Audiophiles of a certain age might know about the Hill Plasmatronics loudspeakers, a unique design that used ionized air, aka plasma, to create sound. Those of us who have heard these rare birds, or speakers and transducers of a similar design like the Ionovac tweeter, know that hearing such a transducer is a remarkable experience. There is a purity and transparency to plasma drivers that leaves an unforgettable impression. However, the implementation of plasma speakers requires what for most people would be a completely impractical consideration: namely, that they require helium gas to make them work.
Impractical for some, but not all of us, like turntable specialist Brian Walsh of TTsetup.com. Brian is a lifelong music and audio enthusiast and former dealer who goes to peoples’ homes around the country to do phono cartridge setups using advanced tools and computer analysis.
Some history: the Hill Plasmatronics loudspeakers were the brainchild of Dr. Alan Hill, a former laser physicist who worked at the Air Force Weapons Laboratory at Kirtland Air Force Base in Albuquerque, New Mexico. He became intrigued by the fact that in certain configurations, lasers could create sound, and interested enough to pursue the development of a plasma-driver-based loudspeaker system. After some engineering hiccups, he introduced the Hill Plasmatronics loudspeakers in 1978 at that year’s Consumer Electronics Show (CES). As one might imagine, they weren’t a commercial success and production stopped in 1984.
A plasma driver works by varying the surrounding air pressure via an electrical plasma, as opposed to the conventional means of moving air via a conventional physical driver diaphragm. The plasma heats the surrounding air, which causes it to expand. Since the plasma is electrically conductive, it can be modulated by an audio signal. The air expands in accordance with the signal applied to the plasma driver, and as most of us know, sound is created by variations in air pressure; hence, the modulations of the plasma is translated into sound.
In order to create the ionized plasma, gas is required. The Hill Plasmatronics speaker system employed two helium tanks, one for each speaker. Helium offers advantages, including heat dissipation, over other types of gases (and obviously nothing toxic or explosive could be used). And yes, you had to rent the tanks and have them refilled when they ran out, making the speaker impractical for all but the most dedicated audiophiles. A pair of Hill Plasmatronics loudspeakers cost $7,000 back in the day, a serious financial as well as logistical commitment. Because of its limited frequency range down to about 1,000 Hz, the plasma tweeter is mated with a midrange driver and woofer, and the speaker system required the use of a dedicated crossover/electronics unit.
The speakers are big and imposing, and, well, most people would consider them ugly if not downright hideous, with their box shape, 1970s wood finish, clunky control panel at the top, and industrial-looking middle section that houses the plasma tweeter, something that seems to have been designed with 100 percent consideration to functionality and zero to aesthetic appeal. Although looking at the plasma flame itself is fascinating. There’s something primeval and alluring about it, aside from the fact that such a device can actually make music. (For more information on the speaker, click on this Stereophile review originally written by J. Gordon Holt in 1980 and republished in 2014, which was an invaluable resource in researching this article.)
Beauty is in the ear of the behearer: one of Brian Walsh's Hill Plasmatronics speakers.
So, right now you’re probably thinking, why bother?
Of course, the answer is, the sound.
Conventional drivers, whether cone, planar magnetic, ribbon, or electrostatic, have mass, which means that they have inertia, however much or little. As a result, it takes them a while to get moving, and to stop moving, which affects their fidelity, their ability to reproduce the audio signal exactly. Plasma drivers, on the other hand, do not have such mass or inertia to overcome. The transient response, clarity, and transparency of the Plasmatronics driver are extraordinary. Extraordinary enough that I had never forgotten the sound of hearing a plasma speaker for the first time. Incredible enough to inspire Brian Walsh to spend more than three years looking for a pair, no mean feat since he told me only around 10 systems are known to exist, and to set about restoring them.
The inner mounting flame: a closeup of the plasma driver.
The back of the speaker reveals its inner workings.
Brian gave some background: “I first heard a pair of Hill Plasmatronics around 1979 at Opus One, a store in downtown Pittsburgh owned by an eccentric, passionate guy named Tasso Spanos, who loved music and people. The lower-midrange drivers and woofers were driven by a Rappaport amplifier. I was young, not long out of college, and couldn't afford these esoteric things but asked to hear them.
What followed was perhaps the biggest audio epiphany of my life. The song “School” on Supertramp's album Crime of the Century played at fairly high volume was life-changing. The kids were in the room! The cone drivers didn't keep up with the plasma tweeters, but OMG, what a revelation!” Brian noted, “I heard a pair at the Hill dealer in San Francisco a year or two later, but the room was way too small, although they sounded great there, too. I hadn't come across any after that but never forgot.”
Then in October 2022, he heard about Tony Salsich, who was coming down to Chicago to visit an audio dealer to demonstrate a pair and talk about them. Brian jumped at the chance to attend the demo, and met Tony and reminisced with him about his memories of hearing the Plasmatronics. Brian told Tony that he wouldn’t mind finding a pair at some point.
