The JBL Professional Series 5231 (single channel) and 5232 (dual channel) are electronic frequency dividing networks designed for use in bi-amplified or tri-amplified studio monitor and sound reinforcement loudspeaker systems. These devices separate an audio signal into high and low frequency bands (or mid and ultra-high frequency bands in tri-amplification) prior to amplification, allowing individual power amplifiers to drive specific loudspeaker components more effectively. This approach aims to reduce distortion, offer greater flexibility in system configuration, and potentially lead to cost savings by eliminating the need for high-level passive crossover networks for each loudspeaker system.

Function Description:

The 5231 and 5232 receive a line-level program signal from an audio preamplifier, studio mixdown console, or portable mixer. They then divide this signal into distinct frequency bands. The outputs from the network feed the appropriate power amplifiers, which in turn drive their respective loudspeaker system components. The 5231 provides a single channel crossover, while the 5232 offers two separate channels with independent crossover action, suitable for stereo installations.

A key feature is the use of plug-in crossover cards to select the desired crossover frequency. These cards determine the 3-dB crossover point and implement a 12 dB per octave filter slope, with high and low frequency outputs attenuated by 3 dB at the crossover point. Standard crossover cards are available for common frequencies like 500 Hz and 800 Hz, as well as specific characteristics required for JBL 4330, 4332, 4340, and 4350 bi-amplified studio monitors. Blank cards (JBL Model 52-5120) are also available for users to construct custom crossover networks for other frequencies (e.g., 900 Hz, 1100 Hz, 1200 Hz, 1500 Hz, 2000 Hz, 2500 Hz, 5000 Hz, 6000 Hz, 7000 Hz, and 9500 Hz) using standard components (five identical resistors and five identical capacitors). The selected crossover frequency is indicated through a front panel window.

The front panel includes a power switch, a pilot light, and a high-frequency level control for each channel, allowing for easy adjustment of the high-frequency output level.

Important Technical Specifications:

Gain: 0 dB in the pass band.
Rated Output: 6.2 volts (+18 dB/Ref 0.775 V).
Distortion:
- Less than 0.5% THD, 20-20,000 Hz at rated output.
- Less than 0.2% THD, 20-20,000 Hz at +10 dB output.
Frequency Response: ±0.5 dB, 20-20,000 Hz equivalent bandwidth.
Crossover Frequency: Selectable by plug-in module, 3-dB crossover point ±10%.
Filter Slope: 12 dB per octave.
Input Impedance: Greater than 50,000 ohms.
Output Impedance: 2,000 ohms minimum.
Input Isolation: Less than 70 ohms.
Output Isolation: 70-2500 ohms (depending on setting of level control).
Channel Isolation: Greater than 60 dB.
Signal-to-Noise Ratio: Greater than 90 dB, 20-20,000 Hz.
Controls: One high frequency level per channel, one on/off switch.
Power Requirement: 5 Watts, 120 VAC, 50/60 Hz.
Operating Temperature: Up to 132°F (55°C).
Dimensions: 1¾ x 19 x 7⅝ inches deep (4.4 x 48.3 x 19.4 cm deep). EIA standard rack space.
Finish: Non-glare baked enamel, light gray.
Weight: 4 lbs (1.8 kg) either unit with card(s) installed.
Optional Crossover Cards:
- 52-5120 Blank
- 52-5123 800 Hz
- 52-5125 500 Hz
- 52-5127 7000 Hz (for JBL 4340 studio monitor)
- 52-5129 7500 Hz (for JBL 4350 studio monitor)
- 52-5130 500 Hz (for JBL 4330 studio monitor)
- 52-5131 800 Hz (for JBL 4332 studio monitor)

Usage Features:

Installation: The units are designed for easy installation in a single EIA standard rack space without additional bracing or ventilation. Mounting hardware is supplied. All connections are made on the rear panel via screw terminals. Shielded cables are required for all input and output signal connections. For cable lengths exceeding 15-20 feet between the crossover and power amplifier, isolation transformers or line amplifiers are recommended to prevent RF interference or hum.
Crossover Card Insertion: Before operation, a crossover card must be inserted into each channel. This involves placing the unit upside down, removing the bottom cover, aligning the card's three holes with mounting pins on the main printed circuit board, and gently pressing the card until electrical connection is made. The components on the card should face the chassis, with the frequency designation label oriented towards the front panel. Operating the unit without a crossover card will not cause damage.
System Wiring Verification: A critical step before operation is to verify correct wiring to prevent damage to midrange or high-frequency loudspeaker components. The procedure involves setting the high-frequency level control to "2" and program source level to minimum with all power off. Power amplifiers, if equipped with level controls, should be set to approximately one-quarter power. After turning on the program source, network, and amplifiers, gradually increasing the program source volume should produce bass from the low-frequency loudspeaker. If sound predominantly comes from midrange or high-frequency drivers, power should be immediately shut off, and wiring checked. Advancing the high-frequency level control should then simultaneously increase the treble component from midrange and high-frequency drivers.
Transducer Phasing: Proper transducer phasing is crucial for natural sound. Two methods are described:
- Objective Method: Using a real-time third-octave analyzer, condenser microphone, and pink noise source. Adjust system volume and individual transducer levels for the flattest overall frequency response. Reversing the polarity of the high-frequency driver should show a change in frequency response through the crossover region; the flattest response indicates proper phasing.
- Subjective Method: If test equipment is unavailable, use pink noise or inter-station FM tuner noise to balance the system. Listen to a male voice recording to become accustomed to the sound. Reverse the polarity of the high-frequency loudspeaker; a change in voice character will be noticeable. When properly phased, the male voice should sound natural and present, not "hollow." For crossover frequencies above 2000 Hz, reversing polarity may yield little perceptible difference; the "correct" polarity is that which provides the most natural quality across various program materials.
Level Control Adjustment: For multi-amplified systems, balance can be achieved objectively (using a real-time analyzer) or subjectively (using pink noise or FM inter-station noise). Each program channel should be adjusted individually from the normal listening position.
- Bi-Amplified Systems (5231 or 5232 channel): With the High Frequency Level control at "0," adjust program source for comfortable listening. Rotate the network control clockwise until a satisfactory high-frequency balance is achieved.
- Tri-Amplified Systems (both 5232 channels): Channel 1 level control typically regulates midrange and high-frequency drivers, while Channel 2 governs only the high-frequency driver. With both High Frequency Level controls at "0," adjust source level for comfortable listening. Rotate Channel 1 level control clockwise for satisfactory midrange. Increase Channel 2 level for satisfactory high-frequency balance. Readjustment of midrange, source, or amplifier levels may be necessary for overall balance.

Maintenance Features:

Warranty: JBL Professional Series electronic products are guaranteed against defects in material and workmanship for two years. JBL will replace defective parts and make necessary repairs if faulty workmanship or material is evident. The warranty does not cover damage from misuse, accident, or neglect. The warranty remains valid only if repairs are performed by JBL or under its authorized procedures, and if the serial number has not been defaced or removed.
Service Notes: The 5231 and 5232 utilize integrated circuitry. Proper operation can be verified by measuring DC output voltages for each integrated circuit, which should be 0, ±½ VDC. If this specification is not met, the integrated circuit is defective and should be replaced. A DC voltmeter with a high input impedance (10 megohms or greater) is recommended for these measurements. Servicing should be referred to qualified personnel.
Product Improvement: JBL continually researches product improvement, introducing new materials, production methods, and design refinements without notice. Any current JBL product may differ from its published description but is always warranted to equal or exceed original design specifications unless otherwise stated.

JBL 5231 - Electronic Schematic Diagram