Fonken in Semi-Trailer Truck - diyAudio
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Old 18th September 2008, 10:51 AM   #1
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Default Fonken in Semi-Trailer Truck

Overview
Overall, this project is way overkill for a loud semi-truck and I would not recommend trying to duplicate the results. This project is simply what an unemployed engineer does to fill the time and avoid looking for work. So read on if you would like some entertainment. Many thanks to Planet10 for making this design available. Please, by all means, humor me with comments!

Design Criteria
This mobile radio/CD player was designed to integrate within the cab of work truck for a daily road driver. The unusual design was necessitated due to the following:

1) No original factory mounts for door speakers or radio.
2) Company trucks cannot be modified permanently.
3) Drivers switch trucks every so often, so gear has to be portable and simple to set-up/breakdown.
4) Truck has a short wheel base and stiff, i.e. very rough suspension requiring a rugged build.
5) System must not attract undue attention, ugly is better.
6) Gear must be secured for safety reasons.

Ideas
The greatest challenge of this project was finding bookshelf size speakers that sounded good on low power at 4 ohms. Unfortunately most car speakers require more power than this and are designed to work infinite baffle in large sealed enclosures (doors 30 liters, parcel shelf 200 liters). The best idea I came up with for car speakers was some efficient Focal 6.5" two-way coax speakers in smaller than optimal aperiodic enclosures. I wish I could have experiment with this idea more, but resources were limited. I also tested a few commercial 2-way bookshelf speakers, but the decent ones were all 8 ohms and had a recommended amp power of 20 - 100 watts RMS. Ideal size would have been a one hand pick up and carry unit with something like used Bose 101 outdoor speakers (4 ohm full range 4.5 drivers in 6 x 9 x 6 enclosures). It would have been far easier to buy something like this and although it would have worked OK, I (not the user) wanted something unique and better. Therefore, I decided to build a set of Fonken speakers.

System
It was a challenge to make everything portable, rugged, and easy to use for someone very non-technical (my father). The head unit is from a 2006 Toyota Corolla junkyard car and rests in an all aluminum enclosure which doubles as a heat sink. I found some shock mounts at a local Ax-Man surplus store and could not resist using them since this truck destroyed a boom box within a week from vibration. They are surprisingly effective. All hardware is stainless steel and aluminum. The head unit assembly is modular, but is designed to rest on top of one of the speakers. Holes for the two x 20 threaded brass inserts do not go all the way through the cabinet so the bolts can be removed without an air leak. One speaker will be secured via rope to the rear cabin wall behind the stick shift within reaching distance of the driver. The other speaker sits where a passenger seat would normally be. The head unit supplies an estimated 7.5 - 10 watts RMS to the Fonkens at 8 ohms, presenting a very easy impedance load for the integrated chip amplifier. The enclosures were built according to Planet10 specs, but the driver being used is the FE126E which was originally for another project that did not get built. The center brace was inset 3+ mm more than was needed for the FE126, which was then filled with the right amount of cotton and felt for a tight fit on the back of the driver.

Results
The system has not been tested in a small truck cab yet, but sounds very good playing in a medium sized living room with the bass boost set to +3 and the speakers set up corner loaded. Without this, they sound thin and hollow below 200 hertz and can run out of excursion on some music so care is needed. However, low notes of the bass cello are clear and deep. Fortunately, for most music they can play way louder than I care to listen too and the midrange is incredible! I did not fully anticipate the work and full cost that goes into the Fonken cabinet. They are not a simple build given the double walls and center brace. I am not quite satisfied with the end result as I would like to hear them with the intended FE127E driver. The system is also larger than anticipated.

I will upload some pictures in following posts. There are plenty of good Fonken builds on this site so I am not attempting to duplicate that here.
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Old 18th September 2008, 10:57 AM   #2
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Baffle with 1/16" dado for center brace guide.
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Old 18th September 2008, 11:06 AM   #3
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Rear with rabbit joint was made to help with glue up alignment and clamping. The trick is to sneak up to the right fit for the rear panel with paper on the fence of table saw.
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Old 18th September 2008, 11:19 AM   #4
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Dry assembly with damping glued on using spray contact adhesive prior to glue up.
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Old 18th September 2008, 11:21 AM   #5
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Glue up was done in two stages for each speaker due to limited assembly time and number of clamps.
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Old 18th September 2008, 11:34 AM   #6
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Normally I like plain wood color, but these are designed to blend in with the black interior of the cabin. The finish is Krylon 3370 wrinkle finish, which seems to be some sort of one part aliphatic polyurethane. It worked OK but requires thick successive coats and heat lamps. Although the finish is extremely durable, it is only a few mils thick, has a heavy harmful VOC content, runs easily, and takes a few days to harden. If I could do it over again, I would have them professionally sprayed with Rhino lining or Line-X, which is a two part polyurethane with chemical catalyst and easily goes on as thick as 1/16" in one application. It is normally used in industrial coatings and truck beds. It is pretty much the same thing as JBL uses on their pro audio cabinets. Cost is like $7-10 per square foot though.
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Old 18th September 2008, 11:39 AM   #7
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Here is the full set up, including the 4 foot Firestik AM/FM antenna. The FM work great, but the AM works much better on my 95 Toyota. The AM tuner on this radio is terrible.
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Old 18th September 2008, 11:40 AM   #8
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Close up view.
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Old 18th September 2008, 11:42 AM   #9
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These shock mounts work great if matched with their optimal load.

http://www.lord.com/Portals/0/Vibrat...se/grommet.pdf
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Old 18th September 2008, 11:45 AM   #10
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The all aluminum enclosure doubles as a heat sink. Rivets were used as much as possible for simplicity and they do not vibrate loose as easily as machine threads. Five 8-32 bolts actually attach to the tapped heat sink of the radio. It was difficult to bend accurately without a brake, but not impossible with some clamps and pieces of scrap wood.
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