Fostex FE206En in Double Bass-Reflex Speaker Enclosure
Flea power valve amps
Those who know me know that I love my flea power single-ended (SE) valve amplifiers. The three valve amps that are in my current rotation are The Silver Dragon - 300B SET, Luciano - 807 SE UL and Paris - SE 6EM7 fully integrated amp. The power output from these amps ranges from 2.5W to 5.8W. With so little available power it is easy to understand why high-sensitivity loudspeakers are required. I have had a pair of Fostex single driver back loaded horns for a few years now. The horn speakers use the Fostex FE206En (8") fullrange driver. The FE206En driver has a rated sensitivity of 96dB/1W/1m. The back loaded horn speakers are large and (according to my wife) ugly speakers. But when it comes to flea power amps they can make "boogie" real loud.
A family friend had built me the back loaded horns and at the same time a pair double bass-reflex boxes also for the Fostex FE206En driver. The horn speakers I kept for myself but the double bass-reflex speakers went to my son's house along with a two-stage KT88 SE UL amp that I built. Kit, my son, had the FE206En double bass-reflex speakers in the corner of the room on a wood floor. The bass from these speakers was always very impressive. So much so that I had to have a pair for myself.
Photograph 1: First pair Fostex FE206En Double Bass Reflex Speakers
Double Bass-Reflex Type Speaker Enclosures
The double bass-reflex (DBR) speaker system is a variation of the standard bass-reflex (BR) enclosure that is intended to achieve further extension of the bass response. The augmentation to the bass output is achieved by the use an additional chamber in the bass-reflex process. Other advantages of the double bass-reflex speaker compared to a conventional bass-reflex system are a reduction of the cone excursion around F3 and a corresponding reduction distortion. The use of an additional chamber in the speaker enclosure also reduces the likelihood of resonances.
The interior chambers of the double bass-reflex speaker box is typically lined with a thick damping material and/or stuffed with loose synthetic or wool fill. The fill material is used to dampen the rear wave from the driver and minimize standing waves and reflections within the speaker enclosure.
Fostex FE206En Double Bass-Reflex Speaker Box Plan
The double bass-reflex speaker enclosure plan is from the datasheet that was included with the Fostex FE206En driver. Figure 1 below shows the speaker box plan from the datasheet.
Figure 1: Fostex FE206En Double Bass-Reflex Enclosure Plan
Fostex FE206En Fullrange Speaker Drivers
The photos below show the Fostex FE206En fullrange speaker drivers. I have added bituminous aluminium foil to dampen the driver frame. The extremely large magnet of the driver is also covered with bitumanized heavy aluminium foil to reduce sound reflections from the back of the box. There are many tweaks and modifications that can be done to improve the drivers. For more ideas, see the Fostex FE206E / FE206En Fullrange Driver Modifications and Tweaks page.
Photograph 2: Fostex FE206En Fullrange Speaker Drivers
Photograph 3: Damping the Frame of Fostex FE206En Fullrange Drivers
More than one skill
Anyone who has tried a DIY audio project knows that you generally need to know more than how to just read a schematic and use a soldering iron. The skills required can vary from simple drilling holes to complex carpentry. When it comes to carpentry I leave that to those who are better skilled than I. But I don't shy from timber or metal enclosure preparation and finishing.
Photograph 4: Unfinished Fostex FE206En Double Bass-Reflex Speakers Cabinets
Luckily when the double bass-reflex speaker boxes arrived they had already been sanded down and that meant I only had to finish the surface preparation with a pass of fine sand paper before applying a coat of primer. I wanted a dull stipple finish so reflections from the big screen TV from the sides of the speaker boxes would be minimized. To achieve this I used a cheap paint-roller and after going once around the box with a white undercoating paint, I went around three more times with a semi-dry roller to really tease-up that stipple finish. This process worked out very well.
After two hours of drying time I applied the top coat to the speaker boxes. This was a satin finish in black. Not quite black more of a dark charcoal. Once more I went back over each surface three or more times using a semi-dry roller. The stipple finish, which had started with the undercoat, was now built on with the top coat. That was the finished I had envisaged, dull and heavily stippled.
Photograph 5: Painted Fostex FE206En Double Bass Reflex Speakers Cabinets
Assembly - Fostex FE206En Doouble Bass-Reflex Speakers
The top chamber of the double bass-reflex speaker boxes is lined (but not stuffed) with Dacron poly fill. The bottom chamber of the speakers remains empty.
Photograph 6: Speaker Damping Materials FE206En Double Bass-Reflex Speakers Cabinets
Gold plated binding posts are used to connect the speakers at the back. Two strands of wire wrap wire (fine 30 AWG silver plated copper wire) is use for both positive and negative of the speakers. A special pair of single strand wire wrap wire speaker leads was made for the speakers knowing the amp of 2.5W would require no heavier wire.
Photograph 7: Wire Wrap Wire and Speaker Binding Posts
In the final assembly steps, most of the slack from the connecting wire (double strains of lightly twisted wire wrap wire) was removed and the drive screwed securely onto the boxes with supplied gasket. The fine metal grills are held on with four rare earth button magnets and are easy to pop on and off.
