Soltanus Acoustics Euridiche Electrostatic Headphones

Match made in heaven between the mighty sound of planar-magnetic transducers and the subtlety of electrostatic earpieces

Introduction

Not unknown to connoisseurs of electrostatic high end, primarily due to Soltanus Acoustics Virtuoso electrostatic loudspeakers (see: publications), the designer has launched a family of headphones based on similar design/circuit principles as its floor-standing “granduncle,” the Virtuoso (12,000 €) and great-granduncle, Virtuoso Grand (48,000 €).

After years of prototype testing which involved a number of music-lovers and High-Fidelity experts, Soltanus Acoustics came out with three headphone models of different weight, size and price, presented first at the prestigious Hi-Fi Show at Budapest Hilton City, in October 2019:

  1. Euridiche Minima, the smallest electrostatic headphones in the world, weighting only 370 grams,
  2. Euridiche 3D, a rather large, but relatively light (450 grams) and
  3. Euridiche, a heavy weight (620 grams) headphones for the uncompromising music lovers.

All of them are very comfortable to wear, ideally adjustable from small to XXXL heads and from small to large ears, which all fit well into the wide and deep soft skin ear pads.

If I would have to describe in short these headphones, I would say, they are a match made in heaven between the mighty sound of planar-magnetic transducers and the subtlety of electrostatic earpieces. They go as deep as the longest organ pipe and as high as 40,000Hz. When one listens to organ music the might of these headphones is amazing, as is in the case of a large symphonic orchestra with eight contrabasses, tympani and big drums. Music is really present, full-bodied. This does not mean the lack in finesse and in transparence, however. Lied-, choir- and chamber-music comes out with all the intricacy one expects from a high-end headphone system.

However, for me, the most prominent aspect of these headphones is the way they present space – space between musicians, the unfathomable air around their instruments, and between the orchestra as a whole and the walls of the end and of the sides of a concert hall. This extraordinary spaciousness accounts for a perfect phase of the signal, free movements of the very thin and large membrane and the extremely sturdy stators, particularly in the case of the top model. The stator is very open, lets music flowing out from the membrane unhindered. Due to a smart arrangement of the circuits and the membrane the listener perceives the orchestra partly in front of him/her, instead of being in middle of the orchestra, which is the usual headphone experience. This relates the Euridiche to the famous, already discounted AKG K-1000, but while the Austrian headphones paid a high price for this spatial experience, namely the lack of bass, the Euridiche delivers spaciousness and spectacular basses at the same time.

Musical experience

These headphones are suitable for all kinds of music generated by acoustic instruments and vocals from early madrigals, Renaissance instrumental music, Baroque ensembles, symphonic music of all kind, operas and oratoria of the 19-20th centuries up to small Jazz ensembles and big-bands. It is presumed that rock music, heavy metal requires sound levels which exceed the 120 dB limit of electrostatics, but people who do not want to destroy their hearing can rely on Orpheus headphones until 120dB also for rock music.

Potential buyers should bare in mind that these electrostatic headphones, as the majority of this type, is open to the outside, thus not suitable in noisy environments. However, it is known that other types of headphones, like dynamic or planar-magnetic also give their best if they are open. For instance, the famous Sennheiser HD 820 would never sound so open and spacious as its open counterparts, the HD 800 and HD 800S.

Compared the Euridiche headphones to other electrostatics, primarily the Stax SRT-009 or the Sennheiser Orpheus, the main difference would be a stronger bass-response due to the larger membrane and larger mass of the whole system of Soltanus Acoustics headphones. In the case of Soltanus Acoustics Euridiche, the top model, the stators are not only larger than their Japanese and German competitors, but also made of sturdier material and geometry that sustains vibration of the membranes much better. These thick metal wires account for the weight difference between the middle-weight Euridiche 3D and the heavy-weight top model, the Euridiche. The difference between these models is slight but audible to the experienced audiophile.

Another difference is the already mentioned space delineation. Save the Euridiche Minima, the other models have a geometry of the membrane and electronic circuity which gives the impression that music comes both from the front and the sides and not only from the sides (feeling of sitting in the middle of an orchestra) as is the case with the majority of headphones, be it dynamic, planar-magnetic or electrostatic.

