Mellow Acoustics  FrontRo is a £7500 loudspeaker of small proportions that employs a novel electrostatic panel for high and mids and a conventional driver for bass frequencies. Janine Elliot plugs them in and takes a listen for Hifi Pig.

Very few things make me fall in love at first sight, but the Mellow Acoustics FrontRo was just one such thing, the most beautiful and unassuming work of art that would look great permanently sat in my living room or Tate Modern. Alas, I only had access to this speaker for several weeks; a triangular bass body and a circular electrostatic head, the whole standing only 80cm from the floor.  Designed in the UK by Mr Tim Mellow, he has been an expert on electrostatic designs for many years, and this now becomes the only truly British electrostatic design out there, being built in Berkshire.

My two heroes of the past were Harold Leak and Peter Walker. The latter, also a hero of Tim’s, is synonymous with electrostatic speakers, and whilst Tim never met him, he worked with his other acoustical hero Leo Beranek. The 2^nd edition of the book which he and Leo have produced, called “Acoustics: Sound Fields and Transducers” is due for publication as I write this review and this new version contains a 43-page chapter dedicated just to electrostatic loudspeakers. Tim initially worked as an electronics design engineer, as an acoustical engineer and then at Nokia at Farnborough until its closure in the UK. When he left Nokia in 2011 he founded Mellow Acoustics. After about two and a half years the first prototype was made, though it had too many compromises to be of any commercial value;

“After a one-year break, I started to completely redesign it using different materials and a more sophisticated delay line. I think the design really matured after another two years (mid 2016) but then there was the problem of finding a manufacturer willing to take on the electrostatic unit”

One major stumbling block for any designer and manufacturer is CE testing. This slowed things up for the first production sample and subsequent modifications. “Hence, the total timescale has been more than seven years”. This delay allowed him to write the book plus do acoustical consultancy work. And now HifiPig have got the chance to audition the end result of all his toils.

CONSTRUCTION

The FrontRo is a hybrid electrostatic loudspeaker employing an LP-sized lollypop shaped electrostatic unit to handle the mid-range and treble frequencies (from 600-20,000kHz), while a tetrahedron shaped box employs the conventional 5.25” dynamic woofer to handle the lower midrange and bass taking it down to 40 Hz. The company are conscious that the lower end is limited due to the size of the enclosure, which is designed to be as unobtrusive as possible in the home, and anything below 40Hz is deliberately filtered to prevent distortion. More on bass later. Of course, deeper sound can be obtained from a subwoofer, and Mellow are indeed contemplating designing one in the future (perhaps called the BackRo?).

For the review, I chose not to use my Wilson Benesch Torus sub, so I could just hear the FrontRo on its own. Speaker placement was not a problem for me in my living room, not least because the electrostatic loudspeaker is usually less sensitive to room placement than a conventional loudspeaker box. However, as the electrostatic diaphragm has a figure-of-eight output, sounds will be heard equally behind as in front, so placement near to a reflective surface is not suggested.  The bass section being infinite baffle made overall positioning easier, though at least a foot away from the nearest object was found to be the best starting point. The unit is finished in light oak, with a choice of non-removable grille cloths in gunmetal, navy or burgundy. Underneath the cloth of the electrostatic unit is a special screen that keeps out dust and moisture.

Like Mr Mellow, I am a big fan of the principle of electrostatics. Not only does it mean doing away with both a cabinet and ‘slower’ conventional drivers that are both prone to adding their own signature to the music, but the electrostatic panels are incredibly light and therefore much more responsive to the signal. The sound has low colouration and low distortion, but generally they are not quite so efficient as conventional drivers. However, trying to get a realistic portrayal of the music from an electrostatic loudspeaker is far from easy. If it isn’t designed well an electrostatic unit can be quite forceful and only sound good at a single sweet spot on the settee.  Martin Logan created their distinctive curve-shaped electrostatic hybrid to enable a far larger dispersal of mid/high frequencies, allowing more than one person to sit on the settee and enjoy the music to the same extent. Mr Mellow considered curving the diaphragm, but this would add distortion to an otherwise distortion-free loudspeaker and curtail the lower notes due to increased diaphragm stiffness. I did feel the small size and slant of the combined unit meant the electrostatic membrane was angled too low for my head, though the screw-in feet do allow some adjustment, but a few back copies of old Hifi mags were finally put to good use propping it up a few centimetres more.

