Fostex FE208EΣ rear loaded horns

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Fostex FE208EΣ rear loaded horn loudspeakers

This was the start of my DIY hi fi journey.

Finished cabinet with T90A supertweeters. Record added for scale...

Background

Whilst enjoying the beautiful hills in the Lakes in 2002 I was inspired by Paul Messenger's writings on some commercially available horn loudpeakers using Lowther drivers, and was drawn to the positive comments about musicality and communication. Whilst Paul said they were not perfect they did possess a super communication.

I started to search the net and quickly came across James Melhuish's Single Driver Website. This was a revelation and I spent quite some time reading the posts on the old forum and studying the projects. It had never occurred to me that I could actually build my own speakers, AND get a decent result.

Lowther drivers seemed to attract strongly polarised opinions, a love-em or hate-em reaction. They weren't exactly cheap either, a pair of EX4s cost over £1100 at the time. This was a little too rich for me - I hadn't even heard a pair of horns yet and it seemed a rather large risk as they might not be for me. Reading further, the Americans rated the Fostex drivers, some even said they preferred them, but opinions expressed on t'Internet should be taken with a healthy dose of salt... But they were much more affordable. Fostex drivers weren't readily available in the UK at the time, but I got a pair of the new FE208EΣ on special order from a Fostex supplier for £320 the pair. Not being used to a drive unit of this expense the construction of the cast frame, size of the magnet and overall weight was reassuring.

Rear view of the FE208EΣ driver

Whizzer-less banana fibre cone

The supplied mounting gasket

Choice of speaker

I thought about design for some time, considering different flare types and horn lengths etc. I also considered Martin King's mass loaded transmission line enclosures but Martin felt the new drivers weren't sufficiently suitable and might give less than ideal results. In the end I decided to play it safe and build Fostex's recommended enclosure. Whilst not perhaps a true horn (more a series of coupled chambers) I figured that the good burghers at Fostex know more about speaker design than I do!

Preparation

So, looking at the recommended enclosure the first thing I did was convert the cutting list to a sheet size of 2400mm x 1200mm which is available in the UK. This worked out at four sheets for the two cabinets, which is a lot of 18mm thick birch ply. The best part of 10 stones, or 60kg, for each speaker! I weighed up using mdf but decided I only wanted to build them once so birch ply it was.

There was a real benefit to rejigging the cutting list. The Fostex plans show a number of panels which are joined: the back, base and sides. Using 2400mm x 1200mm sheets allowed me to use single pieces which I figured had to give better rigidity as well as looking nicer.

Cutting the sheets up at the local timber yard took about 10 minutes and saved no end of work. Definitely the way to go, provided the operator makes accurate cuts... The four sheets cost about £150.

Two rather large piles of birch ply waiting to be assembled...

Back in the shed assembly was pretty straightforward thanks to the square construction. Unlike many horns there are no varying width flares, which means the ends are square. This is supposed to help disperse any unwanted high and mid frequency sound that has entered the horn, so don't radius them. The square ends helped to keep things simple, and fortunately only minimal fitting of the panels was required as the cutting in the timber yard was pretty accurate. However, when working out my cutting list I did allow a few millimeters extra to some of the pieces to allow for trimming to size, particularly the side panels. This gave me the flexibility to make sure that the outside panels were a good flush fit.

Construction

Using the table saw the first job was to cut the numerous lengths of ply to the required lengths. Then the pieces were glued together in small sections using pva before assembling the small sections together. There were only two areas that required a little more thought. The first was the baffle for the driver. The second was the speaker binding posts, more of which later.

The hole for the drive unit marked out prior to cutting out with a jigsaw. The outer circle is for the mounting screws. Beware the filler piece to the top left - used to patch knots. I spent quite a bit of time checking all the pieces of ply to make sure that the impact of any of the filler pieces was minimised. Unfortunately they were quite numerous and their complete avoidance was not possible.

