of efficiency gives the partnering power amplifier a relatively easy life; even low power valve amps should go realistically loud.

Alongside the Supravox drive units, we also borrowed a pair of thick rectangular acrylic open baffles to mount them. Open baffle mounting is an uncommon technique today, and has both positive and negative aspects. The down side includes a reduction in bass output caused by cancellation between the sound energy from the back and the front of the cone (as happens with all open-back or di-polar speakers). The usual closed cabinet traps and dissipates this backward firing energy, creating the low frequency equivalent of a baffle of infinite size. However, internal enclosure resonances and reflections produce their own sounds, so a well-designed open baffle may sound more open and less coloured, especially in the midrange. When open baffle size and the driver positioning on it is optimised, surprisingly promising results can be obtained with fascinating consequences.

For those who don't want to get into DIY, the stylish acrylic 650x1055x20mm (WxHxD) open baffle we used is available for £1,000/pair from UK Supravox importer Real Hi-Fi. The driver cut-out is asymmetrically placed to reduce baffle cancellation dips, with its centre 655mm from the floor and 252mm from the nearest side. The driver sits in a recessed cut out, with bolts to hold the driver in place, and also to mount the baffle on a brushed aluminium stand. This stand is a single 10mm bent sheet with a semicircular top and cut-out for the driver motor. The baffle tilts back 15 degrees, while the stand has a footprint 450mm deep. The effect is contemporary, stylish, and actually far less obtrusive in my room than a normal box loudspeaker.

The speaker requires a DC power supply and connecting cable as well as the normal speaker cable connection. There's no crossover so the amplifier is connected straight to the voice coil, ensuring minimal signal losses and no crossover phase anomalies or delays.

 

Sound Quality
Using a fairly standard lab power supply set to 10V, and with the speakers connected to the Exposure MCS system (Vol5 No3) I was immediately rewarded with a fast, dynamic and very detailed sound. I had to experiment with placement a bit to get an acceptable frequency balance, but that holds true for any loudspeaker. Very quickly the Supravox speakers were giving the sort of sound that makes you want to listen to all the recordings you love, because fresh detail and fine nuances keep appearing.

I have experienced this effect before with other breakthrough products, but never in quite such an effortless fashion. I know from past experience that certain frequency or phase anomalies can result in similar positive psychoacoustic effects, but in my opinion never in quite this way. I ended up listening for a very long time, though towards the end I was becoming fatigued and beginning to be bothered by some tonal balance forwardness and also a degree of image imprecision.

After further reflection I accepted that the Supravoxes were capable of exceptional performance in some respects, and was driven to try and extract even more quality and therefore overcome my reservations. My next session was therefore rather more scientific. I began by varying the field-coil power supply voltage and while this can create subtle but worthwhile differences, I still wasn't satisfied. I was still hearing lots more information than that available from far more expensive, exotic and past favourite loudspeakers, but that forward presentation still worried me.

It might have been that they needed to be run in for longer, but I also had a suspicion that the power supply might be a critical component – the one I was using wasn't particularly low noise and was more industrial than audiophile. Then I remembered a couple of 12V 21Ah batteries in the shed. Hoping they were still capable of maintaining 12V at around 2A for a reasonably long listening session, I put them on charge and meanwhile left the drivers running in for a bit longer.

With fully charged batteries supplying 12V now installed, and no other changes, my reservations over the Supravox presentation had completely evaporated. Not only had the tonal forwardness gone but the Supravox speakers now revealed an additional and extraordinary range of virtues. They just thrived on that clean supply. Their intrinsic abilities meant that the system sounded more dynamically lifelike than I've ever experienced from any hi-fi before. Even old recordings could really surprise, and several times I thought someone had entered the room, as reproduced voices and noises sounded so real and present.

Edits and poor mixing techniques were revealed with a precision that may annoy some people, but I'm learning to accept these as part of the performance of that particular recording. Ultimately the overall result is delightfully interesting and involving, and comes the closest I have yet heard at producing dynamically coherent and convincing music. These speakers are capable of exquisite timing and sonic coherence, somehow always extracting the best from the rest of the system. They even managed to create a level of musical performance that transcends what I previously could have expected from the system driving them.

I tried other amplifiers, cables and sources, and liked the 215 EXCs even more for their abilities to convey the best attributes of all the good electronics I had to hand. But they just as easily reveal any abhorrent characteristics and deficiencies in flawed amplifiers and sources.

Note that this open baffle speaker design is not tonally perfect. In my listening room it can never be called particularly neutral, and I couldn't find a position that produced much deep bass, due to the finite baffle size. However, whereas I'd complain about midrange frequency anomalies with most loudspeakers I've heard, here the Supravoxes haven't yet troubled me – it's as if what they do so well transcends all else. The mid is sweet and open with quite astonishing levels of detail.

Interestingly, I'm more critical of its sound in the room than with many other speakers when I'm not seated in the ideal listening position: this may be something to do with my room and the open baffle mounting, and I'll have to experiment further here. The treble is surprisingly good, far better than I actually expected from a cone of this shape and size, but the high treble does seem a little reticent, lacking some air and sparkle. Focus is also good, with solid and stable images and a surprisingly wide and deep soundstage. Soundstage height reveals some amplifier characteristics, and the Proteus Diamond MkII provided a higher image than the Exposure MCX. There's definitely a sweet spot here, where everything snaps into focus and it sounds more tonally right, but even when sat slightly away from this, I really didn't find that much to complain about; it still has good balance and I can still clearly hear all the startling midrange detail and dynamics.

Irrespective of music genre, the Supravox 215 EXCs remain so communicative I could simply enjoy them on all the recordings I played. They have an ability to reveal the music structure and excite one so effortlessly that the senses are rewarded in an amazingly consistent fashion.

 

Conclusions
Although they are totally exceptional in their ability to reveal information, this loudspeaker isn't necessarily for everyone. Those who insist on a flat and extended frequency response with deep bass should look elsewhere. Some are happy driving a bland people carrier, which is efficient and dull and will get from A to B safely without incident or excitement. Others seek the thrill of dynamically superior, critically tuned sports car which excites the senses with every bump and turn in the journey. The difference between the two is stark, and obvious to those who are interested. The difference between the Supravox 215 EXC and other loudspeaker systems that I have tried which claim a similar performance-oriented purpose is just as stark.

I'm sure this Supravox driver could be built into a more complex system by using it with other units that can supplement its small deficiencies at the frequency extremes. But then you would be adding some crossover components, and I can't guess how much of the essence of its lifelike performance would be lost. David Cathro has been working with this driver in just such a system (HIFICRITIC Vol5 No2), and tells me that the ability to alter the drivers by adjusting the voltage can be very helpful in seamlessly blending drive units together. (He uses a field coil bass driver as well.) I'm told that some simple equalisation will produce a far flatter response and may actually improve the sense of timing and realism, but then that wouldn't be the speaker system that was supplied to me.

Whatever one's ambitions for experimentation, if lifelike dynamics and the ability to create the most impressive combination of realism and structural information are what is important, this speaker is guaranteed to satisfy and enthrall.  

 

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