Sony SS 7400 ULM4 (around 1971)
A passive bookshelf speaker system developed by Sony after fundamental research into speaker design, featuring advanced magnetic circuitry and a tri-amped configuration option.
Overview
The Sony SS-7400 ULM4 is a 3-way, bass-reflex loudspeaker system designed for high-fidelity audio reproduction. Built around 1971 and originally priced at ¥45,000 per unit, it uses a bookshelf or standmount configuration with a walnut-finished enclosure. The system was developed from the ground up by Sony after conducting foundational research into speaker technology, resulting in several innovative design choices aimed at reducing distortion and improving longevity.
Specifications
| Model Rating Method | 3-Way, 3-Speaker, Bass reflex system, Bookshelf Type |
| Unit For low band | 30 cm cone type |
| Unit For Medium Range | 12 cm cone type |
| Unit For high-pass | 2.5 cm dome type |
| Playback frequency band | 40 Hz to 20000 Hz |
| Output sound pressure level | 91dB/W/m (New JIS) |
| Output sound pressure level | 97dB/W/0.5m (Old JIS) |
| Impedance | 6 Ω |
| Maximum allowable input | 100W (weighted pink noise 24-hour guarantee) |
| Crossover frequency | 600 Hz, 4500 Hz |
| External dimensions | Width 415x Height 680x Depth 300 mm |
| Weight | Approx. 21 kg |
| Type | Passive |
| Positioning | Standmount |
| Enclosure | Bass Reflex |
| Way system | 3 |
Design
The SS-7400 ULM4 features a redesigned magnetic circuit intended to eliminate non-linear distortion: a notch in the center pole and a thick copper cap or plating reduce magnetic material distortion. The voice coil uses a special heat-resistant synthetic resin adhesive, and the woofer includes a metal bobbin to increase power handling. Both woofer and midrange drivers use corn paper (pulp) cones, selected after extensive study of materials and processing. The surround is made by applying a thin rubber layer to cloth for airtightness and smooth response, particularly smoothing a dip around 400 Hz. High-purity copper foil, treated with special resin, is used in lead wires to prevent breakage and extend lifespan over tenfold. A special diffuser (equalizer) on the 2.5 cm dome tweeter ensures smooth high-frequency response. The baffle is edgeless, and the Saran frame is brimless to minimize reflections. The system includes continuously variable level controls for tweeter and midrange (±3 dB), plus a multi-selector switch and an LC network disconnect option to enable direct tri-amping of each driver.
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