Altec 604 Duplex (1944–1998)
At 55kg and with a 15" coaxial roar, it doesn’t just fill a room—it commands it.
Overview
The Altec 604 Duplex isn’t just a speaker—it’s the blueprint. When the first production model rolled out in 1944, it redefined what a studio monitor could be: accurate, powerful, and built around a single-point source design that eliminated phase issues before most engineers knew to worry about them. This wasn’t background gear; it was the frontline tool in control rooms from Abbey Road to Sunset Sound, where the clarity of a vocal take or the weight of a bass drum had to be trusted without question. The 604 became legendary not because it was flashy, but because it was faithful—delivering a sound so honest it came to define an era of recording. Owners report that even today, few drivers convey the physicality of music quite like the 604, with its massive 15" cone and coaxial horn working in concert to project studio-grade dynamics into any space.
Despite evolving through numerous iterations, the core idea never wavered: combine a large low-frequency driver with a high-frequency compression driver mounted coaxially, sharing the same acoustic center. That design, branded as the "Duplex®" system, gave engineers a coherent soundstage long before multi-way systems with complex crossovers became common. The 604 served in countless recording studios worldwide and earned its reputation as the first monitor speaker valued for its accuracy. In 2005, its impact was formally recognized with induction into the TECnology Hall of Fame—an honor few vintage audio components ever achieve.
Specifications
| Manufacturer | Altec Lansing |
| Product type | Coaxial driver / studio monitor speaker |
| Size | 15" |
| Type | 2-way coaxial speaker unit |
| Impedance | 8 Ω |
| Frequency characteristic | 20 Hz to 22 kHz |
| Crossover frequency | 1.6kHz |
| Sound pressure level | 103dB/W/m (New JIS) |
| Allowable input | 75W |
| Lowest resonance frequency | 30Hz |
| Voice Coil Diameter | Low : 75 mm; High-pass : 45 mm |
| High-frequency horn type (604-8K) | manta ray Lehorn |
| External dimensions | Width 465x Height 905x Depth 505 mm |
| Weight | 55kg |
| Enclosure type (example) | vented-type enclosure with an internal volume of 150 liters |
| Enclosure material (example) | 30 mm thick birch laminate, finished with a teak veneer |
| Magnets | Alnico V (for specific Alnico variants; ferrite variants also exist) |
Key Features
15" Coaxial Duplex Design with Single Point Source
The heart of the 604’s magic lies in its coaxial layout—a 15" cone driver for lows with a high-frequency compression driver mounted directly in its center. This configuration ensures both drivers radiate from the same point, eliminating time and phase discrepancies that plague multi-driver systems. Engineers could trust what they heard because there was no “sweet spot” to hunt for—the dispersion was consistent, and imaging was locked in. The aluminum die-cast frame holds everything rigidly in place, minimizing mechanical resonance that could color the sound.
Curved Corn Paper Cone with Corrugated Edge
The low-frequency unit uses a cone made from high-quality corn paper, formed into a subtle curve and fitted with a corrugated edge. This design balances stiffness and damping, allowing the cone to move air with authority while resisting breakup modes that lead to distortion. Combined with a 75 mm voice coil and strong magnetic circuit, it delivers deep, controlled bass down to a lowest resonance frequency of 30Hz—remarkable for its era.
Aluminum Alloy Diaphragm with Tangerine Phase Plug
The high-frequency driver features a rigid aluminum alloy diaphragm and a tangential edge that maintains linearity under high acceleration. Heat-resistant voice coil and bobbin materials allow sustained high-power operation without thermal compression. The Tangerine Phase Plug—a signature Altec innovation—sits at the throat of the horn, smoothing high-frequency dispersion and reducing resonances within the path. It’s a critical detail that contributes to the 604’s clear, uncolored highs.
Manta Ray Lehorn (604-8K)
A major evolution came with the 604-8K model, which replaced earlier multi-cellular horns with the distinctive manta ray Lehorn. This wide-mouthed, flared design improved high-frequency dispersion and reduced edge diffraction. The horn throat was changed to a 7/8" cylindrical bore (from the earlier 1" conical flare), and later versions included an open-cell foam plug at the throat and small Helmholtz resonators in the horn sides to tame resonances. This version, based on ferrite magnets, marked a shift toward improved reliability and cost-effective manufacturing.
Reduced Diaphragm Mass in 604-8G Series
In the 604-8G series, transient response was enhanced by reducing the vibration mass of the diaphragm. This Alnico V-equipped variant prioritized speed and detail, appealing to studios that valued precision over raw output. The crossover remained at 1.6kHz, and the M.F. level controller—located on the rear of compatible enclosures—allowed fine-tuning of the midrange by ±10 dB around the crossover point, giving engineers real-time control over tonal balance.
Historical Context
The Altec 604 Duplex emerged from a lineage of broadcast and theater speakers, but its 1944 debut marked a turning point: it was the first monitor specifically valued for accuracy rather than just loudness. It quickly became the industry standard, found in professional studios across the globe. Over its 55-year production run—from 1944 to 1998—it evolved through key variants: the 604B (1949), 604C, 604E, the Alnico 604-8G (1973–1979), and the ferrite-based 604-8K (early 1980s onward). Its influence extended beyond Altec—Urei based their iconic 801A and 801AA studio monitors on the 604-8G and 604-8K drivers, respectively, cementing the Duplex design in audio history.
Collectibility & Value
Market prices for the Altec 604 vary widely depending on model, condition, and enclosure. A complete "Legendary ALTEC LANSING 604 Duplex Speaker" with original 605 residential enclosure listed for $3,950, while a non-functioning 604E driver alone sold for $535.49—indicating demand even for untested units. Matched pairs in original 612A "Abbey Road" monitor cabinets have appeared at $3,995, reflecting the premium placed on authenticity and provenance. However, collectors note that operational integrity is not guaranteed; forum discussions reference "Altec 604-8k driver issues" and "604-8g newbie problems," suggesting that restoration and maintenance are common pursuits. No original MSRP has been documented, and current values appear driven more by nostalgia, studio pedigree, and scarcity than standardized pricing.
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