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AVI Trio Loudspeakers

Background

During the last forty years, "hi-fi" has become a commonplace term, habitually used by people to denote any domestic sound reproducing equipment with two or more loudspeakers and visible pretensions of quality. What has been forgotten, sadly, is that the "fi" stands for "fidelity", meaning "faithful to the original sound". For the pioneers who coined the term, this concept of fidelity was their ultimate goal: they were idealists who wanted to record and reproduce the sound of acoustic instruments as accurately as possible, so that people could listen to music at home with corresponding levels of enjoyment and satisfaction.

Unfortunately, the visible pretensions of quality displayed by most domestic hi-fi systems are just that: visible, but not audible. Such equipment is high only in price, and its sole fidelity is to corporate demands for low-cost, mass production using cheap components. This is why, half a century after the pioneering work of firms like Quad and Radford, most amplifiers do not even have enough power to avoid clipping on the frequent peaks in modern recordings, and most loudspeakers are certainly loud, but sound horrible because they are crudely designed and cannot handle the power from any decent amplifier.

At AVI, we respect the term "high-fidelity", and we respect our customers: sound quality comes first. No gimmicks, no compromises. So, when we designed our Laboratory Series Integrated Amplifier, we aimed for very low distortion under all signal conditions, and we gave it a maximum power capability into 8 ohms of at least 400 watts, so that it could punch to the peaks with ease. The result? Unimaginable success, outstanding reviews from all over the world and some quite extraordinarily flattering testimonials from delighted customers. Many had previously bought expensive "high-end" equipment, only to discover that our amplifier was dramatically better at a fraction of the price. One industry professional wrote to us privately to say:
"The integrated amp is the single most remarkable hi-fi component I've ever had the pleasure to use. As I work through these Beautiful Systems each month with their five figure pre-power combos and frighteningly expensive CD players, I always try the Lab Series just to see how it compares. It invariably (and I mean invariably) sounds better, sometimes embarrassingly so. I can fully understand the feedback you get from your customers."

The same no-compromise approach went into our hugely successful Pro-Nine-Plus monitor speakers, which set new standards for mid-range clarity and dynamics. Both we and our customers believe that they comfortably outperform the products of our competitors, including many of those which are a great deal bigger and much more expensive. For those with modest-sized rooms and a limited budget, the Pro-Nine-Plus is an ideal partner for the Laboratory Series Integrated Amplifier.

Feedback from customers and our own market research suggested that there was a demand for a larger, three-way floorstanding loudspeaker, even if this were much more costly than the Pro-Nine-Plus. We were therefore keen to design and build one, since, in theory, three-way loudspeakers should produce far better sound quality than two-way designs. But we knew there would be difficulties.

The Challenge

At AVI, we listen to our customers as well as other firms' efforts. We analyse people's criticisms of certain models and relate their observations to measurements and technical parameters, in the course of our exhaustive research into loudspeaker design. Below are some of the important lessons we've learned.

(i) Crossover design and drive unit performance
The crossover in a two-way speaker is at a relatively high frequency, so designers can often get away with using quite poor ones. However, in a three-way speaker, the lower crossover point occurs just above middle C, so the device must be near perfect to avoid disastrous effects.

Drive units are another potential source of trouble. Unless the frequency response of a driver extends smoothly by at least one octave beyond the desired crossover point, its phase behaviour will be poor with consequent audible distortions of the music. Even such famous designs as the two-way LS3/5A BBC monitor fell foul of this danger: the LF drive unit had a poor high frequency response, for which the designers tried to compensate with a resonant circuit in the crossover. But some listeners could hear this, and reacted strongly against it.

Unfortunately, poorly designed crossovers and drive units with insufficient bandwidth abound in three-way designs, so much so that those regularly exposed to them seem to become immune to their failings. This explains a curious paradox: we see enthusiastic reviews in the press for many such designs, but our customers, comparing one or two against an outstanding two-way speaker such as our Neutron IV, report hearing distortion and unpleasant artefacts. They dislike what they hear, and no amount of reviewer or dealer praise will persuade them to buy such products.

