The long version...links to part suppliers at the bottom of the page
I needed some "small" speakers to use on my computer desktop, hang in my camper van and occassionally use in a small stereo. I saw the Aurasound 4" driver on sale at Madisound and thought that might be a good place to start. It has a long excursion with a neodynium magnet structure that is compact and shieled. I got the 4 ohm version to get the most output from small amps. The HiVi TN25 1" dome tweeter is also compact with a shielded neodynium magnet and a rear chamber. This is not the TN28 that got a bad review on zaphaudio.com.
This system is great for listening to my favorite radio streams radioparadise.com, gaiaslap.com, magnatune.com or my mp3/ogg player. The height of the enclosure puts the tweeter near my head level. The footprint at the base is a 5-1/2"circle. The best placement is several inches away from the monitor and slightly behind the screen. This reduces sound reflecting off the screen and improves depth and separation.
I've used these "full range" with a small T-Amp and some bass boost but I prefer to use a Keiga 2.1 plate amp. It provides an electronic crossover, 2x25w for the tubes and 1x50w for my 8inch Sub which I put under my desk. OK, I have a big desk. This setup will play very loud with low distortion and can be used with a larger amp if you watch for excessive cone excursion and cross over above 100 Hz.
Construction
Making the tube enclosure was a challenge. The 4" woofer is small and care needs to be taken when front mounting the speaker. A thick baffle with a speaker cutout that has straight sides will restrict airflow from the back of the cone into the enclosure. I used a piece of 1/4" plexiglass (from a Home Depot store) which allowed the woofer basket to extend beyond the baffle so airflow was not restricted.
I cut speaker holes with a circle cutter on a drill press and then glued the baffle to the inside of the tube with clear silicone rubber (next time I'll try pvc cement). When this was cured I carefully cut away the tube from the outside to expose the baffle and then sanded the tube edges flush.
The baffle is 7" long and the sides are cut at a 30 deg angle to approximately match the angle of inside tube walls where they intersect. The front of the baffle is 3-7/8" wide and the back is a bit over 4" due to the angle. The tube is 11-1/5" long and the face is sliced down 6-3/4" to expose the baffle. The baffle extends 1/4" below the slice and a small piece of 1/4" tempered hardboard is glued between the circular part of the tube and the bottom of the baffle.
The endcaps are made from 1/2" baltic birch. There is a smaller piece of 1/4" material attached to the endcap that fits into the tube. The top endcap is permanently glued to the tube. The bottom one is removeable and has 3 angle brackets with threaded rivets attached that face the walls of the tube. Screws from outside the tube come through the wall and thread into the rivets to hold the endcap in place.
The speakers are mounted on the front of the baffle using pan head machine screws. They are secured on the inside with kep nuts, a nut that has an integrated star lock washer. I used 8x32 screws for the woofer and 6x32 screws for the tweeter. Mounting the tweeter was easy when done before the woofer. Mounting the woofer was harder.
The woofer has 4 mounting tabs with screw holes. Unfortunately the part of the frame that makes contact with the baffle is lower than the mounting tabs. I put 1/8" closed cell foam weatherstrip on the part that makes contact and then used 2 washers underneath each tab as standoffs to keep from bending the tabs too much when I screwed down the frame.
I mounted binding posts vertically on the back of the tube. These will accept bare wires or dual bannana plugs. I found some curved, wide mesh gutter screen at Home Depot that was just wide enough to fit across the front of the baffle and fasten to the sides of the tube with screws (not shown in photo). It provides some protection to the drivers and doesn't seem to affect the sound.
Crossover
The high frequency filter is third order (18dB/oct) at approximately 3.5kHz with resistors to adjust the tweeter level. The low frequency filter is first order (6dB/oct) with damped notch filter across the woofer to mitigate a dull ring in the 2kHz region. I tried to achieve maximum efficiency with a minimum component count for this enclosure sitting on a desktop. Other builders might come up with something they like better.
Fitting the crossover into the tube was a challenge. A picture above shows it mounted on the bottom endcap. I glued the coils between the 3 angle brackets and found room for capacitors and resistors wherever I could. The coil L2 in series with the woofer requires a low DC resistance but the ohter ones don't and smaller guage wire could be used for them. Capacitors with a lower voltage rating might be smaller too.
Take note if you try to use this crossover with a different speaker or enclosure. I used the 4 ohm version of this Aura woofer. The 8 ohm version has a different response graph. A wider front baffle will reinforce the upper midrange when sound reflects off it. Putting carpet pad on the outside of the baffle may help. A rectangular box may sound different than a tube with curved walls and require considreable damping material to mitigate standing waves. When I tried this the damping absorbed energy in the low end and required a different crossover to compensate.
Ported or sealed?
If you run this with a subwoofer I would recommend a sealed enclosure with no port. If you don't use a subwoofer you may want to port the enclosure to get some extra bass. Aura recommends a 3 liter enclosure with a 1" dia port (pvc pipe works) that is 4.5" long. They mean 3 liters net volume after you have subtracted the volume of the speakers, port and crossover.
The speaker enclosure in this design is a 5" dia pvc pipe which means that mounting a straight port tube of 4.5" on the front will put the end too close to the back wall. You could make this work by mounting it on the top or using a 90deg fitting to bend the port tube. However, the standard pvc fittings are made to fit the outside of a 1" pipe so they are actually 1.25" dia. This makes the port area larger so the total length has to be longer to achieve the same tuning frequency.
I used two 45deg PVC bend fittings connected together to create a 6" port length. This took up more volume that I had planned and so did the crossover components. The end result is that my speaker was not tuned correctly. I'm currently using the tubes with a subwoofer so I took out the ports and sealed the enclosure. They sound pretty good now. Next time I'll make the enclosure tube a little taller to add volume and use a short 1" dia port and a single 90deg bend.
I recently built some deep rectangular boxes that could handle a long straight port tube. I used the same speaker and crossover components. The bass is definitely better but I still prefer the tubes, they don't have a "boxy" sound. I'm still working on the new box version and will build a page that describes them with I'm happy with them.
Trial and error?
Aura said that a 2 liter sealed enclosure would be 3dB down at 100 Hz. I first tried 2 liters and then 2.6 liters. They both had a large peak around 150 Hz and a rapid decline after that. I got much better results with a 3 liter tube and very little damping material. The peak dropped in frequency and amplitude.
I love these speakers in the midrange even at high volume. This has a lot to do with the Aura woofer. It has an underhung voice coil in a long magnetic gap and a stiff cone that resists breakup. I also like the HiVi TN25 and think it's a bargain for the price. I'm not satisfied with the system response below 150 Hz. I want it to sound "bigger". Further experiments are required.
Some energy is lost at very high fequencies in the crossover. If you miss this you can bridge a very small capacitor across C1 and C2. I tried 0.33 uF and could hear and measure the effect on pink noies but rarely hear it with music so I left it out.
My measuring equipment is pretty crude right now. I have an old 1/3 octave analyzer with a pink noise generator and an impedance meter with a signal generator. After a preliminary crossover design I listen to a variety of well recorded CDs and make adjustments. I would like to have a good computer based system that works with a high end sound card on a Linux desktop. If you know a combination that works well let me know.
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Last update: 4/30/2006