An Electrostatic For Everyman
Acoustat Model Two full-range electrostatic loudspeaker,
with Magne-Kinetic interface. Dimensions: 19 1/2 by 56 1/2 inches (front),
3 inches deep (panel, 14 1/2 inches deep at interface base). Price: $1,200
per pair. Warranty: "limited," five
years parts and labor. Manufacturer: Acoustat Corp., 3101 S.W. First Ter.,
Fort Lauderdale, Fla. 33315.
Full-range electrostatic loudspeakers have commanded a dedicated following for years, but their typically high price and their somewhat quirky electrical properties have conspired to keep their appeal rather limited. Acoustat, a Florida-based manufacturer, would like to change that. It believes it has solved some of the problems inherent in the species, and at a price that dramatically increases the appeal. Its lowest-priced system, the Model Two, consists of two tall, thin panels and two stepup devices that provide the necessary polarizing voltage. Dubbed Magne-Kinetic interfaces, these polarizing units employ a bi-former-essentially two transformers in one-that is said to keep system impedance at or above 3 ohms over the entire audio frequency range.
Proper initial assembly and setup takes some time and patience. Each interface box serves double-duty as a deadweight counterbalance and support for the panels. Three wires from the electrostatic elements are attached to the interface box, which in turn is screwed securely to the bottom rear of the panel. Each speaker must then be plugged into a grounded wall outlet for a charge-up period of twenty-four hours; should AC power ever be interrupted, Acoustat advises a similar charge-up period before further use. A high-frequency level control is located on the back of the interface, as are banana-plug jacks for connection to the power amp.
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The whole assembly is quite handsome and remarkably stable, considering the height of the panels.
Diversified Science Laboratories measured frequency response with the high-frequency control set to its recommended three-o'clock position. Good response is evident down to about 50 Hz-quite a feat, considering that the Model Two consists of just one panel per side-but the response curves can hardly be called smooth through the midrange. Predictably, treble frequencies in the on-axis curve are quite elevated and smooth, though output peaks at around 12 kHz. The off-axis curve depicts a basic truism about electrostatic systems; high frequencies are exceptionally beamy, and response 30 degrees off axis rolls off dramatically above 1.3 kHz. But the specifics of these curves should not be taken literally as indicative of how the speaker will perform with your amplifier. In tests aimed at determining the action of the high-frequency control, DSL discovered anomalies outside the control's intended range that appeared to result from the combination of low overall impedance (about 4 ohms), extreme dependence on the control setting, low minima in the range around 250 Hz, and even lower minima at 20 kHz for some control settings. At the recommended control setting, the midbass minimum measures 2 ohms and that at 20 kHz is 1.6 ohms; turning the control for minimum treble smooths the curve considerably, while the maximum setting exaggerates the impedance minima. As a result, at some frequencies and settings, the source impedance of the lab's test amplifier became the controlling factor in determining the frequency response of the speaker.
The Model Two, therefore, emerges as extremely critical (and demanding) of the amplifier that powers it. (Acoustat suggests the Hafler DH-200 amplifier as a reasonably priced model with enough output to drive the system and a power MOS-FET output stage requiring no current-limiting circuitry because of its immunity to thermal runaway.) And since any maximum-input tests also would be amplifier-dependent, the 300-Hz continuous and pulsed tests were scuttled as relatively meaningless in this case. We turned to the listening room instead to tell us how loud the speakers would play.
Since speaker sensitivity is now measured in a real (i.e., reflective) room at a standardized test voltage, the Model Twos output of 80.5 dB SPL with 2.8 volts input (equivalent to 0 dBW or 1 watt into 8 ohms) must be seen as indicating low efficiency. With our former test method, the "nominal" impedance would have measured 2 ohms and the voltage therefore would have been reduced to deliver 0 dBW across that load-resulting in an output 6 dB lower or 74.5 dB SPL. Harmonic distortion measurements reveal essentially negligible amounts of both second and third harmonic products; measured at drive levels sufficient to produce 85and 90 dB sound pressure levels, total harmonic distortion stayed below 1% from 50 Hz up( In the listening room, we followed Acoustat's placement instructions to the letter. Both speakers were about four feet from a reflective back wall. We adjusted the two small feet on the bottom of each interface for vertical tilt and the panels themselves for toe-in angle to produce the proper "sweet spot" at the listening position. We were delighted by the results. Our first, and lingering, impression was of an airiness, detail, and clarity seldom surpassed by dynamic speakers--the classic virtues of electrostatics--with none of the common boxy colorations we've grown so accustomed to. Voices emerge with crystalline clarity; strings, woodwinds, and brasses are handled with vividness and realism; transients have whip-snap immediacy Deep bass, perhaps, is not up to that of the best dynamic systems, but the musical underpinnings are satisfyingly solid with a wide variety of music. Like other electrostatics, the Model Two is relatively beamy; tone color can change quite radically with only minor shift in listener position-a fact some auditioners found disconcerting.
Also typical of electrostatics is the Two's dislike of levels approaching those of a real orchestra: When prodded hard, it starts to crackle and complain.
Acoustat warns that amplifier clipping or protective circuitry may be the limiting factor, though our experience with a Dynaco Stereo 300/A (rated at 150 watts or 21 3/4 dBW per side) as power source left us in doubt whether the crackle on loud peaks was from excessive diaphragm excursion or from the amp's protection. Again, the behavior of the Model Two depends to an unusual degree on the properties of the amp driving it. Acoustat tells us, incidentally, that it is virtually impossible to destroy the electrostatic elements by overdriving them with too large a power amp.
So here it is: An electrostatic in the tradition of the KLH Nine and the Acoustech, but less subject to idiosyncrasies than they and certainly less expensive than they would be were they available today. Listening to the Model Two is rather like driving a Jaguar XKE-with some modernizing modifications, of course.
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(High Fidelity, USA print magazine)
Also see:
Boothroyd Stuart Meridian M-2 powered speaker
The Critics Go Speaker Shopping [June 1981]