Yelloweye 2.1 speaker system: yellow eyes
I couldn't think of a simpler name for a speaker cabinet with yellow Kevlar cones; although, looking at the way this project turned out I could've named them cooking pots or Mjölnirs (Thor's Hammer) instead. Anyway, looking for an idea to replace the not-so-good sounding Logo speakers, I tossed up the idea of building two bookshelf cabinets and a separate subwoofer, often referred to as a sub+sat or 2.1 system. The excellent, but huge, and moreover retired, Yellowstones would have been way too bulky for our living room.
After tearing apart the Yellowstone project, I was left with an excellent set of drivers. I fired up the good ol' cab sim (WinISD) and decided upon closed 2.5 liter enclosures for the satellites, each equipped with a Monacor Monarch SPH-135KEP and a tiny Dayton ND16FA-6 dome tweeter. One of the SPH-300KE monsters went into my home theater sub-under-the-stairs; I haven't decided yet what to do with the second SPH-300KE. Maybe when I buy a bigger car, it can be put to use on the road.
Here's the real kicker: the Monacor SPH-135KEP drivers have a removable solid aluminium cone in their centers, where other drivers have a dust cap that's usually very hard to remove without damaging the sensitive cone. The Dayton tweeters with their 32.5 mm diameter are so small, they fit nicely inside the 35 mm voice coil of the SPH-135KEPs. Thus creating a coaxial system! So I made two plastic tubes to replace the phase plugs and fitted the tweeters inside it. Unfortunately the woofers have an extra ring magnet on top of their pole pieces, so there's not enough room for the tweeters to sit flush with the woofer cones, but I think the advantage of having them in the center would make a big enough improvement in itself. It should sound better than having them next to the woofers or floating in front of them, like with most coaxial speakers.
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5. The tweeter is wired with two kynar wires running through the pole piece, along a very long thin bolt. Because the bolt is narrower than the original one, there's room for wires. The washer you see has two small holes for the wires. The nut is locked with nail polish.
The Dayton tweeters really are super tweeters. And I don't mean they're of some sort of superior quality or something. I'm saying they have a pretty limited range, with a resonance frequency as high as 2.3 kHz. As a result, I will be crossing them over at a fairly high frequency, where the resonance peak and phase shift can't cause too much trouble. After all, what's the point of a coaxial setup if the tweeter has a phase offset? The woofers give good output up to at least 6 kHz, so it all makes perfect sense.
Now, it's been on my list for a very long time: building a tube shaped cabinet. With the coaxial configuration I'm using, this is easy as cake.
6. I got a 160 mm tube fitting, routed an MDF disk, and glued it into the end of the fitting. I also made a 40 mm hole in the side.
11. Now you can see what the hole in the side was for: attaching a pole. I reinforced the glue joint with a piece of a tube connector that I rounded to the inside diameter of the housing.
12. Now I can start finishing the cabinet with brushed aluminium adhesive foil. This may prove not very durable, but I think it looks good.
13. The coaxial driver fits perfectly (of course it does, I made sure thoughout the whole process).
14. This doesn't look like much, but it's the backside of the front cover. I cut a cardboard ring, wrapped a piece of speaker cloth around it and glued it with hot melt glue. I marked the driver's mounting holes and glued tiny magnets there, so the grill nicely clicks onto the front. Some tinkering was necessary to get the magnets to the correct depth, but it works beautifully.
All the while I have of course been thinking about the crossover. I'm a fan of Linkwitz/Riley (LR) crossovers; to me it's the only crossover that makes sense. Rod Elliot's ESP site has an excellent article about passive crossovers. I will be using a second order crossover at 3.75 kHz with a Zobel correction for the woofer. Some measurements will be needed after that to see if the tiny tweeter needs some form of impedance correction. Thanks to an abundance of online crossover calculators it's not particularly hard to calculate the values for the capacitors and inductors. I find the one provided by ApISC LLC very useful. It even provides a Zobel circuit calculator.
The crossover will not be located in the enclosure. I will be using socalled "bi-wiring", meaning the drivers will be wired separately to an external crossover box near the amplifier. This way it's much easier to tweak the circuit. Another advantage is that the crossover will not be directly subjected to the vibrations of the cabinet, strongly reducing microphonics.
This is the schematic I drew up. Like I said, it's a 2nd order Linkwitz/Riley crossover at 3750 Hz with a Zobel network for the woofer. I attenuated the tweeter slightly by adding a 2.2 Ω resistor in series, because it was a bit too dominant for my tastes. A happy coincidence is that the woofer and tweeter now have the same impedance, and I can use identical inductors and capacitors for both low pass and high pass.
With even order crossovers, every other frequency band is phase reversed. In this case, this means the tweeter has to be connected in reverse to not cause an acoustic phase reversal at the crossover point, which would cause an audible dip in the frequency response. I did this inside the crossover, so I wouldn't have to think twice about how to wire the tweeter.
So now I've set them up in our living room for an extended test drive. They sound completely different from the bar speakers they're replacing, and I think that's a good thing, because the bar speakers were a bit screamish, and backing off the tweeter turned out to be a wise decision.