Altec Lansing 604E (1965)

A 26-pound, 13-ounce magnet structure wrapped around a 15-inch frame - the last great revision of the coaxial driver that American studios built their reputations on.

Overview

The 604E is a driver, not a speaker system. One cast frame carries two magnetically, electrically, and mechanically independent transducers: a 15-inch low-frequency cone and a high-frequency compression unit. They are stacked coaxially, so the horn fires straight through the center of the woofer. Altec's own 1965 literature described the Super Duplex as a two-way coaxial transducer. The high-frequency section uses an edge-wound aluminum voice coil on a 2½-inch aluminum diaphragm with tangential compliance, coupled to a multicellular horn with a 40° by 90° distribution angle. The low-frequency cone runs an edge-wound 3-inch copper voice coil on a high-compliance suspension with a cloth surround rim. The dual magnet structure weighs 26 pounds, 13 ounces and contains oversize Alnico V magnets, so a bare 604E feels less like a component and more like a small engine block.

Frequency response is quoted at 20 to 22,000 Hz. Audio (June 1965, p.42) treated that as the headline claim when Altec issued the 604E Super Duplex as the latest revision of the long-running 604. Altec's spec sheet rated the driver at 35 watts continuous (50 watts peak) and 8 to 16 ohms impedance, nominally 16 ohms. That load behaves gently with tube amplifiers. High-efficiency coaxials like this don't need power, they need quality. Sensitivity is reported at 101 dB, though one spec sheet fragment lists a lower 100 dB SPL/1W/4ft figure. The discrepancy is minor. Either way, a 604E will play at genuinely uncomfortable levels off a handful of watts.

The 604E is the fifth and final major revision of a design Altec acquired and launched as the 604 in 1944. It is the version most often cited as the definitive studio monitor of its era. HiFi-Stereo (January 1965, p.24) announced the Super Duplex as a 15-inch coaxial loudspeaker rated for 20 to 22,000 cycles, aimed squarely at professional engineers rather than hobbyists. That framing was accurate. This driver spent the late 1960s and 1970s in broadcast booths, mastering rooms, and the soffit-mounted walls of recording studios.

Key Features

Coaxial Mounting with a Multicellular 408B Horn

The defining feature is the geometry. The high-frequency driver sits at the center of the woofer and fires through a multicellular horn identified as the 408B, so both bands radiate from effectively the same point. Altec's literature described this as ensuring time-coherent delivery and a coherent soundstage. Most two-way systems of the era simply lived with that alignment problem, and it would not be formally addressed in studio monitoring until UREI's time-aligned work in the late 1970s. The horn's 40° by 90° pattern is broad enough to cover a control room without beaming. The multicellular construction is what gives the top end its openness, rather than the cupped coloration of a simple sectoral flare.

Alnico V Dual-Magnet Motor

Two independent magnet structures share one frame. The low-frequency motor uses a 3-inch edge-wound copper voice coil. The high-frequency side uses a 2½-inch aluminum diaphragm with an edge-wound aluminum coil. According to Resistor Magazine, the 604E's low-frequency motor received a full-thickness top plate and a shorter voice coil than previous iterations, specifically to improve linearity at low frequencies. The trade-off of Alnico V is well documented among owners: over decades the magnets can lose strength, and re-magnetization by a specialist is the accepted remedy. That is not a defect of the design so much as the cost of sixty-year-old metallurgy.

The N-1500-A Dividing Network

Each 604E shipped with a full-section dividing network crossing over at 1,500 Hz, designated the N-1500-A. Altec built in a high-frequency shelving control with a 0 to -10 dB range so the driver could be matched to a given room. The intent was practical: a coaxial driver designed for both a broadcast booth and a living room needs a way to trim its top end. The execution has drawn the most criticism. The stock network is a series-type crossover, and technicians note that series topologies are exceptionally sensitive to the impedance of the drivers they control. Owners who have opened up original N-1500-A units report that the real problem is not the schematic but the parts - cheap potentiometers and low-grade capacitors. Replacing the stock network with a modern film-capacitor design, a Werner Jagusch autorformer crossover, or a Mastering Lab network is one of the most common modifications made to these drivers. It is arguably the single highest-value change an owner can make.

