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                                                                                                                                  Photo: Siegfried Linkwitz (left) with Jean-Marie Liere

Jean-Marie Liere has been a fan of Siegfried Linkwitz’s crossover network circuit topologies and open-baffle loudspeakers for many years. In this article, he recounts how a recent visit to Linkwitz’s home in California came about, and his experiences listening to two of Linkwitz’s most famous loudspeaker designs, the Pluto and Orion.

Whilst preparing for a trip to the USA I stumbled upon an audio site that I didn’t know about and which was going into a great deal of detail about open baffle and point source speaker design: www.linkwitzlab.com. I had been researching open baffle speakers for my blog after recently reconnecting with Alain Wacquet, a friend who designed such speakers in the 1980s under the AW Audio brand. At the time I was building Microphase speakers in France, and he and I were often co-exhibitors at the Paris Hi-Fi Show. Wacquet’s designs were really ‘out there’ at the time—particularly his penultimate design (EA-16) released in 1987 and his last design, the Tranparence (1995).

The funny thing is that the name Linkwitz Lab should have been an instant hit, but it was only after an email exchange with the owner of the website that the penny finally dropped: I was indeed talking to Siegfried Linkwitz, whose name is immortalised in the loudspeaker field (the Linkwitz-Riley filter is an infinite impulse response filter used in Linkwitz–Riley audio crossover networks. It’s also known as a Butterworth squared filter) and famous for teaching KEF engineers about group delay and transient response. The initial contact came about in the early 70s, when KEF’s engineers were shopping around to buy a state-of-the-art FFT analyser and ended up at Hewlett-Packard in California, where Linkwitz was working at the time with his colleague Russ Riley. You can read an amusing account of that story here: www.linkwitzlab.com/AX%20interview.htm. (You can also find an interview with Siegfried here: www.stereophile.com/interviews/503/)

Interestingly enough, I let another penny drop back in the 80s. I was working for Hewlett-Packard at the time, but in my spare time I was designing a speaker system that used second-order Linkwitz-Riley filters. Siegfried and I had lunch together at an HP conference in Amsterdam in 1985, but I didn’t realise who he was and obviously missed out big-time on an opportunity to pick his brain!
So this time around I was not going to miss the chance to meet with him, particularly as I would be in San Francisco for a few days and Linkwitz lives just up the road in Marin County. He was generous enough to spend the whole morning of Boxing Day with me, showing me his laboratory and home office, and talking shop non-stop for a couple of hours. Afterwards we sat down to listen to his unusual creations: the open-baffle Orion loudspeakers and the point-source Pluto system.

Before I go any further, let me explain where Siegfried’s Orion system fits in in my experience of electrodynamic open baffle systems. AW Audio used a series of 120mm drivers and one tweeter in a two-way arrangement that involved some quite complex passive filtering and some physical time alignment between drivers. Emerald Physics uses a Berhinger DCX2496 as a digital filter and equaliser and several amplifiers to get the job done. Kyron Audio has an even more extravagant foray into the ‘box-less’ adventure (10 drivers per channel each driven by its own amp—a total of 700-watts of power per channel)!
Linkwitz takes a more middle-of-the-road approach with his Orion system, now in its Rev.4 avatar, using only five drivers per channel (all made by SEAS) and each one of which is equalised, along with a three-way electronic filter (which he calls an ASP, short for ‘Analogue Signal Processor’). If you are prepared to build a pair yourself, the cost is said to be less than $4,000 for a pair. Just add your own amps, preamp and sources (Siegfried uses an Oppo BDP-95 Blu-ray player as his primary source, otherwise he uses audio files stored on his computer).
Before I tell you about my listening impressions, let’s first talk about Linkwitz’s Pluto speakers. Another favourite acoustic topic of mine has always been the elusive ‘point source’ speaker, the one that is supposed to solve a lot of our listening problems: stable image, absence of a ‘sweet spot’, coherent phase, time-aligned response and so on.

