Altec Lansing Duplex

Duplex was the trade name name given by Altec Lansing to its line of coaxial loudspeakers, beginning with the first model 601 in 1943. However, the name was most commonly associated with the subsequent model 604 which was a seminal loudspeaker that became a milestone in loudspeaker development. Well over a dozen different models carried the Duplex name over a near 50-year period. The vast majority consisted of a high frequency (HF) compression driver mounted to the back of a large diameter (12"-15") paper cone low frequency (LF) driver. However, there were also a few models with small diameter LF cones and direct radiator tweeters.

The Duplex speakers were extremely versatile with the 604 model alone finding application in studio monitoring, home high fidelity, public address, tour sound reinforcement, cinema sound and installed sound. A number of duplex drivers were developed for very specialized applications, such as ceiling speakers. Some of the more significant models in the Duplex lineup were, in order of introduction, the 15" 601, 15" 604, 15" 602, 12" 601, 15" 605, 15" 904 and 12" 920.

The significance of the Duplex loudspeakers is evidenced by the fact that the Altec Lansing 604 Duplex was inducted into the TECnology Hall of Fame in 2005.

The path to the Duplex line started with Altec's acquisition of the Lansing Manufacturing Company (LMCO) in 1941. That acquisition brought Altec into the loudspeaker manufacturing business and brought the loudspeaker design services of its principal, Jim Lansing. Shortly after that, Altec was approached by one of its dealers, named Art Crawford, with a request to develop a compact loudspeaker. He was seeing demand for a quality loudspeaker small enough to be used in radio stations for monitoring. The current two way speakers with separate LF and HF drivers were too large for this application and Art suggested a coaxial design whereby a LF and HF driver could be combined in one unit. Altec saw the merit of this concept and began development under Jim Lansing and John Hilliard.

By 1943, a prototype was rapidly developed with off-the-shelf LMCO components. The LF portion of the driver was developed by taking the field coil motor from the LMCO 287 compression driver and using it to drive the cone from one of LMCO's 15" LF drivers with a 2" voice coil. Both were mounted on a stamped steel frame. An LMCO 801 field coil compression driver with a 1.75" diaphragm was used as is for HF reproduction and mounted coaxially to the back of the 287 motor. A horn throat from the 801 penetrated the center of the LF field coil and exited into a small multicellular horn mounted in front of the LF cone. The horn was designed with a cut-off frequency of 900 Hz and the system was set with a crossover frequency of 1200 Hz.

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