Siegfried Linkwitz, the co-inventor of the Linkwitz-Riley crossover, a favourite of loudspeaker manufacturers and DIY speaker builders the world over, has died, aged 82.

When asked about his many inventions over the years, Linkwitz would often invoke one of his favourite Ralph Waldo Emerson quotes: ‘We are all inventors, each sailing out on a voyage of discovery, guided each by a private chart, of which there is no duplicate. The world is all gates, all opportunities.

Although Linkwitz designed and built loudspeakers his entire adult life, and was associated with several well-known loudspeaker manufacturers, including Audio Artistry, where he was vice-president of engineering, and his own company, Linkwitz Labs, he worked full time in the research and development department of test and measurement company Hewlett-Packard (now Keysight Technologies) for 37 years, developing and helping develop state-of-the-art electronic test equipment including microwave spectrum analysers, network analysers and EMI receivers. In his later years at Hewlett Packard, he travelled the world extensively, teaching seminars on test and measurement. He also helped develop US National and International standards for Electromagnetic Compatibility Test Instrumentation.

Linkwitz was born on November 23, 1935, in Bad Oeynhausen, Germany. His father was a surveyor and his mother a school teacher. He was a second son, his brother being eight years older. After graduating with a Diplom Ingenieur degree in Electrical Engineering from Darmstadt Technical University in Germany, Linkwitz moved first to Hanover in 1957, where he worked for Telefunken, and four years later to Munich, to work for Siemens. Linkwitz married his wife Eike in 1961 while the two were working for Siemens. Both were disenchanted by the situation in post-war Germany and they decided they’d embark on what Linkwitz called ‘a cultural experience’ by moving to California for two years so Linkwitz could do post-graduate studies at Stanford University and become involved in the ‘New Age’ movement. Whilst studying, Linkwitz also started working for Hewlett-Packard, first in Palo Alto and then in Santa Rosa… where he continued to work until his retirement in 1998.

It was while he was at Hewlett-Packard that Linkwitz and his co-worker Russ Riley developed their ‘Linkwitz-Riley Crossover’ in 1975. Later, Linkwitz extended this work to develop what he called the ‘Linkwitz Transform’ circuit. The circuit had nothing to do with work either of them were doing at HP. Linkwitz said that it came about simply because in their spare time the two were both audio hobbyists.

First described in the Journal of the Audio Engineering Society in the paper ‘Active Crossover Networks for Noncoincident Drivers’ (Vol 24 No 1, Feb 1976), a Linkwitz-Riley filter (also known as a Butterworth Squared filter) is an infinite impulse response filter. Applied to a loudspeaker crossover network, it consists of low-pass and high-pass Butterworth filters in parallel. This results in −6dB gain at the cut-off frequency, so that when the low-pass and high-pass outputs are summed, the gain at the crossover frequency is 0dB, so the crossover has a flat amplitude response. (A standard Butterworth crossover has a 3dB peak at the crossover frequency.)

In the ensuing years, Linkwitz wrote many important papers that were published in the Journal of the Audio Engineering Society, including ‘Investigation of Sound Quality Differences between Monopolar and Dipolar Woofers in Small Rooms’, ‘Development of a Compact Dipole Loudspeaker’, ‘ Loudspeaker Design for Reduced Reverberant Sound Power Output’, ‘Shaped Tone-Burst Testing’, ‘Narrow Band Impulse Testing of Acoustical Systems’, and ‘Passive Crossover Networks for Noncoincident Drivers.’ He also contributed dozens of articles and speaker-building projects to electronics and DIY loudspeaker enthusiast magazines such as Electronics (Wireless) World, Speaker Builder, and AudioXpress.

Meanwhile, at Hewlett-Packard, Linkwitz was primarily responsible for developing such famous test instruments as the HP8566 100Hz-22GHz Microwave Spectrum Analyser, the HP8554 RF Spectrum Analyser, HP8405 RF Vector Voltmeter, HP8546 EMC Analyser, HP85650 Quasi-Peak Adapter and HP 85685 RF Preselector.

After retiring from Hewlett-Packard in 1998, Linkwitz was able to concentrate on designing loudspeakers, the designs of which he made available to DIY constructors at low cost, or for free. His most famous designs were the Phoenix (2000), Thor (2001), Orion (2002), Pluto (2005), Pluto+(2006), Orion++ (2007), Pluto2 (2008), Orion3 (2010), Orion4 (2011), LX521 (2012), LXmini  (2014), LXstudio (2015), LX521.4 (2015), LXmini+2 (2016), LX521.5-ASP (2016) and ASP.4 (2016).

He also took the opportunity afforded by his retirement to greatly expand his website (www.linkwitzlab.com) to help educate visitors to that site about loudspeaker design, sound reproduction and recording. On the site you can find detailed plans for DIY construction of state-of-the-art dipole and monopole loudspeakers using active electronics, all of which are optimised for operation in reverberant domestic spaces.

Linkwitz died peacefully at his home in Corte Madera, California, on September 11, 2018, where he’d been receiving hospice care since 2016 for Stage IV prostate cancer, after first being diagnosed in 2002. He is survived by his wife Eike, their son Andras, daughter Monika and three granddaughters, Julia, Anja, and Danielle. In his final years, Linkwitz said of his life: ‘I feel blessed, having been given a life to contribute to understanding, consciousness and joy.’ #