Sony SS 7400 ULM4 (around 1971)
A passive bookshelf speaker system from Sony, developed after fundamental research into speaker design. It has advanced magnetic circuitry and a tri-amped configuration option.
Overview
The Sony SS-7400 ULM4 is a 3-way, bass-reflex loudspeaker built for high-fidelity reproduction. It dates to around 1971 and was originally priced at ¥45,000 per unit. It is a bookshelf or standmount design with a walnut-finished enclosure. Sony developed it from the ground up after doing foundational research into speaker technology. That research led to several 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 has a redesigned magnetic circuit meant 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 has a metal bobbin to increase power handling. Both woofer and midrange drivers use corn paper (pulp) cones, chosen after extensive study of materials and processing. The surround is a thin rubber layer applied to cloth, for airtightness and smooth response. It smooths a dip around 400 Hz in particular. The lead wires use high-purity copper foil treated with special resin, which prevents breakage and extends lifespan over tenfold. The 2.5 cm dome tweeter has a special diffuser (equalizer) for smooth high-frequency response. The baffle is edgeless and the Saran frame is brimless, both to minimize reflections. Tweeter and midrange each get a continuously variable level control (±3 dB). There is also a multi-selector switch and an LC network disconnect option, so each driver can be tri-amped directly.
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