Photograph 8: Fostex FE206En Double Bass Reflex Speakers
Photograph 9: Fostex FE206En Double Bass Reflex Speakers
No speaker crossover, baffling?
The Fostex FE206En drivers are "fullrange" types and their main appeals is that no crossover is required. The advantage is that there is no crossover distortion or suppression of the dynamics which occurs with almost all passive crossover circuits. Now some builders may opt to use a use a baffle step correction (BSC) network. You can use the online calculator to determine the value of choke and resistor required based on the width of the speaker baffle and impedance of the speaker. The circuit is simple, just one inductor and one resistor in parallel with each other and in series with the speaker. Figure 2 shows typical values for use with this double bass-reflex speaker enclosure.
Figure 2: Baffle Step Compensation Circuit - Fostex FE206En Double Bass-Reflex Speaker
What the BSC network does is to tame from the mid-range on up, without affect the lower bass response. Where these drivers can sound overly forward (too hot in the mid-range) this adjusts that forwardness leaving the bass and treble better balanced. I find that the mid-range richness is why these drivers can sound good with acoustic instruments and especially vocal. Now many may find that the BSC circuit can reduce or suppress the dynamics and for that reason I don't really like to use them. With the circuit you also lose a little of the speakers sensitivity, which could be an issue with low power amps. Another way to try and compensate for the natural rising response of the FE206En is to position the speakers off-axis (ie. toe-in the speakers). The amount of speaker toe-in will vary dependent on the listening room, amplifier and personal preference. Experiment by starting with a speaker toe-in of about 30-degrees and adjust for best results. You can also try to wall-load the speakers but pulling them closer to the rear wall to try lift the low-end. You will want to take time to find the best sepaker placement as proper placement will be rewarded with excellent acoustics.
For those who will ask, the impedance equalization circuit (Zobel network) for the Fostex FE206En driver shown Figure 3 can be used.
Figure 3: Zobel Circuit - Fostex FE206En Double Bass-Reflex Speaker
And the sound?
The drivers sound excellent. I don't have a microphone setup but using the Stereophile test CD, bass starts to roll off very slowly from about 100hz until you reach about 50Hz where there is a definite rise and a there is an even bigger rise at 40Hz. That seems consistent with what is described on the enclosure plan, "Internal volume of 45 liters tuned to approximately 50Hz (Fb). Low frequency response from around 120Hz is gently damped with a controlled peak at 50-60Hz." This was quite a surprise! Putting your ear to the port with a 40Hz tone you can hear air rushing in and out. The whole box sounds like it is working real hard. Because of the big bass lift offered by the double bass-reflex type enclosure, test tones at 40Hz, 31Hz, 25Hz and even 20Hz are quite audible. 20Hz is finished in terms of volume but as stated, still quite audible and surprising from a bass-shy speaker driver. While the rise at 40Hz does not make itself present in general music listening, it does add drama when listening to movies which is the primary role these speakers are intended to fill. Because these seapkers are so fast and dynamic it really is a challenge to find a subwoofer that can keep up.
All-in-all, this has been a very pleasing project and to get the stippled finish I wanted with a $5 roller and tray is doubly pleasing. With only 2.5W or so on tap and the speakers at a full 96 dB / 1W /1m, there is plenty of volume. Not only do these speakers round-out the 2 channel TV end of the house but it also gives me an excellent third music listening area with quality DIY gear (don't tell the wife). Now I'm triply happy. See the Fostex FE206En in dual chamber box (double bass reflex) thread for more information or to ask any questions.
Mark Houston, Classic Convenor, Melbourne Audio Club.
UPDATE - 29 October 2012
I made up the BSC circuit using a non-inductive 2.2 ohm 5W resistor and 0.82 mH air coil inductor (a little over 2 dB of attenuation). To evaluate I listened to the speakers with and without the BSC circuit, with and without the grill coverss and all possible combinations in between. The test music was a Steinway legends/digipack which contains heavy solo piano with massive dynamics and a thundering sound. The best combination turned out to be with the BSC circuit and no grills covering the drivers. The Ashkenazy/Steinway digipack sounds its very best when played through the Oppo BDP-93 and Paris (6EM7) amplifier. On my main system which consists of a Oppo BDP-95, Silver Dragon (300B SET) and large 3-way Osborn loudspeakers the dynamics are no where near as "startling" nor the sound as rich and natural (even raw).
An interesting observation with the Fostex FE206En in Double Bass-Reflex Speakes is that there appears to be a volume related sweet-spot. Play the volume too softy and the speakers can sound quite plain and flat - play them too loudly and they start to sound compressed and the bass lacks detail while the mid-range sounds shoutty. But if you find just the right volume the spekaers can really sing.
Fostex FE206En Links
- Fostex FE206En Fullrange Speaker Driver (Transducer)
- Fostex FE206E / FE206En Fullrange Driver Modifications and Tweaks