Brian continues: “At AXPONA 2023, Tony was in dealer Joe Galanti's room (Superior Audio), demonstrating the same system I'd heard at Joe's place a few months before. What's more, inventor Alan Hill himself was there with his wife and son. Alan had a great time, although he was in poor health.”
Dr. Alan Hill (center) with wife Carol at AXPONA 2023.
Not long after the show, Brian received an e-mail from Tony, saying that someone from Minnesota had spoken with him at the show whose friend had a pair he wanted to sell. Brian decided to buy them. The seller was Mark Balkowitsch of Audio Perfection in Minneapolis, who had been a Hill dealer and had moved a pair into his storage area, where they sat for about 40 years. “A true audio barn find,” Brian noted.
Except for the foam woofer surrounds which had rotted away, very little needed to be done to restore the speakers. A speaker repair person in Chicago replaced the woofer surrounds, and a couple of noisy pots in the interface/crossover unit were fixed. “The speakers pretty much fired right up,” said Brian. “Ultimately a few diodes, capacitors, and fuses needed replacement, and the plasma modules will be fully recapped soon as a precaution – after all, things can go bad after over 40 years. Most parts [for the speakers] are available. The 6MJ6 tubes [for the electronics] are no longer made, but can be found. The parts quality is high.”
About the need for helium tanks, Brian explained: “I didn't like the idea of having to haul helium tanks up and down the stairs, so with a little correspondence with Tony and some planning, I now have a single full-size remote tank in my garage, feeding up to the speakers in the room. You rent helium tanks from industrial welding suppliers such as Airgas and take them there to exchange them when they're empty. A full-size tank supplying both speakers will give up to 150 hours of playing time but probably less, and a refill is a bit over $300. So, the cost may be three to four dollars per hour.”
A closer view of the connection for the helium supply, and other parts of the speaker.
The front-panel meters indicate the status of the plasma currents and gas flow.
The manual contains detailed hookup and operating instructions.
A few weeks before AXPONA 2025, Brian, along with Tony Salsich, who worked at Hill Plasmatronics company for around 12 years, and Richard Braine of tonearm manufacturer Supatrac invited a group of people to his home to have a listen to the restored Hill Plasmatronics system. The speakers were complemented by a serious audio system with a Dohmann Audio Helix One turntable (Brian provides factory authorized setup and service for Dohmann) as the source.
As soon as I walked into Brian’s generously-sized, dedicated listening room, I knew. Instantaneously, there was the sound. Astoundingly clear and natural, with remarkable rendering of instrumental texture. There was an ease and effortlessness to the sound that reminded me of the best electrostats, ribbons and planar magnetics. The sense of reality was spooky. The system did all the audiophile “stuff” well – the soundstage was wide and deep, the imaging was there, the spatiality, especially on classical music, was convincing – but my efforts to try to translate sonic and musical impressions into words aren’t really all that convincing. All I can tell you is that there was a magic in hearing jazz musicians, vocalists and…well, everything…sound eerily close to reality in a way that was almost disconcerting at first, then spellbinding.
Yeah, there’s a reason why my first listening to plasma drivers left a lifelong impression. I kind of knew what I thought I’d expect to hear at Brian’s house, but actually hearing that sound again was thrilling. Moments like this don’t happen in audio…or in life…and when they do, they are to be savored and reveled in.
Well, no audiophile listening session would be complete without some kind of sonic spectacular demonstration track. For the end of the listening session, and believe me, no one wanted it to end, Brian put on The Power and the Majesty, the classic 1978 audiophile disc from Mobile Fidelity Sound Lab. Side one’s track is called “The Coast Daylights Featuring Steam Locomotive No. 4449.” The track starts out with the train literally building up speed, chugging and clacking over the rails. Sounds like an audiophile cliché, but make no mistake, on this system it was incredible. The speakers played at a room-shaking volume that really did sound like you were riding on that train, with astounding dynamics, and when the train let out a whistle that was even louder, without any strain whatsoever, I’m here to tell you it was scary.
I’m a jazz aficionado and the jazz cut that the guys played was equally if very differently impressive. What was the word I’m looking for again? Magic. Yes, that’s it. I left Brian’s room dreaming of helium tanks, glowing pyramids, and magical sound.
Some of Brian's demo records: test pressings of Doug McLeod, Bring it on Home, and Joan Baez, Diamonds and Rust; Doc Powell, The Doctor on VTL Records; Roger Waters, The Lockdown Sessions; Satchmo Plays King Oliver on Audio Fidelity records, and a trio of original Mobile Fidelity Sound Lab UHQR discs. Courtesy of Brian Walsh.
Header image: the Hill Plasmatronics speakers in Brian Walsh's listening room. All images courtesy of Brian Walsh and the author.
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