JBL 128H / 128H-1 Woofer
1978-1985
The 128H was JBL's first high-performance ferrite magnet 12" woofer. It used the SFG (Symmetrical Field Geometry) magnetic circuit and a Faraday loop to cut distortion. It marked JBL's move 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
Cobalt prices rose in the late 1970s, and Alnico magnets got more and more expensive. JBL's engineering team, led by Ed May, developed the SFG magnetic circuit so that ferrite-based woofers could match or exceed their Alnico predecessors.
The Faraday loop was a simple copper ring. It stabilized the magnetic flux in the gap and dramatically reduced modulation distortion. JBL kept using it in its high-performance woofers through the 1980s.
Enclosure Design
The JBL Professional Enclosure Guide gives 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 →
Market Value
┌──────────────────────────────────────────────────────────────────────────┐ │ JBL 128H WOOFER Nov 2026 │ ├──────────────────────────────────────────────────────────────────────────┤ │ High $541 Latest $410 │ │ Median $410 Trend — │ │ Low $90 Volume ~1/mo │ ├──────────────────────────────────────────────────────────────────────────┤ │ $600 ┤ │ │ │ . │ │ │ . │ │ $364 ┤ .│ │ │ │ │ │. │ │ │ │ │ $50 ┤ . │ │ ├─────────────────────────────────────────────────────────────────│ │ vol │▄ ▄ █ ▄│ │ └─────────────────────────────────────────────────────────────────│ │ 2026-02 2026-04 2026-06│ └──────────────────────────────────────────────────────────────────────────┘
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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