GRAVITRON

LOUDSPEAKERS

The Gravitrons are AVI's "regardless of cost" loudspeakers using the latest and best technology available to produce as good a sound as it is possible to make.

We are fortunate to have considerable experience in the design and manufacture of CD players, preamplifiers, tuners and poweramplifiers and this has given us a unique opportunity to study the interaction between these and loudspeakers. As a result we have been able to design loudspeakers to get the best from power amplifiers and, power amplifiers that are unaffected by the worst that our loudspeakers will do to them.

The Gravitrons are the culmination of approximately eight years research into this subject, the ground work having commenced eighteen months ago. Very few are in such a strong position to guarantee real system synergy.

DESCRIPTION

Low frequency enclosure: sealed 120L, 25mm, MDF cabinet braced with a 50mm section Tulip wood frame and 6mm bitumastic damping pads. Drive units are 2 X 10" high end , each capable of handling up to 600 Watts! Advantages are a more gentle LFroll-off, useful output down to 20Hz and easier coupling into smaller rooms where large ported designs are more likely to excite room resonances. The Gravitron's bass is punchier, tighter and more detailed.

Two 10" drive units were chosen to keep the baffle width to a minimum to avoid impeding the dispersion of the mid and treble units. The enclosure is stuffed with carefully selected acoustic wadding which lowers the system resonance and, in conjunction with the tulip wood frame, helps to break up internal resonances.

Mid and treble unit: 15mm MDF flat-topped pyramid stuffed with acoustic wadding. The "pyramid" ensures good dispersion, time alignment and the break up of internal cabinet resonances which occur across parallel surfaces. Tweeters, in particular sound different depending on the width of the baffle - the narrower the better.

The tweeter which, in our opinion, is the best available, is the same 28mm soft dome as used in all our loudspeakers. It has a smoothness, clarity and ambience retrieval that we were unable to find on any others of the many that we tried. The 5" mid range has a 32mm voice coil wound onto an aluminium former to give it substantial power handling and dynamic range. Its diaphragm is soft doped paper.

Variations in harmonic distortion between drive units is comparatively small but the differences in sound are substantial and our research has shown that the primary explanation is the choice of diaphragm materials. Soft well damped diaphragms sounding by far the most natural whilst the harder ones, even those made of harder paper, have obvious and unacceptable colourations.

The mid and tweeter cabinet "plugs" into the top of the bass cabinets with a coating of velvet to ensure some decoupling and avoid resonances. Gold plated Speakons are used to connect the two cabinets together electrically.

The crossover: Phase is the most important aspect of crossover design. You do not get predictable phase behaviour unless the crossover conforms to the theoretical ideal. Therefore drive units must, not only sound good, but also have a flat amplitude response way beyond the chosen crossover points. That way they will be phase linear and the only impact on the phase of the system will be the crossover. We model second order filters on a circuit analyser to ensure absolute accuracy. The crossovers are then built into the system and measured with different test equipment to ensure that the theoretical and the practical agree. Using this method we have been able to obtain a level of phase accuracy previously only possible in active systems.
Only electrical measurements are made in the laboratory, all acoustic measurements are made with traditional Bruel and Kjaer equipment in anechoic conditions, that way absolute accuracy can be guaranteed. We do make measurements on computer based test equipment that we have modified, but we do not consider this type of measurement provides the resolution necessary for original design work.

Impedance is a nominal 6 Ohms and the phase angle imposed on the amplifier never exceeds 30 degrees - the Gravitrons are an easy load. Any deviation from a flat impedance other than at both the low frequency resonance and crossover regions is an indication of a resonance which will damage the overall phase response destroying stereo image and clarity. It may even upset the amplifier.

The three crossovers are housed in a separate compartment in the base of the lower cabinet and each section has its own PCB positioned as far as possible away from the other two. This minimises interference between inductors and ensures that the huge drive unit magnets are as far away as possible.

The Gravitrons have the best stereo image you will hear from a big speaker, drive unit integration is absolutely seamless, they are an easy load and have the lowest possible distortion.

A great deal of trouble has been taken to make sure that the Gravitrons stand up well against the most highly regarded competition. With the assistance of several famous journalists, recording engineers, acousticians and retailers, a number of comparisons have been made in ideal listening rooms. The Gravitrons appear to have a substantial dynamic range, very much better stereo image, more neutral sound (very obvious on voice) and exceptional clarity and smoothness. They seem to work in smaller rooms without booming and yet have the bass extension of the best of the others but with better bass dynamics.

In short the Gravitrons are as good as it is possible to buy and, exceptional value for money.


SPECIFICATION



Sensitivity 88dB/W/M

Nominal Impedance 6 Ohms

Amplitude linearity +or- 1.5dB 80Hz-30kHz

Bass extension -6dB per octave useful to 20Hz

SPL capability 110dB +10dB Threshold

Standard finish Bass cabinet: Cherry

Mid and tweeter pyramid: textured black

Rosewood and piano black lacquer available at
extra cost

Weight approximately 60 Kilos each side

Size 1.2M X 320mm X 540mm HWD.