Altec 604 Duplex Series

1944 - 1990s

The Altec 604 Duplex is the most famous coaxial loudspeaker ever made. Recording studios all over the world used it, and it went into the TECnology Hall of Fame in 2005. Production ran over 50 years.

The Duplex Concept

The Duplex design came in 1943 with the model 601 and was refined as the 604 in 1944. A high-frequency compression driver sits coaxially behind a large 15" woofer, which makes the whole thing a point-source loudspeaker. This arrangement provided:

Point-Source Radiation: Both high and low frequencies originate from the same location
Phase Coherence: Improved time alignment between drivers
Wide Dispersion: Consistent coverage pattern across the frequency spectrum
High Efficiency: Compression driver on a horn provides exceptional sensitivity

Evolution of the 604

604 (1944-1952)

First permanent-magnet Duplex speaker
Alnico V magnets (5.2 lbs LF, 2.5 lbs HF)
Multicellular horn (2x3 cell design)
2000 Hz crossover
25W power handling
60 Hz - 16 kHz response

604B (1952-1957)

Lower crossover: 1000 Hz
Improved response: 30 Hz - 16 kHz
30W power handling
Still used multicellular horn

604C (1957-1964)

New sectoral horn with vanes (40° x 90°)
1600 Hz crossover
35W power handling
30 Hz - 22 kHz response
Green hammertone finish introduced
Iconic "fins" or "vanes" in horn

604D (1964)

Short-lived transitional model
Quickly replaced by 605A and then 604E

605A (1959-1964)

Modified magnet structure (shared flux path)
95 dB sensitivity (3 dB less than 604C)
Market rejected it in favor of 604

604E Super Duplex (1964-1973)

Restored full magnet structure
Improved suspension and cone
98 dB sensitivity
Available in 8Ω and 16Ω

604-8G Super Duplex (1973-1979)

Final production 604 version
Improved transient response
Tangerine phase plug
Most sought-after by collectors

604-8H, 604-8K, 604-8L (1980s-1990s)

Later production versions
Manufactured by Altec and later by GPA
Continued refinements to design

Technical Specifications Summary

Feature	Typical 604 (8G)
Woofer	15" with 3" voice coil
Compression Driver	1.75" diaphragm
Crossover	1,500 Hz
Sensitivity	98 dB/W/m
Frequency Response	30 Hz - 22 kHz
Power Handling	50W continuous
Impedance	8Ω or 16Ω

Applications

The 604 found use in virtually every audio application:

Recording Studios: Main monitors at Western Electric, CBS, RCA, and thousands more
Broadcast: Radio and television facilities worldwide
Home Hi-Fi: Serious listeners used 604-based systems
Sound Reinforcement: Portable and installed PA systems
Cinema: Smaller theater and preview room applications

Legacy and Modern Production

The Altec 604 went into the TECnology Hall of Fame in 2005. Today, Great Plains Audio (GPA) makes the 604-8H-II, built with modern materials and aimed at keeping the original's sound.

Individual Model Pages

604-8G Super Duplex (1973-1979)
604E Super Duplex (1964-1973)
605A Duplex (1959-1964)

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