 Cabinet size, shape and relative driver positions are all part of this equation and it is believed that all these limitations would disappear if (and it is a big ‘if’!) we could design a single, very small full-range driver with high efficiency and high power-handling. I guess you could call it the Nirvana of speaker design!
Many attempts have been made at the point-source ideal, and the coaxial driver ‘a la Tannoy’, and the Uni Q from KEF (and even more recently, KEF’s ‘Blade’) are good examples. The Elipson spheres back in France in the 70s were another elegant solution to this problem. Other examples that spring to mind are made by Visme in France (the Cube), Audel in Italy and here in Australia by VAF (the i90)—all good examples of small point-source designs. I was,

Pictured at front is a pair of Linkwitz Pluto V2.1 active loudspeakers, with the piping left ‘unfinished’. Amplifiers are in the base. At the rear is a single Linkwitz Orion Rev.4 dipole loudspeaker.

however, completely unaware that Siegfried Linkwitz had designed a similar system in the 70s and abandoned it to concentrate on open-baffle speakers.
Fast forward to 2002 and Siegfried, in the course of another assignment, discovers a 50mm tweeter (made by Aura) that can reproduce frequencies down to 200Hz at the same time that he was also was toying with the idea of using a 127mm midrange driver from SEAS as a woofer. Put the two drivers together and the result was the Pluto: an omnidirectional speaker system, very close to being a point source, with built-in amplifiers, two-way electronic crossover (1kHz) and equalisation. As of 2012 the Pluto is in its 2.1 revision. Siegfried was always clear about his design intentions for the Pluto. ‘My goal was to obtain an acoustically small source with wide and uniform sound dispersion over most of its frequency range. Next to dipole radiation, as with the Orion, I consider omni-directional behaviour desirable, provided that either the room has neutral acoustics or that one listens to such speakers from a close distance,’ he says.
As you can see, the WAF (Wife Acceptance Factor) of the Siegfried’s Pluto design is appalling (though their small size and light weight means one could tuck them away in a cupboard and have them deployed when needed. Simply put a blindfold on, and listen!) but since these are a ‘Do It Yourself’ design, many DIYers around the world have come up with far more ‘spouse friendly’designs. [www.tinyurl.com/pluto-photos]

We started listening to the Plutos at Siegfried’s place (searching for familiar material, I found my favourite test track on Siegfried’s hard drive: Private Investigations from the Dire Straits album ‘Alchemy’) and I have to say that sound-wise, these are mind-blowing little marvels. The only time I noticed them at Siegfried’s place was when there was a bit too much bass and the piped woofer excited the floor-boards. I have not yet had a chance to listen to the Plutos on a concrete floor, but my guess is this problem could disappear… although remember we are talking about a 127mm driver in a sealed enclosure.
As an aside, to those of you who’ve looked at the design specifications for the Pluto I should point out that I too am generally suspicious of speakers with a crossover point at 1kHz or so, but after auditioning the Plutos, I would be the first to admit there is no way you would pick that the crossover point is so high and, even knowing beforehand, I found the integration between the two drivers to be astonishing, the stereo image very stable and the so-called ‘sweet spot’ was certainly fairly wide.

We moved on to audition the Orions, and managed to listen to some of the same tracks we had used on the Plutos, and sound that had already been quite extraordinary when listening to the Plutos became almost miraculous when played on the Orions. It was like there were no speakers at all, as we were transported into the recording venue: no distortion, a wide and stable image, high dynamics… in a word, pure music!

I can only compare this experience to my recent audition of hi-res digital recordings made by AIX Records via an Oppo BDP95, Bryston amplifiers and Thiel speakers at the CES: there was the same absence of ‘hi-fi-ness’; the same impression of being there, the same sheer musical pleasure...
Merci beaucoup Siegfried, for spending some time with me and letting me share your world: it will feed some long lasting memories and maybe some more speaker designs on my part… Jean-Marie Liere