Product: IPL S3MTL DIY kit floorstander
Manufacturer: IPL - UK
E-mail: Ivan Leslie
Approx. price: 233 UKP - 400 S$ (pair)
Reviewer: Geoff Husband

The IPL TL kits-parade, S3MTL third from left

Over the last ten years I've built seven of Mr. Ivan Leslie's kits and so I suppose it's time I wrote something about them for TNT.

IPL is a one-man band, Mr Ivan P Leslie does it all. I like his designs because they're not run-of-the-mill, he believes in what he does and his prices are ridiculously low.
To give you an idea of the sort of value I'm talking about, I recently built a pair of his small sealed-box monitors for a friend.
The cost of the full kit, drivers, crossover, connectors and PAF was less than the retail cost of the Morel drivers alone. One well-known British speaker manufacturer uses the same driver line-up in one of their speakers and it retails for over UKP 1000, the cost of the kit? Just under UKP200. That is a huge saving by any standards.
So even if you don't like the end result you have a basis for further experimentation using good quality drivers as a starting point. This value also means that when you come to sell the speakers you will loose less money, not always the case with kits. But for this article I've not chosen one of IPL's small kits, but something much dearer to Mr Leslie's heart - a Transmission Line (TL).

In a world of ported and sealed boxes, three other designs rumble away in the background, never being mainstream but never going away. I'm talking about Horns, Electrostatics and TL's (Transmission Lines).
Fifteen years ago you'd have been pushed to find a horn manufacturer, now a cottage industry has built up as low powered valve amps have finally been rediscovered. The horn has massive efficiency and dynamics, all from a couple of watts - hence its continued existence. The electrostatic has stunning transparency on its side. Both types are doing well, but TL's? Apart from TDL very few people make them, so what makes them so rare, and why should we bother with making one from a kit?

First a very quick bit of (very) basic theory... A sealed box used a trapped volume of air to act as a spring and load the main driver. It results in a gentle 12 dB roll-off in the bass. A ported box uses (crudely) the air in a tuned port bouncing in and out, in phase at resonance to load the bass driver. This adds a wadge of bass as the port obviously makes a sound, hopefully augmenting the bass just as it would normally begin to roll off. This increases efficiency by about 3 dB's, but the bass roll off is steep at about 24 dB below this point. So sealed boxes appear less bassy but have more very low bass, ported boxes are more efficient.
TL's on the other hand work on totally different principles. The idea is that behind the bass driver is a long tapered tube lined with absorbing material. In theory this absorbs all the back radiation from the driver and loads it by being resistive i.e. the driver has to push the *heavy* column of air back and forth. In practice it is impossible to absorb all the back radiation so rather than terminating in a point the *line* exits as a thin slot.
Now here's the clever bit. If the line is 1/4 of the wavelength of the resonant frequency of the driver, the sound exiting the slot will be in phase and around this frequency. You see the idea? The result should be that the driver acts like in a sealed box with a gentle slow roll-off, but the slot will augment this output for a relatively wide bandwidth, compared to a port, around resonance. All output above this is effectively absorbed by the line - neat huh...

So what's the catch? Well first with so much of the back radiation absorbed TL's are no more efficient than sealed boxes. Secondly unlike ported boxes where you stick you driver parameters into a computer and get a *ball park* design, with TL's it's much harder to get it right. The Loudspeaker design cookbook devotes 30 pages to ported boxes and 3 to TL's. The most telling phrase being Tuning TL's is generally done by ear...
Basically there is some theory with a lot of *suck it and see* involved and of course experience which is where IPL come in. Thirdly - making a domestically sized box containing an 8 foot tapered pipe requires the line to be folded rather like a backwards horn, involving a lot of expensive woodwork and a heavy, expensive-to-ship speaker at the end. Horns cost a fortune but do things no other speaker will, TL's cost almost as much to make, but the advantages - especially considering the design difficulties - are slight.

Despite this there are a few people out there who believe that the bass quality of a TL cannot be matched by a ported speaker or a sealed box, and Ivan Leslie is one of them. All his larger speakers use TL loading, and it is perhaps the most interesting of these that this article is about.

