Fostex FE 208 Σ
![Pair of Fostex FE-208 Sigma 8 inch Fullrange Drivers](images/Fostex_FE_208Sigma_FS.jpg)
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](images/fostex_fe_208_sigma_vs_fe_208_esigma.jpg)
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
My Measurements
![Fostex FE-208 Sigma SPL measuerd in the anechoic chamber](images/Fostex_FE_208_Sigma_BK.jpg)
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](images/Fostex_FE_208_Sigma_0-15-30.png)
![Fostex FE-208 Sigma SPL Responce at 1m on axis in the Universal Horn](images/Fostex_FE_208_Sigma_UNI_0_15_30.png)
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](images/Fostex_FE-208_Sigma_THD.png)
![Fostex FE-208 Sigma Second, Third and Fourth Harmonic in the Universal Horn](images/Fostex_FE-208_Sigma_UNI_THD.png)
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](images/Fostex_FE_208_Sigma_BD.png)
![Fostex FE-208 Sigma Burst decay Responce at 1m in the Universal Horn](images/Fostex_FE_208_Sigma_UNI_BD.png)
Burst decay response, measured at 1m on axis
![Fostex FE-208 Sigma The impedance in free air and in 18 ltr seled box](images/Fostex_FE_208_Sigma_Z.png)
![Fostex FE-208 Sigma The impedance in Universal Horn](images/Fostex_FE_208_Sigma_UNI_Z.png)
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](images/Klang_Ton_Xover_Crossover_Schematic.png)
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](images/Klang_Ton_Xover_Measurement.jpg)
![Measurement of the correcting networks for Fostex FE-208 Sigma](images/Fostex_FE_208_Sigma_Xover.png)
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](images/Fostex_FE_208_Sigma_Z_Jericho_Xover.png)
![Impedance with Rutcho Crossover](images/Fostex_FE_208_Sigma_Z_Rutcho_Xover.png)
Impedance graphics for both crossovers above, compared to the directly connected driver
Conclusion
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.