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High-Performance Audio Reviews
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September / October 2004
Oskar Heil Kithara Loudspeaker
During the '70s and '80s, a line of loudspeakers incorporating the AMT design was manufactured in Sacramento, California and marketed under the brand name ESS Heil. While the ESS Heils had their admirers, many listeners (this writer among them) found them deficient in, among other things, bass response and tonal balance. Physicist Jack Bybee (Mr. Quantum Purifier), who was a friend and neighbor of Oskar Heil in San Mateo, Calif., remembers Dr. Heil expressing frustration with the ESS Heils, citing the front-firing woofers and what he felt were inferior cabinets, crossover designs and parts. Dr. Heil built he original Kithara loudspeaker at the age of 80.
What Is A Kithara? The Kithara is a moderately sized two-way floorstanding loudspeaker measuring approximately 16 by 16 inches square and 44 inches high. The up-firing 10-inch woofer is mounted at the top of the bass cabinet, covered by a black grille cloth; the bass-reflex port is on the bottom. Wooden feet at each corner maintain the necessary clearance for the port. Bi-wire five-way loudspeaker terminals on the rear are supplied with single-wire jumpers. With no driver cutouts, the hardwood-veneered bass cabinet reminds me of a planter receptacle such as one might see in an upscale shopping mall.
How The AMT (AVT) Operates The air movement is five times greater than the movement of the diaphragm; therefore the velocity is also five times greater. With a total moving mass of approximately one gram, the AMT/AVT is theoretically an almost perfect transducer system. NOTE: The following two sections (nine paragraphs), redacted from Heil literature, address key theoretical concepts underlying the Heil AMT design. The impatient bottom-line-oriented reader may choose to skip this discussion and pick up with the resumption of the review narrative.
Concepts Underlying The AMT/AVT Driver Volume (Intensity) Variations: The ear has little sensitivity to sound level "jumps," or to the relative loudness of different sounds that are audible at the same time. For a loudspeaker, sound output levels (amplitude) over a range of frequencies are valid criteria, but they are of lesser importance for our ears. The physical construction of the ear makes it relatively insensitive to amplitude changes. The difference in amplitude between a whisper and normal speech is not just 1:2 or 1:4, but can be as much as 1:100,000. The relative loudness of different sounds, within certain limits, is therefore not too important to us, since the ear has the ability to adjust to different levels. This explains why street noises do not necessarily disturb conversation. It also explains why we can hear an opera singer even though the sound level of the orchestra is many times that of the voice. Frequency Variations: In contrast to its relative insensitivity to amplitude variations, the ear is extremely sensitive to minute fluctuations in the frequency of sounds, especially in the mid-frequency range. A half tone in the musical scale represents a frequency change of 6 percent, while the frequency shift in the vibrato of a violin is approximately 0.5 percent. In the critical midrange of 250Hz to 6000Hz, we can differentiate between two tones even when the frequency difference is as little as 0.06 percent. This sensitivity to frequency variations enables us to identify different voices. When we speak, we do not produce constant tones, but constantly varying tones. We can usually recognize a familiar voice immediately, even over the telephone, and we can often judge the mood of the other party by differences in speech pattern produced by the changing tension of the loudspeaker's vocal cords. Frequency Variations vs. Amplitude Variations: It is commonly accepted that the smallest change in amplitude that the ear can detect is 1dB, which represents a power difference of 26 percent. Compare that to the ear's sensitivity to frequency variations of 0.06 percent. Contrasting this relative insensitivity to amplitude changes with the ear's extreme sensitivity to frequency variations, it is difficult to understand the loudspeaker industry's obsession with minor loudness variations of 1 or 2 dB in the frequency response of a loudspeaker, while ignoring the audible shifting or fluttering or high frequencies which can result from changes in materials stiffness as a sound wave spreads transversely across a transducer's diaphragm. Phase Differences — The Ability to Localize Sounds: A listener's ability to localize sounds is made possible by phase differences (time delays) resulting from the differences in path lengths from a sound source to each ear. This ability is frequency-dependent and is more pronounced in the critical range of 500Hz to 3000Hz than at lower and higher frequencies. This is why the speed of response of a loudspeaker diaphragm is extremely important to the faithful and realistic reproduction of music. If the loudspeaker's diaphragm cannot respond fast enough to reproduce these transients, or if it distorts them, the listener's ability to recognize and localize the sound source is greatly diminished, and the realism of music reproduction and pleasure of listening are seriously reduced.
Problems Of Loudspeaker Design However, diaphragm material that is too thin and light will not be sufficiently rigid to prevent it from flexing and producing its own audible resonances. If the deformation occurs between the center area and the edges of the diaphragm, that portion will vibrate independently of the music signal and produce standing waves — bell-shaped vibrations — that are clearly audible as distortion. In addition, the diaphragm will store the resonant energy, and when the music signal stops it will continue to move in order to dissipate that stored energy. The continued vibration of the diaphragm will damp (absorb) the sharp rising transients of the ensuing music and seriously affect the accuracy of the music reproduction. Efforts to Eliminate Unwanted Resonances: Attempts by designers to minimize diaphragm resonances usually involve coating (damping) the diaphragm with silicon, rubber or other substances to increase rigidity and prevent flexing. There is a trade-off, however: while the damping material may help to reduce resonances, it adds to the mass of the diaphragm, increasing inertia and resulting in slower response to the transients of complex musical waveforms. These damping methods can reduce to a mere 0.25% the ability of many loudspeaker diaphragms to move air efficiently. Large Diaphragms and Differentiated Driving Force: Efforts have been made to minimize unwanted diaphragm resonance by applying the driving force more evenly over a large area of the diaphragm. Electrostatic loudspeakers distribute the driving force over a large, flexible plastic panel suspended on a framework. EMIT and magnetostatic loudspeakers utilize a differentiated driving force applied to different areas of the diaphragm to compensate for the varying flexibility of its surface. However, when a flat or conical diaphragm supported at its edges is caused to vibrate, only part of the diaphragm oscillates in a direction perpendicular to its surface. At the outer edges where it is suspended, the diaphragm cannot oscillate in the same manner, since the surface of one side will stretch with each + oscillation, while the reverse side will be compressed — and vice versa. Thus the entire diaphragm does not move uniformly like a rigid piston, but vibrates like a suspended flexible membrane, producing a self-resonance with a pitch.
