Hi-fi speaker build
Can’t afford those top-shelf units? Well make your own
I’m probably showing my age but I still believe that much of what passes for audio hi-fi today is vastly inferior to some of that which went before. I may be quite wrong but I can remember truly awesome sound that made you feel that the band was in the same room and the sound was coming from their own monitors. Sound with clarity and definition. While I appreciate that for enormous sums of money you can still get that quality I still believe that it was more common in the ’70s from off-theshelf equipment. To prove the point I wanted to build a set of speakers in a style that was popular at the time.
There are three basic kinds of speaker enclosure. Sealed units, ported units — which have a port somewhere in the cabinet, and transmission line (TL). TL speakers are similar in some respects to ported enclosures; they also have an opening into the cabinet. Reputedly, TL cabinets provide better bass replication across the range without overemphasizing various frequencies.
Choosing my kitset
One of the most accessible TL designs is the Tritrix, designed by Curt Campbell (speakerdesignworks.com), which is is supplied in kit form via Parts Express (parts-express.com) in the US. You can buy either the full kit, including precut panels for the speakers, or simply the drivers, crossovers, and stuffing. I didn’t see the need to transport so much MDF through the post so I elected to buy the parts only. The good thing is that the cost of the parts is now reduced so they can be imported easily under the $400 limit before GST gets charged.
The kit comes with Two Dayton DC28FS-8 shielded silk-dome tweeters,
This type of speaker was more popular in the ’70s but fell out of favour with manufacturers because it was too expensive to build
four Dayton DC130BS-4 classic shielded woofers, two Jantzen 2.50mH 18-gauge air-core inductors, two Jantzen .60mH 18-gauge aircore inductors, two Dayton DMPC-10 10μF 250V polypropylene capacitors, two Dayton DMPC-10 20μF 250V polypropylene capacitors, and two Dayton DNR-2.4 2.4Ω 10W resistors and two bags of Acousta-Stuf to stuff the speakers.
The good news is that they are even cheaper than when I bought them a year ago. The last time I looked the component kit was retailing for only US$149.
Transmission lines
A ‘transmission line’ speaker is effectively a speaker attached to a long tube. The tube length has to be long enough to ensure that the sound out of the tube end is in phase with the sound from the front. The sound at the rear of the speaker cone is 180 degrees out of phase to the sound from the front; however at one-quarter of the resonant frequency wave length the sound will be 90-degrees phase shifted and in phase with the front so the two waves will sum to boost the lower frequencies. The tube is lined or stuffed with acousticabsorbent material to attenuate the higher frequencies that might cause interference problems. A taper in the pipe also helps to attenuate the higher frequencies; the taper is usually in the order of 3:1 or 4:1. This type of speaker was more popular in the ’70s but fell out of favour with manufacturers because it was too expensive to build. It is still popular with audiophiles though.
Speaker design
Designing speakers to get the best performance from the drivers is not a straightforward process as there
A driver with a resonant frequency of 40Hz will not reproduce frequencies of 30Hz well but will perform well at 45 and 50Hz
are so many parameters to take into account. Fortunately due the pioneering work of two eminent audio engineers in Australia, Neville Thiele and his colleague Dr Richard Small, this is made so much simpler. The Thiele/Small parameters (usually abbreviated as T/S) cover all aspects of a driver and make it possible to predict with some certainty how the driver will perform in various cabinets and configurations (see panel).
Resonant frequency
The ‘resonant frequency’ (Fs) of a driver is the frequency at which the driver moves with minimal effort. After this point the response of the speaker starts to roll off. The lower the resonant frequency (Fs), the better it is for a woofer.
‘Compliance’ is the measure of the stiffness of the surround and the spider, this affects the Fs. The stiffer the surround, the less compliant; as the compliance increases — that is, gets less stiff — the Fs goes down.
Sd is the effective area of the cone that actually moves the air. The surface area of the cone together with Xmax directly affects how much sound pressure level (SPL) the speaker can output.
SPL is measured in decibels. It is a measure of the efficiency of a speaker. An
SPL of between 88 and 90dB at 1W/m is good. An SPL quoted at 2.83V/1m takes the impedance of the driver into account. There is no difference at 8Ω but at 4Ω the 2.83V/1m is equal to 90dB measured at 2W/1m which is equivalent to 87dB at 1W/1m.
