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a.k.a... The Big Fun Box

[Italian version]

Product: TNT BFB - fullrange floorstanding loudpeaker
Producer: not for sale, TNT-Audio DIY design
Adopted drivers: Fostex FE 204
Drivers made by: Fostex - Distributed worldwide
Approx.cost: 300 Euro/$ or less
Author: Geoff Husband - TNT France
Reviewed/built: January, 2003


[Fostex FE 204]

Often people ask me "what is it that real Hi-Fi speakers do that mass market designs don't do?" It's a tough question and there are many answers, but for me one fundamental stands out - the ability to reproduce realistic dynamic swings and to do so at high levels - for example realistic volume for a Saxophone.
In my experience you can get some quite impressive 3-D imaging from a 150e bookshelf speaker, and deep bass is available cheap, though not bass quality. Ditto flat frequency response should be within the reach of any budget two-way. But take such a design and put it into a big room and turn up the wick and hear it compress, get flustered, distort and generally fall apart at the seams - ultimately this trick needs decent drivers and a big box. p align=justify> Which of course means big money, and so here we are once more plunging towards the high-end, except this time we're going to do it all for under 300 Euro/$...

And yes the only way is to build it - NOW HOLD ON! All you DIY virgins who have never considered building a pair of speakers just bare with me because this is the easiest speaker project you will ever see on TNT and the results, if you are used to a budget miniature, will blow you away.

Hopefully the curious will still be with me so here goes. With the 'Big Fun Box' I wanted something cheap that would do dynamics and volume, and be perfect for a first speaker project. That meant is had to be simple, I also wanted it to be easily tuned for a novice, a tough brief.

What set me off were my current speakers, the Loth-X Polaris horns. At 8000e they are firmly high end and their horn is very complex to build, but they sure do dynamics! OK this is partly down to the horn, but also the very light, full-range driver, with it's powerful magnet, stops and starts like nothing else.
The problem is that the driver isn't available separately and it's Lowther equivelent costs over 1000e each. But the idea of a single full-range driver had another appeal - it needs no crossover so in one step we have removed the cost and construction difficulties of the crossover. There are a lot of people out there who say that running crossoverless is the only way to audio nirvana, avoiding the phase distortions etc that a crossover introduces.

So I trawled the net for full range drivers and how to use them. It turned out that quite a few are quite happy working in a ported box, so my desire for a simple tuneable box was achieved.

With dynamics as a high priority the box would have to be big, and the driver of sufficient size to shift air. At the same time I wanted decent high frequency extension so the classic 8" driver size seemed to fit the bill. I tried two, Audax make a cheapie. It sounded dreadful. Fostex make a much nicer one, the 204, or better still the new 206 at about 85/110e respectively. That's quite an expensive driver, what you'd expect in a pair of 1000e+ speakers.
The dual cone is in classy fine pulped paper, the magnet massive. It works fine in a ported box, and in such you can expect an efficiency of about 95 dbl, good enough to go loud on 5 watts. Fostex make bucket loads of them for the pro market so they're good value.

The Box

I said this would be easy.

Make a 45 litre Box. Cut a hole for the driver, it doesn't need to be neat, a jigsaw will do. Cut a 3" (80mm) hole (cheap hole saw) and shove a length of 3" plastic pipe in to it for a port - this can be at the front, back or whatever depending on where you plan to site them - for a freestander I put it facing forward under the driver. Drill a small hole for one run of CAT 5 computer network cable. Take the four coloured wires from the CAT5 to the +ve terminal and the others to the -ve. Now make it all airtight with a bit of bath sealant and screw the driver in. This is an afternoons work.

The Details

45 litres? It's what Fostex recommend. Work it out for yourselves, and the shape is up to you. You want to avoid standing waves so a cube is a poor idea and ideally the three dimensions should be different, and best of all with non-parallel sides. I was in a hurry so I made a dumpy cuboid stand-mount, but 45 litres is big enough to make a small floorstander where the front leans back to point the speaker at the listener thus saving the cost of the stand. In fact you could make any shape, curved, spherical, whatever, which of course brings us to the question of what to make the 'box' from.
Personally I used 19mm exterior grade plywood because it's heavy, stiff and has a nice veneer. You could use exterior grade chipboard, MDF, solid hardwood, or even composites like concrete or fibreglass - the choice is yours, all will sound different. In my case I kept it simple - all 90 degree cuts, butted up together and glued and screwed - quick, airtight and efficient. The point is that if you want you can go to the trouble to make a very beautiful piece of furniture, alternatively you can use some scrap chipboard from your shed and stick record covers on it - your choice.

