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Fostex FX120 Fullrange Speaker Driver (Transducer)
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Fostex FX120 Fullrange Speaker Driver (Transducer)

The Fostex FX120 is a 125 mm (5") cone type full-range speaker driver. Consistent with most of the higher-end Fostex drivers, the FX120 is well built, performs well and sounds excellent. Unlike the lower cost Fostex drivers which use a stamped frame, the FX120 is constructed on a fancy cast frame. The cone is constructed out of KENAF paper which is a fiber composite and is suspended with a rubber surround. The dust cap is metallic. The magnet is large, weighing 330 grams. Of the Fostex fullrange line speakers, the FX120 has the greatest Xmax at 2 mm. The FX120 can be used in a wide variety of enclosures, but is best suited to bass reflex, Transmission Line and Quarter Wavelength enclosures.

Fostex FX120 Full-range Speaker

Photograph 1: Fostex FX120 Fullrange Speaker Driver

Thiele/Small (T/S) Parameters - Fostex FX120 Driver

The Thiele/Small Parameters of the Fostex FX120 driver have been measured using the Dayton Audio Test Systems (DATS). The T/S parameters from the small signal speaker testing as well as the parameters noted on the Fostex FX120 datasheet are summarized in Table 1. The T/S parameter measurements were made at a room temperature of about 20 degrees Celsius. For those who are looking to design a Zobel / Impedance Equalization Circuit for the FX120, in addition to the inductance measurements shown in Table 1, I have measured the voice coil inductance (Le) of the pair of drivers at 1 kHz. The inductance measured on the two drivers using a Sencore LC75 meter was 0.11 and 0.12 mH.

Table 1: Thiele/Small (T/S) Parameters - Fostex FX120 Fullrange Driver

 Parameter  Fostex Driver 1 Driver 2
Fs 70 Hz 70.7 Hz 72 Hz
Vas 8.2 L -- --
Qts 0.45 0.69 0.68
Qes 0.47 0.77 0.75
Qms 8.4 6.3 6.6
Re  7.3 Ohms   6.9 Ohms   6.9 Ohms 
 Le @ 10kHz  n/a 0.28 mH 0.29 mH
SPL 89 dB 87.6 dB 87.4 dB

Figure 1 shows the impedance sweep of the Fostex FX120 speaker driver produced using the Dayton Audio Test Systems (DATS).

Fostex FX120 Measured Impedance Response Plot

Figure 1: Fostex FX120 Measured Impedance Response Plot

The frequency and impedance response plot from the Fostex FX120 driver datasheet is show in Figure 2.

Fostex FX120 Frequency Response and Impedance

Figure 2: Fostex FX120 Frequency Response and Impedance (from Datasheet)

Datasheet and Enclosure Plans - Fostex FX120

Speaker Projects Links - Fostex FX120