Greenie512
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Greenie512 |
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Formatube Subwoofer [Project No 1]NOTE: please read the Subwoofer No 2 page (above) if you are thinking about building a similar Formatube sub. This clarifies my reserves below about port tuning etc.
I’ve had many a fine speaker after the trannies but never such bass. Today’s system consists of Musical Fidelity CD, tuner, pre/power amps (I’m a big MF fan) and Mission 753 with a small room system of Voyd Valdi deck, MF A100 amp and stand mounted baby Tannoy’s for the occasional LP. So searching the Net for local supplied of REL subwoofers I came across Allan Clarks fine article on his DIY subwoofer, this seemed a admirably cheaper solution. So I used his basic plan to build my own Formatube unit. Before reading the rest of this page, please check out Allan’s site for pictures, very clear explanation and further links: - http://www.users.tpg.com.au/adsltqc3 T The idea of using a tube for the speaker enclose instantly hit me, knowing the work involved with making really good cabinets. However I was a bit sceptical about using a cardboard tube even if it was 9mm thick and used for concrete column formwork? Allan used a Jaycar subwoofer amp but as I was buying the 10” Peerless subwoofer fromWES Components (WES - http://www.wescomponents.com ) so I decided to use their S200W Subwoofer amp. I also decided to mount the Subwoofer amp into the top end cap and also to put a 475mm diameter smoked glass shelf, mounted on legs over the amp. Amp - http://www.wescomponents.com/DataSheets/S2 00W.pdf The enclosureI live in Brisbane, Australia so I sourced the 457mm diameter Formatube from Boral but unlike Allan could not get an off cut so had to buy a full 3m length (off cuts for sale?). I decided to tune the unit to 20 Hz so decided to cut the tube to a 700mn length for slightly more volume. I followed Allan’s instructions and peeled off the internal thin plastic membrane that prevents the stick oil finish seeping into the cardboard. Cutting such a wide diameter tube to length is also tricky, my cut wasn’t perfect but after finishing (see below) you wouldn’t know? The only bit of woodwork? However this requires 10 perfectly cut circles (Four for the actual end caps (two laminated/glued together), two 240mm dia for the speaker and four 90mm dia for the ports). I can see Allan’s logic in getting a cabinetmaker to do this for you (see his pictures). But being a “cheap skate” I thought I’d have a go and borrowed my brother’s Festo jig saw (what a gem). I practiced on a couple of pieces of plywood first and once the radial arm was set correctly - success, when I placed them; they ever-so slowly drifted down the tube, perhaps a millimeter too small. So I adjusted the cut and did the four end caps out of 16mm MDF (I couldn’t find small sheets of 18mm, so “cheaped” out again). I then clammed two circles together and drilled the centre pilot hole for the speaker circles, then cut each out separately. I had previously laid out (with dimensions) the speaker, ports and legs positions in Visio I had to cut the 90mm port openings by hand as it was too small a diameter to use the radial arm, so I practiced on the MDF off cuts. The final technique was to use a short bit of 90mm dia PVC pipe and using it as a template, trace both the internal and outside diameters then cut carefully and slowly between these two guide lines. The end result wasn’t perfect but when you placed both end caps together it formed a tight fit for the ports. I also used one cap as a ports cut out template for the second. I also cut the amp section out of each of the top caps before glueing and clamping them up. I wanted my chrome legs holding the top glass shelf to look like pillars coming out of the top cap (arty, hey). So I drilled three 25mm dia holes at an equilateral triangle spacing; and screwed the shelf leg plates internal to the cap (there was a internal hole though the plates into the screw-on legs so I “bunged” the holes with speaker gasket material). [I'd still recommend using 18mm MDF for the caps - thicker] The portsI downloaded a copy of WinISD and based on a 100 litre cabinet volume and 20 Hz frequency, calculated the lengths of the ports to be 806mm long. I made this length up of 90 mm (OD) PVC pipes with two 90 degree bends to create the two “J shaped” ports using standard pipe glue. Because the ports were a very tight fit into the end caps I glued/laminated the two end caps and fitted the ports and wood glued them all in one go and left the cap clamped up for a day. I had glued the top cap pair earlier. NOTE : see final comments at end of page. Cabinet fabrication
