Fostex FE 208 Σ
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.
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
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.
Frequency response at 1m with 2.83 V input RED - on axis, BLUE - 15 º off axis, GREEN - 30 º off axis
The second, third and fourth harmonic curves measured at 2.83V input RMS.
Burst decay response, measured at 1m on axis
The impedance curves
In the Klang+Ton Jericho project a frequency response and impedance equalization network was provided. Here is the schematics:
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.
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 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.
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