Another aspect of speaker design provoking strong customer reaction is the common use of metal dome tweeters. Given their poor technical performance, this isn't surprising. Drive unit diaphragms do not retain their shape as they are required to move backwards and forwards with increasing speed. At a critical frequency dictated by their size, they begin buckling and twisting, creating sounds of their own as well as the music. With soft diaphragms, this tends to happen over a wide bandwidth and at a low level. Metal diaphragms, in contrast, appear more stable until they reach a higher frequency. At this point, as shown in the example above, they enter a state known as catastrophic resonance, producing a vicious spike of unwanted noise. (One critic described the tweeter of a speaker costing $38,900 as "going off like a car alarm" at 17kHz.) This vicious peak has damaging effects upon sounds at frequencies lower down, lending to the music that harsh, bright, metallic quality that disturbs and fatigues the discerning listener.

(ii) Porting
Porting is a method for boosting bass in many narrow floorstanding designs. In a ported speaker, a direct radiating driver and an exit tube are fitted to a resonant cavity. The bass resonance thus created is quite convincing in small speakers with limited bass response, but both we and our expert listeners find that it becomes increasingly unpleasant as the diameter of the drive unit increases. Certainly it does not work well with half-cycle or impulse signals, such as those produced by drums. As the driver cone moves forward, much of the air just pumped out is drawn back through the port, softening the sound and reducing its dynamic range to produce offensive booming and smearing effects. This in turns veils or obscures important mid-range detail.

The Solution: The AVI Trio Loudspeaker

Our approach to the process of developing a new product is necessarily long and meticulous. To help us, we rely on a wide range of experts who listen to all kinds of music. Some are musicians and producers, others are broadcasters, acousticians and architects. We also consult some of our customers, expert listeners with huge music collections. All these people have differing sensitivities, and sometimes they notice something we've missed, in which case we do further work until everyone is happy with the result.

In addition, we engage in much consultation with the companies whose products we use to ensure that everything is done correctly and with their blessing. Therefore, by the time an AVI loudspeaker reaches the shops, we are confident in its excellence and know what it will sound like in most people's homes. And our new floorstanding monitor is certainly no exception to this rule.

The AVI Trio Loudspeaker is a highly accurate, three-way monitor, with the drive units mounted in two completely separate, sealed enclosures and the crossover in a third. The top one, filled with acoustic wadding, contains the mid-range and tweeter. In order to ensure impeccable phase behaviour, we have provided a large volume behind the mid-range to keep the resonance as low as possible. The central compartment houses the bass driver and the lowest contains the three crossovers. Dimensions of the single external cabinet are 40" X 9" X 12". The top, bottom and sides are made from 25mm MDF, veneered on both sides and damped with bitumastic pads, whilst the front and rear are 18mm thick to avoid resonances across the drive unit openings.

The 8" bass driver is made by Volt Loudspeakers Ltd, the 5" mid-range from Vifa and the 28mm tweeter is from Scanspeak. Each one is the finest unit in its class, hand-made, and with all metalwork precision machined rather than stamped out. The poles are extended and the diaphragms damped for neutrality. Despite a slightly lower sensitivity than average (88dB/W/M), the ensemble is capable of about 110dB with a good threshold for peaks because of the large voice coil diameters (38mm, 32mm and 28mm respectively). The lower sensitivity helps to minimise coloration and ensures that the reproduction of voices, natural sounds and acoustic instruments is utterly convincing.


Crossover and drive units: the AVI approach
Our crossover consists of 48 elements on three separate PCBs, manufactured with 2oz copper on fibreglass. Capacitors are metallised polypropylene and inductors have 1.25mm copper wire and huge ferrite cores. Every part is of the highest available quality and the three boards are mounted some distance apart in their discrete enclosure.

As stated earlier, the crossover in a three-way system is vitally important and very difficult to get right. As electronics engineers, AVI design not only amplifiers but also electronic crossovers and all our speakers have been developed using these as a means of precisely checking the impact of a passive alternative. We have evolved a design of passive crossover which, while much more complex than conventional types, gives a completely seamless integration of the drive units and a stereo image as good in width and stability as the best two-way designs, but with far greater depth of soundstage because there is less intermodulation distortion.

