David Hafler

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David Hafler (February 7, 1919 – May 25, 2003) was an American audio engineer. He was best known for his work on an improved version of the Williamson amplifier using the ultra-linear circuit of Alan Blumlein.

In 1950, Hafler founded Acrosound with his colleague Herbert Keroes. This company was primarily in the business of designing and manufacturing transformers for tube amplifiers. Around this time Hafler and Keroes popularised the ultra-linear output-stage for audio amplifiers. However, the partnership did not last.

In 1955 he founded Dynaco with Ed Laurent. He was instrumental in bringing affordable, high-quality audio kits to hobbyists, and his name was a household word in the international audio community for many years.

In the 1970's Hafler promoted "passive pseudo-quadraphonics", an inexpensive method of recreating ambient sound at the rear from ordinary stereophonic recordings. Known as the "Hafler hookup", this consisted of two similar additional rear speakers, connected in series (typically 8 + 8 or 16 ohms total) between the live feeds to the front speakers. The crosstalk or loss of stereo separation in the front speakers is less than 2 dB while the rear sound level in a typical recorded live performance is about 7 dB below the front, but clearly audible. This passive method is arguably as good as any of the expensive active matrix electronic decoders which attempt to reconstruct ambient sound from a stereo recording. It has been observed that ambient sounds in a concert, such as applause or even coughs from the audience, are generally received in out of phase by the stereophonic microphones, while sound from the musicians is generally in synchronous phase. Thus, if rear speakers are fed with the difference between the stereo channels, audience noises and echoes from the auditorium can be heard from behind the listener.

Hafler died May 25, 2003 in Philadelphia, Pennsylvania, USA.