Good mistake!When positioning the port outlets in the base end cap I didn't take into account the thickness of the dampening material on the inside of the tube wall? Hence, when I came to fix the end cap the port pipe pushed (not too tightly - luckily) against the dampening material and made it difficult to fit the end cap in to the tube. However, a spin-off to this is - it should help to dampen any resonance in the pipes? Final enclosure finish
I fixed the fabric square to the edge of amp on the top - I trapped and screwed small aluminium strips under the material but ended up with small ruffles in the corner (I can fix that later). I did the same on the base and used the leg plates and more strips to fix the material. Although I did not cover the drive unit as it sits proud of the base I did cover the port outlets to stop anything (bugs) getting inside. Currently I'm using a spare piece of 10mm rectangular plate glass for the top shelf, I will eventually get a circular version cut. Final costAs of January 2004 in Australian dollars and 10% GST: - AU$915.00 This doesn't include the glass top I have yet to buy, so I'd say a neat $1,000 would be the final cost (see below of comments on cost) Final resultsWell as Jamie Oliver would say “I’m well chuffed”. The unit looks good even without its smoked glass shelf. I’ve only being using the subwoofer for two days before writing this but so far I’m really pleased with the sound/musicality. I had read before that bass is non-directional and that certainly seems to be the case as you would swear the front speakers were producing the extra frequencies, so the extra box does not distract from the stereo. Currently I have positioned the woofer about 4m at 90 degrees up the side wall to the Missions, which is just behind my sofa position which sits (basically) in the middle of the room. I also tried about 2m out from the corner where I could see it but being closer to the front speakers seemed to null some of the effect so it went back to the original position. The subwoofer amp is adjustable for Gain (volume) and Frequency (10-250 Hz). I'm not brave enough to turn it any higher than half volume and have it set about a third or less at about 80 Hz; any higher (freq's) and the box draws attention to itself. The amp also has an auto mode so if it doesn't see a signal after 15 mins it turns itself off and on if there is a signal. So this project cost a grand! (Aussie dollars) you could built it cheaper if you can find a Formatube off cut as Allan did and don't do the top shelf detail. The above is a bit “wordy” so if I haven’t made myself clear or you’d like to ask a question please drop me an email see my contacts page. Final commentsHaving delved into the WinISD software a bit further, I am now in two minds whether I calculated the port lengths correctly. So I would recommend you do your own calculations to be sure you get it right. Whether or not my design is technically correct - the final test is I can I live with the end results. Well let me say that I'd rather have the subwoofer switched on that off. I can not hear any port "noise" and the unit only adds to the bass of the system and does not distract from it i.e. it seems fast enough to keep up with the front speakers. Project No 2 will be to make a traditional ported box for the speaker and amp and compare the two enclosures - but that will be a while away yet [Now complete - actuallyProject No 3 Other LinksThese are some other links you might find useful (in no particular order). http://www.netlynx.us/audio/sonotube-calc.html - tube volume |
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I blame all this on my late father who got me interested in Hi-Fi as a youngster in the 60’s. So, in the early 70’s when I wanted a system and didn’t have much money we built a pair of transmission line speakers based on a Seas tweeter, Peerless mid and Kef bass and passive bass radiator. It took us about a week to hand build these monsters with hundreds of screws per cabinet but the end result was superb at the time and the bass “ hummm“ heaven. The rest of this system was a Connoisseur chassis desk in my own box, I’ve forgotten what arm/cartridge I used, mounted on squash balls with a Armstrong amp (happy days).
hanks to Allan for answering a couple of my questions.
Allan’s idea of using “black dress knit ribbing” from Spotlight is a stroke of genius. Pulling this tube of material over the enclosure hides any little mistake you’ve made (a few I’m afraid to say). He’s correct, it does look like speaker grill material when fitted.