I chose The S3MTL because of its mid/bass driver. Remember IPL are a one-man band selling kit's to a niche market. How then is it that IPL have designed and had built their own aluminium coned woofer? According to Ivan the story is simple. Firstly finding good woofers with the correct Qts for TL's is difficult, and secondly he was very impressed by the Magnesium coned Seas drivers but not by their price! He reckoned he could have a more suitable driver built for a lot less and so the IPL/P/8M was born. It looks a little *hand made*; the odd spot of glue here and there and a less than perfect finish, but it's spec is impressive.
A spun curvilinear aluminium cone 6 1/2 inches across, a hefty 102 mm magnet assembly and cast magnesium chassis certainly make it look the business... Mated to this is one of Seas' metal dome tweeter's - the H398.

So apart from the drivers what else is in the kit? All the crossover components, polyprop caps, hi-power resistors and air cored inductors, mounting hardwear in the form of bolts and T-nuts, banana sockets, profiled acoustic foam (PAF) and long haired wool. The builder supplies the wood, glue, glue gun, wire and sweat... The finished speaker is a nicely proportioned floorstander 810x275x400 mm.

The recommended material is 18mm MDF. IPL can supply you with all the panels cut and ready to use and at UKP80 extra this would be a good option for most people without access to a decent sawbench. Being a masochist I decided to build the main carcass out of 30mm thick hardwood, an undertaking I'll not relate here though suffice to say it added a few more grey hairs...
Construction is then pretty straightforward, the box being built up on one side and the box sides and inner panels fixed along with the PAF before the final side is added. There are no complex joints, the pieces are simply butted up against each other and located with the help of small hardwood blocks fixed in place by a glue gun. This might seem a bit dodgy, but with modern glues and the huge solidity offered by the internal baffles you end up with a VERY solid piece of MDF. I'd personally veneer it using iron on veneer which is pretty simple, but you could paint it or even stick old LP covers on it - up to you...

This will take a weekend - tops, it really is that easy and the instructions are clear and simple. However here I made a modification I would strongly recommend to anyone taking on any IPL TL. The standard set-up for the crossover is to have them mounted high up on the rear directly behind the woofer. This is so both can be removed through the woofer cut-out. I made up a removable panel at the base of the speaker which held the optional bi-wire connection panel and all crossover components. This has several advantages.
It keeps the crossover away from the vibration and magnetic field of the woofer, makes removing and fiddling with the crossover without de-soldering a doddle, and it allows access to a part of the line otherwise closed to you when the box is glued up. Soldering the crossover was very easy, IPL provide a schematic and it has few components. One note here - the Achilles Heel of metal drivers is a sharp break-up peak, in the case of the IPL driver at about 6 kHz.
To kill this off there is a second small bass inductor to steepen the roll-off of the driver before this frequency. The tweeter has a level matching resistor in series and there are two to choose from. I would always start with the smaller but your mileage may vary.

So bolt up (8 bolts!) the woofer and tweeter and plug in...

Yuk! Muddy. Slow. Recessed. Plodding... all that work!

Deep breath - now the real (fun!) work starts... IPL give details on how to tune the speakers and it is here that one of the TL's biggest plus points for the DIYer comes into play - its tuneability. First don't touch anything. Just thrash the speaker for a week.
You know the kind of thing - Nirvana, 1812, Barry Manilow... "Great - much better" but still a bit slow, recessed, plodding. Drop down the rear panel (glad you took my advice?) and just unwind three turns off the large bass inductor (as per instructions). Now we're getting somewhere, much more lively and exciting. Another couple wound off - hm! Starting to sound clattery and metallic. One wound back on - that's it! Now cut the loose inductor wire off and resolder. (NOTE: As of now all S3MTLS will be supplied with 3 turns less on the inductor as standard so you're initial reaction should be more favourable than mine!). So the mid's OK and I think there's plenty of treble, now for the bass. With a sealed box you'd be very limited here. With a ported speaker cutting and changing ports makes a difference but unless you use toilet roll inners you're going to use a lot of ports! With a TL you have much easier control.
Putting long-haired wool in the port damps everything down a bit, but if overdone can load the woofer so it sounds a bit shut in. Much more effective is to place teased out wool behind the woofer. The more you put in the more midrange emphasis, the more apparent detail, more *hi-fi* if you like.
Start with say 5 grams and work up to 15grams. It takes minutes and the result can be quite an improvement. In extreme cases you can add/remove PAF from the line though the tweeter/bass/terminal block holes (bit like being a vet...) but to be honest in most cases the sound is pretty good *straight from the box* with only mild tuning needed.
Nevertheless some people will take months before they're are happy, any analogue addict will understand this tinkering, it's just like fiddling with arm settings! In my case I think I've got them sounding as good as they are going to get in my room. Worth remembering that this sort of tuning would be much more difficult with a commercial speaker, especially if it was under guarantee...