I mentioned my Hifi hero Peter Walker, the original CEO of the Acoustical Manufacturing Co. Ltd, later known as Quad. I even once owned an ESL63.  What makes Walker’s ESL63 different from the earlier ESL57 is largely the use of concentric anode rings with tappings from a delay system from the centre working its way to the edges to replicate a conventional speaker to imitate a point source behind the diaphragm. Tim’s approach is to create a spherical waveform – to mimic the natural sounds we hear – by  employing six rings fed from a tapping on a delay line that progressively increase from the centre towards the outer edge, so that by the time the audio comes from the edge, the sound from the centre has already had a head start, and since it is a distance from the diaphragm it creates a curved wave-front. Incidentally, Peter Walker was to do this a different way in what became known as ‘Peter’s Balls’; a pre-production spherical creation built just before his untimely death that had three diaphragms and four plates in front of each other with a time delay going forward in order to create a spherical waveform (Indeed, it actually had two sets of time delays, a rear-ported cardioid design that the company thought wouldn’t be popular due to its revolutionary shape, so was shelved). A spherical wave is ideal because it has constant directivity and a perfectly smooth frequency response. The perfect dipole sound source is a rigid sphere oscillating back and forth in free space because it spreads the sound out in the same pattern across the entire musical spectrum. If Tim could construct the perfect diaphragm no attenuation would be needed, as was required in the ESL63. Conventionally, the delay has to be attenuated, or “windowed”, due to the electrostatic diaphragm’s fixed size. However, Mellow’s dipole behaves like an oscillating sphere and so is ‘self-windowing’.

“In summary, whereas the ESL63 delay line has resistance losses added to compensate for the finite size of the diaphragm when reproducing a “virtual point source” behind it, the FrontRo doesn’t need this because it is imitating an “oscillating” sphere. In both designs, the outermost rings roll-off naturally due to the analogue nature of the delay lines which smooth out any irregularities that would occur due to the discrete rings.”

He explains much better than I ever could his designs in his thesis the FrontRo unit, largely due to its size,  is not as efficient as other electrostatic speakers I have reviewed (84 dB @ 1m for 2.83 VRMS) so any intentional attenuation would be unwanted as they need to hang on to every decibel they can! However, I didn’t find this a drawback in my evaluating, and indeed, the speaker worked better at low sound levels than many speakers I have reviewed. As an aside, the pattern of holes on the diaphragms is apparently the same as the seeds on a sunflower head, showing that a link to nature is perhaps possible. Sweet. And because the membrane is flexible, each part can move more-or-less independently from the rest according to the signal on the nearest ring.

The triangular woofer box containing the 5.25” driver is made of half-inch thick birch plywood that is heavily damped with special lining material, just as was used in BBC LS3/5a’s. And like that iconic speaker, this is an infinite baffle design meaning positioning is not quite so critical as with a rear-ported design. As Tim explained;

“Originally, there was a gap under the front cover so that the woofer could fire through the floor of the box, which looked cool, but this didn’t really work acoustically. Removing the gap has enabled the box volume to be increased and improved the bass, although the recess below the cover still gives the impression of a gap”.

The box has separate connectors for the dipole and woofer, should you want to bi-wire it, and a green light in the ear indicates you have powered it up. My only gripe was the connectors allow bare wires and spade connectors, but bananas need to be inserted in the holes reserved for bare wires. Not an issue in practice, and one that will be rectified. Inside the box are all the electronics and power unit to operate the speaker, with no need for an external wall-wart unit, rather a figure-of-eight socket to connect directly to the mains. The unit has a green LED on the rear of the unit.  Two internal circuit boards do all the important things like generating the 2000V polarizing voltage to charge the electrostatic membrane, and two audio stepping-up transformers to drive each of the stators of the electrostatic unit, plus a pair of delay lines which are mounted on a separate delay board. Tappings from each delay line are connected via one of two ribbon cables to the copper rings on each stator.

THE MUSIC

How appropriate that I should start the review with Voyager ‘Eye Contact’ since the FrontRo was delivered by their PR manager Chris Hook who was the bass player in the late-70’s pop-rock band, that also originated in Berkshire. The opening track in this 2006 album is “B.A.B.E. Babe” which starts with a deep bass intro to announce the track, covered well by the speakers down to 40Hz. Mid vocals were clear, and tops excelled with the dated analogue synthesisers. Only the bass drum could have been a little more forceful, but when you consider the size of the bass unit it was acceptable. Whilst the company might suggest using a sub – and I hope they do design their own one day – I decided not to use mine for the review so I could get an accurate impression of the FrontRo. The top end was particularly where the speaker showed its laurels. It was lovely to hear the speed of initial transients and clarity of placement and positioning of musical instruments when I was sat correctly.

Playing Syd Lawrence Orchestra “Hawaiian War Chant” (Mike Valentine – Chasing the Dragon) gave the speaker a chance to excel, especially now that the angling of the unit was adjusted for my ideal sitting position. At that point, the sound just opened up. “Too Darn Hot”, featuring vocalist and broadcaster Clare Teal, was quick, exciting and precisive with the music ably filling all the room. Since the electrostatic membrane creates sound behind as well as in front, I could hear a very spacious and natural rendition of the music.

The piano in Chasing the Dragon II track 5 (Mozart piano sonata no. 15) was clearly and cleanly delivered. Bass was also well delivered. A real test for that bass, however, was Bach’s Toccata and Fugue in D minor, excellently performed and presented on the same album, and only missing out on that lower octave.

