The power output from these amps ranges from 2. 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 FEEn 8" fullrange driver.

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For their size and cost the Fostex FEEn double bass-reflex speakers really sound great. The speaker enclosure plans for the double bass-reflex cabinets were provided with the FEEn driver documentation which also included speaker cabinet plans for a large back-loaded horn speaker enclosure.

Being impressed with the great sound from the smaller double bass-reflex speaker boxes I decided I wanted to get horny and have a pair of the large back-loaded horn loudspeaker enclosures made up. Photograph 1: Fostex FEEn Back-Loaded Horn Loudspeaker Enclosure Back-Loaded Horn Type Speaker Enclosures The back-loaded or rear-loaded horn loudspeaker system is like bass-reflex and double bass-reflex speaker types in that the sound radiated from the back of speaker transducer is utilized to enhance the bass response.

The difference however is that back-loaded horn BLH speaker enclosures make among the most efficient use of the back wave from the speaker driver.

The back-loaded horn speaker enclosure operates by developing the bass response using the back wave from the speaker driver and an expanding horn section. The remainder of sound spectrum mid and high-frequency ranges is radiated directly from the front wave produced by the speaker driver. Back-loaded horn speaker enclosures are generally amongst the most efficient enclosure systems and respond well to the subtlest components of music signals.

For "tighter" sound reproduction, you can reduce the interior volume of the horn speaker cabinet using fill material. Sound absorbing material should be used directly behind the driver and also at the mouth of the horn to smooth out the frequency response.

Figure 1 below shows the speaker box plan from the datasheet. The magnet of the FEEn driver is extremely large so it is also covered with bitumanized aluminium foil tape Photograph 2 to help reduce sound reflections from the back of the speaker box.

Better yet would be to cover the magnet with a more sound absorbing material like felt or wool. Note that there are many tweaks and modifications that can be made to improve the performance from the Fostex FEEn drivers. For more tips and ideas see the Fostex FEEn fullrange speaker driver tweaks and modifications page.

The back of the drivers really has to be treated in some fashion due to the large size of the magnet surface and the shallowness of the first compression chamber in the horn cabinet.

Bituminised aluminium foil is layered on the magnet and and also wrapped around the spokes struts of the speaker basket. Felt is layered on top of this. In Photograph 4 you can see the complexity of design and construction of the horn speaker cabinets. There are thirty-six pieces of timber in each speaker box. Stainless screws, glue, dowelling and clamps were employed to assemble the horn speaker boxes.

The timber used for the speaker cabinets is 21 mm thick pine plywood. The front baffle board and bottom are doubled up in thickness. The plywood horn speaker boxes are largely resonance free Photograph 5. The speaker boxes arrived fully sanded so all I had to do was seal the surface of the timber. To do this I applied two coats of Australian beeswax to the outside of the horn loudspeaker cabinet. To the mouth of the horn I applied a heavy cedar oil.

Both brought the pine grain forward and made the boxes look quite attractive and smell delicious! Photograph 8 The horn speakers are quite heavy so carpet sliding feet were fitted to the bottom of the speaker cabinets so they can be easily moved around. Gold plated binding posts are used to connect the speakers at the back. Remove any excess slack from the connecting wire and the FEEn drivers are securely fastened to the horn speaker boxes using the supplied gasket. I only added dampening material to the compression chamber immediately behind the driver.

Originally I packed the compression chamber with Dacron poly fill. Later I realized I had stifled the bass a little. The mids though were stunning.

I finally settled on a single layer of Dacron directly behind the driver and a small amount at the bottom of the first chamber. The advantage here is that there will be no crossover distortion or suppression of the dynamics which tends to occur with almost all passive crossover circuits. Now the Fostex FEEn datasheet includes a frequency response plot of the back-loaded horn speaker system, shown below in Figure 3.

You will notice that above a response of about 2 kHz you begin to get "mid-range shout" from the single mm driver. This will often make the speakers sound bright and perhaps lean on the bass end. One way to try and compensate for this natural rising response of the FEEn is to position the speakers off-axis ie.

The amount of speaker toe-in required will vary dependent upon the listening room geometry, amplifier and personal preference. Begin experimenting with a speaker toe-in of about degrees and adjust from there to your liking. With the horn speakers can also try use the room boundaries to extend the mouth of the horn by pulling the speakers into wall corners.

You will want to take time experimenting to find the best speaker placement as proper placement of the horn loudspeakers will be rewarded with excellent acoustics with pin point accuracy. The BSC network tames the response from the mid-range at the baffle step frequency on up, without affecting 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 vocals. Furthermore, with the BSC circuit you also lose a little of the speakers sensitivity, which could be an issue with low power amplifiers. 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. I would say that the use of low frequency augmentation is required if you really like a solid bottom end as the horn speakers begin to roll-off steeply below about 60 Hz. The bass from the horn speakers is cleanly defined and very articulate but not overly extended.

The treble is good but once more not overly extended. The back-loaded horn speakers can draw an extremely accurate soundstage. This is due to the fact the all the music is coming from a single point and is not smearing the sound over multiple drivers. Like elctrostatic speakers, the voices really come to life. The mid-range performance from these horn loudspeakers is excellent. Excellent sounding they are also.

For me, with my Paris amplifier dual mono SE 6EM7 integrated with 4S preamp in triode mode, the combination of a high sensitivty speaker, razor sharp sound stage and extended and very clean mid-range, can lead to a whole new listening experience. And this is at a total system price that many can afford.


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