The use of CAT5 is a personal choice, call it pseudo-litz-4-twisted-pair-solid-core if you like, but if you do it'll be 50e a metre. As it's basic computer network cable get twitchy if anyone asks more than 1e a metre. The relatively small cross-section won't be a problem with such efficient speakers, and of course it's fine solid core construction is very purist...
To save money just run it direct from the Fostex drive unit to the amp, why pay for banana sockets and plugs which only add to losses anyway? Just make sure you put a bit of bath sealant around the cable exit hole so things are kept airtight.

The port is critical. The length determines where the port's frequency reinforcement comes in. In my case a 3" (80 mm) long piece of pipe gave a very (amazingly) smooth roll-off. A longer port will give more low bass re-enforcement but at the expense of a peak in bass response. The point is that with a metre long piece of pipe costing 2e you can afford to play...

Now comes the time to wire up the driver. You could use the push on tags, but why not get the soldering iron out and go purist? Then just screw the sucker in tight and plug inJ


I said I wanted this to be a design (I flatter myself with the term - it's just public knowledge that you put a 204 in a 45 litre, ported box) to be tuneable. Altering the port length will alter the bass rolloff - longer to lower the frequency of bass reinforcement, shorter to raise it - have a play.

With a dobbing great hole for the speaker, you have access to the interior which allows you to easily add or remove damping, add braces to reduce panel resonance etc. Here you've just got to start 'naked' and go on from there, added damping will reduce the 'shout' but chez moi I liked them raw and exposed. BUT one very important point.
The BFB will sound pretty dreadful and peaky at first. High efficiency drivers with light cones take a long time to break in so let the thing put say 100 hours on it before you fiddle, otherwise you'll find yourself undoing it all as the driver smooths out.

The Results

Here I am referring to my own pair. Made of 19mm exterior grade plywood, 3" diameter port 3" long. No damping just the bare box and wired with one run of CAT5.

Punch. Immediacy. Impact. DYNAMICS. These jump out and grab you by the throat. I'm not in any danger if I say that in this respect these babies will comprehensively mug any commercial speaker under 1000e. These are the loud boorish guy at a party, the dog barking in the night, party next door at 3.00am. You can't ignore them - they shout and stamp their feet and demand to be listened to. Dynamics 'r' Us'.

Putting them through a frequency 'in-room' they are quite amazingly even through the midrange, and have decent extended treble though the whizzer rolls off after 15 kHz leaving cymbals sounding a little soft. The bass rolls off early, a gently slope from 150 Hz down, but it's very even with no humps or bumps.
The only glitch is a hump around 8 kHz which give the immediacy and 'PA' quality to the sound. However you may well find your ears adjust to this and see it as 'correct', in which case the immediacy of the design will leave other speakers sounding slovenly.

What measurements don't tell you is the speed and impact of the bass - though rolled off this really kicks. Lovers of the concert organ will miss out, but for rockers the bass lines fire out with speed and impact. That light cone and big motor mean that though far from subtle the unit is more than capable of extracting fine detail a more conventional woofer (slugged by its crossover) might miss.
The other thing, and it's hard to quantify, is the coherence single drive speakers have - though the frequency plot isn't perfect everything seems to hold together so well, regardless of level.

For a big bluff box imaging was surprisingly good with reasonable depth, remember this is a 'point source'. Ideally toeing-in the speakers to cross over just in front of the listener takes a little sting from the 8 kHz hump as the whizzer is very directional. This causes an interesting phenomenon of such full-range drivers, shared with my own Polaris, in that if you move to the right of the center-line the image remains in the center or even moves to the left away from you - the opposite of a conventional speaker.
The reason is that as the treble is 'beamed' towards you, if you move to the right, the left-hand speaker will point right at you and so up its treble (and thus spatial) output whilst the speaker nearest you reduces this output. As a result the off-axis imaging is excellent giving a real 'hear-into' 3D soundstage. With that smooth rolloff they are also tailor made for a big meaty sub, however it will need to be a good one otherwise the BFB will make it sound very slow and lumbering - such is its speed...

But as for my original brief the speaker delivers in spades. It's not even, bass is curtailed but this speaker gives High-End' dynamics and coherence. Put a value on it? Well the Omega Speaker Systems Model TS2R reviewed here on TNT recently costs 800e and is in essence the 'Big Fun Box' in miniature.

But the BFB has one more ace up its sleeve. AT 95 dbl efficiency not only can the speaker be driven by a small valve amp, it can also transform one of those little compact systems. In fact the review pair went to a friend with a 10 watt per channel cheapo that sounded hopeless. The BFB utterly transformed the system - suddenly it was effortlessly powerful and dynamic - I was gob smacked.


If you fancy having a go at a DIY speaker none come easier than this. They'll also give you more than a glimpse of the High-End, and if in the final analysis they are just too much for you then you can always save them as your "party" speaker - you'll have fun.

© Copyright 2003 Geoff Husband - http://www.tnt-audio.com

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