Altec Lansing Duplex

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The Duplex was a coaxial loudspeaker driver made by Altec Lansing beginning in 1941. The product was incorporated into various loudspeaker enclosures, and supplied with various crossovers to make studio monitors and public address system loudspeakers for professional audio.

Starting with the 601 model, Altec Lansing mated a 12-inch (300 mm) low frequency woofer with a 2-inch (51 mm) high frequency compression driver, the two drivers sharing a common center for better point-source clarity in sound. With the introduction of an Alnico magnet in the woofer and a larger 3-inch (76 mm) voice coil in the compression driver, the 604 model was well received in the marketplace. Many recording studios used the Duplex 604 model for studio monitoring.

In 2006, the Altec Lansing 604 Duplex was inducted into the TECnology Hall of Fame.[1]

Altec 601[edit]

The first Altec Lansing duplex speaker was the Altec 601 which was first manufactured in 1941 or 1943. It used a 12” paper cone for the bass driver with a two inch voice coil. It was designed with the magnet for the bass driver mounted on the back (as with any conventional loudspeaker) and the driver for the horn mounted on the back of that. There was then a hole up the middle of the magnet for the bass driver allowing the high frequency sound to pass through the magnet, through the horn and out to the listener. The 601 used an electromagnet (known as a field coil speaker) for the bass driver instead of a permanent magnet which is used on all speakers today. This was because strong permanent magnets were not readily available at the time. Soon after the release of the 601, Altec began working on a permanent magnet version. During the war Altec was involved in manufacturing detection equipment for the Marine Airborne Detection program. It was through this that they discovered a new type of permanent magnetic material; Alnico V. This was much stronger than any other Alnico magnet at the time. After the war, they used this technology to create a version of the 601 the 601a with a permanent magnet.

Altec 604[edit]

A 15" version of the loudspeaker was released in 1944, known as the 604.[2] The 604 was the same basic design using a 15-inch low frequency driver and a permanent magnet made out of Alnico V, but a larger 3-inch voice coil was used.

When making the horn for the 601/604, it had to be designed so the sound coming from it would not interfere with the sound from the bass driver. To overcome this, a 2x3 multicellular design with a dispersion of 40°×90° was used. This allowed the sound to leave the horn and be dispersed largely without coming into contact with the cone of the bass driver. The horn was also coated with a damping compound to reduce high frequency resonance from the plastic. The original 601 horn was designed with a cut-off frequency of 900 Hz and the system was set with a crossover frequency of 1200 Hz. The first 604 had a crossover frequency of 2000 Hz.

Both the 601 and the first 604 had a frequency response of 60 Hz to 16 kHz with a power rating of only 25 watts, relatively low but acceptable due to the speakers' efficiency. As amplifiers of the time did not produce much power, the speakers had to be extremely efficient to produce adequate loudness. The next speaker in the 604 line to be released was the 604B, with a slightly higher power handling at 30 watts and a better frequency response ranging from 30 Hz to 16 kHz. Also, this speaker had a lower crossover frequency of 1000 Hz.

Following the 604B came the 604C. Again, the power handling capacity and frequency response were increased to 35 watts and 30 Hz to 22 kHz, respectively. Following the 604C was the 604-8G, released in 1973 and produced until 1979. The power handling capacity was substantially increased to 65 watts and the frequency response was improved, bringing the bass response down to 20 Hz and treble respones up to 22 kHz. This speaker was constructed with a die cast aluminum frame instead of a stamped metal frame, making the speaker more rigid and eliminating undesirable resonances.

The next big step in the 604 family was the 604-8K, which was similar in most respects to the 604-8G, but used a ferrite magnet instead of an Alnico V magnet. Ferrite magnets are much more resistant to being demagnetized, so they are a much better choice for the magnet for these speakers. Also, the 604-8K used a Mantaray horn design, rather than the multi-cell horn design of the older models, producing smoother high frequency dispersion.

Altec 904[edit]

After 604-8K, Altec released the 904-8A. This was basically a high powered 604-8K. It could handle up to 150 watts.

The 604 series was used as a standard in studios up until the late 1980s and used by some up until 1998.


  1. ^ "1944 Altec Lansing 604 Duplex Speaker". Mix. September 1, 2006. Retrieved June 26, 2013. 
  2. ^

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