Oskar Heil Kithara Loudspeaker
Greater Than the Sum Of Its Parts.
Review By Wayne Donnelly
here to e-mail reviewer.
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.
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.
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 Enjoy
the Music.com Review Magazine in 2001. Since then, Precide has continued to
refine the design, and the version reviewed here reflects upgrades to the
woofer, cabinet bracing, crossover capacitor and internal wiring.
What Is A Kithara?
Let's begin with the name, hardly a household word in this household.
Kithara is the name of an ancient Greek stringed instrument, somewhat
resembling a lyre. Now on to the loudspeaker.
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.
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.
How The AMT (AVT) Operates
The unique design feature that distinguishes the OSKAR AVT (AMT) from
all other loudspeakers is its extremely lightweight diaphragm, folded into
accordion-like pleats to which aluminum foil strips are bonded. This
diaphragm is mounted in an intense magnetic field, and is driven from its
edge so that its folds move the air. This design does not require a huge
magnet and voice coil, and it allows the driver's resonant frequency to
be outside the frequency band it reproduces. When a music signal is
applied, the pleats alternately expand and contract in a bellows-like
manner, forcing air out of the pleats on one side and sucking in on the
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
The Ear's Ability to Differentiate Sounds: A principal
function of the ear is to identify voices. For this use it has developed
an extraordinary ability to differentiate sounds. We can separate a single
sound source such as a distant voice from other sounds by concentrating
our hearing apparatus on the voice and ignoring noise or other voices that
we don't want to hear.
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 6,000Hz, we can differentiate
between two tones even when the frequency difference is as little as 0.06
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
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
Spurious Diaphragm Resonances: Any solid material made to
vibrate by striking it or otherwise setting it in motion will produce a
unique pattern of resonances characteristic of that particular material.
If made to vibrate at a specific frequency by an external driving force it
will introduce, in addition to that frequency, its own resonances. In
music, the pattern of these resonances (harmonics) is peculiar to each
instrument and enables us to distinguish, for example, between the sound
of a saxophone (metal) and an oboe (wood), even when both instruments are
playing the same fundamental note. This characteristic, useful in
recognizing musical instruments, constitutes a major problem for the
loudspeaker designer, since spurious resonances generated by a diaphragm
will distort and mask the musical signal. In order to move a large amount
of air with minimum loss and provide fast transient response, the
diaphragm must be extremely lightweight.
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
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
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.
The Kitharas displaced my Egglestons in the downstairs reference rig,
in my somewhat unusually proportioned living room. (System and room
are described in detail in the reviewers' bio section.) In
addition to my monster VTL MB 750 Reference amplifiers, the solid-state
150 wpc Coda S12 (a very good match) and my hot-rodded 40 wpc Jolida 202A
integrated tube amp also spent time driving the Kitharas.
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
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
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!
After working through the various issues described above, I had a
wonderful month of enjoying the sound of the Kitharas. Essentially,
their sonic presentation is relaxed and laid-back, blessedly free of
aggressive in-your-face behavior, either tonally or spatially. The
Kitharas are as un-loudspeaker-like as any loudspeaker I have ever lived
with and that's a compliment!
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.
Type: Bass reflex two-way loudspeaker
Drivers: AMT midrange/tweeter and
1 25 cm (10-in.) woofer
Frequency Response: 28Hz to 23kHz (+/- 5dB)
Crossover Type: second order (12dB per octave)
Crossover Frequency: 650Hz
Impedance: 4 ohms minimum
Amplification Requirements: 50 to 200 Watt
Cabinet Finish: Oiled walnut, cherry or black, maple wood veneer
Dimensions: 16 x 16 x 44 (WxDxH in inches)
Weight: 77 lbs.
Warranty: 5 years
Price: $4,900 per pair
Via V. Vela 33
6834 Morbio Inferiore
Voice: 0041 91 6831734
Fax. 0041 91 6836679
United Home Products
10820 Guilford Road
Annapolis Junction, MD 20701
Voice: (866) 482-8346