Altec 604 Duplex (1944–1998)

Name: Altec 604
Brand: Altec

It weighs 55kg and has a 15" coaxial cone. It fills a room.

Overview

The Altec 604 Duplex is the blueprint for the modern studio monitor. The first production model rolled out in 1944. It was accurate and powerful, and it used a single-point source design that eliminated phase issues before most engineers knew to worry about them. It was working gear in control rooms from Abbey Road to Sunset Sound, where a vocal take or a bass drum had to be trusted. The 604 became legendary because it was faithful, not flashy. 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 together.

The 604 went through numerous iterations, but the core idea never changed. A large low-frequency driver and a high-frequency compression driver are mounted coaxially and share the same acoustic center. Altec branded this the "Duplex®" system. It 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 it was inducted into the TECnology Hall of Fame.

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 604 is a coaxial layout. A 15" cone driver handles the lows, and a high-frequency compression driver sits directly in its center. Both drivers radiate from the same point, which eliminates the time and phase discrepancies that plague multi-driver systems. There was no "sweet spot" to hunt for, because the dispersion was consistent and the imaging was locked in. The aluminum die-cast frame holds everything rigidly in place and minimizes 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 balances stiffness and damping, so the cone moves air with authority while resisting the breakup modes that lead to distortion. Paired with a 75 mm voice coil and a strong magnetic circuit, it delivers deep, controlled bass down to a lowest resonance frequency of 30Hz, which was remarkable for its era.

Aluminum Alloy Diaphragm with Tangerine Phase Plug

The high-frequency driver has 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 part, sits at the throat of the horn. It smooths high-frequency dispersion and reduces resonances within the path, and it contributes to the 604's clear, uncolored highs.

Manta Ray Lehorn (604-8K)

A major change came with the 604-8K, which replaced the earlier multi-cellular horns with the distinctive manta ray Lehorn. The wide-mouthed, flared design improved high-frequency dispersion and reduced edge diffraction. The horn throat changed to a 7/8" cylindrical bore, from the earlier 1" conical flare. Later versions added an open-cell foam plug at the throat and small Helmholtz resonators in the horn sides to tame resonances. This version used ferrite magnets, which meant improved reliability and cost-effective manufacturing.

Reduced Diaphragm Mass in 604-8G Series

In the 604-8G series, transient response was improved by reducing the vibration mass of the diaphragm. This Alnico V variant favored speed and detail, which appealed to studios that valued precision over raw output. The crossover stayed at 1.6kHz. The M.F. level controller, located on the rear of compatible enclosures, adjusts the midrange by ±10 dB around the crossover point.

Historical Context

The Altec 604 Duplex came out of a lineage of broadcast and theater speakers. Its 1944 debut was the first monitor specifically valued for accuracy rather than just loudness. It quickly became the industry standard and turned up in professional studios across the globe. Over a 55-year production run, from 1944 to 1998, it went through several variants: the 604B (1949), 604C, 604E, the Alnico 604-8G (1973-1979), and the ferrite-based 604-8K (early 1980s onward). Urei based their 801A and 801AA studio monitors on the 604-8G and 604-8K drivers, respectively.

Collectibility & Value

Market prices for the Altec 604 vary widely with model, condition, and enclosure. A complete "Legendary ALTEC LANSING 604 Duplex Speaker" with original 605 residential enclosure listed for $3,950. A non-functioning 604E driver alone sold for $535.49, so there is demand even for untested units. Matched pairs in original 612A "Abbey Road" monitor cabinets have appeared at $3,995. Collectors note that working order is not guaranteed. Forum discussions reference "Altec 604-8k driver issues" and "604-8g newbie problems," which suggests restoration and maintenance are common. No original MSRP has been documented, and current values appear driven more by nostalgia, studio pedigree, and scarcity than standardized pricing.