The Subwoofer DIY Page v1.1 - Projects
The "Chick-Run Blastoramas"

last updated: 22 April 2014

Introduction
My eldest daughter had a few friends over a few weeks ago, and after watching in horror as she cranked the volume on my HT system for her "dance music", I thought it was time to put together some simple speakers for just that purpose, ones which could be easily swapped in when "a few friends visiting" turned into a party and the main speakers were at risk because they simply were not loud enough.

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. 

This is the measured frequency response (down to 1kHz) with speaker mounted on a stand that brings the tweeter to ear-level:


The "full" response of the Blastorama extends down to below 100 Hz.

..and here's the impedance curve:



Minimum impedance is 4 ohms at 8kHz.  This should be an easy load for most amplifiers.

So far, the parts tally is as follows:

- Qty Unit Cost Total Cost
Drivers
Eminence Beta 8A 2 $59.99 $119.98
Goldwood 2x5 piezo tweeter 2 $1.96 $3.92
Crossover Components
0.22 mH 18 AWG inductor 2 $4.51 $9.02
0.20 mH 20 AWG inductor 2 $3.41 $6.82
2.2 uF capacitor 2 $1.98 $3.96
20 Ohm 10 Watt resistor 2 $0.98 $1.96
Circuit Board 2 $5.91 $11.82
Other

18 mm ply

$0.00

Polyester fiberfill

$0.00

Speakon Terminal blocks

$0.00

Total

$166.50

 

Build Results
Fb came in at about 60 Hz, which is slightly lower than I expected, but it worked out well in the end as the drivers Fs was around 60 Hz as well. If I had to rebuild these, I'd probably try clipping about a cm off the height and depth but leave the vent as-is (Fb should go up a bit in the smaller enclosure). There's also a small bump in the frequency response around 4 kHz that can be address with a more expensive x-over design, but the rest of the curve seems pretty decent. At the moment though it does sound pretty good, with no sign of that "zinginess" that can be caused by a bad implementation of piezo tweeters. And they can reach 110dB when driven by my 25W Denon PMA-250 amp - not bad at all.
 

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