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GENIUS! #28: Peter Thomas and the Advanced Transmission Line

Dave Robinson 20 April 2015
Peter Thomas, PMC, Genius!

Peter Thomas was born to be a designer of audio equipment. A passionate music lover and hi-fi fanatic from an early age, his fascination with audio technology endures in his current day job as owner and chief designer at UK monitor manufacturer PMC. Visitors to the company’s former factory in Bedfordshire got used to sidling past additions to his ever-growing archive of vintage speakers, recording devices, microphones and other recording ephemera (in their new HQ, the collection has a barn to itself).

Moving after engineering college to the BBC, Thomas eventually became responsible for the technical upkeep of Maida Vale studios, home of the legendary Radiophonic Workshop, and was tasked with sourcing a high-output, low-distortion main monitor for the studio. He met many loudspeaker designers to discuss designs, but became convinced he could improve on their products – a common milestone on the path from engineer to designer. Eventually, he decided to try to design the required monitors himself. The clean highs and mids of electrostatic panels were appealing, but lacked power and level, while ported designs offered the bass response, but were too distorted for reference monitoring. Drawing on the best acoustic attributes of these designs, Thomas returned to the concept of the transmission line loudspeaker, first within the BBC, and then at PMC, which he co-founded in 1991 to continue his design work commercially.

PMC QB1-A, Genius!Transmission line speakers were popular in hi-fi circles in the 1960s and ‘70s for their low-distortion, high-output characteristics, but fell from favour by the ‘80s for being too complex to realise practically. Thomas’s vision was to recognise their potential, while simultaneously coming up with engineering solutions to remedy their past deficiencies. For years, different speaker designers had been focused on honing sealed-cabinet and ported designs, but no-one had attempted to reimagine the transmission line with modern materials or design principles. “They were cabinets with labyrinths inside, crudely damped with long-haired wool,” explains Thomas today. “They sounded OK – not great. But the theory made sense. I thought that if we got rid of the wool and got the absorption right with some with some properly engineered bespoke damping, we might get somewhere. It was the start of a long, expensive journey…” (Pictured right is the PMC QB1-A system.)

There’s a reason PMC’s first speakers and active electronics – built by Thomas himself and still in use at Maida Vale 24 years later – were called the BB5s. “The BB1, BB2, BB3 and BB4 didn’t really cut the mustard…” explains Thomas today with characteristic modesty.

The Advanced Transmission Line (ATL), PMC’s refinement of the concept, features in all their designs, from large-scale studio monitors to hi-fi models. Integrating crossovers and drivers into an ATL cabinet requires high-specification engineering, and still has to be tailored specifically to each product. “That’s probably why the concept failed in the 1970s,” comments Thomas. “Put simply, it’s easier and cheaper to build speakers another way! Perfecting transmission line designs demands persistence, a holistic approach, and obsessive attention to detail. But after 23 years, we must be doing something right…”

www.pmc-speakers.com

Hail to the boffins! Genius! is all about celebrating those clever people whose inventions have transformed the world of professional audio. Mailed out with the February print edition of PSNEurope, the 36-page supplement is also available to read in handy digital-edition form. Read it online, or download as a PDF, at www.psneurope.com/introducing-genius.

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