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Fostex FX120 DIY Transmission Line Speakers

 Martin Paul   German Flag   To email Martin, type out the email address.

Introduction - Fostex FX120 MLTL Speaker Project

My first introduction to fullrange speakers was about 4 years ago in a German diy audio magazine which featured a small transmission line speaker project using a Vifa 10 BGS 119/8 driver. I decided to give them a try and was surprised with the sound quality! Of course they did not play as loud as the bigger two-way speakers that I built before, but something else was fascinating. Fullrange drivers seem to play music with more coherence, more life and they sound more direct. However, there are compromises with fullrange drivers; little drivers have disadvantages at the low end and bigger ones at the top end.

Looking for more information about fullrange drivers, I found the Mathcad worksheets by M.J. King and decided to try a speaker design of my own. My first try was a speaker using the Fostex FE167E, but I never made proper cabinets for them because my wife said "No way, they are too big". Searching a smaller driver the Fostex FX120, a 12 cm fullrange unit caught my eye. There is not much information available for this driver, but the feedback from people who have used was positive. On the Fostex data sheet they show a flat frequency response without great peaks and drops.

Enclosure Design - Fostex FX120 T-Line Project

The FX120 will need a little help on the bottom end. The goal was to let them go down to about 50 Hz. With the Mathcad worksheets I designed a Mass Loaded - Tapered Quarter Wavelength Tube (ML-TQWT). Afterwards I contacted M.J. King and asked him to have a look at the simulation I had completed. With his feedback "I think you have a design" (I want to thank Mr. King for his kind help and advice) I made some test enclosures out of chipboard. Figure 1 shows the calculated frequency response for the FX120 for the enclosure shown in Figure 2.

Calculated Frequency Response - Fostex FX120 ML-TL

Figure 1: Calculated Frequency Response - Fostex FX120 ML-TQWT (T-Line)

Enclosure Plan - Fostex FX120 ML-TL

Figure 2: Enclosure Plan - Fostex FX120 ML-TQWT (Transmission Line)

At first they were a little bit harsh but after about 100 hours they smoothed out. I began to tune them by ear until I was satisfied with the music coming out of the speakers. I played with the length of the port and tested several values of inductors and resistors for the baffle step correction (BSC) circuit. The BSC is necessary, otherwise the sound isn't balanced.

Fostex FX120 ML-TL Baffle Step Compensation Circuit

Figure 3: Fostex FX120 ML-TQWT Baffle Step Compensation Circuit

Fostex FX120 ML-TQWT Enclosure, Cutting and Building Plans

Enclosure Construction - Fostex FX120 MLTL

The enclosures were made using 15 mm birch-multiplex, the front baffle and the top is 19 mm beech, and the inner plates are 16 mm MDF. Naturally you can use material of a different thickness, but be sure to adjust the dimensions in the plan.

Fostex FX120 ML-TQWT Enclosure Construction

Photograph 1: Fostex FX120 ML-TQWT Enclosure Construction

Fostex FX120 Baffle Camfer

Photograph 2: Fostex FX120 Baffle Chamfer

Before I closed the box with front and top there were four things to do. First I attached nylon strings between the sides to keep the damping material from falling to the bottom. Another thing is to put in the damping between the inner and back plate as you cannot do this through the driver cut-out once the cabinet is closed. The wiring must be in place and the hole in the back for connecting the wire with the BSC has to be sealed. Also, I placed some felt behind the driver and at the end of the line to avoid reflections. I donít know if it helps, I thought it couldnít be bad doing this.

Fostex FX120 ML-TL Baffle Step Compensation Circuit

Photograph 3: Fostex FX120 ML-TQWT Baffle Step Compensation Circuit

The cavity in the cabinet was filled with gravel to add some mass to the enclosure and prevent resonance. Be sure to seal the box carefully.

Fostex FX120 ML-TQWT Speaker Box

Photograph 4: Fostex FX120 ML-TQWT Speaker Box Internal

For the base-plate I found a granite plate at a stonecutter's store, he made them for a moderate price for me. The two panels out of aluminum on the front and top are anodized in black and covered with a shiny clear coat. The enclosures finish consisted of sanding 60, then 120 and then 180 grit sandpaper and four treatments "hard wax oil" that fires up the color of the wood and protects the surface.

Fostex FX120 Transmission Line Speakers

Photograph 5: Fostex FX120 Transmission Line Speakers

Conclusion - Fostex FX120 ML-TQWT Speaker Project

The Fostex FX120 speaker driver is well balanced, with a relaxed sound, detailed midrange and good highs (also very well assembled). The TQWT enclosure gets good low end from this little driver. Of course the bass doesn't blow you away, but 50 Hz is there and it fills my room with music. Small group and acoustic music sounds wonderful. When the recording is good (for example 'Songs from the last century' by George Michael and the recordings from Norah Jones) the artists seem to be standing in front of you. I was surprised with the performance of this driver. I have no measurement equipment, but what I hear seems to correspond with the simulation. In my opinion the FX120 is worth every cent you pay for it. High resolution photographs are available in the DIY Audio Projects Photo Gallery: Fostex FX120 ML-TQWT (Transmission Line) Speakers.

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