JBL 128H / 128H-1 Woofer
1978-1985
The 128H was JBL's first high-performance ferrite magnet 12" woofer, featuring the innovative SFG (Symmetrical Field Geometry) magnetic circuit and Faraday loop for reduced distortion. It marked JBL's transition from Alnico to ceramic magnets.
Specifications
| Nominal Diameter | 12" (30cm) / 10" (25cm) for 128H-1 |
|---|---|
| Magnet Type | Ferrite (Ceramic) |
| Magnetic Circuit | SFG (Symmetrical Field Geometry) |
| Voice Coil | Edgewound with Faraday loop |
| Impedance | 8 Ω |
| Power Handling | 80-100W continuous |
| Sensitivity | 88-89 dB/W/m |
Key Technologies
- SFG Magnetic Circuit: Symmetrical Field Geometry for reduced distortion and improved flux distribution
- Faraday Loop: High-conductivity ring attached to pole piece to stabilize magnetic flux and reduce second harmonic distortion
- Ferrite Magnet: Ceramic magnet offering cost stability and consistent performance over time
- Edgewound Voice Coil: Improved heat dissipation for higher power handling
Applications
- L112 (1980-1983)
- L150 / L150A (1979-1982)
- L96 (128H-1 variant)
Historical Context
Rising cobalt prices in the late 1970s made Alnico magnets increasingly expensive. JBL's engineering team, led by Ed May, developed the SFG magnetic circuit to ensure that ferrite-based woofers could match or exceed the performance of their Alnico predecessors.
The Faraday loop was a particularly innovative addition - a simple copper ring that dramatically reduced modulation distortion by stabilizing the magnetic flux in the gap. This technology became a hallmark of JBL's high-performance woofers through the 1980s.
Enclosure Design
The JBL Professional Enclosure Guide provides specific recommendations for the 128H as a hi-fi low-end driver or small subwoofer:
| Application | Hi-fi low-end or small subwoofer |
|---|---|
| Chart Reference | #3 |
| Enclosure Volume | 2.5 cubic feet (70.8 liters) |
| Tuned Frequency | 30 Hz |
| Internal Dimensions | 20.2" H x 16.5" W x 14.3" D |
| Vent Area | 12.6 sq in (1x 4" duct) |
| Duct Length | 11.9 inches |
View the complete JBL Professional Enclosure Guide →
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Documentation
- Lansing Heritage - Comprehensive JBL specifications, photos, and historical documents
- HiFi Engine - JBL manuals, service documents, and brochures
- Audio Karma JBL Forum - Community discussions and restoration guides
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