There was a time in the 1960s and 70s when the add-on electrostatic tweeter was commonplace. In particular, the Janszen Z-130 and the RTR ESR-15 come to mind as stellar examples of this genre. Multiple angled panels were used to improve dispersion though with limited success. This type of tweeter has become nearly extinct over the years, the only commercial version I could locate that is still in production being KingSound’s Hummingbird II super-tweeter.
It would be correct to characterize the EnigmAcoustics Sopranino as an add-on electrostatic super-tweeter. But that’s where the similarity to past designs ends. For starters there is no external bias supply—nothing to plug into the wall! That’s right, the Sopranino is self-biased and should be thought of as an electret tweeter. The name electret is a concatenation of sorts of electrostatic and magnet, and refers to a material that is permanently polarized. An electret diaphragm is typically manufactured by melting a PTFE plastic (e.g., Teflon), sometimes with additives to increase the polar molecule loading, and allowing it to solidify in the presence of a strong electrostatic field. The field aligns the material’s polar molecules producing a permanent electrostatic bias, which in the case of a high-resistivity material such as Teflon can be stable for up to several hundred years.
Electret diaphragms have been used in microphones for many years. In fact, electret mikes are heavily used today, not only for measurement and studio recording but also for mass-market applications such as cell phones and laptop computers. However, turning an electret diaphragm into a tweeter demands a technological breakthrough. This new technology was developed with research contributions from several Taiwanese institutions, including National Taiwan University, and mainly the Taiwan Industrial Technology and Research Institute. EnigmAcoustics and others have been licensed to use this technology. One of the other licensees is Taiwan Electrets Electronics Corporation (TEEC), which has been marketing a low-end electret speaker. But according to EnigmAcoustics, the TEEC device lacks several key elements, making it unsuitable for high-fidelity applications. These are said to be high-THD, fragile stator materials, susceptibility to environmental factors, and inadequate SPL and dynamic range for high-end audio applications.
The challenge before the Enigma R&D team was to forge this new technology into a form that would deliver high-fidelity performance. The stator structure and materials were modified and patented. Spacer materials were replaced and the driver circuits were completely re-designed. The Sopranino employs a custom transformer for stepping up the audio signal and for maintaining the nominal impedance at about 4 ohms, the minimum being around 3 ohms. There is also a protection circuit for enhanced reliability. To be clear, this is a push-pull design with the diaphragm sandwiched between front and back stators. The key issue was to create a PTFE-based material that, according to Enigma, “would be able to hold as many electrons as possible and also to keep them there for as long as science and the laws of nature allow. Hundreds (if not thousands) of formulas were concocted and experimented with during years of R&D before we came up with the current patented commercial version.” The dimensions of the diaphragm are 9cm x 12cm, and at a thickness of only 25 microns its moving mass is only about 1.6mg.
The tweeter is horn-loaded in order to increase its efficiency. The horn’s flare profile is said to be non-standard. The rationale given was as follows: “We indeed started by first using a standard exponential-flare-type horn with the purpose of eliminating back waves and avoiding standing waves. However, we soon discovered its shape also had an adverse effect on the dispersion angle of front waves, and we subsequently made a conscious decision to adjust the exponential flare in order to more effectively control the overall dispersion angle. The finished product, therefore, doesn’t technically represent the standard exponential-flare shape.”
A crossover frequency selector switch is located on the back panel of the enclosure. Available options are 8, 10, or 12kHz. These settings provide second-order high-pass filters for matching the Sopranino to the main speakers. The nominal sensitivity is 90dB; however, a gain switch on the back panel provides a -3dB setting for level-matching. The Sopranino is intended to be placed on top of or next to the main speakers and is to be connected in parallel with the main speakers. Hookup is simple enough, and involves installing short cable runs between the main speaker inputs and the Sopranino input terminals, with care taken to maintain correct polarity.
What is not so simple is the task of integrating the Sopranino into an existing system. Lacking a volume control (save for its -3dB switch), there’s the practical problem of gain-matching the Sopranino to a range of speakers of varying sensitivity. It turned out that this is not as serious an issue as it sounds, since an indirect adjustment of volume level is possible by use of the toe-in angle. The fact that output level falls off with angle, though not as dramatically as is the case for a large flat panel, can be put to good use in controlling the tonal balance at the listening seat. The most serious set-up complication arises from potential interference between the super-tweeter and the main speaker’s tweeter. The latter is not rolled off by an external crossover network and depending on its relative distance from the super-tweeter, response dips and peaks may result in a frequency overlap region. Granted that with two level settings and three crossover frequencies, there is fair amount of flexibility built-in for experimentation, far more than is offered by other add-on tweeters, but that of course is no guarantee of success.
