Hill Plasmatronics Type 1 loudspeaker
Dr. Alan Hill, president of Plasmatronics Inc., was previously employed by the US Government in laser research. His assignment: To increase the efficiency of lasers so that they could do something more impressive than produce holograms, mend leaky retinal blood vessels, and punch pinholes in steel blocks. Dr. Hill earned his keep, thus advancing laser technology a giant step closer to Star Wars, and then retired from government service to design. . . a loudspeaker?!!!?
How could laser research qualify someone to design a loudspeaker? The connection is really much more direct than it seems. Twenty-odd years ago, Dr. Hill envisaged a loudspeaker that would use a field of ionized air as the transduction element, but didn’t feel enough was known about plasmas (footnote 1) to perfect such a device.
At about the same time, a firm called the Dukane Company started producing such a device anyway: The “Ionovac” tweeter. It was not a huge commercial success, partly because of its (for those days) outrageous price and partly because add-on tweeters have never been big sellers. (The Ionovac was subsequently made by ElectroVoice until phased out in 1963.) Nonetheless, the Ionovac is still considered by the knowing to be the best supertweeter ever made, and there are few audiophiles who would sniff at its 2–40kHz (±2dB) response.
While developing the high-efficiency laser, Dr. Hill found it was necessary to control the shape of the plasma of ionized gas that does the lasing. And it occurred to him that shaping might be the key to a high-efficiency, wide-range “Ionovac.”
His first efforts, using a relatively low-temperature plasma (and an absolutely Mickey Mouse mockup), were disappointing. It produced sound, over a respectable part of the audio spectrum, but at ridiculously low levels of efficiency. Using higher ionizing voltage, and a mixture of air and helium as the plasma medium, he was able to sustain a much larger plasma field (thus significantly extending the low-end range) and to yield practical efficiency figures. Then it was necessary to do additional trimming of the system to produce the flattest possible frequency response across the board.
All Photos Courtesy John & Amelia Mayberry
In the final production version, flat response is maintained (with 1dB) down to around 700Hz. The upper limit is claimed to produce “significant acoustical power” out to beyond 100kHz. It was deemed impractical to try and carry the low end because of cost and power-supply considerations. Even in the production version, the required driving amplifiers (built into the system and all tubed) are rated at 500 Wpc.
The range below 700Hz is handled by conventional cone drivers: a 5″ midrange and a 12″ woofer, which must be driven by their own (choice optional) amplifier.
The speakers connect to the main system preamplifier via a 30′ cable and an “electronic interface”—a small box housing the system’s electronic crossover circuitry, balancing controls, and a series of LEDs that display the system’s output level at any given instant. The interface unit is located at the main preamp end of the interconnecting cable.
Gas Beside the plasma driving amplifier and the transduction device, each speaker enclosure also houses a large bottle of compressed helium gas (footnote 2), which is fed on demand to the plasma field when the speaker is operating. (When the system is off, the helium flow is automatically turned off.) The bottles must be recharged after each 300 hours or so of operation—representing s little under 6 months of 2-hours-a-day listening sessions. Refills cost around $30 per bottle, which translates into an operating cost of 20¢ per hour for helium alone.
For people living within convenient delivery distance of a major city, there should be no trouble locating a helium supplier. (You’ll find them in the Yellow Pages, under “Gas—Industrial and Medical—Cylinder and Bulk,” or under “Welding Supplies and Materials.”) For those people who live ‘way out in the boonies, recharging may involve shipping the empty bottles to some distant supplier and waiting, perhaps for weeks, for their return. (Anyone who can afford a pair of the Plasmatronics should certainly also be able to afford a second set of gas bottles to be put into use when the other set is away being recharged.)
Practicalities Each speaker weights about 300 lbs with its fully charged bottles. And when both amplifiers have been running for an hour or so, their combined heat dissipation dumps about 3500 BTUs (just over 1kW) into the room—dandy on those chilly winter evenings but a dubious blessing on a hot August afternoon.