Vocals in my listening sessions were clearly and realistically delivered, due in part to the 1st order crossover point being 600Hz. I don’t like crossovers in the important 3kHz area. Listening to Mark Knopfler’s ‘Get Lucky’, his vocals were cleanly and naturally delivered and the clarity from the mid/high unit enabled me to hear details in the recording I have previously missed out on. Indeed, a distortion in the right leg in the track “Hard Shoulder” showed up an unintentional hard shoulder on the record groove. I hadn’t heard this before.  I decided to turn to a double-album I know has good audio engineering. Eagles ‘Long Road to Eden’ starts side 3 with an eerie wind blowing before keyboard and guitar take it on its journey. I did, however, feel there was a need to sit in exactly the right position to get the perfect 3-D experience. Where the speakers were very good in the mid/high range that bass really needed some help, especially with the prominent bass-tom in the title-track. Adding my excellent Wilson Benesch Torus subsonic generator would have helped enormously. I was so tempted. The Mellow might be mellow in the lowest octave but that top-end does give great depth of sound, positioning instruments in front and behind, especially from the percussion. The strings in “I Dreamed There Was No War” were so clear I could position each violin in the soundstage.  This was highly absorbing, and I could hear what Tim was trying to do in creating a realistic soundstage. Only that I could hear a slight bump around 2.2kHz spoilt the enjoyment for me, plus of course missing that lowest octave. The former was mostly rectified by careful placement. Tim did try a larger bass unit but found it didn’t actually add any bass extension. A larger box would also take away the “cute” look of the speakers.

Listening to my favourite British classic work ‘The Planets’ by Gustav Holst was a chance for me to explore the depth of soundstage if perhaps missing out on the unfortunate sounds of lorries driving past the Lieu d’enregistrement in Vienna, where it was recorded in 1961 (Herbert Von Karajan, Vienna Philharmoniker, DECCA). I once had the pleasure of sitting at the piano where Holst put pen to paper and composed this great work at the London school where he taught. It was also the last location-recording I did for a BBC Radio 3 feature on the composer before I left the corporation. Just as I needed that to sound good, I needed this performance to be good, too. The mid-band still required me to do some adjustment of placement to keep under control that mid-range so that it was as clear and precise as Karajan would want it to sound. What a good listen, though. Whilst glockenspiel and triangle were perfectly formed, I still found brass bursts, particularly in “Jupiter”, were less than clear when put against the string passages. Once all hell settles into the famous British theme the strings sounded as good as I would expect. Jupiter, the bringer of jollity lived up to its name. Despite not hearing the lorry rumbles passing the cello section I didn’t feel bass was unduly missing; The repeating riff from the cellos and bass in “Saturn” were all there. To complement this the final phrase from strings was beautifully performed on the FrontRo. I really was on the front row of this brilliant performance.  The clarity from individual instruments in this performance was just as I had hoped the electrostatic membranes would give out, even to the point of hearing page turns on the first desk of the violins, and someone’s foot hitting a mic stand! I remember reviewing Flare Audio’s first headphone for HiFi Pig and though it wasn’t the best headphone by any means I could see what a future the company in terms of what they were trying to achieve. Now they are leading the world in IEM design. I can see a similar future for Mellow. This is a highly commendable product, and am excited at what could be next.

CONCLUSION

For a first attempt, this is a superb offering. The ideal source is a rigid sphere, and Mellow’s delay line and rings bring this closer. Where Quad failed to realise it in production, Tim Mellow now brings the aim of perfect reproduction of soundwaves that much closer. What makes the FrontRo special is that I wasn’t listening to loudspeakers but rather I was listening to music.  Accepting the bass limitation this is a very able product and well worth listening to, working particularly well on classical and jazz music.

AT A GLANCE

Build Quality: Well-built unassuming design that will interest visitors to your house

Sound Quality:  Excellent detail in mids and highs. Bass could be a little more forward. Better at lower levels.

Value for Money £7500 is not cheap but this takes dipoles to a new age. 

Pros: Very low distortion. Excellent 3-dimensionality of sound. Works particularly good at lower levels.

Cons: Mid frequencies can sound obtrusive largely due to the lack of lower bass. Not so flat at louder listening levels.

Price: £7500 (Price correct at time of publication)

Janine Elliot

Review Equipment:

Krell KAV250a and Leak Stereo20 amplification, Music First Audio Baby Reference pre, Pre-Audio turntable/AT33sa cartridge/Manley Steelhead Phono-stage, Krell KPS20i CD, Ferrograph Logic7 15ips/½ track Reel to reel.

Technical Specifications

Overall dimensions (without feet): 762 mm (H) x 494 mm (W) x 291 mm (D).

Weight 10.1 kg

Recommended amplifier power rating: 25 to 100 W

Frequency response: 40 Hz to 20 kHz

Input impedance: nominal 8 ohms.

Sensitivity: 84 dB @ 1m for 2.83 VRMS.

Maximum output: 98dB SPL from electrostatic unit for 14 VRMS (input protected).

Crossover frequency:  600 Hz, 1st-order, time-correct

Mains connection: 240 V, 4 mA