JBL 4320 Studio Monitor (1970)

A 97-pound control-room workhorse with an 800 Hz crossover and a horn that throws sound across a 120° arc. This is the monitor that put JBL inside the glass.

Overview

Set a JBL 4320 on a console bridge and you can tell right away it isn't a home speaker. The 15-inch woofer's cone is terminated by a molded bubble-ring edge JBL developed to allow excursions greater than 1.27 cm, and free-air cone resonance sits at 25 Hz. This driver is built to move air. Above it, a horn-loaded compression driver takes over and runs smoothly through a range greater than four octaves. The whole package weighs 44 kg, and it was never meant to be moved once it found its spot.

JBL's own manual describes the 4320 as a professional monitor loudspeaker system designed to reproduce the extended frequency and dynamic ranges of master recordings, equally suited to small auditoriums, theaters, and other high-quality sound reinforcement work. That's manufacturer language, but it matches how the speaker was actually used: control rooms, editing suites, mastering facilities, and broadcast booths where accuracy mattered more than warmth. The published on-axis response is ±3 dB from 40 to 15,000 Hz, with dispersion of 120° horizontal and 40° vertical. Per JBL's own measurements, a listener sitting 45° off axis hears almost exactly the same overall response as someone directly on axis.

Pricing in the period ran to ¥392,800 per unit around 1973, with the 4320 WX variant listed at ¥413,400. That was serious money for a single monitor, which explains why 4320s tended to show up in facilities with budgets rather than in bedrooms. The 4320 is the larger, more expensive sibling of the 4310 and 4311, which became the better-known home and small-studio monitors. It's not as common as either, and collectors hunt it when they want the full-size version.

Key Features

15-Inch Woofer with 4-Inch Edgewound Ribbon Voice Coil

The low-frequency section is built around a 15-inch driver, the 2215B in the configuration most commonly documented. It uses a 4-inch edgewound copper ribbon voice coil in a magnetic field of 11,000 gauss, with 450,000 Maxwells of total flux in the gap. JBL's manual makes a specific claim about coupling efficiency: 10 watts of DC across the voice coil produces roughly 5.5 pounds of force on the cone. The magnetic circuit alone weighs 19.5 pounds. That mass and motor strength shows up as control at high levels rather than as boom, and it's why these speakers survived sessions that would have cooked lesser drivers.

Power handling is rated at 60 watts continuous sine wave and 120 watts program material. JBL's own documentation includes an unretouched oscilloscope photo of the woofer reproducing a 75-watt continuous sine wave at 30 Hz with an almost perfect waveform. The manual notes this test signal is far more difficult than the rated program capacity, and says the driver could sustain it indefinitely without failure.

Horn-Loaded Compression Driver and Slant-Plate Acoustic Lens

Above the 800 Hz crossover, a compression driver with a 0.002-inch duralumin diaphragm, pneumatically drawn to shape, takes over. Its voice coil is an edgewound aluminum ribbon measuring 1.75 inches in diameter, running in a 19,000 gauss field. A pure silver impedance-controlling ring counteracts voice coil inductance at high frequencies. That shows up as an extended, unstrained top end rather than a rolled-off horn sound.

The driver couples to a heavy cast aluminum exponential horn and a slant-plate acoustic lens, the 2391. The lens is built from 11 plates set at a 38° angle and spaced 0.25 inches center-to-center. It works like a divergent optical lens, spreading sound evenly across 120° laterally while restricting vertical dispersion to roughly 40°. That's why the 4320 could sit on a meter bridge and still cover a room without the beaming that plagues narrow-dispersion monitors.

Computer-Designed Dividing Network with Autotransformer Attenuation

The crossover, set at 800 Hz, is a parallel L-C type with additional reactive components to compensate for the complex impedance characteristics of the transducers, maintaining a 12 dB per octave slope in terms of actual acoustic output. A 3-position control lets the user balance high-frequency intensity to the room. Unusually, the attenuation circuit uses a tapped autotransformer rather than resistive pads, so the coupling between amplifier and high-frequency driver isn't degraded by where you set the control. Crossover components in the era included air-core inductors and metal-film resistors.

Brace-Built Enclosure with Ducted Ports

The cabinet is made of 3/4-inch stock with lock-mitered, wood-welded joints and heavily braced, padded interior surfaces to suppress panel resonance and internal reflections. There are no removable panels. Drivers mount directly from the front of the baffle board, which is a structural choice as much as a service one. A pair of ducted ports provides acoustic loading and increased efficiency in the 30-50 Hz region. Overall dimensions are no greater than 31 inches high by 24 inches wide by 21 inches deep, finished in oiled walnut or textured gray with a charcoal fabric grille. The system is nominally 16 ohms, with minimum impedance of 12.5 ohms at 175 Hz. Period documentation also lists the speaker as designed to operate from 8 to 16 ohms.

Historical Context

The 4320 was introduced in 1970, though at least one reference lists a 1971 release date, so the record is not entirely settled on the exact year. It arrived as multitrack recording was becoming standard, and facilities needed monitors that could hold up to long sessions without drifting or failing. The 4320 was part of JBL's 4300 series of studio monitors, which also included the 4310 and 4311. It became a fixture in studios across the U.S. and Europe, heard on countless recordings from the 1970s and early 1980s.

JBL's own advertising in Recording Engineer (June 1973, p.28) made the pitch directly to the trade, positioning the 4320 as the monitor of choice for control room, studio, and laboratory playback, and citing its extremely smooth frequency response and full 120° horizontal, 40° vertical coverage. The company ran similar copy in DB Magazine (July 1973, p.28). The 4320 also appeared in Studio Sound (May 1971, p.20) with a specification listing that matches JBL's own documentation: 60 watts RMS and 120 watts program, 800 Hz crossover, and a nominal impedance designed to operate from 8 to 16 ohms with minimum impedance around 12.5 ohms at 175 Hz. A later Studio Sound listing (December 1972, p.47) gave the 4320 a price of £117 in the UK market.

The 4320's success fed directly into JBL's expansion of the monitor line. One account notes that with the great success of the 4320 Studio Monitor, JBL management began exploring ways to extend their offerings in the same territory. That path led to the 4310 and, ultimately, the 4311 that became a home-audio icon. The 4320 was also a popular upgrade platform: owners of the era added the JBL crystal slot tweeter (077/2405) to convert a 4320 into a 4333, and examples with that modification still circulate.

Collectibility & Value

Working pairs of 4320s typically sell in the $400-$800 range depending on cosmetic condition and whether they've been serviced. Fully refurbished examples with reconed drivers and upgraded crossovers can command $1,000 or more. Asking prices on individual listings run considerably higher - one pair listed on eBay at $11,999, and a modified pair was advertised at €1,750 (about $2,027) - but those are outliers rather than the going rate. The 4320 is not as common as the smaller 4310 or 4311, which keeps supply tight and prices firmer than you'd expect for a speaker this large.

The usual vintage-monitor problems apply. The original foam or treated cloth surrounds on the woofer degrade over time, causing bass distortion or outright failure. Re-foaming or reconing is the standard restoration step, and many surviving examples have already had it done. The compression driver's diaphragm is generally tough, but prolonged exposure to high SPLs or clipped signals can damage it, and some owners replace aging units with modern equivalents. Crossover capacitors dry out and drift, and swapping in modern film capacitors is a common refresh. The MDF cabinet is solid, but older examples show wear on the veneer or loose joints, so refinishing and reinforcement are routine. Replacement parts and active community support make the 4320 a realistic restoration candidate rather than a display piece.