So, to start gluing up...

First the front baffle and base of the back chamber. I decided to drill pilot holes for the driver mounting screws all the way through the wood to ensure the screws would drive home. I'm glad I did as the screws are a tight fit even so.

Next the rear wall of the back chamber (face down on the bench) with the bracing to the first chamber sticking upright.

The rear panel. The piece sticking up forms the third and fourth chamber. Note the two holes for the binding posts. I decided I didn't want to use a binding post tray, rather keep the natural birch look. It seemed like a good idea at the time...

The internal vertical panel.

And then the base panel and bottom front (on the bench).

Glued up to one side panel. Note the circle cut from the outside panel for the binding posts. I did this for aesthetic reasons, and I think it looks very nice. 'Course, I'm stuffed if I ever want to change the binding posts or cable inside the cabinet... No, I wouldn't do it exactly the same again but I'd still wouldn't use a black plastic tray.

The cabinet construction is easy to see now. Internal wire is two lengths per channel of HU1 - silver plated ofc with ptfe dielectric from IPL. This is the same stuff I use for speaker cables. It's small so fits behind the carpet, and it sounds significantly better than the Linn K400 bi bire cable I used to use with the Keilidhs. I only had a puny 18W Antex soldering iron which just wasn't big enough to heat the binding post to get the solder flowing. Fortunately, as I was building the speakers round at my parents I was able to press my mum's cooking blowtorch into service, and without burning too much of the wood either!

First speaker almost complete except for the second skin to both sides, the bottom base panel, and proper cleaning up.

Finished. Note the over-zealous sanding at 5 o'clock. Fortunately most of this is covered by the drive unit. Phew!

Initial impressions

Well, the first thing to note is that these are not small loudspeakers. Okay, that may sound obvious but they certainly make a statement in a room. Birch ply isn't everyone's choice, though Mr. Ikea has made a healthy living selling forests of the stuff. But it has the advantage over mdf that it only needs a varnish finish to protect it. And I can live with.

Being a pretty impatient person, as soon as both cabinets were finished I moved the speakers in to my parents living room, mounted the drivers, connected my little Teac all-in-one job, and stuck a couple of test CDs on. The drive units were brand new and hadn't loosened up yet, and whilst this was quite audible, so was the open presentation and inherent "rightness" of the sound. My parents living room is a good size and I'm sure this helped the speakers to "breathe", but I was impressed straight away. Much better than the Keilidhs.

Setting them up at home

Lugging the speakers home was no mean feat at nearly 10 stones each. The speakers are positioned in the corners of my room, either side of a fireplace. There is little alternative, but fortunately they work pretty well here. I have them hard up against the corner, in theory giving better bass response, although the form of the speaker with the mouth in the middle of the baffle perhaps means the speaker doesn't get the full benefit of their location.

The recommended enclosure helpfully refers to "sound absorbents" which should be placed in the back chamber, inside the bottom of the speaker, and inside the bottom front baffle. Whilst I thought about what to use for sound absorbents I connected them up the Exposure amps to see what they sounded like. I was still impressed, though the presentation was a little different from my parents' house as my room is half the size and I listen much closer to the speakers at home. Still much better than the Keilidhs.

For the "sound absorbent" I got some Dacron-type fibrous pillow stuffing from the market for a couple of quid and filled the bottom of the speakers between the bracing ribs. But all this did was rob the life from the speaker, not at all good.

There was an effect on the bass too. By the time I was experimenting with stuffing the drivers had just over 15 hours use. I remember this because at this point I was listening to David Gray's White Ladder and nice deep bass started to come from the horn mouth. When I put the stuffing in the cabinets the newly found bass was severely curtailed. So I took the stuffing out and had a think.

I had some 2mm thick laminate floor underlay left over from the kitchen floor which I thought might be worth a try. I cut a number of strips and placed one thickness between each brace. As they were cut slightly ovesize they wedged in nicely without any need for fixing. This cleaned the sound up a touch and preserved the life and bass. A good result that I was happy enough with to live with and forget about.