Our answer to the problem of metal tweeters is simple: we don't use them. Some years ago, we conducted exhaustive tests of all the tweeters, metal and otherwise, on the market. It was quite clear to us then, and remains so today, that the Scanspeak 28mm fabric driver stands head and shoulders above the competition. And our thousands of satisfied customers and expert advisers clearly agree. It avoids the tendency of most fabric diaphragms to sound like a gazoo. Unlike metal rivals, it does not rely on catastrophic resonance, with its associated distortion of the music, to prop up its top end. Furthermore:

  • it has a smoother, flatter amplitude response to over 35kHz, while rivals struggle to 17-20kHz
  • there are no glitches in its impedance plot that would indicate a resonance
    Ferrofluid in the magnetic gap stops the diaphragm from rocking around and reduces distortion
  • an extended pole piece gives a symmetrical magnetic field on fore-and-aft movement of the diaphragm

And the downside? Only that it costs three times the price of most tweeters on the market.

Our enthusiasm for the Scanspeak tweeter is mirrored by that for the 8" Volt bass driver. Since a fundamental at 60Hz can produce quite high levels of energy as 2nd and 3rd harmonics (120Hz and 180Hz), we wanted an ultra-low distortion driver which would not cause the kind of harmonic interference that ruins voice reproduction, for example. In this respect, the Volt unit certainly doesn't disappoint: it boasts a very rare triple suspension system to eliminate rocking motions under high drive conditions, greatly reducing both harmonic and other kinds of distortion. Other key features of this superb driver include:

  • greater than normal linear excursion capability, by virtue of a very long coil in a short gap
  • superior power handling through venting and use of a 38mm voice coil

The Critical Mid-Range
Because the Trios are quite compact, some customers will want to use them in relatively small rooms and sit fairly close to them. This made it imperative for us to achieve mid-range performance that is as faultless as technology allows.

Our research showed that the best drive unit for the purpose would be a cone type of either 5" or 6.5" diameter. In recent years, a number of such drivers have appeared with diaphragms made of supposedly revolutionary materials, usually with some gimmick such as a bright colour to draw attention to them. However, what matters to the audiophile is not appearances and industry hype but lab and listening room performance. So, after severe critical testing, we selected a Vifa 5" full-range drive unit with curvilinear doped paper diaphragm. This is capable of massive linear excursion and is remarkable for its very broad bandwidth (100Hz - 10kHz), allied to a relatively flat amplitude response. We chose a 5" driver because the break-up modes occur at a higher frequency than in a 6.5" and the sound is marginally less hard, making it more suitable for mid-range purposes in a speaker of the Trio's size. But in a larger speaker, where higher sound pressure levels are demanded, a 6.5" unit would have the edge.

Of course, a few famous manufacturers have tried using 3" dome drivers for the mid-range frequency band. This is a fatal error, since, whatever such manufacturers say, science determines that these cannot perform as well as a cone loudspeaker. They have a very narrow bandwidth and very steep roll-off characteristics, together with a resonance that tends to coincide with the lower crossover point. By the time a crossover is added to the already unsatisfactory amplitude and phase responses of these units, everything is wrong: it is impossible to integrate the drive units seamlessly, the stereo image is poor and the sound appears veiled and harsh.

The flat amplitude response of our Vifa drive unit occurs over a much wider bandwidth than we actually use in our loudspeaker, and it is linear phase. Because the crossover points are 300Hz and 2.5kHz, we thus only need to consider the impact on phase of the crossover itself. Our crossover design is completely inaudible, so we can put the higher crossover point at quite a low frequency, thus reducing intermodulation distortion. This hugely improves the natural sound quality of violins and massed choirs, but also dramatically enhances the electronic sounds in high energy pop and rock music, where there is much greater high frequency content than with natural sound. Congestion is virtually non-existent and so is the awful harshness that often accompanies complex passages.

Sealed Enclosure

By using a sealed enclosure, we avoid the boomy, ill-defined bass that characterises larger speakers with ports, plaguing their performance at higher frequencies with interference and distortion. As one critical listener commented, "with AVI speakers, you only hear the bass when it's supposed to be there, whereas, with other speakers, it seems to be present all the time and slowing down the music." Inferior ported products of a similar size may appear to have more bass on first audition, but our experience shows that the critical listener quickly tires of this artificial, distortion-laden sound, preferring the purity and dynamics of the Trios.