Driver-Only Format and Enclosure Dependency

The 604E is a driver only. Its bass performance depends on what it gets bolted into. Altec's own documentation recommended the 857A studio monitoring enclosure for broadcast and recording work, the 858A "Carmel" for home use, and the 855A "Malibu" furniture-style cabinet as a domestic alternative. The published driver dimensions are roughly 17.5 inches in diameter and 9.5 inches deep, so it will not drop into a bookshelf. This component dictates the cabinet, not the other way around. Owners pairing 604Es with large ported A7 or A8 enclosures report deep, punchy low end, while sealed or undersized boxes leave the bottom octaves thin. Free-air resonance is quoted at 32 Hz.

Phase Plug and Diaphragm Serviceability

Altec's specification language for the 604E was unusually explicit about serviceability. The high-frequency diaphragm and voice coil assembly were required to be field-replaceable without special tools or skills, using self-centering dowels to guarantee proper spacing and alignment. For a driver living in a working studio, that was an operational requirement. A machined phasing plug with two exponential acoustic slots maintains the phase relationship between the center and outer edge of the high-frequency diaphragm. That is the mechanical reason the midrange stays smooth rather than combing.

Historical Context

The 604 series traces back to the Western Electric 594 coaxial driver, developed in the 1940s. Altec acquired the design and launched the 604 in 1944, and the model quickly became the reference coaxial for American studio monitoring. By the time the 604E arrived in 1965, the line had been through four prior revisions, and this one refined the compression driver, the horn geometry, and the crossover network. Altec's 1965 catalog copy was blunt about the lineage: for twenty years, 604-type Duplex speakers had been the standard coaxial speaker for the audio industry, and the 604E was the contemporary playback version of that famous design.

The 604E's working life spanned the late 1960s and 1970s, and it is widely associated with the recording output of that period. The source material for this page names Motown and Capitol Records among the environments where it was used, though those specific claims are single-source and should be treated as reported rather than confirmed. The coaxial studio monitor concept was eventually challenged. In 1977, UREI introduced the 813 Time-Aligned Monitor, built around the Altec 604 but using a crossover with passive delay to move the apparent position of the woofer back to that of the tweeter, following Ed Long's 1976 AES paper on the subject. The 604E was the benchmark that time-alignment was measured against. Forum comparisons have even pitted a circa-1971 604E directly against Great Plains Audio's 604-8H-III from 2009.

Collectibility & Value

The 604E is a serious collectible. The single source page rates it 9 out of 10 on a vintage audio collectibility scale, with market value for a pair running roughly $1,500 to $3,500 depending on condition, originality, and completeness. Units with original Alnico drivers, intact horns, and matching numbers command the top of that range. The driver was widely produced, but fully original and well-maintained pairs are increasingly rare. Asking prices can run higher: a single 604E coaxial unit has been observed listed on eBay at $7,159, and a pair of Alnico drivers with spare high-frequency horns has been offered at CA$2,850. Those are asking prices, not sale prices.

Condition is everything, and the failure modes are well known. Original foam surrounds on the 15-inch woofer often disintegrate and get replaced with modern foam or rubber. Paper and electrolytic capacitors in the original crossover fail, and replacement with film capacitors is standard practice. The multicellular horn should be checked for cracks or delamination. Minor damage is repairable, and severe damage may need professional restoration or replacement. Alnico magnets lose strength over decades and can be re-magnetized by a specialist. Because the 604E is a driver only, the cabinets it lived in - original A7 and A8 enclosures - often need refinishing, re-grilling, and port restoration on top of whatever the driver itself needs.

Prospective buyers should also account for the crossover question. If a pair comes with an original N-1500-A network, expect to either rebuild it with quality capacitors or replace it outright. Great Plains Audio has provided re-magnetization and reconing to 604E specification for at least one documented pair. On amplification, owners report that high-output-impedance designs, single-ended amps with little or no feedback among them, tend to sound richer and more blooming with these drivers, injecting more power where the impedance curve rises at resonance and at high frequencies. One documented demonstration ran a 604E with an N-1500-A crossover off a 0.75-watt Darling amp.