I placed a piece of the same foam underlay on the back wall of the compression chamber but it didn't make much difference. I did, however, attach some self adhesive bitumen pads on the back of the magnet. Available from car parts places for a couple of quid a sheet, their principal use is for damping car panel resonances, but with the surface area of the driver's magnet being so large and the high pressures within the compression chamber they really helped damp vibrations reflecting back into the drivers. Although the pads are not easily removed once applied I think this was one of the biggest improvements I made, cleaning the sound up and revealing more detail.

And that was as much tweaking as I did initially. I sat back and enjoyed the music.

Listening impressions

After listening almost exclusively to box loudspeakers for many years the Fostex horns came as quite a revelation. I suddenly (and shockingly) realised how coloured the box loudspeakers I had heard were. Not just the Keildhs, but other well known speakers costing the thick end of £2000. What was so surprising was that I hadn't really noticed the boxy colouration of the speakers when I had heard them, just that the horns stripped it away with a wonderfully organic sound. Bass was fast as you would expect from a horn, the driver coupled directly to the air behind it, though not particularly deep or extended, although the size of my room means I'm unlikely to get very deep bass anyway.

Being a single driver there is no crossover. This has the obvious advantage of making the speakers more efficient, and I have come to the conclusion that more efficiency is generally a good thing (especially for SET valve amp enthusiasts). But I think the biggest advantage is having no components in the signal path - this seems to preserve the immediacy, detail and life of the speaker. And not spending money on any crossover components is an added bonus!

There are some downsides though. The FE208EΣ is maybe better described as a "wide band" driver rather than "full range" as the treble rolls off from about 7kHz. I lived with the treble quite happily for a couple of years. It's amazing how much the brain can accommodate aural deficiencies, and my enjoyment of the speaker wasn't impaired at all during this time. Eventually though, as I heard other high quality speakers, notably Steve Shiels' Lowther EX4 horns and James D's Quasar open baffles with AER and Supravox drivers I became more aware of the missing treble and eventually added a supertweeter.

The other downside is that due to the large 8" driver the speakers beam, and there is a very narrow sweetspot for the best stereo imaging, literally a few millimeters makes a large difference. How much this might affect you probably depends on whether you can sit in the sweet spot between the two speakers. That said, I can quite happily sit in the next room and listen to the music as the sound is, to my ears anyway, fundamentally right. Instruments just sound like they should. It would take a very good box loudspeaker to tempt me back.

So, for under £500 these are a pair of speakers that sound better than anything I have heard commercially for £2000. Not perfect, but a nice balance of compromises that I can easily live with, and have done for nearly four years so far. And perhaps most significantly allowed me to start playing in the world of low powered single ended valve amplifiers...

Improvements

Well, I did end up adding a supertweeter, and as good as the speakers were the supertweeter improves them no end. See here.

Probably the best value improvement is time. Time allows the drive units to run in and the performance does improve, quite markedly in some respects. Bass can thunder witht he right material, given it's only an 8" wide band driver.

What has been interesting is that as other components in my system have improved the Fostex horns have responded. For instance, connecting my WE91 type 300B monoblocks revealed lots of bass that hadn't been there before. At a recent meet in a decent sized living room the wooden floor really shook due to the bass energy produced by the speakers. Scottmoose was sufficiently surprised to model the speaker using Martin King's 1/2 space back loaded horn mathcad sheet...

The red line shows the response has excellent sensitivity, nominally flat to 38Hz. There is quite a lot of ripple above 100Hz but this is not noticeable in practice: a combination of room acoustics and the internal square edges scattering the medium and high frequency waves in the cabinet helping here. Thanks Scott.

The longer I listen to these speakers the more capable they seem to be. It will take something very special to replace them.

 

 

 

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