Behind The Scenes (Oct. 1974)

I ONCE READ somewhere that over 2,000 different makes of automobiles were produced in this country since the dawning of the "horseless carriage" era. I don't know if the audio industry has spawned 2,000 different makes of loudspeakers, but in the last 25 years we certainly have seen (and heard) a great many variations on the original theme. Speakers seem to be peculiarly the "better mousetrap" of our industry, and hardly a month goes by without an announcement that the "Zilch Company" has been formed to produce their "revolutionary new Zilchophonic Mark One, the ultimate speaker which obsoletes conventional designs." Well, more power to these venturesome souls! The rate of attrition with these loudspeaker companies is appalling, but every once in a while a genuinely worthwhile new design emerges and a company survives and becomes a factor in the industry. The West Coast is home base for such old guard stalwarts as Altec and JBL, and in the last few years the focal point for a number of new loudspeaker companies.

Earlier this year, at the Philadelphia Hi-Fi Show, I had visited the demonstration rooms of Infinity Systems, the California-based speaker manufacturer noted for their innovative designs. I was very impressed with several of their speakers, and when I told Arnold Nude11, the dynamic young president of Infinity, that I expected to be in Los Angeles in May for the AES convention, he kindly invited me to visit his plant in suburban Chatsworth.

The Infinity plant is located in a pleasant industrial park, and my first sight of their parking lot, replete with 2 Panteras, a BMW 3.0CS, and an Alfa roadster, told me that I would be dealing with "my kind of folk." The Infinity plant has the usual complement of offices and unusually well-equipped labs (Hewlett Packard Fourier harmonic spectrum analyzer, for example) and extensive production facilities, which nonetheless are bulging at the seams, and the whole complex is due for expansion. The plant also has a sound room, which needless to say, Arnie and I gravitated to in short order. Fortunately, the sound room had decent acoustics, and, with the 15 ips Dolby A tapes we were playing, really did justice to Infinity's premier speaker system, the Servo-Statik 1. This system has been on the market since early 1970, and in general it received glowing reviews. However, the first time heard it at a hi-fi show in Newton, Mass., I was singularly disappointed.

It had neither the vaunted bass nor super definition in mid-range and treble that I had expected. I thought something surely must be wrong, and as I eventually learned, the system had been set up incorrectly (not by Infinity) with 3-way balance all askew, and grossly audible distortion. The next time I encountered the Servo-Statik was at the aforementioned Philadelphia show. There the balances were correct and the sound very clean, but the acoustics of the room added unpleasant colorations, especially in the bass frequencies.

In the Infinity sound room I finally was able to hear the Servo-Statik properly, and could understand why the system was praised so extravagantly. The Servo-Statik can be categorized as an electro-static/ dynamic hybrid, a configuration gaining in favor these days with versions available from Crown, RTR, SAE, and Janszen, to name a few. However, the Servo-Statik differs from these units in several respects, not the least of which is that it is a 3-way, tri-amplified system. At this juncture, I should point out that the Servo-Statik I system I heard at Infinity in early May was succeeded a little over a month later at the CES in Chicago by the Servo-Statik 1A. This is an updated version with some significant improvements in a number of areas to provide a higher quality of performance. I will point out the differences between the original and the "A" as we go along.

A four-cubic-foot bass "commode" houses a specially designed 18-in. woofer driven by a massive 26-lb. ceramic magnet. A small sensing element on the voice coil of the speaker is connected in a feedback loop to its own 110-watt rms bass amplifier utilizing an IC operational amplifier at the front end. The output signal of the voice coil sensor indicates the motion of the cone and moving system and any non-linearities are corrected by the driving amplifier and the feedback loop . . . hence the "servo" designation. The servo-control amplifier also functions as a three-way electronic crossover and is housed in a separate decorative cabinet, with sliding level controls for the adjustment of bass and treble relative to the fixed gain mid-range. The bass commode handles frequencies below 100 Hz, with a crossover rate of 12 dB per octave. In the Servo-Statik system, the commode acts as a common mode bass speaker, based on the non-directional character of frequencies below 100 Hz.

Incoming left and right signals are matrixed and then amplified and further controlled by the feedback/servo system. I must confess that I have never been kindly disposed to a common-mode, mixed-bass, single speaker. However, in this case, I could not fault it and, in fact, was greatly impressed with the rock-solid fullness and uncommon smoothness of the bass response. We were listening to a tape of the Sibelius 1st Symphony, and in several sections there are huge bass drum punctuations around 30-35 Hz, which were reproduced not only cleanly but with gut-thumping impact.

In the updated Servo-Statik 1 A system, the servo amplifier and the crossover now employ high voltage FETs instead of op amps, the crossover rate is now 6 dB per octave and because the mid-range electrostatic diaphragms have only half the mass of the original units, enabling higher output at a slightly lower frequency, the bass commode now operates from 70 Hz downward. This lower frequency makes the bass commode even less critical in positioning it in a listening room. I've covered the bass end of the Servo-Statik system, now on to the mid-range and treble descriptions.

The electronic crossover directs frequencies from 70 Hz to 2 kHz to an external stereo power amplifier, which in turn drives electrostatic mid-range panels. These panels might be regarded as the heart of this Infinity speaker. They are a proprietary design, and I watched them being fabricated in a special room at the plant. The ultra-low mass mylar diaphragm is sandwiched between two grids, and then this element is made up in panels approximately 8 by 12 in. The cements and sprays that bond the panels together are highly volatile, and the workers must use breathing masks. Four of the mid-range modules are placed on each side of the 28x37x6 1/2 in. deep screens which comprise the left and right sections of the system. The modules are angled to pro vide broad dispersion. Arrayed down the middle of each screen, between the mid-range modules, are narrow, rectangular electrostatic strip tweeters.