With all the design complexity, the question of reliability must inevitably come up. As of now, the speakers haven’t been around long enough to establish ay sort of reliability record, although their ability to withstand accidental overloads and foolhardy listening levels has already been demonstrated. They seem to be very rugged, but whether or not production samples will be inadvertently sabotaged by a parts vendor remains to be seen.
Those of us who have read alarming things about the toxic effects of ozone may wonder how much of a problem it is with this system. Well, the Plasmatronics do generate ozone, but in such small quantities that after three hours of continuous operation, it could barely be smelled at a distance of 12″ from either speaker. This concentration of ozone is so far below the toxicity (or of potential damage to rubber and plastics) that to worry about it may be symptomatic of some degree of neurosis.
There is provision for biamplifying the two lower-range cones, but this is one of those rare instances where biamping is not recommended. The built-in crossover has phase-correction circuitry; electronic crossovers do not. As a result, biamping the Plasmatronics speakers introduces audible frequency-response irregularities (which are absent when their own crossovers are used), neatly shooting down the system’s remarkable blending of drivers.
Listening We auditioned two versions of the Type 1 speaker over a 3-month period. The first was early production, and while that part of the audio range covered by the plasma driver was impressive (more details subsequently), we were unhappy with the low end. The cones blended superbly with the upper range, but the bass was somewhat loose, floppy, and ill-defined. We were inclined to blame that on the driving amplifier, which was one we had never been enamored of: the Audio Research D-100.
Subsequently, Dr. Hill made changes in the cone portions of the system and also found what he felt to be a better drive amplifier for them (the Threshold 4000A), and that was the version of the system we auditioned for this report.
So, how does the current version sound? Quite simply, mind-boggling! One’s first reaction is that there is just no transducer there at all. You seem to hear through the system to the program source. Stereo imaging and depth are as well reproduced as form any system we have heard, and the most immediate response to all this is that the system sounds incredibly alive.
Footnote 1: To a physicist, a plasma is a volume of ionized gas. (An ion is an atom having more than or fewer than its usual complement of electrons.) The gas within a plasma has an extremely low density, relative to the gas surrounding it, Thus, when cool gas is heated to the plasma state, it expands in volume and imparts a pressure wave to the surrounding, cooler gas. Using an audio signal to vary the volume of the plasma produces the alternating compressions and rarefactions of a soundwave.
Footnote 2: Helium is inert, odorless, and completely harmless. Deep-sea explorers have breathed a 50/50 mixture of oxygen and helium for days at a time without any effects other than a comical raising of the voice pitches that makes grown men sound like Donald Duck. (Excluding nitrogen from the “air” prevents a nasty diving disorder called “the bends,” which results from the formation of nitrogen bubbles in the blood stream when a diver returning to the surface undergoes rapid decompression.) The raising of voice pitches is due to gaseous helium’s very low density, which provides less acoustic loading the vocal cords than does normal air, causing them to vibrate more rapidly.
Sonic details are reproduced with clinical clarity, which is dandy with superb source material but a liability with the majority of recordings. Bass is deep, tight, and gut-shaking, and the seams between the drivers are virtually imperceptible—quite an accomplishment in view of the fact that two of them are cones, with appreciable inertial mass, while the other, widest-range one is completely massless (footnote 3).
