Active Three Way Speakers Project Electronics & Audio
This is an older project, one of my better audio-related ones. Some of the photos are not great – the majority were taken with my old 2.1 mega pixel camera and a few have those beautiful ‘water spots’ – illuminated specks of dust in the air from the flash. Still, not too bad and get the details across well enough.
I’m not very good with dates – it’s just how it is. Luckily, these pictures are encoded, and the oldest ones are from 2006 – hard to believe it’s six years later.
At that time, I was heavy into do-it-yourself audio, to the nearly complete exclusion of everything (and everyone) else. I had several projects in various stages of completion, and the biggest one was to build this pair of speakers: three-ways that would be actively driven. For those that are not familiar with the language, ‘three-way’ is the speaker configuration – woofer, midrange, tweeter. ‘Actively driven’ is to separate the frequency ranges for each driver (bass, midrange and treble) and separately amplify each one.
This is much more complex and costly than a regular speaker system, with a passive crossover network, but I believe the results are worth the extra time, money and effort. It’s more challenging also, and that is what I was looking for at the time, something that would be an extended project. I got what I was looking for, as I’ll show here!
This was a time before SketchUp (for me, at least) and I rarely made any kind of plans, just rough sketches and ideas. Buy some lumber and start cutting. This strategy worked out reasonably well for most of the things I made.
For this, some more detailed planning would have been a better idea, since I did make a mistake or two that a well thought out construction plan could have avoided. It all worked out well in the end though.
The majority of the box is made with 5/8″ thick MDF. The front is in two layers, to form the curve. I didn’t have much of a problem bending the MDF, since it is a fairly large radius. I used an ample quantity of glue and 3″ long screws to secure the first layer to the box, as shown in the picture below. I then let this dry over night and glued and clamped on the second layer the next day. No screws were used for that layer – it completely relies on the glue to hold it.
After the glue had fully set, I rounded over the top and side edges of the baffle (front, where the drivers are) with a 1/2″ round over bit in the router, I then laid out the locations on the baffle for the drivers. The holes through the baffle were cut with a jigsaw, and it was while cutting out the hole for the midrange (centre hole) that I found a problem. There was a divider installed between the upper part of the speaker and the lower part. This is to keep these chambers separate, and not have the back pressure from the woofer cone act on the midrange. The divider was not in the right place, actually it was just oriented incorrectly: it should have been perpendicular to the baffle, not the back panel of the enclosure.
The picture is not great (terrible, actually), but it’s fairly easy to see how I cut into the divider panel:
To mount the woofer flush with a curved surface presented a difficulty. I had to make a template from MDF in order to route out the mounting flange recess. I used a pattern bit in the router and clamped the template on in the exact location, wedged up off the face of the baffle to steady it over the curve. Tricky.
For the finish, I wanted the baffles to be “piano black”, an extremely high gloss and ultra smooth paint job that is typically done with lacquer. I had done a reasonable facsimile of it of an earlier pair using cans of ordinary spray paint. This was expensive and really didn’t give the results I wanted.
To get the speakers ready to paint, I brought them out back and sanded the baffle up to 400 grit. I knew that if you want a perfect finish, you need to start with a perfectly smooth surface:
I then gave each baffle about ten coats of primer, let each coat of primer “flash off” (tacky, but not wet) before spraying it again. I used an oil based primer and sprayed it with a gravity feed sprayer.
It takes this many coats to build a hard surface over the MDF, something that can be sanded smooth to receive the finish coats of paint. I then let the primer cure for nearly two weeks before moving on to the next step.
The paint I used was gloss black Tremclad rust paint. Here’s how it looked after it was sprayed:
To complete the paint finish, it would have to be smoothed and rubbed out. This involves wet sanding up to 2000 grit and buffing with polishing compound – a lot of work.
It’s at this point where the painting takes a few twists and turns, and I wind up going with another product: a water borne polyurethane paint from Sherwin Williams. The new paint turns out to be a good idea, since it has qualities that make it very desirable for this finish: it dries extremely fast and hard, much harder than the Tremclad. Water clean up and easy to spray. My one complaint would be that it was not a true black, but more like a really dark grey.
The speaker sides, top and back were to be two layers, separated with a flexible layer of silicone. This is known as constrained layer damping, and the concept is that as the interior panels vibrate from the sound energy, a portion of this energy will not be transferred to the outer panels. Some of the sound energy will be used up in the flexible layer between panels. My version of this was untried, untested, so I would be taking the chance that it would not work at all. To be honest, I wasn’t overly concerned about the effectiveness, I just wanted to try something new.
The inside panels of the box are 5/8″ MDF and for the outer layer I used 1/2″ particle board core veneered plywood. The plywood would have to be very accurately scribed and cut to fit the curved front baffle. Also, the corners had to be mitered, to avoid having the end grain show. Add to this the complication of the silicone layer between the inner and outer layers, and I had my hands full.
The first panel fitted, and glued on:
“Glued” is not the correct term, since it is being adhered with the silicone. The clamps are applying very light pressure, just enough to keep the panel flat against the side.
The silicone is clear, from a tube and applied in beads like this:
The second panel is put in place. The panels have to be done one at a time. It takes a few hours for the silicone to set enough before I can proceed:
The top is fitted and glued on:
The green tape is acting as a clamp to hold the panel in while it dries. The glue I used for the corners, polyurethane construction adhesive, doesn’t need a lot of clamping pressure to create a strong bond.
The outside panels complete.
The rectangular cut out near the bottom on the back is for the connection terminal plate. The plywood I used has a maple veneer, but is not the best grade:
I stained the sides, top and back to the colour I wanted and applied three coats of clear satin polyurethane.
The baffle, getting it’s final polishing before I install the drivers:
The drivers are mounted with foam weather-stripping tape as a gasket. This is self adhesive, about 1/8″ thick and makes a very good gasket material:
The terminal plate gets a gasket too:
The terminal plate is made from 1/8″ thick aluminum plate and I’ve used banana jacks instead of binding post. My cables will have banana plugs on the ends to match this:
And finished, for now…:
After using the speakers for a few weeks, I decided to change the enclosure from sealed to vented, to increase the bass performance. Luckily, the woofer I chose was suitable for a vented alignment, and I calculated the port size and length, based on the woofers specs and the enclosure size. I used 2″ ABS plumbing pipe for the vents and carefully drilled holes through the back, at the bottom. The hole saw I used was very close to the size of the pipe, but slightly smaller. I used a 1-1/2″ drum sander attachment in my hand drill to make the holes the right size.
The ports were then glued in using clear silicone:
Another change came a bit latter, when I decided to open the back of the upper chamber to improve the tone of the midrange. Although this was quite a lot of work, it was definitely worth the time and effort:
Nearly two years after starting these, they were finally finished.
I made a grill from 1/4″ hardboard to cover the opening:
Also notice the different veneer. I was not happy with the grain and colour of the plywood veneer and ordered a sheet of jatoba veneer to cover it.
These last three photos are new, taken as this article was being written.