All three models share the same membranes, but the rest of the headphones make up for the acoustic difference. The extraordinary spaciousness of the two larger models is achieved by distributing treble and bass in a special way. The treble comes more from the front and basses, which are less directional, more from the sides, but without sharp cutting, known in the case of traditional loudspeaker crossovers. However, it is important, that there is still one membrane per side, which ensures the same phase for the whole acoustic spectrum, contributing to the extraordinary coherence of the sound.

There is a slight difference between the top model and the middle-way Soltanus Acoustics Euridiche 3D.

The top model with heavier, metal stators contributes to deeper basses, although this difference cannot be measured. In high end audio measuring instruments do not entirely reflect the actual musical experience. Moreover, the top model gives a bit more air around the instruments and the venue as a whole. Certainly, due to the substantial hand-made stators this model is more difficult to manufacture and, thus, it is a bit more expensive.

Customizing the sound

The two larger models offer two ways of customizing:

  1. A separate lever on each side for recordings with too much treble due to short distance between the instruments and microphones has a special, simple provision on these headphones. The audiophile can take off 2 or 4dBs from the biting upper-midrange and lower highs. This filter is also a great help if the high-fidelity system tilts to be hot, either due to solid state technology or digital jitter. Certainly, these headphones give their best with impeccable high-end gear, but until one acquires it, these filtration options offer an acceptable solution.
  2. The careful choice of cables for the headphones also allows to customize the sound to different amplifiers or sources, be it analogue or digital. The materials of these cables, silver, copper, gold, chromium, nickel and their combinations, the mixture of variable diameters and different geometry of assembly offers a wide range of fine-tuning. This is mainly the perceived frequency response – it cannot be measured with the instruments today – but one can subtly shape the sound. Soltanus Acoustics offers different ready-made cables or for special requests even customized cable for some audio system that may require that.

Moreover, if one uses the driver unit, the cables between the amplifier and the energizer also allows good customization of the sound. Basically, for this purpose one can use loudspeaker cables from different manufacturers, but again, if needed, Soltanus Acoustics can provide users whose system requires stronger customization than what is usually needed.

The Euridiche Minima could be tuned only by the clever choice of cables between the headphones and amplifiers and between the driver unit and the amplifier.

Amplification for Euridiche headphones

The Euridiche headphones give their best with high-end tube amplifiers. However, the constructor has provided these headphones with the possibility of attenuation of the high frequencies for the case of using cheaper amplifiers or sources with less refined sound. The attenuation in three steps helps enjoying music even with modest solid-state amplifiers and digital sources with some moderate jitter. The sound of these headphones could be also tuned slightly with the choice of cables that Soltanus Acoustics offers, as mentioned earlier.

Soltanus Acoustics offers two solutions for feeding the Euridiche headphones. The simpler solution is a high-grade Headphone Driver Unit that could be connected either to traditional headphone amplifiers for dynamic or planar magnetic headphones, or to power amplifiers’ loudspeaker or headphone socket. The combination of Euridiche electrostatic headphones and the driver unit is highly efficient: a 1 W pro channel tube or solid-state amplifier is more than enough, thus, quite simple tube or hybrid amplifiers can reach good sound levels. If one has limited amount of money, it is better to spend more on these headphones and spare on amplification than the way around. One gets better sound with the Euridiche, the Driver Unit and a simple headphone amplifier, like Little Dot, than with an expensive headphone amplifier and planar-magnetic headphones. Amplifiers can be upgraded later. Fine-tuning of the set of amplifier and Driver Unit is by selecting proper cables between them.

The more advanced solution for feeding Euridiche headphones is the Soltanus Acoustics Fortissimo headphone and integrated amplifier which can drive also highly efficient loudspeakers. The Fortissimo amplifier has sockets for the Euridiche headphone family, i.e. two 3-pin XLR sockets on its front for the optimal separation of left and right channel’s signal and for the optimal bias-current transfer to the left and right ear speaker. Also, on the front plate there is a socket for the 5-pin Stax electrostatic headphones that share electrical characteristics with Euridiche headphones – bias voltage and signal current. If one owns a high-grade tube amplifier it will give with the driver unit a very close results to the Fortissimo amplifier, but the latter offers better safety from overloading of the headphones. Therefore, on a long run, the Fortissimo amplifier is the safer and slightly better sounding alternative. Last, but not least, it can feed high grade highly efficient dynamic speakers in a smaller room.

Durability

These headphones are practically indestructible. None of the prototypes failed in the two years long testing period. Some testers used it over 10 hours a day, including bedtime, sometimes falling asleep and lying on the unit. Still, the producer supplies each pair of headphones with a spare pair of membranes, easily replaceable by the user. The lambskin ear pads are also replaceable.

