Main Channel Left & Right Loudspeakers

These Speakers are totally unique in that no other set of speakers in the world uses the same two drivers... yet. However I think that there were some speakers with the carbon fiber mid-bass drivers at this years CES show.

After months of planning, changing my mind &tc, the project finally came to a head in the first few days of January 2000. Many thanks to Mark and Dave, without who I would certainaly a leauge or two furthur up shit creek when it comes to woodwork. Cheers Lads!

The Box

I decided to use a double box configuration, with an internal 1/2" mdf box shrouded by 1/2" softwood. What I didnt realise at the time was that due to humidity changes, the softwood expands and contracts with tremendous force, in a direction at right angles to the grain. This is used to mine slate in Africa, where they hammer bits of wood into the rockface, pour water over them and then 15 minutes later 70 or so tonnes of rock comes crashing down.

I screwed the MDF to the Softwood in two places on the front baffles, down each side, and when the softwood dried out it split down the middle. It made quite a bang - enough to wake me up anyway!

Also the softwood that they gave me was much more of the order of 7/8ths, so my boxes were much bigger than expected.

The box is an SBB 4 with an internal volume of 15 litres which is brought down to the required 14 by the internal bracing / driver displacement, tuned to 64 hertz.

I can tell you that I used close to 100 screws on the boxes. I believe that is overkill. I also used 21"x1" bracing rods to connect the sides and the top and bottom, hopefully reducing wall movement. I think next time I will try for A B&W-esque "matrix" arrangement. I screwed these from the outside, but all other screws are from the inside.

At a later date I purchased an electric plainer and got the back, sides and top down to about 1/3". This makes the boxes much smaller and lighter, but does make the surfaces uneven.

Lessons learnt from this are that I wouldnt use softwood to cover the internal box ever again. Next time ill probably go for "door skins" or veneer. I do like being able to round over the edges of the box though (plained) so I would probably use quatered dowel for this purpose.

The Tweeter

The tweeter that I decided to use was the Morel MDT-30. It seemed to get good press, and was somewhat of a compromise. It has a hand treated fabric dome, which usually mean that they are not fatigueing to your ears. This is quite important seeing as how much time I spend listening to music. They are rated at 200W RMS and 1000W for 10 Ms transients, which means that Im not very likely to blow them up. Very important! (charlie :)

There are relatively few tweeters in-between the Super-tweeters (ravens, MDT-33, etc) and bog standard tweeters. I could get them from within the UK from the same place (IPL) that I was getting my middbasses and crossover components from. Im happy with them, but dont have anything for comparison. Perhaps Ill shop around a bit before I make my next tweeter upgrade, but I dont think that will be for quite some time.

With regards to IPL accoustics, I found mr Leslie very freindly and helpfull, not at all pushy, and would reccomend his services to any DIY-er in the UK. The drivers work out cheaper than madisound equivalants once you add on import tax. Contact him on 01271 867 439. btw, Im not related / getting paid for this plug or anything.

When playing the tweeter alone suprisingly little of the signal is in the 3K + range, so Ill concerntrate on bass, and midrange before I upgrade these again. Also they do a shielded version for the center channel.

The Woofer / Midrange

This was the most important decision in the design process. I wanted a 6.5" driver that had decent power handling, decent bass response in resonable sized boxes, decent frequency response, good midrange clarity, and a good overall appreciation by those who have used them.

This carbon fibre driver is fairly new on the market and you can see them just starting to use them in commercial designs. I forget which ones now, but Im sure theres one in the LDSG. The carbon fiber is both light and stiff, making it an ideal tranceducer. However IM distortion of the backwave from the edge of the driver was a concern at first. I havent noticed any problem along these lines though.

The carbon fibre mesh is pretty cool and often reflects like a bit like mother of pearl does... "Fat cones" someone remarked :)

I was initally slightly concerned that there were lots of holes in the carbon fibre mesh... I thought that this would effect bass response... but they seem to be small enuogh to resist the air.

I use these drivers without a LF crossover, and therefore full range. They yeild a theoretical f3 of about 65 hertz. I have only over-excursed them once, and that was when I was messing about with a tone generator and the bass knob on my amp.

There are two things that I dislike about the driver. It isnt a circle (hence a pain in the ass to rout an insert for, and a bit ugly) and that there is a big step between the edge of the surround and the edge of the cast iron faceplate. I keep on meaning to blue tack this to a 45 degree slope, as one chap claims that this made stereo imaging far, far better.

The Crossover

This is a simple Linkwitz-Reily second order design, essentially straight out of the Loud speaker design cookbook. It is crossed at 3 Khz, and incorporates a L-pad resistor to damp the tweeter's efficiency, so that it matches the woofers efficiency. I attempted to match the phase, by setting the frequency such that half of one wavlength was the same as the time delay offset caused by the center of the two drivers being offset could be cancelled by reversing the polarity of one of the drivers.

Above picture is the final Schematic and capacitor / inductor values. I have yet to measure the frequency response of my speakers, so I have not been able to perform any sort of optimisations of this crossover. I can say that when listening to frequency sweeps that cover the crossover range, the output (to me) seems to be very flat. I do notice a bumb at about 140 hz though.

The Port

This is approx 3" long by 1" in diameter. I had to buy 2 meters of pipe though. doh. Chiseled around the opening to make a flare, and filled over this, being carefull to leave the filler bumpy and uneven. Anyone who has seen a fluid flow trace of air over a flat ball and a golf ball will understand the importance of this - uniform airflow. Next time round I would like to get hold of some of the contoured purpose built ports, as seen on high end speakers and subs by B&W.

The diameter of these ports is not sufficient to keep the airflow linear at high SPL low frequencies. I would go for 2" or perhaps 3" next time, and try to get propper flares for both ends.

When plaing the speakers hard, you can smell the glue / fresh wood smell as air in the box is pumped in and out. When I smell this too much thats my queue to turn things down a little :)

More Pictures

Update - Comparison

I recently took the speakers into a HIFI dealer in Manchester when a friend who liked my speakers wanted to compare them with a set he was thinking of purchasing (Dynaudio Audience 52s). The two speaker systems are essentially comparable, both being 2-way stand mount bass reflex designs. After listenting to both the lad in the shop commented that mine had exellent bass detail, and good treble, but that he did not like the midrange.

I am obliged to agree with him - when I listen to mp3's I use winamps EQ function to attenuate the midrange section, using my Rat Shack SPL meter for calibration. A little work on the crossover is required I think! However that is not to say that the Dynaudio's were better, just different. I would peronally be happy with either pair.

I am currently planning on an active crossover project for these speakers, using the rear channel amlification on my yamaha to power the tweeters, and the standard main channel amplification for the mid/bass drivers. This setup should be far superior, using ajustable 24db/octave crossover points - at the line level stage, thus minimising noise, and allowing me a bit of experimentation of where to cross the drivers over.

Update - Stain and varnish

The speakers have been stained and varnished now. I thought I'd better get some practice with this before I build my next pair, because to be honest, I am not too happy with the finish on these! They are my first effort though, and it could be worse!