The development of the Duplex traces its roots to Jim Lansing's involvement in the Shearer Horn project of 1934. That system set the standard for wide-bandwidth, accurate, sound reproduction. After its introduction, Jim Lansing began a decade long quest to bring the same level of sound quality to monitoring applications. His first attempt was in 1936 when Lansing Manufacturing introduced a scaled down version of the Shearer Horn known as the Monitor System 500A. He followed that up in 1937 with an even more compact system called the Iconic It retained the multicultural horn of previous designs but used a smaller, ported bass section. It became the standard of its day for monitor quality, but even its relatively small size was inappropriate for many control rooms.
The solution to high quality sound in a compact package lay in a coaxial speaker driver. The idea of a coaxial system for monitoring was suggested by an Altec Lansing dealer named Art Crawford. James Lansing immediately picked up on his suggestion as he recognized a number of benefits to this approach. Not only did it allow reduced size, but equally important in his mind, was the opportunity to develop a loudspeaker that would act as a point source. He had identified a deficiency of using two-way systems in nearfield listening where two distinct sound sources could be perceived from the widely spaced bass and high frequency units.
Lansing's design concept for a coaxial speaker was to mount a modified version of the small-format Lansing 801 compression driver behind a 15" cone bass driver. He envisaged having the horn throat of this driver inside the voice coil of a 15" bass driver and terminating in a miniaturized multicellular horn. He recognized that the horn would need to have a controlled dispersion to prevent interference from the cone and outer rim of the speaker. For this reason, he settled on a 2 x 3 cell horn that held the horizontal dispersion to 60 degrees. The horn was coated with a rough-textured, damping compound, that not only eliminated unwanted horn resonances, but also ensured that there would not be regular reflection patters for sounds emanating from the surrounding cone. The use of a miniaturized multicellular horn dictated a relatively high cross-over point. The cutoff frequency of the horn was 900hz, and the cross-over frequency was set at 1200hz to accommodate a 12db/octave slope.
The bass driver used a 15" seamless paper cone attached to a 2" voice coil. The pole piece of this driver was inordinately deep to allow an underhung coil topology that kept the voice coil inside a constant magnetic field, even at maximum excursion. Both drive units used heavy field coils encased in thick-walled pot structures and very small coil gaps to generate extremely high flux densities. It resulted in a very efficient driver with minimal distortion.
The field coil Duplex was introduced in 1943 and was originally given the model number 601 (it should be noted that this design bore no relation to the 601 introduced in the 1950's that was a 12" permanent magnet coaxial driver). The original 601 was available as a component kit or could be ordered installed in a utility cabinet as the model 602 or in a furniture cabinet as the model 605.
Almost immediately after its introduction, Lansing began work on the development of a permanent magnet version that was to become the famous model 604 Duplex. At this time, Altec Lansing was involved in a wartime development effort to produce equipment for the Marine Airborne Detection program. Work on this program introduced Altec Lansing to the newly introduced Alnico V magnetic material. This magnet type was significantly more powerful than previous formulations of Alnico that had been used in permanent magnet speakers. George Carrington, the President of Altec Lansing, recognized the applicability of this new magnet to speaker design and authorized research on its incorporation into their current product line.
In redesigning the magnet structure, Lansing decided to increase the bass driver voice coil to a 3" diameter using flat wire. This was the largest diameter coil yet used in a 15" driver. Initial tests proved, that for the first time, a permanent magnet speaker could be designed that matched or exceeded field coil units of the same dimensions. (Note: the illustration to the right shows a cross-section of 1975 604-8G that illustrates the permanent magnet topology. There are very few physical differences from this speaker and the original 604 from 1944)
Jim Lansing succeeded in producing the Alnico V, 604 Duplex in 1944. This speaker actually outperformed the original 601 in efficiency and maximum output. Lansing recognized that this loudspeaker had application beyond monitoring. A series of systems were developed that installed the 604 in various cabinets for public address and even home use. It was an immediate success that would result in the 604 being the ubiquitous monitor in recording studios. Even as late as the 1980's, the 604 was the standard for main studio monitors as OEM drivers in UREI's famous 813 system. There were still loudspeaker companies making state-of-the art consumer designs using the latest 604-8K into 1998. That year, production would cease as Altec's owners, Telex Communications, closed the Oklahoma City plant that manufactured this speaker.
© 2000 Don McRitchie