The Subwoofer DIY Page v1.1 - Projects
Blastoramas V2

last updated: 17 August 2017

 

Introduction
This is basically an update of my Chicken-Run Blastoramas, where the peizo has been replaced with a constant-directivity horn (the Eminence APT150s), driven by a cheap Pyle neodymium compression driver.  The goals of the design remain the same as the ones for the original Blastoramas - 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"). Of course they are not as cheap as the original Blastoramas (very hard to beat a US$4 cost for a piezo tweeter), but the Pyle PDS222 driver cost only $18 or so on Amazon, so... 

The most difficult part of this build was selecting a suitable x-over point.  I obviously wanted to take advantage of the horn's ability to reach a bit further down, but at the same time I didn't want to end up feeding too much power to the cheap driver. I also ran into issues with cancellations happening above or around the x-over frequency if it was set too low, due to the z-offset between the horn-loased tweeter and the woofer. And of course there's the issue of the horn-loaded tweeter not reaching much lower than 2kHz anyway. I eventually settled on a 3kHz acoustic x-over, which ended up being pretty simple to implement. I used the free software tool XSim to design the x-over, and a screen-capture of the x-over is shown below.

The predicted frequency response and impedance curves are given below.  Note that the response does not roll off below 500 Hz as shown - I limited the measurements to 500 Hz and above to ensure that the tweeter did not get damaged during the raw driver measurement phase.

One issue that I ran into with the Pyle PDS222 drivers is that the response isn't very consistent from driver to driver.  I ended up purchasing four and selecting the two with the closest matching impedance and frequency response curves. Luckily they are pretty cheap to begin with! Anyone considering duplicating this design should consider doing the same as well - purchase four units, performe frequency response and impedance curve measurements on them, then choose the two that match the closest.

Construction
As I built the Blastoramas with removable speaker baffles, replacing the piezos with horns was pretty simple - I just removed the old speaker baffles and replaced them with ones designed to hold the horns.

 

Results
Here's a frequency and distortion measurement for one of the completed V2 Blastoramas.  As the response curves suggest, the speaker sounds pretty clean.  I'm quite happy with the results.  There is a small dip in the response around 4kHz, but it it will take considerably more filter elements to address this, so it may not be worthwhile for a cheap PA speaker build. The measurement is not gated, so there's quite a bit of room interaction below 400 Hz or so.

The parts tally is as follows:

-

Qty

Unit Cost

Total Cost

Drivers

Eminence Beta 8A

2

$64.99

$119.98

Eminence APT-150s horn

2

$10.99

$21.98

Pyle PDS-222 neodymium driver

2

$18:85

$37.70

Crossover Components

0.62 mH 18 AWG inductor

2

$8.28

$16.56

12 uF capacitor

2

$5.24

$10.48

4 Ohm resistor

4

$1.38

$5.52

7 Ohm resistor

2

$1.38

$2.76

2.2 uF capacitor

2

$2.79

$5.58

Crossover circuit board

2

$3.70

$7.40

Other

18 mm ply

$0.00

Polyester fiberfill

$0.00

Speakon Terminal blocks

$0.00

Total

$237.96

 

 

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