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.