Connectivity
Scientific fact and properly-conducted tests show that bi- and tri-wiring of loudspeakers serves no useful purpose, so we recommend the standard arrangement with single cables. For the same reason, we advise our customers to use standard multi-stranded 2.5 sq mm speaker cable, rather than needlessly expensive exotica with silly names.

However, since we are about to announce 500 watt monobloc amplifiers as an upgrade to the Laboratory Series integrated, we believe that standard 4mm banana plug terminals are no longer adequate or safe. Therefore, we are departing from convention by fitting two 6mm banana sockets. These, and the corresponding plugs, are completely different from the 4mm type: the plugs are multi-fingered, providing a much greater contact area and maintaining a better connection for a longer period of time. As they boast a maximum current capability of 80 amps at 600 volts, customers will be pleased to know that it is impossible for them to touch any part that conducts electricity! Also, given that the plugs are unusual, customers can rest assured that AVI will supply them if required.

Conclusion
By using the best drive units available for the purpose with a superb crossover design in a sealed enclosure for its new Trios, AVI has produced a loudspeaker that overcomes all the pitfalls of three-way designs. Its key qualities are:

  • the finest possible performance at high frequencies, yielding accurate reproduction of such difficult sounds as those of violins, singers and complex electronic music
  • seamless integration of drive units across the frequency range, yielding exceptional accuracy, clarity and smoothness
  • a mid-range unrivalled for its resolution of detail with minimum distortion and phase linearity
  • excellent stereo imaging, allied to unrivalled depth of sound stage
  • firm, precise, dynamic bass performance

By uniting these qualities in one loudspeaker, we are fully confident that the AVI Trio surpasses even the finest electrostatics.

Suitable applications, availability and demonstrations

The behaviour of any loudspeaker is greatly affected by the kind of amplifier used to drive it. Amplifiers with limited power that clip on peaks in the music can produce all kinds of unpleasant effects and cause damage. Room acoustics can have even more marked consequences.

For the above reasons, we have designed the AVI Trio loudspeaker specifically for use with AVI amplifiers, in domestic listening spaces typical of the regions in which we sell them. There are IEC standards defining the characteristics of such spaces, and we adhere strictly to these definitions in our designs.

Through adherence to IEC standards and our own exhaustive research, we can predict with reasonable accuracy what our loudspeakers will sound like in most people's homes. By merely talking to customers, we can often help them to achieve the best performance from our products, or even advise them not to buy if we suspect that their listening room has an acoustic anomaly that renders our speakers unsuitable.

Industrial premises and shops often have features that make them inappropriate for testing and demonstrating loudspeakers destined for domestic use. Strong lateral reflections from hard, undamped surfaces destroy stereo images; big office spaces with large areas of glass and partitions have little or no bass; while, in contrast, converted storerooms and cellars often have a single door and no windows, making them susceptible to bass booming. Under these circumstances, an excellent domestic loudspeaker will sound dreadful.

For the above reasons, AVI will only distribute the Trio loudspeaker during the next few months to selected dealers with listening rooms that accurately represent typical domestic spaces, as defined by the IEC. The same restrictions will apply to hi-fi magazines seeking to review the Trio. Prototypes are already on permanent demonstration at AVI, so if you would like to hear them in suitable surroundings, please contact us for an appointment.

The Trio's classic impedance plot showing absence of any unwanted resonances

AVI TRIO SPECIFICATIONS

Size (HWD) 40” X 9” X 12”
Sensitivity 88 dB/W/m
Amplifier requirement AVI Integrated or ask
Weight 35 kilos approx (unpacked)
8”Bass drive unit Paper diaphragm, 38 mm voice coil, symmetrical magnetic field.
5” Mid As above with 32 mm voice coil
Tweeter 28mm voice coil viscous damped fabric
Crossovers 3 with 48 elements metallised Polypropylene capacitors and 1.25 mm wired inductors on massive ferrite cores.
Amplitude response < +or- 2 db 100 – 35kHz
Bass extension (anechoic) -6dB @45Hz
Connectors 1 pair special 6mm banana plugs (available from AVI)