JBL 2121H (1980)

Little is documented about the JBL 2121H beyond its basic specifications. It was a 25 cm cone-type mid-woofer, released in 1980 and sold in Japan for ¥47,000 per unit around 1981. JBL built it for the middle and low range of four-way systems, and specifically for the 4343B studio monitor. The model number appears in JBL's own professional systems reference chart as the mid-frequency driver of the 4343B, paired with a 2231H low-frequency unit and 2420/2405H high and ultra-high drivers. The "2121" (without the H suffix) is also named in connection with the 4343B, and aftermarket recone kits list both designations. That suggests they're closely related or the same driver.

What's on record: an 8-ohm unit, 95 dB/W/m sensitivity, a 250 Hz to 2 kHz band, and a 35 Hz resonant frequency. JBL rated it for 75W continuous program input, measured by sweeping 100 to 500 Hz. The magnet is Alnico, with a flux density of 10,500 Gauss. It's 11.5 cm deep and weighs 5.5 kg, and when used it sits in a small sealed enclosure of 11.3 liters or more. Aftermarket recone kits describe a 10″ cone with a 3″ voice coil opening, foam surround, and an edge-wound aluminum voice coil (8 ohms, Re: 6 ohms). Those kits are the main reason these drivers are still serviceable today.

Beyond that, the record is thin. No period reviews, owner commentary, or detailed production history have surfaced — no documented end-of-production year, no original US price, no published Thiele/Small parameters. What survives is essentially a spec sheet and a parts ecosystem. If you're restoring a 4343B, the 2121H is the part you're hunting; treat any single-source figure with the usual caution.

Type: 25 cm cone mid-woofer
Impedance: 8 Ω
Sensitivity: 95 dB/W/m
Frequency band: 250 Hz – 2 kHz
Resonant frequency (fo): 35 Hz
Continuous program input: 75W
Magnet: Alnico, 10,500 Gauss
Depth: 11.5 cm · Weight: 5.5 kg
Enclosure volume (when used): 11.3 L or more, sealed