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Here the Fostex FE127E fullrange driver is used in a small vented (bass reflex) box. This speaker system is suited for small to medium sized room with low to medium powered amplification. The speakers are well suited for use with Valve (Tube) Amps as well as ChipAmps (Gainclones) or Class D / T Amplifiers. The FE127E driver has limited power handling (15W RMS) and a Xmax of only 0.67 mm, so it is not the speaker for thumping, but is suited to moderate levels which are still loud.

Used in the right location, wall loaded and in a smaller room, this speaker has very special sonics which can bring out spectacular mids. The enclosure design has a deliberate peak in the bass to add a bit more punch to the bass (F3 = 61 Hz) which is surprisingly good for the small size of the driver. There is no rule to say the response has to be flat and in this instance, sounded much better with the 1 dB peak.

The enclosure design has taken the golden ratio into consideration which helps with the final sonics. In addition to the cabinet bracing, small blocks of various sizes and shapes were glued to the inside panels (in random and non symmetrical locations) to further alter resonances and helped to use up the 1.07 L volume allowed for bracing.

12.5 L Bass Reflex Enclosure Plan for Fostex FE127E
Figure 1: 12.5 L Bass Reflex Enclosure Plan for Fostex FE127E

Fostex FE127E Cabinet Bracing
Photograph 1: Fostex FE127E Cabinet Bracing


The speaker design needs baffle step correction / compensation (BSC) as the top and upper mids were not tonally balanced. The following circuit was added to the speaker input terminal.

Fostex FE127E Baffle Step Compensation Circuit
Figure 2: Baffle Step Compensation Circuit

The above calculations are for inductor and resistor values to suit the FE127E driver (DCR of 6R9 on the sample used) and the baffle width. With wall loading (within 25 mm), 2 dB was found to be adequate, 4-6 dB was required in free space, but this speaker really needs wall loading. I found that R1=2R2 and L1=0.56 mH worked well (just over 2 dB BSC) and happened to be what I had on hand.

A bi-amp speaker terminal was used with one set to the BSC and the other directly to the speaker. This way I have the option of using the BSC or by-passing it.

Fostex FE127E Baffle Step Compensation Circuit
Photograph 2: Fostex FE127E Baffle Step Compensation Circuit

Finished Fostex FE127E Bass Reflex Bookshelf Speaker
Photograph 3: Finished Fostex FE127E Bass Reflex Bookshelf Speaker