VBS-6.2
Value Buster Series 6" Two Way
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The Design:
A value oriented compact 6" 2-way with a horn loaded compression driver and excellent sound quality and output above 100hz. Best used as surrounds or compact mains in a home theater or where a small high output speaker is needed when subwoofers will be used to fill in the bottom end. Did I mention it costs less then $100 each* to build! *Drivers & Crossover.
This speaker wasn't something I had been planning but rather a spur of the moment build, basically said hey I've got enough MDF scraps to throw that quickly together! The drivers measured well and I was quite pleased with the performance of the finished speaker and thus the econowave-esque VBS-6.2 was born.
Quick Subjective Impressions:
Very clean and clear with little coloration to the sound, it feels like you can just keep cranking the volume and it takes all you throw at it, a 200-400w amp would not be unreasonable with these if listening to fairly dynamic content. Not the widest dispersion in the top octaves but the off axis tracks well so there isn't a big tonal shift off axis, just a drop in output. Definitely needs a subwoofer unless you are listing to podcasts or talk radio. The bass gets better when placed near a wall but a subwoofer is still needed. Overall very pleased with the performance from these budget drivers. The harmonic distortion through the midrange is among the lowest of the speakers I have measured, THD ~0.2% @ 100dB 800Hz - 2kHz!
Driver selection:
I had picked up the new GRS 6PT-8 from Parts Express recently and the small inexpensive pro style woofer seemed fairly decent after running some tests. The Qts I measured was a little higher then spec but otherwise it looked good. Thinking about about how it could be used I naturally gravitated towards pairing it with a small horn and compression driver. I think ideally this driver would be best suited to midrange due to the exceptionally low distortion in that range for it's price. It leaves you wanting in both bass extension and distortion performance down low but scrapes by as the distortion isn't as readily noticed there and subwoofers are there help fill in the missing extension.
Wanting to stick with value oriented parts I've found the JBL/Selenium HM17-25 bi-radial horn is one of the better measuring small horn/waveguides that's easily available and very inexpensive (my tests of that waveguide here). It's a little on the narrow side in terms of it's rated dispersion but that seems to be at the top octaves as it starts around 90 degrees towards the bottom of it's usable range which is fairly decent.
In that round of testing I did with the HM17-25 I found the PRV D230Ti-S showed excellent potential, a smooth response without any major warts from 2-18kHz in that small horn and clean distortion behavior as well.
Enclosure Design:
I wanted to try and keep things compact and with the high Qts of the woofer knew that meant going sealed. When modeling the woofer in anything approaching what I was considering it left the response with a large peak towards the bottom with the QTS of the system over 1. To get around that I made sure to use heavy damping which would help lower the Q of the system but the main solution would be a large capacitor in series with the woofer. When tuned right the capacitor eliminates the peak in the response and shifts the knee downwards increasing bass extension as seen in the response graph to the right.
I used polyfil damping in this enclosure, the loose pile was about twice the size of the enclosure before stuffing it all in there. A light fill with fiberglass, rockwool or recycled denim would also be adequate.
I did flush mount both the woofer and the horn but surface mounting both should not introduce too many issues and is obviously much easier.
Cabinet Cut List:
The dimensions for the cabinet I ended up with are 14" tall x 8" wide x 8" deep using 3/4" baffle and 1/2" for the rest of the box.
Baffle - 14" x 8" x 3/4"
Back - 14" x 8" x 1/2"
Sides - 14" x 6.75" x 1/2"
Top/bottom - 7" x 6.75" x 1/2"
Braces - 7" x 1.25" & 6.75" x 1.25" - Or you can use a single window style brace - 7" x 6.75"
If you want to use 3/4" thick enclosure material all around just increase the depth to 10".
Baffle and back stay the same size,
Side panels increase from 6.75" to 8.5"
Top/bottom now 6.5" x 8.5"
Braces now 6.5" and 8.5" long.
Crossover Design:
I was shooting for a crossover in the in the 2kHz range in knowing that range would likely provide the smoothest directivity hand off between the woofer and horn. In order to achieve a good time alignment/phase overlap and the filter shapes needed for a smooth response I ended up with 4th order filter on the woofer and 3rd order on the tweeter/compression driver. As spoken about above a large series capacitor is placed on the woofer to flatten the in box bass peak and also helps to increase extension. The high frequency network also has a few extra parts that help with general response shaping/padding the of the compression driver.
Parts List:
C1 (1uF) - C2 (3uF) - C3 (1.5uF) - C4 (2.2uF) - C5 (12uF) - C6 (17uF) - C7 (200uF)
L1 (0.40mH) (Alt) - L2 (1.0mH) (Alt) - L3 (2.0mH) - L4 (1.5mH)
R1 (9.1 Ohm) - R2 (4.3 Ohm) - R3 (0.5 Ohm)
Woofer (GRS 6PT-8) - Compression Driver (PRV D230Ti-S) - Horn (JBL HM17-25)
The final crossover ended up just below 2000hz with some very steep acoustic slopes.
Impedance is 8 Ohm nominal, dipping to 6 Ohms at ~250hz but otherwise a very easy load over 8 ohms for the rest of the range.
Measurements taken at 2.83v/2m and scaled up 6dB to approximate 1m level - 1/48th octave smoothing - 14ms gate used.
11-7-21 - measurements now updated to included diffraction adjusted neafield response below 200hz for more accurate low frequencies:
I purposely left area around 3k slightly recessed to maintain a smoother and more linear listening window/power response as that spot fills in as you move slightly off axis which can be seen in the horizontal response chart below.
Horizontal:
Horizontal Normalized:
Horizontal Polar:
Horizontal Polar Normalized:
Vertical Above Axis:
Vertical Below Axis:
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