The idea for this project began with the drive unit, which was inherited from a friend. It originally came from Radio Spares (part number 433-309), but it hasn't been stocked there for many years now. It's described as a 3 inch 20W 4 ohm full range speaker, but that's all I knew - RS don't even suggest a manufacturer. It's quite well-made, having a large magnet and a treated paper cone with a foam surround and a plastic dust cap to extend the h.f. output. Before proceeding any further, I had to measure the basic TS parameters:
| FS | Qms | Qes | Qts | Vas |
|---|---|---|---|---|
| 93Hz | 2.89 | 0.69 | 0.55 | 1.57 Litres |
Next I measured the volume of several of my spare likely-looking diecast boxes, and spent some time putting these values and the above TS parameters into a calculator, deciding that the best box to use was one with a volume of 1.6 litres. In theory, this would provide a -3dB point of 120Hz, with the system Q ending up at around 0.78. Not bad. While writing up this project some years later, I took the time to confirm these numbers with WinISD, which was useful confirmation of the earlier calculations. I wish I'd known about WinISD back then!
The next problem to consider was treating the box. Try the "knuckle test" with the next diecast box you find and you'll find it rings like a bell! Clearly no good for a loudspeaker enclosure. To stop this, I applied a layer of self-adhesive bitumen based damping material, available at car accessory shops. This transformed the box into an acoustically dead enclosure - I must admit that I was rather surprised at just how effective this was...
Next I mounted the speaker into the box. It took a while to get a good clean circular cut-out, but it worked out well in the end. As you can see from the pictures, the box was pre-finished with a grey nylon coating, so I had to take care not to scratch or damage this. I also mounted a phono socket so that I could connect the speaker to an amplifier.
At this point, the system resonance was 140Hz, not too far from the prediction (which didn't include the volume taken by the driver). I added some stuffing, which lowered the resonance to 132Hz - this works because it causes the driver to "see" a larger volume of air. Next I sealed the box, as I thought I could hear some tiny air leaks, which brought the system resonance down to 128Hz.
Listening to the driver directly from the workshop hi-fi, I was surprised at how good this cheap drive unit sounded. Don't get me wrong; I'm not claiming this was hi-fi or anything, but it was certainly better than you might expect. Happily, there was no hint of extraneous noises coming from the enclosure - it seemed pretty "dead". So the next stage was to experiment with the electronics - for this I just used solderless prototype board and played around until I was happy with the sound.
And here is the final result. From left to right, there's the input connector (gold-plated, so it must sound good ;-), the volume and tone controls, the green power LED and the red clipping LED.
The rear panel is very simple - just an IEC mains inlet and a fuse holder. There is no power switch as the quiescent current consumption is tiny, and the whole bench gets turned on and off by the workshop power switch anyway.
And here is an internal view, showing the damping material and stuffing. Some years later, I removed the foam and replaced it with more of the white stuff, as the foam was starting to rot. Note the gasket around the edge of the box - as mentioned, this proved to be very necessary - the bass quality can be heard to improve as the screws are tightened! There are similar gaskets surrounding the loudspeaker opening, and the IEC mains socket.
This is a closer view of the electronics. The TDA2030 power IC is bolted to a 6mm piece of aluminium, which is held to the front of the box by the potentiometers. The expensive blue pots were the only things I could find with long enough bushes to reach through the aluminium and the front panel - luckily they were "recycled". The two LEDs are connected to the brown wires, and held in place by a small piece of damping material which also stops air leaks.
The next page discusses the circuit design in detail.