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Fostex FE 208 Σ

Pair of Fostex FE-208 Sigma 8 inch Fullrange Drivers

This driver is something special to me. My first backloaded horn - the "Jericho" horn, published in the German DIY magazine Klang+Ton was based in the FE-208 Σ and I used it as a reference speaker system for many years.

The Difference Between FE-208 Sigma and FE-208 E Sigma

First - do not mistake this loudspeaker with the FE-208 EΣ, which has completely different design, as is shown in the picture above. In fact the only common thing is the same basked.

FE-208 Σ was introduced in 1992 and the production ended at late 2001. Its predecessor was Fostex FE-206 S and the successor was Fostex FE-206ES-R.

This is the manufacturer's datasheet Link To Foste FE-208 Sigma Manufacturer's Datasheet

My Measurements

Fostex FE-208 Sigma SPL measuerd in the anechoic chamber

Above is my first measured frequency response of FE-208 Σ in 1999 in the anechoic chamber of the Institute of Radio Electronics and Technologies (IRET) in Sofia. Sadly to say, the new owner of the building destroyed this facility as non-profitable..

Driver Properties Thiele - Small Parameters
Chassis material Vacuum-molded aluminum alloy   Parameter Measured Datasheet
Membrane material Banana Pulp Paper   Fs 45.7Hz 45 Hz
Magnet System Ferrite Re 6.8 Ω 6.7 Ω
Surround Impregnated Cloth Qms 2.76 4.03
Voice Coil Diameter 35 mm Qes 0.20 0.23
Nominal Impedance 8 Ω Qts 0.19 0.21
Sensitivity 96.5 dB/W/m   Vas 44 ltr 36.9 ltr
Rated Power 34 W   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.

Fostex FE-208 Sigma SPL Responce at 1m on axis and 15 30 deg off axis Fostex FE-208 Sigma SPL Responce 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

Fostex FE-208 Sigma Second, Third and Fourth Harmonic in 18 ltr sealed box Fostex FE-208 Sigma Second, Third and Fourth Harmonic in the Universal Horn

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

Fostex FE-208 Sigma Burst decay Responce at 1m in 19 ltr sealed box Fostex FE-208 Sigma Burst decay Responce at 1m in the Universal Horn

Burst decay response, measured at 1m on axis

Fostex FE-208 Sigma The impedance in free air and in 18 ltr seled box Fostex FE-208 Sigma The impedance in Universal Horn

The impedance curves

In the Klang+Ton Jericho project a frequency response and impedance equalization network was provided. Here is the schematics:

Equalization network provided by Klang+Ton magazine

The components have the following values:
R1=10 Ω
L1=1.0 mH
C1= 2.2 µF
This part of the network is tuned at 3400Hz where a measurement shows a pick in the midrange. In fact, during a decade, very few listeners of BLH with Fostex FE-208 Σ marked this slight  prevalence of vocals as a negative effect.
R2 = 10...33 Ω
C2 = 15 µF
This is the impedance equalization network, that makes the system more "tube friendly".  Actually it causes an audible lack of detail, regardless of the amplifier - tube or solid-state. 

Equalization network measurement of FE-208 Sigma as givenin the Klang+Ton Magazine Measurement of the correcting networks for Fostex FE-208 Sigma

Above are the equalization graphics as are given in the Klang+Ton magazine and the graphics I measured.
The red curve is without the crossover. The blue one is with a Klang+Ton recommended crossover.
The crossover I developed for listeners who prefer less midrange presence, is in green - with omitted R2 and C2.  R1 = 4.7 Ω L1 = 0.68 mH C1 = 2.2 µF

Impedance with Klang+Ton Crossover Impedance with Rutcho Crossover

Impedance graphics for both crossovers above, compared to the directly connected driver


FE-208 Σ will remain one of the finest fullrange speakers ever made. Placed in a proper backloaded horn, this driver performs with an excellent accuracy, linearity and micro detail. For best utilization I strongly recommend a SE tube amplifierwith a DHT. My favorite matched amplifier is the classic WE91 and its clones.

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