Tritrix design
The Tritrix uses most of the typical TL criteria. It has a long tube created by the internal baffle. The optimum length of the tube for a TL speaker is a quarter wave length of the resonant frequency. In this case the resonant frequency is 51.8Hz. To convert the frequency into a wave length, the frequency (in hertz) is divided into the speed of sound in metres per second to yield the wave length. This is then divided by four to find the quarter wave length:
The tapered baffle in this design serves to decrease the necessary line length by changing the acoustic impedance. So it’s not exactly apparent that the quarter wave length is translated into the cabinet dimension.
This design has two woofers instead of one in what is called an ‘MTM’ configuration. That simply means midwoofer-tweeter-midwoofer — it refers to the layout of the speakers, which are set out in that manner. MTM is a means of correcting the vertical polar response making the sound spread out horizontally.
The build
The actual build is quite straightforward. I worked out the most effective way to cut the MDF sheet to get the most value out of it and in spite of the inevitable miscut I had material left over. I elected to make one small change to Curt’s design and decided to make the front panel a complete piece rather than that captured by the walls. I wanted to round off the
edges on the front on all sides. Ideally speakers shouldn’t have hard edges, and you might notice on high-end units there are often no sharp edges. In fact the ideal shape for a speaker is a sphere; they’re just a bit problematic to make. Hard edges allow eddy currents to build up and can affect the SPL of the speaker. How much that affects the perceived performance is another matter of course, but I wanted to give this build every chance of blowing my socks off. You don’t have to be as extreme as I was with radiusing the corners — 12mm would be ample. With all the pieces cut I worked out the mitre of the baffles. The angle of the baffle is at 78 degrees to the top so I cut the mitre at 12 degrees.
Router jig
To cut the holes in the front panel I bought a Jasper Pro circle-cutting jig from Parts Express. I contemplated making one but for the time it would take me it was far easier to order the jig. The jig is calibrated in imperial but they come equipped with a formula for working out the metric equivalents. I had changed all the imperial measures into metric but it was easier to stick with the imperial for the front panel cut-outs.
The jig is calibrated in 1∕16-inch increments and it is intended to be used with a ¼-inch bit. You can use other shank sizes but you have to do a bit of calculating to do so. To set up the jig, centre your router using a supplied ¼-inch rod in the router chuck and then find the appropriate mounting holes. It is set up for most common routers. Then it is simply a case of finding the centre point and drilling a 1∕8-inch hole to fit the 1∕8-inch pin supplied. There are two supplied pins — one of 25mm (1 inch) and another of 38mm (1½ inches) — find
Tuning the speakers involves adjusting the stuffing until the sound is more or less how you like it
the appropriate-diameter hole and insert the pin. Fit the pin in the jig to the hole in the timber and cut the circle. It is wise to take things slowly so don’t try to cut the whole depth at once. It is only a ¼-inch bit. For the shoulders I had to cut several passes, reducing the radius by a ¼-inch each time. It also helps to preset the depth you want to cut for the shoulders. I cut the shoulders first, then cut the disc out. The tweeter cut-out is very close to the woofers so it pays to get things very precise. The tweeter is offset from the centre line by ¾-inch (19mm). You will also have to cut a small area out of the tweeter hole to fit its terminals through.
With the cut-outs finished and tidied up I set up a router bit in the spindle moulder to cut the curve on the edges of the faceplate. The curve is 20mm to match the size of the MDF. While the spindle moulder was set up I also rounded over the ends of the back panel and the baffle in the cabinet.
Redrilling
To make it easier in future to remove the woofers — for example, to adjust the stuffing — I used M6 socket-head
machine screws that fit 6mm threaded inserts. However, the holes in the speakers were slightly less than 6mm. I had to drill out each of the holes in the drivers to 6mm. I was then able to use one of the drivers as a template to drill the mounting holes. It’s worth paying attention to this stage because I did drill a couple of the holes too close to the edge and the subsequent 9mm hole to fit the threaded inserts broke through the edge. The inserts are simply hammered in.
