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
|
External Links
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