Without running any curves, we would guess the low end to be effectively flat to around 35Hz in a room of adequate proportions. (The one we listened in was not. The tightest, deepest low end was only audible in an adjoining room, which did at least prove that the system was capable of producing that kind of bottom.) Our only cavil about the sound concerned the system’s brightness, which was too much. Dr. Hill assures us that the system measures flat out to the bat’s radar region, and indeed it sounded flat when we listened with the cartridge of his choice (a GAS Sleeping Beauty Shibata). But with original tapes, and a cartridge we have found to provide comparable brightness, we felt the sound from the Type 1 to be brilliant almost to the point of stridency (although without the teeth-setting edge that betrays the presence of spurious odd-order harmonic content). For this reason, the system never quite captured the correct musical timbres of most musical instruments—an attribute few audiophiles seem attuned to anyway. (Take an audiophile to a concert and his first reaction is, almost invariably, “My God, where are all the highs?“)
With most speaker systems, some degree of exaggerated treble is necessary to help overcome the innate deficiency of detail. It is not necessary with the Plasmatronics, although we can well understand how that brightness may be necessary to sell these speakers to the kind of listener willing to pay $7000 for speakers alone (many of whom are locked into cartridges whose own brightness range is attenuated). If we had our druthers, we would like to see (and hear) this system equipped with a switch that would provide, in one position, the kind of sound that we heard (by which we cover ourselves against the possibility that it may sound less bright in other rooms), and in the other position,, a more neutral musically felicitous sound.
Considering the current chaotic state of the high-end” audio field, few listeners will get any real idea of what these speakers can and cannot do until digital program sources become more widely available. Anyone endeavoring to evaluate the Plasmatronics at a dealer’s will be at the mercy of the dealer’s often-misguided choice of associated equipment. There is, in fact, more than just a possibility will sound better than it really is, because of the current popularity of deadish but tipped-up phono cartridges that will tend to offset the speaker’s brightness and underscore its remarkable detail.
The Type 1’s literature specifies a maximum output level of 107dB, which looks pretty good but not outstanding. In truth, we found it possible to achieve clean signals up to a peak SPL reading of 116dBA before overload became audible. That may not be disco-type output level, but to any other listener it is one hell of a lot of noise—particularly when we consider that live music form acoustical instruments rarely exceeds 100dB when heard from an audience seat, even a very close one.
Incidentally, “overloading” the Type 1 system does not cause the usual startling snap or crackle of amplifier clipping or voice-coil bottoming. When overloaded, the system—literally—runs out of gas and progressively limits the amplitude of signal peaks, in much the same unobtrusive manner as the peak limiters used for years by virtually all commercial record companies.
Conclusion Is this loudspeaker worth its $7000 price tag? Maybe. There is no doubt but that there is $6500 worth of technological know-how and constructional hardware in a pair of Plasmatronics Type 1s, but whether or not they are worth that much to you as a consumer depends on what you value, and how much. If you are hyper-critical of imaging, inner detail, transient response, and high-end openness, be assured that this system will give you more of those things than any other currently available system. If you are a bass freak, these won’t disappoint you, though they may not make you as happy as a large transmission-line system or a monumentally dimensioned horn system.
But if you are more of a music listener than a detail fanatic, you may well find that there is much in the grooves of most discs that is best left unheard. And if you are picky about the accurate reproduction of timbres, you may also—depending on the characteristics of your program sources—be more or less put off by the Plasmatronics’ brightness. We suspect, though, that most audiophiles will find these speakers to provide the most mind-blowing listening experience they have ever known.
Further Thoughts Although not the perfect transducer, the Plasmatronics Type 1 represents a significant advance in the state of the audio art because it eliminates, once and for all, the need for detail “enhancement” in the program material an ancillary electronics. If, and when, this standard of detail reproduction filters down into the lower-priced equipment areas, multimiking and the use of “hot” microphones to offset detail smearing in playback systems will no longer be necessary. This could pave the way for a new kind of audiophile recording, in which performing groups can be presented at a natural distance, to provide blending of the sounds without loss of definition. The result will probably be what we’ve all claimed to be seeking: The sound of live music. Whether or not we will all like that sound is moot. . .
Footnote 3: Well, not quite completely. The gas mixture has some thermal inertia, which causes a gradual rolloff of frequencies above about 30kHz. However, the rolloff is much less rapid than the rolloff that occurs above the resonance frequency of a mechanical transducer.