Assembly
To assemble the speakers I laid out the pieces on one side to determine that I had the appropriate positions. The end of the baffle is set six inches or 153mm from the sides, including the deflectors that occupy the bottom corners. The deflectors are set at 45 degrees to the centre line of the tube and are intended to deflect the sound around the corner but also to maintain the dimensions of the tube. They align with the centre line of the tube and ensure that the tube itself doesn’t extend into the corners but maintains a more or less consistent cross section — albeit that the first part is a taper. I cut them out of leftover scrap.
With the pieces laid out I marked all around each piece and then set out the screw holes. The holes were drilled from the inside out and the piece turned over and redrilled with a countersink bit. In MDF it’s possible to drive the screw heads well below the surface, but the countersink bit makes a tidier job.
Each piece was then glued and clamped in place and screwed through from the other side. Near the ends make sure you predrill through to the corresponding piece or you will get splits in the MDF. I clamped the pieces until the glue had time to dry. With the clamps removed I also sealed all the joints again with filler. It’s important to keep the box as airtight as possible. Before mounting the last side I attached the front, again drilling through from the rear and then countersinking the holes. It’s necessary to drive the screws well below the surface on the curved parts.
Crossovers
At this point I made the crossovers. All the parts are included in the kit: two 2.5mH coils, two .6mh coil, two 20μF capacitors, two 10μF capacitors, and two 2.4Ω 10W resistors.
The coils should be laid so they cannot crosstalk with each other. Set them in different planes and offset them. I mounted mine on a small piece of ply using hot-melt glue and cable ties for good measure. In hindsight I could have made them a bit tidier but they will be hidden in the depths of the box. To ensure that I could reach them again if I needed to, in the not unlikely event that I had in fact wired them incorrectly, I chose to place them directly behind the drivers. I don’t think it will cause any major disruption to the back wave.
Stuffing
With the crossovers placed in position it only remained to stuff the chambers. The kit comes with two bags of Acousta-Stuf, a product designed for stuffing speakers. The Tritrix is mainly stuffed in the tapered portion immediately behind the drivers down to just below the lower driver. Stuffing the speakers is a bit of an art in TLs. Some other builders have added foam and other dampening products to the top and down the baffle.
In this instance I have opted to see what the original design sounds like first before I resort to more drastic measures. Curt Campbell recommends leaving the last third to quarter of the line clear and altering the density of the stuffing one handful at a time. Tuning the speakers involves adjusting the stuffing until the sound is more or less how you like it. Ideally you should add stuffing until the bass is just beginning to be attenuated and adjust from there. It’s something that I will have to do over time as the speakers get worn in.
Finishing the cases
With the crossovers fitted I closed up the boxes by adding the last side. I now needed to finish them. Although I would have preferred to finish the cases in wood veneer or even woodgrain vinyl at a pinch that wasn’t going to happen as I haven’t been able to find anything that looks remotely usable. So I opted to paint them.
I first filled all the nail holes with builder’s bog and sanded them flat, then refilled the holes again where necessary. I masked up the holes and applied a coat of primer filler. I left the filler to dry and then refilled all the imperfections that stand out when you apply an undercoat, and sanded the boxes and sprayed another coat of filler undercoat. I then applied three coats of top coat to the boxes at the side, rear, and top. I masked off the front and sprayed that black, mainly because I can’t apply a grille cloth.
With the boxes finished I connected up the drivers and fixed them in place. I added the terminal posts to the rear of the baffle and connected the wires inside.
Performance
Time for the big reveal. Would they work as planned; will I be overwhelmed? I set the speakers up together with my existing Wharfedale CR 2-40s and played a track through them, then switched to the new speakers. The difference was immediately obvious.
The TLs are clearer, louder, and the bass is certainly enhanced. Decent bass tracks like Charles Mingus’ classic Mingus Ah Hum album have a totally new feeling through these speakers, and they haven’t even been played in yet. To be fair I’ve never really liked the Wharfedales — they seem to lack something in the mid range, they have no real presence, and they certainly aren’t the top of the line — but my speakers are both better and considerably cheaper. I’m looking forward to how much they improve as they get played in. I can’t hear anything that would make me redo or alter the stuffing but that may change as they wear in. I’m also pleased with the colour. It was selected in haste but it has turned out well and it suits the room … matches the drapes.
Thanks to Curt Campbell for his assistance in preparing this article and permission to reproduce his designs.
Time for the big reveal. Would they work as planned; will I be overwhelmed?