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Fostex FP 203

Pair of Fostex FP-203 8 inch Fullrange Drivers

The Fostex FP series is unique with its large voice coil diameter, in fact the largest one of all known eight-inch full-range drivers . It looks a bit unusual with its distinctive whizzer cone made of a special paper with thin carbon fibers. In Europe it was branded ACR.


This is the manufacturer's datasheet Link To Fostex FP-203 Manufacturer's Datasheet

Fostex FP-203 Label Early version Fostex FP-203 label later version

The early production has rated power of 15W, whilst the later drivers were labeled 35W. In fact both versions are completely the same.  

My Measurements

Driver Properties Thiele - Small Parameters
Chassis material Vacuum-molded aluminum alloy   Parameter Measured Datasheet
Membrane material Paper   Fs 37.8Hz 40 ± 8 Hz
Magnet System Ferrite Re 7.4 Ω N/A
Surround Impregnated Cloth Qms 4.6 N/A
Voice Coil Diameter 42 mm Qes 0.22 N/A
Nominal Impedance 8 Ω Qts 0.21 0.30
Sensitivity 95 dB/W/m   Vas 61 ltr 40 ltr
Rated Power 15/35W   Le 0.24 mH N/A


This driver is suitable for back-loaded horn applications.
That's why I included a measurements in so-called "Universal" Horn.
More details about this project can be found here.

SPL Responce of Fostex FP-203 at 1m on axis and 15 30 deg off axis SPL Responce of Fostex FP-203 at 1m on axis in the Universal Horn

Frequency response at 1m with 2.83 V input RED - on axis, BLUE - 15 º off axis, GREEN - 30 º off axis
This driver has flattest membrane of all the speakers in the measured BLH group and it is no surprise that it has the best off-axis response.

Second, Third and Fourth Harmonic of Fostex FP-203 in 18 ltr sealed box Second, Third and Fourth Harmonic of Fostex FP-203 in the Universal Horn

The second, third and fourth harmonic curves measured at 2.83V input RMS.

Burst decay Responce of Fostex FP-203 at 1m in 19 ltr sealed box Burst decay Responce of Fostex FP-203 at 1m in the Universal Horn

Burst decay response, measured at 1m on axis

The impedance of Fostex FP-203 in free air and in 18 ltr seled box The impedance of Fostex FP-203 in Universal Horn

The impedance curves


Fostex FP-203 is a driver for audiophiles, who love bright and in the same time non-intrusive midrange performance. Some audiophiles describe its sound as noble and delicate. It is suitable not only for back-loaded horns, but also for BVR and even bass-reflex constructions. It is a good idea to support FP-203 with a supertweeter like Fostex FT-96 or similar. This driver is well compatible with most of SE tube amplifiers.

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