Review Setup In the initial listening session I was struck by the unusually wide soundstage, but at the same time I was finding the presentation lacking centerfill and solidity. Bringing my reviewer superpowers into play, I eventually noticed that the AMT drivers are mirror-imaged. (I could, of course, have tried reading the manual, but being both a male and a reviewer, that would have been quite aberrant.) After switching the left and right loudspeakers so that the sound-radiating diaphragms of the AMTs were to the inside, I immediately heard the sound field develop depth and image solidity. I spent a couple of weeks playing the Kitharas at highish levels, with the loudspeaker terminals jumpers in place and a single set of loudspeaker cables, in order to break in the woofer, which initially sounded somewhat tight and slow. The AMT driver sounded pretty fast and open from the beginning, but it too showed considerable improvement after about 100 hours. At the end of that initial period, the balance between the two drivers had improved, but I was still conscious of some slight discontinuity between the exceptional speed of the AMT and the low-frequency output. I switched to a bi-wire configuration, which helped but did not eliminate the phenomenon. Since I had on hand one pair of Bybee Ultra Loudspeaker Chargers, I connected them to the woofer terminals, and that seemed to reduce the discontinuity to a point where I was no longer troubled by it. A note on the 94dB sensitivity specification: I find it curious that a loudspeaker with this rating also has a power recommendation of 50 to 200 watts. At various listening levels with the Kitharas, the digital volume level display on my VTL 7.5 preamplifier was typically very close to what I see when driving my 87dB-sensitive Egglestons at similar volumes. Not having on hand a truly low-powered amplifier such as a 10-watt SET, I brought the aforementioned 40 wpc JoLida integrated into play. With the JoLida driving them the Kitharas still played loudly, but I was able to induce clipping at very robust levels where the larger amps sail along easily. The JoLida/Kithara combination sounded lushly beautiful, but the bass was notably looser and seemed to lose a bit of extension. The Kitharas, from what I hear, work best connected to an amplifier with strong low-frequency “grip,” and that characteristic may perhaps account for the minimum 50-watt amplifier recommendation.
Baby, That's What I Like! The Kitharas recreate the performing space — especially on naturally mic'ed recordings — extremely well. Specific vocal and instrumental images are stably located within a nicely dimensional sound field, but without the laser-beam imaging precision and hyper-detail retrieval that are characteristic of imaging “champs” such as the Wilson WATT/Puppy and ambitious minimonitors. As someone who attends dozens of live concerts every year, I can testify that listening to the Kitharas is much closer to what I hear in the concert hall than with most audio systems, even those with much more expensive loudspeaker systems. A similar naturalness is evident in the Kitharas' tonal reproduction. Whether it's the fresh young voices on the stunning new Harmonia Mundi Marriage of Figaro, the gorgeously smooch but muscular strings of the St. Louis Symphony on the Vaughn Williams Tallis Fantasia (Telarc), Bob Dylan's raspy intensity in Bringing It All Back Home (Sundazed mono LP reissue), or countless other listening epiphanies, my attention is consistently drawn to the music, almost never to the system. Readers who are familiar with my reviews may recall that I sometimes characterize equipment in terms of left-brain (analytical, quantifiable) and right-brain (intuitive, pleasure-seeking) appeal. There is necessarily a strong left-brain element in any equipment review and these loudspeakers did quite well. But in the course of this review, I found myself again and again slipping easily into right-brain mode. The Kitharas are real right brain loudspeakers — as emotionally seductive as any loudspeaker I have heard. And in that sense they are, as declared in the title of this article, greater than the sum of their parts.
Specifications
Company Information Voice: 0041 91 6831734
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Dr. Oskar Heil was one of the
leading scientists of the 20th Century, breaking new ground in
a variety of physics and engineering disciplines. Perhaps most
notable among his numerous inventions was the Field Effect Transistor,
which he patented in 1934. The unusual loudspeakers that are the
subject of this article are descendants of his 1973 patent for the Heil
Air Motion Transformer (AMT), a mid/high frequency transducer for which
the radiating surface was an accordion-pleated paper diaphragm. The latest
iteration of this driver has been re-christened the OSKAR Air Velocity
Transformer (AVT), although the two acronyms seem to be used
interchangeably in product literature.
The Swiss company Precide SA had a long association with
Dr. Heil, and has continued to build the Kitharas after his death.
Greg Weaver reviewed an earlier version of these loudspeakers for the 
The aluminum-clad Heil AMT/AVT driver, which is inherently
bidirectional, is held in place by a wooden bracket, located toward the
rear so that it is clear of the direct output of the woofer. In place of
typical domes or cones, it features a large corrugated diaphragm,
described in detail In the following section. Sensitivity is rated at 94dB, and overall frequency response at 28Hz to
23kHz. The crossover frequency is 700Hz, which means the AMT driver
is covering an impressively broad frequency spectrum.
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