When the review project was arranged it was agreed that the Sopranino should be evaluated in one specific context. The framework chosen for this review involved mating the Sopranino with the QUAD-57 ESL. The rationale was based on three factors. First, the QUAD needs help in the treble. Second, it made sense to extend the response of an ESL with another electrostatic driver. And third, since there are over 60,000 QUAD ESLs out there, this coupling would be of interest to a large audience. There is already an established history of extending the QUAD-57’s treble response via the use of an add-on tweeter. The JansZen Z-130, Decca Ribbon, and even the Dukane Ionovac plasma tweeter have been used for this purpose. I recall Bob Graham describing such a setup many years ago, where he used a Dukane Ionovac tweeter crossed over at about 8kHz to very good effect.
Stands were provided by EnigmAcoustics to allow positioning of the tweeter behind and a few inches above the top of the QUAD, the bottom of the stand being at about 39 inches off the floor. The stands were machined out of high-grade acrylic stock and were beautifully finished. Optimal placement is with the Sopranino centered behind the QUAD’s treble panel. As you might have expected, level-matching the 83dB-sensitive QUAD necessitated use of the Sopranino’s -3dB gain setting. In order to obtain a natural brightness level, it was also necessary to toe in the Sopranino such that the tweeter axes intersected in front of the listening seat. To be a bit more precise, a toe-in angle between 5 and 10 degrees (with zero degrees being straight out) should do the trick. A side benefit of such a toe-in angle was an apparent increase in soundstage width.
It turned out that the best way to hone in on the optimal crossover frequency was to focus on female soprano voice. It’s easy to be seduced by an overabundance of presence region energy. Initially, I was wowed by an 8kHz crossover setting, but it quickly became clear that tonal colors just weren’t right, being too hot, while the treble range developed some rough spots texturally. It has been said that if something measures good but sounds bad, you’re measuring the wrong thing. But this was a case of measuring bad and sounding bad. There were severe interference effects measured at the 8kHz setting, fewer with the crossover shifted to 10kHz. Moving the measurement mike several inches vertically also resulted in significant response dips. The most synergistic and stable blend was observed at the 12kHz/-3dB settings, and all subsequent listening tests were conducted with these settings dialed in.
If ever a speaker needed a super-tweeter, the QUAD is it. It is a prime example of British sound, being laid-back and a bit polite though the midband. Well, with the Sopranino in the system, the presentation was fundamentally transformed. Imagine, if you can, a QUAD that is much more neutral in character, whereby instruments are brought a bit more forward in the mix. Where the QUAD sounded somewhat muffled and lacking immediacy playing solo, the Sopranino enhanced transient clarity and soundstage transparency. There was now a genuine sense of treble air, and, most importantly, tonal color saturation improved significantly, especially when it came to soprano voice and violin timbres. It was hard to believe that a driver that outputs such a small fraction of the total acoustic energy could have such a dramatic impact on the perceived sonic impression. But that, ladies and gentlemen, was what happened. I also noted a consistent improvement in image focus. Image outlines coalesced into palpable organic wholes that made for much more secure spatial resolution of complex orchestral passages.
There’s no doubt in my mind that the QUAD and the EnigmAcoustics Sopranino are soul-mates. Despite my initial reservations about driver blending, the Sopranino proceeded to strikingly refresh and extend the QUAD’s performance. Once experienced, there’s no going back. I can’t imagine enjoying the QUADs in the future without the Sopranino super-tweeters. It’s fair to say that performance-wise the Sopranino catapults the venerable QUAD-57 into the 21st century. If you’re a QUAD owner, be sure to check out the Sopranino—it’s a QUAD’s best friend!
SPECS & PRICING
Frequency response: 8-40kHz (+/-3dB)
Nominal impedance: 4 ohms
Sensitivity: 90dB (1m/2.83V), with 0/-3dB switch
Weight: 6 lbs.
Dimensions: 7.13″ x 7.60″ x 8.15″
Irvine, CA 92618
PrimaLuna Dialogue Premium and JWN 807 amplifiers, Yoshino-EAR DAC, April Music Stello U3 digital converter, Sony XA-5400 SACD player with ModWright Truth modification; Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Clearaudio Da Vinci V2 MC phono cartridge; Pass Labs XP-25 phono stage; Pass Labs XP-30 line preamplifier; FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Acoustic Zen Hologram speaker cable; Sound Application power line conditioners
By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.More articles from this editor
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