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| Mark's Project Pages/Audio Projects/Audax Mini-Monitors/Crossover | |
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Crossover design: For conventional passive speakers, by far the hardest element of the design is the crossover. In my experience, it's quite easy to get something that sounds OK, but making something that sounds special is very, very difficult without a measuring setup or at least a good reference speaker to perform a subjective comparison. With this project, I was under time pressures - my house was on the market, and could sell at any time. So, in the interests of actually completing the project, I decided to "cheat", and use John Krutke's design. This was rigorously designed using comprehensive software and a decent measurement setup. It's quite possible that I'll revisit this in the future but this will really be with a view of improving my knowledge in this area. For for now I'm happy to trust John here. I had some problems finding the exact components specified, so had to make a number of substitutions. The required 5u6 capacitor was made from a 2u2 and 3u3 in parallel, making 5u5 in theory. However, I measured the individual components and by selecting examples that were towards the higher end of their tolerance, I was able to get to 5u6. The 25 ohm resistor had to be 24 ohms, and the 8 ohm was 8.2 ohms. As you can see, I used the same scrap of MDF that I tested the circle-cutting jig on! Everything else came from the scrap pile. The crank handle is simply some offcuts of 12mm birch ply, driving a 10mm section of dowel. The second piece of dowel has a small hole drilled through it, and this is to guide the wire onto the former. I left space in the middle for a mechanism to drive this dowel from side to side, but would need to find an adjustable way of gearing down the rotational speed of the main crank. The final section of the machine is concerned with regulating the tension of the wire from the spool - I'm not sure if or how this would be done on a proper machine, but I remembered that supply tension was a problem before.
The only answer was to drill the hole before turning the timber into a former. I'm sure that this isn't a terribly safe working practice, so don't try this at home! A short length of dowel was used to hold the stock, with some hot-melt glue to stop it rotating. The formers were turned from 2 inch cubes of beech.
The next stage is to test John Krutke's crossover using these inductors. As you saw above, the rear panel initially had 4 binding posts, enabling the crossover to be built externally on a prototype board. I must admit to being very impressed with John's work - the integration between the drive units is absolutely seamless.
Building the crossover onto the rear panels: One thing that I really hate about commercial speakers - those cheap and nasty plastic connection terminal plates. You know the thing - those rectangular moulded plastic blocks that are seen on some very expensive models - shame! Why build a rigid box and spoil it with a thin resonant plastic tray? You can buy these from suppliers like CPC (e.g. part number LS-SRP8 or CN0246066) for buttons, but don't even think about it!
The circular hole was cut using the same circle-cutting jig as the drivers. I used the same hole as the tweeter, but in conjunction with a larger bit to give a diameter of just over 50mm. The 1/4 inch round-over bit was used to highlight the birch ply. Then, a scrap of 6mm birch ply is glued to the inside of the panel:
Final details: Having sanded the enclosure and removed all the dust, it was treated to three coats of Danish oil. I generally prefer this finish to varnish as it doesn't form a skin that can get chipped or discolour. Also, it doesn't raise the grain so you don't need to sand between coats. The finish gives a nice sheen, while allowing the natural beauty of the wood to show through. It also allows the wood to darken naturally with age and exposure to light. To secure the drive units, I bought a variety of fixings including M4 machine bolts and T-nuts, but in the end I decided that wood screws were more than sufficient. I found some from CPC that had a hex head, which gives the illusion that real machine bolts were used. I first saw this trick many years ago on KEF Q55's. Next step is to install the acoustic foam. This stuff was recycled from the Musical Fidelity Reference 2's mentioned previously, and is around an inch thick. It is normally used to line the walls of an enclosure, or rolled up and stuffed in, but I cut it into layers as this box was so small. This sequence of pictures tells the story:
Something that I might consider installing is some self-adhesive bitumen pads on the side walls as the walls are a bit thin (10-12mm). But the box responds very well to the "knuckle test", so perhaps this isn't strictly necessary. These are all options for the future, but it would be nice to have some sort of measurement setup to test for any changes.
On to the final section - conclusions and sound quality...
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