Sound Quality

The bass didn't muddy the midrange, though if you add 30% of really low frequency over lesser beasts you expect some midrange to be masked. The tweeter, not hugely expensive, matched in nicely to give reasonable detail and air. bi-wiring with FFRR helped with its light balance, stranded cables sounding slow and harsh. Imaging was OK but the speakers did *disappear* which is the least I expect from quality speakers. Overall the impression is of a full, but balanced speaker.

At this time I had the Cabasse Sloops on test. How would a kit speaker costing at most UKP350 to complete, compare with a top commercial speaker more than four times as expensive? Well they're about as different as you could wish.
The Cabasses were crisp and clear, excellent images - jazz bands spread in front of you, realistic female vocals, fine detail and hugely efficient. The IPL's? The detail wasn't quite there. The tweeter a little thin - once a triangle leapt into the room in the middle of a piece of Elgar - the imaging inferior and the bass seemed a little overblown at times.
This remember, compared with a floorstander built with these goals specifically in mind. On the other had they have musical, warm balance, easy to listen to at low and high volumes, then turn the wick up - and - THE POWER! Nirvana's *Territorial pissings* slammed me back, Marley's *Real Situation* drove along with drive and weight.
When you hear bass this good, and fast with it, you find it very hard to go back to something less no matter how correct. Thorsten goes on about dynamics and I'm right with him, but power in the bass is a big part of this - if a kick drum doesn't hit you in the chest there's something missing. A speaker with this kind of depth does *menace*. *Dies Irae*, *Mars*, Hooker's *Like Jessie James*, Prince's *Batman*, NEED that bottom octave to make your flesh creep. I know this is beginning to sound a lot like my Shure V15 review and perhaps this is why the IPL's work for me.
You might expect these two *bassmeisters* to overdo it in combination but no. The Shure pulls those bottom notes off the disc and the IPL fires them out at you. The bass performance of the V15 wasn't so apparent through the Sloops, and they made the Shure's failings in mid range detail a little more obvious. But in the same mould as the Shure, the IPL's are fun speakers.
They'll impress your friends, frighten your pets and annoy the neighbours. At the same time you can listen to them at silly volumes for hours on end without fatigue. They are a solid, meaty advocate for the TL concept. Are they better than the Sloops? No, just very different. But they do things that I'm increasingly finding it hard to live without - we're talking *fun factor* again. If the Sloops are like *Sugar Ray Leonard* the IPL's are like *Mike Tyson*.

As for compatability they're only just efficient enough for my 30 watt Audion EL34 monoblocks but they were helped by the smooth impedance curve TL's produce, the 8 ohm tap giving enough gain for all but the wildest party. If you want a clearer more refined treble IPL will sell you the S3TLM Ribbon, which uses the AR ribbon tweeter, but it'll add UKP150 to the cost of the kit though the result could be spectacular. Got a huge room and listen to Organ recitals? IPL do a BIG three-way TL that's *flat* to 25Hz. And if after all that you still don't like them you can always ring IPL and they'll give you the box dimensions and port details so you can make a ported box - just don't expect Ivan to be pleased about it :-)

You can tell I'm pleased with the IPL's. They give you things for UKP300 that make many four-figure speakers sound anaemic. The downside is that for lighter stuff they can sound a little slow, and they swallow very fine detail with impunity. This pushes the live performance further away than the Sloops, though that low bass is quite important in catching the atmosphere of a venue. These are minor criticisms.
It's hard not to concentrate on the IPL's bass performance but the rest of the scale is in fact handled to a pretty high standard - and here I'm talking in absolute terms, not relative to cost. Add to this that they are probably the most tuneable speaker on the kit market and excellent value and you cannot help but be impressed. Right now I want a pair of speakers like *Cassius Clay* and Ivan assures me his 3-way will fit the bill, but for now I'm too busy listening to music to get the saw out...