Accompanying equipment during the test

Source:

  1. Ayon NWT tube network drive via LAN, Synology Server connected with AudioQuest Diamond network cables
  2. Ayon Stratos tube DAC, connected to the network drive via I2S also with AudioQuest Diamond network cables

Amplifiers:

  1. Soltanus Fortissimo, connected to the DAC via 99,99% solid gold Interconnect cables
  2. XDUO hybrid headphone amplifiers
  3. Solid state McIntosh MA 5300
  4. Custom made single ended tube amp

Cables:

Cables between the amplifiers and headphones were Soltanus Acoustics varistrand silver-gold hybrid cables, each wire in separate Teflon tubing.
The driver unit was tested with the dynamic headphone output of the aforementioned amplifiers connected via 99,99% silver and 99,99% copper hybrid cables.
All plugs were manufactured by Furutech.

Power Conditioner:

Dunaudio, custom made for this setup

Power cables:

AudioQuest NRG 10
Custom made varistrand silver-copper cables terminated with top-class Furutech plugs

Comparison with other headphones:

Audeze LCD-4, with custom made 99,99% silver-gold alloy cables
Audeze LCD-X, with custom made 99,99% silver-gold alloy cables
Sennheiser Orpheus (new) with its tube amplifier/DAC
Sennheiser HD 800 with custom made cables that brought its hot sound close to the timbre of Audeze headphones
Stax SRT-009
AKG K-1000
AKG K-340, both with custom made cables in order to bring the sound close to the aforementioned headphones
I used tube-rolling in my headphone amplifier of the 12AU7/ECC82 driver tubes from different vintage Mullard, Amperex Buggleboy, Siemens, Telefunken, GE, Tesla, Brimar offerings to level the different timbre of the aforementioned headphones. I also played with a combination of different cables for the same purpose. Certainly, one has to tune the system when comparing different headphones in order to filter out differences in frequency response in order to be able to concentrate on coherence of sound, speed, space and other subtle characteristics. Perhaps the best example is the HD-800 or HD-800s which with the supplied cable gives an aggressive sound and spoils the great subtlety of this system. The same applies for the LCD-X of which the supplied cable give a maffled, remote sound. When taken care off these headphones give good results. The sound of Euridiche headphones are a bit less cable dependent, but requires careful choice, nevertheless.

Recordings

I tested the Soltanus Acoustics Euridiche headphones for about 18 months with classical music and acoustic Jazz, of which the most outstanding experience came from the following recordings:

Classical

  1. Brahms, Piano Concerto Nr. 2. III Andante, Duisburger Philharmoniker, Anna Malikova / Jonathan Darlington, 192kHz, 24bit
  2. Brahms, Ein Deutsches Requiem 45. I. Selig sind die Tote, Nicolas Harnoncour, RCA Read Seal, 96kHz, 24bit
  3. Beethoven, Sonata No. 23 ‘Appassionata’ in F minor Op 57, Allegro Assai, Artur Pizarro, 96kHz, 24bit
  4. Brahms, Trio for Piano, Violin and Violoncello, No.1, Finale, Allegro, Pires, Dumay, Wang, DGG
  5. S. Bach: St. John Passion BWV245 (1724 version), Part I: Aria: Von den Strikken meiner Sünden (Alto), Academy of Ancient Music, Sarah Connolly, Richard Egarr, 96kHz, 24bit
  6. Shostakovich, Symphony No. 15 in A major op. 141 – I. Allegretto, Duisburg Philharmonic Orchestra dir. Jonathan Darlington, 192kHz, 24bit
  7. Medtner, Piano Concerto in F sharp minor, Op.20 – I. Allegro, Yevgeny Sudbin, Bergen Philharmonic Orchestra, Andrew Litton, 192kHz, 24bit

Jazz

  1. Paquito D’Rivera, La Bella Cubana, Portraits of Cuba, 96kHz, 24bit
  2. Yo-Yo Ma, Songs of Joy & Peace, Kuai Le (feat. Wu Tong, The Silk Road Ensemble)
  3. Keith Jarrett, Paris London – Testament, Part III – Royal Festival Hall, London, 96kHz, 24bit

Rudolf Klein

B. Arch., M. Phil. Dr. Eng. Dr. Phil. Habil.
Professor of Architectural History