The Subwoofer DIY Page - Projects
The "Chick-Run Blastoramas"
28 November 2018
The goals of the design were simple - the speakers had to be loud, reasonably clean and flat, decent response down to about 100 Hz or so (the subwoofer would deal with anything below that), relatively small and cheap (cheap being defined as " if any of the drivers blew, it should not cost an arm and a leg to replace them").
So, with these design goals in mind I searched the Parts Express site and came up with the following: - a two-way system based on the Eminence Beta 8A 8" pro audio driver and the Goldwood 5x7 piezo tweeter.
Oh, no, the horror! Did he say piezo tweeter? Well, yes, I did! Piezos have gotten a pretty bad rap, some say for good reason, but just like any other drivers, if you use them under specific conditions that are best suited for them, you can get decent results. For example, they are certainly NOT at their best when connected directly to an amp without a suitable x-over, no matter what the marketing material says. A proper x-over designed for piezo tweeters IS required. Plus they are cheap (like about US$2 - try finding other 92dB tweeters at that price) and finally I wanted a slightly retro look for these speakers and the piezos of course deliver this in spades.
Having selected the drivers, I played around with a few alignments, including some 'quarter-wave resonator' types (e.g. transmission lines), but finally settled on a simple vented alignment with a shelf vent, primarily because I wouldn't have to purchase any extra ports, and the vent would also serve to reinforce the cabinet. Using a shelf vent is also one of the easiest ways to get maximum port area on the front baffle because they tend to be low and wide.
The build process is illustrated in the images below:
Note that the baffle is removable, just in case I want to tweak the design at a later date.
The final box worked out to be 43 cm high by 27 cm wide by 32 cm deep, and the vent is about 21 cm long. The area behind the diagonal cross-brace is lightly stuffed and a piece of wire mesh was inserted above the entrance to the vent to ensure no stuffing material made its way out of the box through that way. Finally, I had a few spare Speakon terminal jacks on-hand (salvaged from some Cerwin-Vega pro audio speakers) so I used those for this build. The Speakon connecters make it very easy for anyone to set up these speakers very quickly.
I settled on a simple x-over for this design - just a 0.22mH coil on the woofer and a 12dB/octave filter consisting of a series 2.2uF capacitor and parallel 0.20mH coil on the tweeter. There's also a shunt 20 ohm resistance across the tweeter. This combination produces a peak in the response that's a good match for dip in response that the Goldwood 2x5 tweeter displays at that point in its response. A schematic of the x-over is given below:
(my thanks to David Henderson for providing this image)
Below is the measured frequency response (down to 1kHz) with speaker mounted on a stand that brings the tweeter to ear-level:
..and here's the impedance curve:
So far, the parts tally is as follows:
Blastoramas - Version 2