These are also an Infinity design, but are subcontracted and manufactured elsewhere. The electronic crossover directs frequencies from the 2 kHz to be yond 30 kHz to another external power amplifier, and the signal then fed into the tweeters. The mid-range modules are of moderate efficiency and can handle efficiency and can handle exceptionally high power without arcing. In fact, Infinity recommends the use of amplifiers such as the Crown DC-300A to drive the mid-range, while the tweeters can handle a maximum of 50 watts rms and a suitable unit would be the Crown D-40. The screens containing the electrostatic elements are open to the rear, and since the elements are bi-polar, some 50 percent of their radiation is reflected, if the screens are within several feet of a wall.

The time I spent listening to the Servo-Statik system was necessarily limited, but I heard a considerable variety of music through them . . . the aforementioned Dolby A tapes, the new Vol. 3 of the direct-disc-cut Sheffield Records, the superb percussion recording by Mark Levinson, and a number of very high quality pop/ rock jazz recordings. As with any speaker system, to really know it you have to live with it, in your own particular acoustic environment, with your own selection of demanding recordings. When the production of the new Servo-Statik 1 A gets underway, I'll be eager to put the system to such a test. In the meanwhile, on the basis of what I heard at the Infinity sound room, I am mightily impressed . . . and I don't impress easily! Some speakers excel in some section of the frequency spectrum. The Servo-Statik delivered an utterly clean, convincingly natural and uncolored reproduction of everything from bass drum to triangle.

Such superlative sound doesn't come cheaply . . . the Servo-Statik with the recommended quality of external amplifiers will leave you with very little change from $3,000.00.

Infinity established their reputation with the Servo-Statik system, but recognizing the limited market for such an expensive speaker, they have produced a series of speakers aimed at the pocketbooks of the average audiophile. In spite of their lower prices, two of the models, the 2000AXT and the Infinity Monitor, offer some interesting innovations.

Common to both speakers is the use of a transmission line tweeter for treble frequencies of 5 kHz up wards. Manufactured by Infinity under license from Ohm Acoustics, it uses the same principle discovered by Lincoln Walsh (of Brook amplifier fame many years ago). However, in the Ohm F speaker, the cone is 12 in. in diameter and is a full-frequency-range device.

In the Ohm speaker, the cones are placed with their apex facing up, the Infinity tweeter has its apex pointed down, with the steep sided cone flaring up like a tulip. Acting as a vertical, pulsating cylinder, the metallic tweeter is said to produce coherent sound radiation, much as a laser does in optics. Sound velocities much higher than the speed of sound in air travel up the metallic cone and sounds are emitted from various parts of the cone.

Sounds are propagated in true 360 degree omnidirectional fashion, and at the same instants in time, thus there is no time delay distortion, and theoretically at least, it is said to have perfect transient response. In any case, the frequency range of the tweeter is beyond 30 kHz, and the sounds of cymbals, snares, triangles, gongs, etc.

I heard from several of the percussion recordings we played were exceptionally clean, with razor-sharp definition. Also common to both of these Infinity speakers is a bass transmission line, with special woofer whose cone has been treated to increase its stiffness-to-mass ratio. Both speakers crossover to their respective mid-range drivers at 500 Hz. In the 2000AXT, the mid-range driver is a special 4.5-in. cone, whereas in the "Monitor" system the mid-range is a 1 1/2-in. strengthened dome, with a 5-lb. magnet. In the Infinity sound room, both speakers ... the $299 2000AXT and the $429 Monitor, were exceptionally well-balanced per formers, with extended solid bass, a mid-range that gave fine definition and projection without coloration, and of course the superb transient response and transparent top end of the Walsh tweeter. I must add that the splendid sound I heard from these speakers was partly because they were being driven by Infinity's newest and most exotic product, their Class D switching amplifier. Infinity has gone even further into electronics, with an FET preamplifier, which was not yet operational while I was there, and this new-to-consumer-audio switching amplifier.

Class D amplification is pulse-width modulation. Instead of using transistors as ordinary linear amplifiers, the transistor is used as a switch . . . either it is on, or it is off. By having the audio signal control when the transistor switches large voltages and currents, the signal is amplified. In this way all the non-linearities in bi-polar transistors are circumvented. Although this amplifier is capable of 250 watts rms per channel, with the switching technique making the unit nearly 96 per cent efficient, the unit runs slightly warmer than when cold, without external heat sinking! A d.c. to d.c. power supply for the amplifier weighs a mere 2-lb., as compared to the 40-plus-lb. behemoths usually found in amplifiers of this power. In fact, the entire switching amplifier looks no larger than a somewhat undernourished preamplifier and weighs in at about 14 lb. total.

Engineering vice-president John Ulrick, went over this remarkable switching amplifier for me in his lab, including showing me a memory hold display of the harmonic distortion on the Fourier harmonic spectrum analyzer . . . there was nary a spike to be seen! Obviously a company of considerable technical depth, and with the present backing of Eastern Air Devices, Infinity Systems is an organization of great potential. I enjoyed my most educational visit with them, as well as the ride back to the L.A. Hilton in John Ulrick's finely tuned Pantera!

(Source: Audio magazine, Oct. 1974; Bert Whyte)

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