Alan Dower Blumlein, the Forgotten Engineer With 128 Patents

The story of the man who invented stereophonic sound and helped the Allies win the WW2.

Ultra-linear amplifiers, stereophonic sound, and the radar system that helped Britain win World War II have something in common —they’re all inventions of the same man, English electronics engineer Alan Dower Blumlein.

Blumlein was born in 1903 in London. It is said that his passion for electronics started very early in his life, having supposedly repaired his father’s doorbell at the age of seven. He got a bachelor’s degree in Science with first-class honors in 1923 and started working for the Western Electric Company in 1924. There, he designed weighting networks and improved analog telephony with a renewed load coil that avoided loss and crosstalk in long-distance lines.

This is one of Blumlein’s earliest patents — and the beginning of a journey full of inventions that finished prematurely with his tragic death at 38 years old.

However, his final 128 patents remain as highly-valued contributions to the electronics behind a wide range of technologies.

Blumlein’s Amplifiers

Blumlein’s name is on the earliest patent of the long-tailed pair, also known as differential pair circuit, registered in 1936.

LPT is a type of electronic amplifier that works with a symmetrical circuit built with two bipolar transistors (originally vacuum tubes).

Long tailed pairs
Long-tailed pair with cathode auto-biasing. Figure 3 from A. D. Blumlein's US Patent 2185367 (British 482740).  Source: Wikimedia Commons.

This way, the circuit can generate two inputs and one output that’s proportional to the difference between the inputs’ voltages. This is differential gain (Ad) that produces the amplification.

V out = Ad (V+in - V-in)

Long-tailed pairs are currently used in integrated circuit amplifiers and operational amplifiers (applied in precision rectifiers, oscillators, filters, buffers, and others).

By mid-year 1937, Blumlein presented another circuit technique called “distributed loading”, and in 1938, he patented in the U.S. the ultra-linear amplifier, built with this technique. When paired with a pentode or tetrode electron valve, the ultra-linear circuit can enhance the linearity of audio amplifiers, increasing the power output and reducing distortion.


Stereophonic Sound

Alan Blumlein is key in the history of stereophonic sound (initially called “binaural sound”). The idea supposedly came to him when he was watching a movie at the cinema with his wife in 1931. Movie theaters had only one set of speakers at the time, and he felt that the sound didn’t follow the actor properly across the screen.

To solve this, he invented an audio recording technique currently known as or Blumlein pair or the Blumlein technique. It’s carried out by placing two dipole microphones at 90º angles from each other. Dipole microphones can record audio from the front and the rear of the sound source, but when placed at the right angle, they can combine their efforts to capture audio in four directions. Therefore, in the final recording, the listener could perceive the sound directionally.

blumlein stereo
Blumlein stereo recording technique. Source: Galak76/Wikimedia Commons.

Blumlein built a "shuffling" circuit to keep that directionality when sound from a spaced pair of microphones was reproduced using stereo loudspeakers. He also adapted his technique to gramophone records by creating a stereo disc cutting head that enabled the recording of two audio channels in a single groove.

This led to the first stereo recording in 1934 at Abbey Road Studios.

H2S Airborne Radar System

During World War II, Alan Blumlein was a senior engineer at EMI. At the time, the company put their best workers at the service of the British military.

Soon, Blumlein started to develop the H2S airborne radar system, a secret project for aiding bomb targeting in poor visibility conditions, like in bad weather and/or during nighttime. The radar also covered a wider range than radio navigation aids, detecting landmarks at greater distances.

H2X radar
 B-17 Flying Fortress bomber as seen from another B-17's H2S display. Source: Unknown USAAF photographer/Wikimedia Commons

He was killed in a plane crash while testing the H2S airborne radar system on a Halifax bomber in 1942. He was only 38 years old.

memorial stone on halifax crash site
Memorial stone in the Halifax crash site. Source: Wikimedia Commons

Although he couldn’t finish his work in the H2S radar system, he inspired other engineers to continue it and helped Britain win (and shorten) the war. Furthermore, he managed to invent the line-type pulse modulator that’s still used in high-powered pulse radars nowadays.

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