Fostex FE126E + Fountek NeoCD3                                                       GO TO DIY LOUDSPEAKER INTRO PAGE
Copyright 2007 Troels Gravesen


Fostex FE126E fullrange drivers.

Fostex has a large program of fullrange drivers and from searching the web, they appear to have a mixed reputation. Lightweight paper cones and large magnets strongly indicate we're into a special corner of hifi where opinions easily divide into anything from praise to rejection. At least it seems we can agree on one thing: These drivers can do things few other drivers can do. For better or for worse. I think that one of the problems may be that these drivers actually are used fullrange. Drivers in this category often have a 5-10 dB rise in response towards higher frequences and the use of a whizzer cone on some of the drivers may produce a harsh beaming treble.

I've seen the FE126E driver used in huge horn-loaded cabs - and I wonder how it sounds. As announced in the heading, the term "high-efficiency mini" has to be taken with a grain of salt. Yes, it can be tuned to high SPLs, but the price to be paid is low-end extension. Subjectively it has a nice bass presentation, but it doesn't go deep. However, placed on a bookshelf, some room gain is achieved a more balanced presentation is possible. The apparent 93-94 dB sensitivity cannot be achieved in a small vented construction. It would be much too bright. Thus the overall response is tuned for a ~89 dB sensitivity, but this is not bad at all for a mini. Many similar constructions may only reach 80 dB/2.8V/1m.

As for the W11XT mini, I have to warn that 64 square centimetres of membrane, do not - and should not - move a lot of air. This is not a party speaker, but placed 1 metre apart on stands for optimum three-dimensionality, it delivers details in spades and some enjoyable moments can be expected. I like the way it handles vocals and most importantly, piano. The FE126E has a crisp, speedy presentation with no ringing and from the tuning shown below it has lots a presence and "let-me-shown-you-what's-on-this-record". If you need more bass, ad a sub at 80-100 Hz and possibly a 1st order high-pass filter for the Fostex, 200 uF or more. I'll try this later.
This construction should also fit well into the small reflex/horn hybrid described in the Fostex paper, which you can download below.

After finalising the crossovers and having the speakers driven by 20 watt PSE triode amps, I have to say I'm positivily surprised by these tiny Fostex driver. The speaker handles female vocals very well. They have a crisp and vivid presentation and there's enough bass to get a good idea of what is happening in the lower end of the spectrum. Don't play them excessively loud, because they can't. It's only 65 cm^2 membrane area.


Fountek NeoCD3 ribbon

PartsExpress cabs

Fostex FE126E + Fountek NeoCD3 ribbon tweeter in 7.5 litres
PartsExpress cabs.

Driver placement on baffle

Port = 35 (ID) x 55 mm, or
40 mm (ID) x 75 mm, 45 mm (ID) x 100 mm, 50 mm (ID) x 120 mm.

Do not forget to chamfer well around the FE126 driver hole.
Due to the large magnet, it needs space to breathe.

Fostex FE126E data sheet
Fostex FE126E cabinet suggestions
PartsExpress cabinets

Box simulation

I've tried cross-calculation on the Fostex data (right) and something juts doesn't fit.
Cross-calculation on own data looks fine, but Mms is significantly higher and Qt fortunately much higher
making a vented cabinet more appropriate.

Selected TS data.

Box simulation in LspCAD, upper from Fostex data in suggested 10.5 litres cab, lower from own data in 7.5 litres cab.

The FE126 driver

Fostex frequency response and impedance profile.

Now, comparing manufacturer's frequency response data with real-world measurements is always fun - or sad - because quite often we're presented a lot of dreamwork and it takes some decent smoothing to create the graph from Fostex. Well, from Fs to 5 kHz it really does look good, but the peak at 6.7 kHz?? A nice little bump on the Fostex graph and some 7-8 dB sharp peak on the CLIO graph. I would have been highly surprised if the FE126E had looked as good as claimed because we're so used to manipulated marketing frequency response graphs. Listening to the FE126E fullrange, this peak is clearly audible and creates at lot of sibilance on vocal recordings. The smooth decline from 10-20 kHz appear to be a sharp dip starting around 16 kHz.
Sensitivity is around 93-94 dB in mid to upper midrange, making this driver suitable for high-efficiency 3-way systems with target efficiency around 96 dB/1W/1m.

The crossover

No big surprises in the crossover layout. Point of crossover is a little above 3.5 kHz making ideal working conditions for the ribbon tweeter. A midrange notch filter is needed to smooth the 700-1000 Hz range and the bass series coil is paralleled by an RC circuit further reducing the 7 kHz peak.

Crossover Kit

For crossover kit, please contact Jantzen Audio at

All technical questions at

A 80 x 240 mm board will take all the components and fit into the rear part
of the PE cabinets, that is if.......

- you cut the bracing. No big deal. Can be glued in place after fixing
the crossover board to the side panel.

Here we have the crossover board well away from the driver magnets.
I add two rubber strips under the board and fasten it with two screws.


Left: Impact on frequency response of mid LCR circuit (red and blue) and the RC circuit in parallel with the bass series coil (yellow).
Right: Individual and summed response of drivers through crossover and response from inverting tweeter (yellow).

Left: Impedance of final system, minimum impedance = 4.5 ohms at 400 Hz. Right: Frequency response again from 2.8V/1 meter (red) and minimum phase (blue).

Left: Frequency response again and port response (blue). Right: Step response showing almost time-coherent performance and a smooth decline from the FE126E driver. This looks really good.

Left: The response from left and right speaker after installing the final crossovers. The minor peak at 5 kHz is the leftover from the FE126E peak, damped by the 0.33 uF + 33R circuit in the low-pass section.
CSD of final set-up.

No need to flush mount the Fostex driver contrary to the ribbon tweeter that certainly must be to perform optimally.