JBL Speaker System

JBL Speaker Units A man who has an audio hobby and loves jazz music usually wants to buy JBL speaker. But it is too expensive even though it is used one. Therefore I decided to get JBL speaker units by net auction and to make their enclosure. The speaker system shall be a combination of a woofer of 38cm (15-inch) and a compression driver with horn. After one month's auction battle, I got a pair of four kinds of JBL speaker units as shown in the photo. All units were manufactured 25 years before.
Woofer: 2231H　38cm　8 Ohms　12,000Gauss　Weight 10.1kg　25〜2,000Hz　Sound pressure 94dB/W/m　New edge
Driver: 2421A　one inch　8 Ohms　15,000Gauss　Weight 5kg　800〜20,000Hz　Sound pressure 110dB/W/m
Horn: 2345　Horizontal directivity 90 Degrees　Virtical directivity 40 Degrees　Weight 6.6kg
Network: 3110　Max. input power 100W　Low range 8〜16 Ohms　High range 16 Ohms　Cross-over frequency 800 Hz

If you put the mouse on the image, it will be enlarged. (Same below)

Characteristic of 3110 I measured the frequency response characteristic of 3110 network. The cross-over frequency seems to be 750 Hz. The 3110 network has an attenuator switch for high frequency range, Max (4dB), Med (6dB) and Low (9dB).


Wooden Material These are wooden materials of the enclosure. The dimension of main wood panel is 91cm x 45cm x 1.8cm. Wood panel for the bass reflection duct is 91cm x 20cm x 1cm.


Hole Opening This is hole opening scene for the woofer with 35cm diameter. Since I have no money to buy a zig-saw, it is manual labor. The rectangler hole (20cm x 8cm) for bass reflection is made similarly.


Assembling of Enclosure It is adhesion of the main part by right-angled tools. Its strength seems to be satisfactory although it was a method of not using nails.

Assembling of Duct It is adhesion of the duct part by right-angled tools. The dimension of duct is 20cm (W) x 8cm (H) x 15cm (D). The resonance frequency is expected to be 35 Hz from the calculation.

Oil-Finished Painting This is enclosure after oil-finished painting.

Absorption Material This photo shows glass-wool for the sound absorption.


Front Grill This is the front grill with navy color to cover the woofer.


Fixing Plate This is the fixing plate of the driver with horn. Its dimension is 35cm x 30cm x 2cm.


Completed Speaker The volume of enclosure is about 150 liters. I got red JBL emblem by net auction, but it was too small.


JBL System Overview The sound of JBL speaker system with compression driver type is very powerful and splendid as I expected.


Measuring Microphone This is measuring scene with a hand-made microphone. The frequency response of the microphone will be 30 Hz to 15 kHz.


Overall Frequency Response This is the overall frequency response of JBL speaker system with 3110 network. The cross-over part at about 800 Hz seems to be smooth.


Individual Frequency Response I measured the frequency response of individual speaker unit, woofer and driver. The cross-over frequency will be about 790 Hz. The part of less than 200 Hz is not the characteristic of driver, but it is background noise derived from PC fan and hum noises.


Duct Frequency Response This is the frequency response of the bass reflection duct. The resonance frequency seems to be 40 Hz.


Standing Wave Response This is the frequency response of inside enclosure. There is no outstanding peak by standing waves.


Impedance Curve This is the impedeance characteristic of JBL speaker system with 3110 network. The minimum impedance in the alternate frequency is read as 8 Ohms. The DC resistance is 7.6 Ohms. The resonance frequency is read as 30 Hz.


Subsequent reconstruction: Improvement of the high region characteristic by tweeter addition

FT90H Although my ear could seldom be heard the sound of 10kHz or more with aging, I thought that the speaker system would be improved if the tweeter is added. I got two pieces of FT90H of Fostex through the online auction. Although I wanted JBL tweeters in fact, since I need much money, I gave it up. The major characteristics of FT90H are as follows:

Maximum input power：50W
Impedance：8 ohms
Sound pressure：106dB/W/m
Frequency response：5kHz to 35kHz


NW circuit This is the circuit diagram of the 3-Way system. Since the efficiency of FT90H is close to the driver, the HPF by L and C was only connected to the driver for simplification. C is 3.3 μF and L is 300 μH, thus the crossover frequency is about 5kHz on calculation.


NW elements These are the network parts for super-poor men. It is the coil made from the enameled wire of 0.4mm in diameter, and 10m in length which I bought. The number of turns was well adjusted so that the series resonance frequency might become to 5kHz and it bundled with thread.


NW frequency response This is the frequency response of LC network for tweeter under 8-ohm load. The minimum insertion loss is about 1dB at 10kHz. The graph is drawn considering 10kHz loss as 0dB. The insertion loss at 5kHz is about 3dB just as designed. The inclination of the curve also serves as about 12dB/octave.


Wood base This is the wood base of the tweeter I made and painted with vanish.


3-Way system This is new 3-way speaker system after the addition of tweeter.


Total characteristic of 3way This is the total frequency response of 3-Way speaker system. It is clear that high frequency region is improved. Also me in whom hearing ability declined, it turned out that the range of sound became wide.