Altec 604 Duplex Series
1944 - 1990s
The Altec 604 Duplex is the most famous coaxial loudspeaker ever created. Used in countless recording studios worldwide and inducted into the TECnology Hall of Fame in 2005, the 604 represents a milestone in loudspeaker development that spanned over 50 years of production.
The Duplex Concept
The Duplex design, introduced in 1943 with the model 601 and refined as the 604 in 1944, was revolutionary: a high-frequency compression driver mounted coaxially behind a large 15" woofer, creating a true 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 was inducted into the TECnology Hall of Fame in 2005, recognizing its profound impact on professional audio. Today, Great Plains Audio (GPA) manufactures the 604-8H-II, continuing the legacy with modern materials while maintaining the original's essential character.
Individual Model Pages
- 604-8G Super Duplex (1973-1979)
- 604E Super Duplex (1964-1973)
- 605A Duplex (1959-1964)